Ebook Clinical anatomy and physiology for veterinary technicians (3rd edition): Part 2

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Part 2 book "Clinical anatomy and physiology for veterinary technicians" includes content: The respiratory system, digestive system, the urinary system, the reproductive system, nutrients and metabolism, pregnancy, development, and lactation, avian anatomy and physiology, amphibian and reptilian anatomy and physiology.

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The Cardiovascular System 14 Joann Colville OUTLINE INTRODUCTION, 339 ABNORMAL HEART SOUNDS, 351 THE HEART, 340 CARDIAC OUTPUT, 351 Location, 340 BLOOD VESSELS, 352 Size and Shape, 340 Arteries, 352 Coverings of the Heart, 340 Capillaries, 353 Wall of the Heart, 341 Veins, 353 Chambers of the Heart, 341 BLOOD CIRCULATION IN THE FETUS, 354 Valves of the Heart, 343 PULSE, 357 Skeleton of the Heart, 343 Pulse Points, 357 Blood Supply to the Heart, 344 BLOOD PRESSURE, 357 Nerve Supply to the Heart, 344 CARDIOVASCULAR MONITORING, 358 Blood Flow Through the Heart, 345 Electrocardiography, 358 CARDIAC CONDUCTION SYSTEM, 348 Echocardiography, 358 NORMAL HEART SOUNDS, 350 VENIPUNCTURE, 359 LEARNING OBJECTIVES When you have completed this chapter you will be able to: 6. Understand cardiac output and what conditions can 1. Describe the external and internal anatomy of the affect it. heart. 7. Describe the anatomy of arteries, veins, and capillaries 2. Follow the flow of blood through the heart, pulmonary and understand the function of each type of blood circulation, and systemic circulation. vessel. 3. Explain how the heart valves keep blood flowing in the 8. Understand the difference between fetal and newborn proper direction through the heart. circulation. 4. Describe the components of the cardiac conduction 9. Understand the different methods used to evaluate the system and explain how it works to keep the heart cardiovascular system. beating in an organized fashion. 10. Know the common pulse points and venipuncture sites 5. Explain what happens during one cardiac cycle. for common species of animal. VOCABULARY FUNDAMENTALS Afterload ahf-tɘr-lōd Autorhythmic aw-tō-rihth-mihck Aorta ā-ohr-tah Base of heart bās of hahrt Aortic valve ā-ohr-tihck vahlv Bicuspid valve bī-kuhs-pihd vahlv Apex of heart ā-pehcks of hahrt Blood pressure bluhd prehsh-ɘr Arteriole ahr-teer-ē-ōl Bundle of His buhn-duhl of hihs Artery ahr-tɘr-ē Capillary kahp-eh-lahr-ē Atrioventricular node ā-trē-ō-vehn-trihck-ū-lahr nōd Cardiac cycle kahr-dē-ahck sī-kuhl Atrioventricular septum ā-trē-ō-vehn-trihck-ū-lahr Cardiac output kahr-dē-ahck out-puht sehp-tuhm Cardiovascular system kahr-dē-ō-vahsk-ū-lahr sihs-tehm Atrioventricular valve ā-trē-ō-vehn-trihck-ū-lahr vahlv Carotid artery kahr-oht-ihd ahr-tɘr-ē Atrium ā-trē-uhm Cephalic vein seh-fahl-ihck vān Auricle ohr-eh-kuhl Chordae tendonae kohr-dā tehn-duhn-ā Auscultation aws-kuhl-tā-shuhn Coccygeal vein kohck-sehj-ē-ahl vān 338
Coronary artery kohr-ah-nār-ē ahr-tɘr-ē Pericardial fluid peɘr-ih-kahr-dē-ahl floo-ihd Coronary sinus kohr-ah-nār-ē sī-nuhs Pericardial sac peɘr-ih-kahr-dē-ahl sahck Coronary vein kohr-ah-nār-ē vān Pericardial space peɘr-ih-kahr-dē-ahl spās Cusp kuhsp Pericardium peɘr-ih-kahr-dē-uhm Deoxygenated dē-ohck-seh-jeh-nā-tehd Polarization pōl-ɘr-ih-zā-shuhn Depolarization dē-pō-lɘr-ih-zā-shuhn Preload prē-lōd Diastole dī-ahs-stō-lē Pulmonary artery puhl-muh-neɘr-ē ahr-tɘr-ē Diastolic blood pressure dī-ah-stohl-ihck bluhd prehsh-ɘr Pulmonary circulation puhl-muh-neɘr-ē Doppler echocardiography dohp-lɘr sɘr-kyoo-lā-shuhn ehck-ō-kahr-dē-ohg-rah-fē Pulmonary valve puhl-muh-neɘr-ē vahlv Ductus arteriosus duhck-tuhs ahr-teer-ē-ō-suhs Pulse puhls ECHO ehck-ō Pulse wave puhls wāv Echocardiography ehck-ō-kahr-dē-ohg-rah-fē Purkinje fiber system pɘr-kihn-jē fī-bɘr Elastic artery eh-lahs-tihck ahr-tɘr-ē sihs-tehm Electrocardiogram ē-lehck-trō-kahr-dē-ō-grahm QRS complex Q-R-S kohm-plehkx Electrocardiography ē-lehck-trō-kahr-dē-ohg-rah-fē Repolarize rē-pō-lɘr-īz Endocardium ehn-dō-kahr-dē-uhm Saphenous vein sahf-uh-nuhs vān Endothelium ehn-dō-thē-lē-uhm Semilunar valve seh-mē-lū-nɘr vahlv Epicardium ehp-ih-kahr-dē-uhm Serous pericardium seer-uhs peɘr-ih-kahr-dē-uhm Femoral vein fehm-ohr-ahl vān Sinoatrial node sī-nō-ā-trē-ahl nōd Foramen ovale fohr-ā-mehn ō-vah-lē Sphygmomanometer sfihg-mō-muh-nohm-uht-ɘr Heart rate hahrt rāt Starling’s law stahr-lihngz lahw Interatrial septum ihn-tɘr-ā-trē-uhl sehp-tuhm Stroke volume strōk vohl-ūm Interventricular groove ihn-tɘr-vehn-trihck-ū-lɘr groov Systemic circulation sihs-tehm-ihck sɘr-kyoo-lā-shuhn Interventricular septum ihn-tɘr-vehn-trihck-ū-lɘr Systole sihs-tuh-lē sehp-tuhm Systolic blood pressure sih-stohl-ihck bluhd prehsh-ɘr Jugular vein juhg-ū-lɘr vān Systolic discharge sih-stohl-ihck dihs-chahrj Mean arterial pressure mēn ahr-teer-ē-ahl prehsh-ɘr T wave T wāv Mediastinum mē-dē-ah-stīn-uhm Tricuspid valve trī-kuhs-pihd valv Mitral valve mī-trah vahlv Umbilical artery uhm-bihl-ihck-ahl ahr-tɘr-ē Murmur mɘr-mɘr Umbilical vein uhm-bihl-ihck-ahl vān Muscular artery muhs-kyoo-lɘr ahr-tɘr-ē Valvular insufficiency vahl-vyoo-lɘr ihn-suh-fihsh-ehn-sē Myocardium mī-ō-kahr-dē-uhm Valvular stenosis vahl-vyoo-lɘr steh-nō-sihs Oscillometric aws-uh-lō-meh-trihck Vein vān Oxygenated ohck-suh-jehn-ā-tehd Vena cava vē-nah kā-vah P wave P wāv Ventricle vehn-trihck-ehl Papillary muscle pah-pihl-leɘr-ē muhs-uhl Venule vehn-yool Parietal layer of the serous pericardium pah-rī-eh-tahl Visceral layer of the serous pericardium vih-sɘr-ahl lā-ɘr lā-ɘr of the seer-uhs peɘr-ih-kahr-dē-uhm of the seer-uhs peɘr-ih-kahr-dē-uhm INTRODUCTION Imagine the world blood lives in. Think of it as a water circulatory system). It is responsible for the movement of world. The plasma is the fluid all the elements live and blood and everything it carries throughout the animal’s swim in. The outer limits of this world are the walls of the body. It is made up of the heart, all the blood vessels, and blood vessels where blood resides. The erythrocytes the blood itself. Normally there are no external openings (red blood cells) are the planes, trains, and automobiles to the cardiovascular system so it is considered a closed that move oxygen and other substances from place to system. Electrolytes, waste materials, nutrients, hormones, place. The leukocytes (white blood cells) are the military antibodies, and drugs are carried by blood contained in vehicles ready for battle at a moment’s notice. The throm- the structures of the cardiovascular system to every living bocytes (platelets) are the EMTs, the first responders to the cell in the animal’s body. scene of a vessel wall injury. Blood is continuously flowing around the animal’s body So far this is a static world; nothing is moving. Enter the and through the heart in a circuit propelled by the beating heart. Each time the heart beats blood is propelled through (pumping) heart. Arteries carry blood away from the blood vessels throughout the animal’s body. This is heart; veins carry blood toward the heart; and capillar- the world of the cardiovascular system (i.e., the ies form the transition between arteries and veins. 339
The cardiovascular system is divided into two parts from throughout the animal’s body (carried in veins) that all blood cycles through in a “figure 8” configuration: and pumps it into the lungs where it becomes oxygenated. the pulmonary (lung) circulation and the systemic The left side of the heart controls the systemic circulation. (body) circulation. One side of the heart controls It receives oxygenated blood from the lungs and each part. The right side of the heart controls the pulmo- pumps it out to the rest of the animal’s body. More on nary circulation. It receives deoxygenated blood this later. THE HEART COVERINGS OF THE HEART The heart is contained in a fibrous sac called the pericar- LOCATION dium. The pericardium is divided into two parts: the fibrous The heart is located in the middle of the thoracic cavity in sac called the pericardial sac and the serous pericardium. the mediastinum, the space between the two lungs (Figure The pericardial sac is a little loose so the heart can beat inside 14-1). The mediastinum in bounded by the thoracic inlet it but it is not elastic so it cannot stretch if the heart becomes crainially, the diaphragm caudally, the sternum ventrally, abnormally enlarged. and the spinal column dorsally. In addition to the heart, the mediastinum also contains blood vessels, the thoracic portion of the trachea, the esophagus, the thymus in young animals, lymph nodes, and nerves. Generally speaking, when viewing a standing animal the heart is located between the elbows (Figure 14-2). SIZE AND SHAPE The heart is sort of heart-shaped (Figure 14-3). It has a rounded cranial end called the base of the heart. The more pointed caudal end is the apex of the heart. This is just the opposite of what we normally think of base (wide bottom) and apex (narrow top) but if you go strictly by shape and forget orienta- Lung tion the wide end is the base and the narrow end is the apex. The heart doesn’t sit straight along the median plane in Heart an animal. The base is shifted to the right and faces more Elbow dorsally. The apex is shifted to the left and sits more ventrally (Figure 14-4). Left side L Right lung Left lung Aorta Esophagus Trachea bifurcation Heart Lung Heart Elbow Right side FIGURE 14-1 Transverse section through the thorax at the level of the heart showing structures in the mediastinum. (Redrawn from Dyce KM, FIGURE 14-2 Location of the heart between the elbows in a standing Sack WO, Wenseng CJG: Textbook of veterinary anatomy, ed 4, animal. (Redrawn from Dyce KM, Sack WO, Wenseng CJG: Textbook St Louis, 2010, Saunders.) of veterinary anatomy, ed 4, St Louis, 2010, Saunders.) 340
CH APTER 14 The Cardiovascular System 341 R L Base Base Spinal column representing midline Heart Apex A Apex FIGURE 14-3 Shape of the heart showing the base and the apex. The serous pericardium consists of two membranes. A se smooth, moist serous membrane called the parietal layer of Ba the serous pericardium lines the pericardial sac, and the Heart visceral layer of the serous pericardium lies directly on the surface of the heart. The pericardial space is the area between ex the two serous membranes. It is filled with pericardial fluid Ap that lubricates the two membranes and prevents friction as they rub together during contractions and relaxations of the R heart muscle (Figure 14-5). B WALL OF THE HEART FIGURE 14-4 Radiograph showing the position of the heart in the thoracic cavity. A, ventral view; B, lateral view. (A, From Evans H, The wall of the heart has three layers (Figure 14-6). The Lahunta A: Miller’s Anatomy of the Dog, ed 4, St Louis, Saunders. middle and thickest layer is the muscular layer called the B, From Brown M, Brown: Lavin’s Radiography for Veterinary Techni- myocardium because it is made up of cardiac muscle. cians, ed 5, St Louis, 2014, Saunders.) Remember that cardiac muscle fibers are joined side-to-side by multiple branches and end-to-end by intercalated discs. the epicardium is the visceral layer of the serous pericar- These two anatomic characteristics mean that the myocar- dium. Two names; same membrane. dium is made up of continuous muscle sheets that wrap The endocardium is the membrane that lies on the inter- around the chambers of the heart. These muscle sheets make nal surface of the myocardium. It is composed of thin, flat a greater force of contraction possible. simple squamous epithelium and forms the lining of the Two other advantageous characteristics of cardiac muscle heart chambers. The endocardium is continuous with the are that it is autorhythmic and it doesn’t fatigue. This endothelium that lines blood vessels. The endocardium also means that without outside stimulus it can start beating covers the valves that separate the chambers of the heart. (contracting and relaxing) in a steady rhythm before an The inside surface of the myocardium is not smooth. It animal is born and continue beating through birth, adoles- forms ridges and nipplelike projections called papillary cence, adulthood, middle age, and old age without taking a muscles that are covered by the endocardium. break. When the heart stops beating and it isn’t restarted the animal dies. CHAMBERS OF THE HEART The epicardium is the outermost layer of the heart There are four chambers or cavities in the heart: two atria wall. It is a membrane that lies on the external surface (singular: atrium) that receive blood into the heart, and two of the myocardium. Sound familiar? Another name for ventricles that pump blood out of the heart (Figure 14-7). The
342 C H A P T E R 14 The Cardiovascular System 1 3 2 Head of animal B A L R 5 FIGURE 14-5 The heart enclosed in the pericardium. A, The wall of 4 the right ventricle visible through the pericardium; B, the wall of the left 6 ventricle visible through the pericardium. The heart is sitting in the area of the mediastinum. (Modified from Clayton HM, Flood P, Rosenstein D: Clinical anatomy of the horse, London 2005, Mosby Ltd.) Serous pericardium OUTSIDE (visceral layer; epicardium) OF HEART FIGURE 14-7 Section of the heart exposing the four chambers. 1, Right atrium; 2, interatrial septum; 3, left atrium; 4, right ventricle; 5, interventricular septum; 6, left ventricle. (Modified from Dyce KM, Sack Pericardial WO, Wenseng CJG: Textbook of veterinary anatomy, ed 4, St Louis, INSIDE sac OF 2010, Saunders.) HEART Serous pericardium blood to the heart. When the atria have filled with blood (parietal layer) their walls (composed of myocardium) contract and force Pericardial space Papillary blood through one-way valves into the ventricles. Myocardium muscles The atria are identified on the outside of the heart by their Endocardium auricles. These are blind pouches that come off the main FIGURE 14-6 Section of the wall of the heart showing the pericardial part of the atria and look like earflaps. sac, the parietal and visceral layers of the serous pericardium, the peri- That’s Interesting: Auricle means “ear flap” or “ear.” In cardial space, the myocardium, and the endocardium. (Modified from humans the auricle also refers to the external ear flap. Since Huether SE, McCance KL: Understanding pathophysiology, ed 4, the atrial auricle looks somewhat like an earflap it seems to St Louis, 2007, Mosby.) make sense to name it accordingly. The auricle is part of the atrium but is not the entire TEST YOURSELF 14-1 atrium so the two terms cannot be used interchangeably. 1. Which type of blood vessel carries blood away from The myocardium of an atrium is not very thick because it the heart? Toward the heart? only has to contract with enough force to move blood into a 2. What are the two parts of the cardiovascular system? ventricle. The same is not true for the walls of the ventricles. Which part carries blood to and from the left rear leg of a pony? VENTRICLES 3. List three structures found in the mediastinum. The left and right ventricles are separated by the interventricu- 4. Which is located more caudally in a standing pig, the lar septum, which is a continuation of the interatrial septum. apex or the base of the heart? Together they form the atrioventricular septum. The area of 5. What is the difference between the endocardium and the pericardium? the interventricular septum is visible on the outside of the heart as the interventricular groove (Figure 14-8). The groove con- tains coronary (heart) blood vessels and is frequently filled with two atria sit on top of the two ventricles and their walls form part fat. When the ventricles have received blood from the atria the of the base of the heart. The two ventricles sit below the two atria myocardium of the ventricular walls contract and force blood and the wall of the left ventricle forms the apex of the heart. through one-way valves into arteries. The right ventricle pumps blood to the pulmonary circulation through the pulmonary ATRIA artery; the left ventricle pumps blood into the systemic circula- The left atrium and the right atrium are separated by the tion through the aorta. Since blood from the right ventricle interatrial septum that is a continuation of the myocar- doesn’t have very far to go the right ventricular wall is thinner dium. The atria receive blood from large veins that carry than the left ventricular wall. The left ventricular wall has the
CH APTER 14 The Cardiovascular System 343 most work to do pumping blood to the rest of the animal’s body came from. In order for the heart to function properly blood so it has a thicker wall that will contract with greater force. In must flow through it in one direction only. fact the left ventricular wall is so thick that it pushes the right The atrioventricular valves (AV valves) are located ventricular wall to the right. Consequently the left ventricular between the atria and the ventricles. The right AV valve con- wall makes up the apex of the heart (Figure 14-9). sists of three flaps or cusps of endothelium and is called the tricuspid valve. It opens when the pressure from the amount VALVES OF THE HEART of blood in the right atrium forces it open and allows blood There are four one-way valves that control blood flow through to flow into the right ventricle. When the pressure from the the heart (Figure 14-10). Two of the valves are located between blood in the right ventricle exceeds the pressure of blood in the right and left atria and their respective ventricles. The the right atrium the tricuspid valve is forced to snap shut. The other two are located between the right and left ventricles and valve is prevented from opening backward into the atrium by the arteries they eject blood into. The valves close at specific collagen fiber cords that are attached to the edge of each cusp times to prevent backflow of blood into the chamber it just and to papillary muscles in the wall of the right ventricle. These cords are called the chordae tendonae (Figure 14-11). The left AV valve has only two cusps and is called the bicuspid valve. It is also known as the mitral valve because of its resem- blance to the headgear, called the miter, worn by Roman Catho- lic bishops. This valve also has attached chordae tendonae. The two valves that control blood flow out of the ven- tricles and into arteries are the semilunar valves, so named because they have three cusps, each of which resembles a crescent moon. The right semilunar valve is the pulmonary valve because blood from the right ventricle flows through it into the pulmonary circulation. The left semilunar valve is the aortic valve because blood from the left ventricle flows through it into the aorta, which is the major artery that is the beginning of systemic circulation. A SKELETON OF THE HEART The skeleton of the heart is located between the atria and the ventricles (Figure 14-12). It is made up of four dense fibrous connective tissue rings and has four primary functions: • It separates the atria and ventricles. • It anchors the heart valves. FIGURE 14-8 Left view of the heart showing the interventricular • It provides a point of attachment for the myocardium. groove filled with fat. (Modified from Clayton HM, Flood P, Rosenstein • It provides some electrical insulation between the atria D: Clinical anatomy of the horse, London 2005, Mosby Ltd.) and the ventricles. Vena cava Pulmonary trunk Auricle of left atrium Aorta Auricle of B right atrium Left ventricle Cr A R L Apex Right ventricle Ca FIGURE 14-9 Ventral view of the heart showing A, the wall of the left ventricle forming the apex of the heart and B, the interventricular groove. (Modified from Brown M, Brown L: Lavin’s Radiography for veterinary technicians, ed 5, St Louis, 2014, Saunders.)
344 C H A P T E R 14 The Cardiovascular System Aortic (semilunar) valve Pulmonary (semilunar) valve Left atrioventricular (mitral) valve Chordae tendonae Right atrioventricular (tricuspid) valve Chordae tendonae A Right AV Right AV (tricuspid) Left AV (tricuspid) Left AV valve (mitral) valve valve (mitral) valve Aortic SL valve B Pulmonary SL valve FIGURE 14-10 Valves in the heart. A, Longitudinal slice through the heart. B, Cross section of the heart. AV, Atrioventricular; SL, semilunar. (B, From Thibodeau D: Structure and function of the body, ed 14, St Louis, 2012, Mosby.) BLOOD SUPPLY TO THE HEART blood to the circulation the coronary veins join together near Like any living tissue, the cells of the heart need nourishment the right atrium and form a channel called the coronary and oxygen brought to them and waste materials carried sinus that drains directly into the right atrium. away. Coronary arteries and coronary veins are responsible for these functions (Figure 14-13). Coronary arteries branch NERVE SUPPLY TO THE HEART off the aorta just past the aortic valve (left semilunar valve). Cardiac muscle is autorythmic and can create its own con- They continue to branch around the heart until they com- tractions and relaxations through its internal conduction pletely encircle it. The left ventricle gets the largest blood system. But there are times when the heart needs to beat supply because it has the most work to do pumping blood faster to increase the oxygen supply to certain tissues. For through the systemic circulation. After the blood from the example, an animal being exercised needs an increased coronary arteries has passed through the capillaries in oxygen supply to its muscles. To help accommodate this the myocardium it enters the coronary veins. To return the increased oxygen demand the heart receives some external
CH APTER 14 The Cardiovascular System 345 Left auricle Right auricle Right ventricle Right atrioventricular valve Chordae tendonae Papillary muscles Left ventricle FIGURE 14-11 Interior view of the right ventricle showing the chordae tendonae of the right AV (tricuspid) valve. Only two cusps of the valve are visible. (Modified from Evans H, Lahunta A: Miller’s anatomy of the dog, ed 4, St Louis, 2013, Saunders.) Root of pulmonary trunk CLINICAL APPLICATION Root of aorta Hardware Disease In cattle, the reticulum (the most cranial stomach compart- ment) rests directly behind the heart, and the two organs are Ring for right separated by the muscular diaphragm. Cattle are not very atrio ventricular valve selective when eating, and it is not uncommon for them to ingest wires, nails, and other foreign metallic objects along with their feed. These bits of “hardware” are ingested into the rumen, from which digestive contractions move them Ring for left forward into the reticulum. Continued ruminal contrac- atrioventricular valve tions, particularly when combined with factors that increase abdominal pressure, such as pregnancy and parturition, may FIGURE 14-12 Skeleton of the heart. (Modified from Evans H, Lahunta A: Miller’s anatomy of the dog, ed 4, St Louis, 2013, Saunders.) push pieces of wire through the cranial wall of the reticulum. Puncture of the reticulum wall by a foreign object often motor stimulation. The nerve fibers enter the heart and ter- results in traumatic reticuloperitonitis, also called hard- ware disease, which is an inflammation and infection of the minate primarily in the right atrium near the area of cardiac reticulum and abdominal cavity. More severe disease occurs muscle cells that is the control center for the cardiac conduc- when a wire is pushed even farther cranially, through the tion system. Some nerve fibers will stimulate an increased diaphragm and into the pericardium. This can result in heart rate and others will stimulate a decreased heart rate. septic pericarditis, which is an infection of the pericardium The nervous system is discussed in detail in Chapter 9. The that usually progresses to heart failure and death. nerve supply to the heart serves a purpose, but is not essen- Hardware disease can be prevented by the oral adminis- tial. Transplanted hearts lose their nerve supply and usually tration of a magnet about the size of a 5 ml blood tube. The continue to function well. magnet stays in the rumen or reticulum, usually for the rest of the animal’s life. Wire or other metal objects ingested by the animal stick to the magnet instead of being pushed crani- TEST YOURSELF 14-2 ally through the wall of the reticulum and beyond. 1. Which sits closer to the base of the heart, the left atrium or the right ventricle? 2. What is the name of the structure that is a continuation of the myocardium that forms a wall between the two BLOOD FLOW THROUGH THE HEART atria? The two ventricles? You’ve read about heart chambers, one-way valves, and uni- 3. Why is the wall of the right ventricle thinner than the directional blood flow so now it’s time to follow the blood wall of the left ventricle? as it moves through the heart. The entire purpose of the 4. What is another name for each of these valves: right heart is to receive deoxygenated blood from the systemic AV valve, semilunar valve in the right ventricle, left AV circulation (right atrium), send it out to the pulmonary valve, semilunar valve in the left ventricle? 5. What is the function of the chordae tendonae? circulation for oxygenation (right ventricle), receive the freshly oxygenated blood back from the pulmonary
346 C H A P T E R 14 The Cardiovascular System (Ru and Ca) (Eq and Su) A B FIGURE 14-13 Coronary circulation of the heart viewed from the right. A, Ruminants and carnivores, B, horse and pig. Ru, Ruminants; Ca, cat; Eq, equine; Su, pig. Red blood vessels, coronary arteries; blue blood vessels, coronary veins. (Modified from Dyce KM, Sack WO, Wenseng CJG: Textbook of veterinary anatomy, ed 4, St Louis, 2010, Saunders.) heart sits in the middle pumping blood through both loops CLINICAL APPLICATION (Figure 14-15). Pericardial Effusion and Cardiac Tamponade We’ll arbitrarily start our journey through the heart in the The heart is able to expand and contract in the chest thanks vena cava, the large vein that brings deoxygenated blood to the layer of fluid that provides lubrication between layers from the systemic circulation (but not the pulmonary circu- of the serous pericardium. Normally, only a small amount lation) to the heart. The vena cava enters the right atrium of of fluid is contained within the pericardial sac. A number of the heart. The deoxygenated blood passes from the right conditions such as infection, inflammation, or hemorrhage atrium through the tricuspid valve into the right ventricle. may cause excess fluid to accumulate in the pericardial sac. At this point the pulmonary valve in the right ventricle is This condition is called pericardial effusion. Sometimes peri- closed. When the right ventricle is full the tricuspid valve is cardial effusion is idiopathic, meaning it may occur sponta- forced closed and the pulmonary valve is forced open. The neously with no known cause. right ventricle contracts and the deoxygenated blood The outer layer of the heart, called the fibrous pericar- dium, is not elastic, so when the pericardial space is overfilled leaves the right ventricle through the pulmonary valve and with fluid, the heart becomes unable to expand normally enters the pulmonary circulation via the pulmonary artery. between contractions. This condition, called cardiac tam- After the deoxygenated blood has been oxygenated in the ponade, leads to less complete cardiac filling, decreased pulmonary circulation it comes back to the heart via the stroke volume, and decreased cardiac output. Pericardial pulmonary vein that empties into the left atrium. The mitral effusion, with or without cardiac tamponade, may be treated valve opens and the blood from the left atrium enters the left by inserting a needle into the pericardial sac (usually through ventricle. When the ventricle is full the mitral valve is forced the chest wall) and withdrawing the excess fluid. closed and the aortic valve is forced open. The left ventricle contracts and oxygenated blood leaves through the aortic valve and enters the systemic circulation via the aorta, the circulation (left atrium), and put it into systemic circulation largest artery in the body. Once in the systemic circulation (left ventricle) (Figure 14-14). Note that each chamber the blood is distributed throughout the animal’s body has its own specific function. Think of blood flow as a through arteries that become progressively smaller until they figure 8. One loop represents the pulmonary circulation, transition to become capillaries. At the tissue capillary level, the other loop represents the systemic circulation, and the blood gives up its oxygen in exchange for carbon dioxide and
CHAPTER 14 The Cardiovascular System 347 right atrium right ventricle y od to the lungs deoxygenated for oxygenation eb blood (arrow) to th from the body left atrium left ventricle from the lungs oxygenated after oxygenation blood (arrow) myocardium from the body FIGURE 14-14 Cardiac blood flow. Arrows indicate the direction of blood flow. (From Colvile J, Oien S: Clinical veterinary language, St Louis, 2014, Mosby.) Pulmonary Systemic circulation circulation Systemic veins Pulmonary veins Heart Pulmonary Right atrium Left atrium Systemic A capillaries capillaries Tricuspid valve Mitral valve Pulmonary Systemic arteries arteries Pulmonary Heart Systemic Right ventricle Left ventricle arteries arteries Pulmonary Systemic Pulmonary valve Aortic valve capillaries Heart capillaries Pulmonary Systemic C B veins veins FIGURE 14-15 Schematics of the circulation. A, Pulmonary and systemic circulation, B, vessels of the pulmonary and systemic circulation, C, blood flow through the heart plus the pulmonary and systemic circulation. other waste materials in tissue fluid. From this tissue level valves open and close at the same time; the right and left the capillaries converge to form small veins that become ventricles contract at the same time, and so forth. When the progressively larger as the deoxygenated blood is carried ventricles are full the pressure from the blood in them forces back to the heart. The veins eventually join the vena cava and the AV valves closed and the semilunar valves open. When the deoxygenated blood flows into the right atrium. The the pressure in the pulmonary artery (right side) and aorta cycle is complete. (left side) exceeds the pressure in the right and left ventricles To make sure blood is entering and leaving the heart in the semilunar valves are forced close. At this point the pres- an orderly, coordinated fashion, the heart has to have rhyth- sure in the atria exceeds the pressure in the ventricles so the mic, coordinated contractions. The actual rhythm pattern is AV valves open and ventricular filling begins again. To keep simple. Even though the two sides of the heart pump blood blood flowing, the ventricles are emptying while the atria are to different areas they do so at the same time: deoxygenated filling and the atria are emptying while the ventricles are blood enters the right atrium and oxygenated blood enters filling. The valves closing produce sounds that can be heard the left atrium at the same time; the tricuspid and mitral with a stethoscope: the heartbeat.
348 C H A P T E R 14 The Cardiovascular System Outside cell TEST YOURSELF 14-3 Na+ Na+ Ca2+ 1. List, in order, the structures an erythrocyte will pass through (including valves) to move in a complete circuit from the left ventricle back to the left ventricle. K+ Cardiac cell CLINICAL APPLICATION Depolarization Defibrillation If you’ve watched a few hospital shows on television, chances Outside cell Na+ K+ are you’ve seen someone grab a couple of paddles, yell “clear” or something like it, place the paddles on the patient’s chest, and “shock” them to restart the heart. In real life, this process is called defibrillation, and it has to do with the electrical conduction system of the heart. Na+ Ca2+ Sometimes a diseased heart will develop one or more Cardiac cell ectopic pacemakers. The word ectopic means out of place, and Repolarization an ectopic pacemaker is located outside the heart’s normal FIGURE 14-16 Depolarization and repolarization of a cardiac pacemaker, which is the sinoatrial (SA) node in the right muscle cell. (From Wanamaker BP: Applied pharmacology for veterinary atrium. Despite the presence of any ectopic pacemakers, the techinicians, ed 4, St Louis, 2008, Saunders.) SA node continues to fire, meaning that the cardiac muscle cells receive electric currents from more than one direction. The synchronized contraction of the heart, which begins in the atria and flows through the ventricles, is lost. If there is First, sodium (Na+) and calcium (Ca2+) ions move through enough ectopic pacemaker activity, a condition called ven- channels in the cell membrane from the exterior to the inte- tricular fibrillation may develop, in which heart muscle cells rior of the cell. This reverses the polarity of the cell mem- in different areas contract independently of one another. In brane. Second, potassium (K+) ions move through channels ventricular fibrillation, all coordinated pumping activity of in the cell membrane from the interior of the cell to the the ventricles is lost. exterior. This restores the original polarity, but now the Na+, The defibrillator sends a large electric current of short duration through the heart, with the objective of repolariz- Ca2+, and K+ ions are on the wrong sides of the cell mem- ing all of the cells at the same time. If defibrillation is brane. During repolarization the ions are pumped back to successful, the SA node and the heart’s normal conduction their original locations and the cell is ready to depolarize system will resume control over depolarization of the heart again. In this way, the heart automatically keeps going after the cells have been “reset” by defibrillation. through the cardiac cycle of cellular depolarization (contrac- tion) and repolarization (relaxation). The impulse generated by the SA node travels from the base of the heart to the apex and back to the base of the heart. The impulse passes through the muscle fibers of the wall of CARDIAC CONDUCTION SYSTEM the atria. When the impulse passes through these cardiac Even though cardiac muscle is autorhythmic it does need muscles, the muscles contract. Remember, unlike other some sort of control to keep it beating at a steady rate. Areas muscle fibers, cardiac muscle can transmit an impulse from of cardiac muscle cells have developed that initiate an one muscle cell to another, so impulses and the resulting impulse that moves through the myocardium, stimulating muscle contractions spread across the atria in a wavelike first the muscle cells in the walls of the atria to contract, then fashion. moving through the walls of the ventricles to cause them to The structures that make up the primary cardiac conduc- contract. After each contraction the muscle cells relax. One tion system are the SA node, the atrioventricular node (AV cycle of atrial and ventricular contraction and relaxation is node), the bundle of His, and the Purkinje fiber system a cardiac cycle. The cardiac cycle produces one heartbeat. (Figure 14-17). The impulse for each heartbeat comes from the sinoatrial As stated above, after the impulse is initiated in the SA node (SA node) located in the wall of the right atrium. It is node in the right atrium, it spreads in a wave across both the pacemaker of the heart. The SA node is an area of cardiac atria, causing them to contract and push blood through the muscle cells that automatically generate the impulses that AV valves into the ventricles, which are still relaxed. The trigger each heartbeat. impulse generated by the SA node travels through the walls At rest, a cardiac muscle cell is polarized. Sodium and of the atria to the atrioventricular node located in the atrio- calcium ions are located on the outer membrane of the cell ventricular septum that separates the left and right sides of and potassium ions are located inside the cell. In order for the heart. The only route of conduction the impulse can take the cell to contract it must depolarize. During cardiac muscle from the atria to the ventricles is through the AV node. When cell depolarization (Figure 14-16) two sets of events occur. the impulse from the SA node reaches the AV node it delays
CH APTER 14 The Cardiovascular System 349 Atrial Excitation Beginning Complete AV node SA node A V A B Ventrical Excitation Beginning Complete Common bundle of His Apex D C Purkinje fibers FIGURE 14-17 Cardiac conduction system. A, The SA node has initiated an impulse; B, the impulse has spread through the walls of both atria and reached the AV node (atrial depolarization); C, the impulse has traveled down the interventricular septum through the bundle of His, and the Purkinje fiber system has initiated ventricular contraction beginning at the apex, D, the impulse has spread through the walls of both ventricles (ventricular depolarization). (From Cunningham JG: Textbook of veterinary physiology, ed 4, St Louis, 2007, Saunders.) for a fraction of a second. The delay permits the atria to BOX 14-1 Try This At Home complete their contraction before ventricular contraction begins. If atrial and ventricular contraction took place at the As mentioned in the text the two atria contract while the two same time, the pressure in the contracting ventricles would ventricles relax and vice versa. To get a better sense of how be so high that the weaker, thin-muscled atria could not push this works make a fist with both hands. Put your right fist blood into the ventricles. representing the atria on top of your left fist representing the After the delay at the AV node, the impulse resumes its two ventricles. First squeeze your right fist in a rolling move- ment starting with your thumb and forefinger and going down. speedy journey, this time through the bundle of His and the This represents the systolic wave of the atria, which begins at Purkinje fiber system. The fibers of the bundle of His travel the cardiac base and moves to the atrioventricular septum. down the interventricular septum to the apex of the heart. Now as you relax your right fist (atrial diastole) squeeze your Here the Purkinje fiber system picks up the impulses, makes left fist (ventricular systole) in a rolling movement starting with a U-turn, and carries them from the bundle of His up into your little finger and moving up. The systolic wave through the right and left ventricular myocardium. Because the the ventricle starts at the cardiac apex with the Purkinje fiber impulse is delivered to the apex more quickly than it can system (i.e., your pinkie finger), and moves toward the base spread from cell to cell in the ventricular myocardium, the (i.e., your thumb). Relax your left fist and squeeze your right ventricles actually contract starting from the apex and fist again. Each time you go through one squeeze and relax- moving toward the base of the heart. This apex-to-base ation of each fist you have completed a cardiac cycle. Once direction of ventricular contraction facilitates ejection of you have mastered the squeezing and relaxing sequence, try and set up a rhythm of contractions of one “cardiac cycle” blood into the aorta and pulmonary arteries, which are per second. This represents a heart rate of 60 beats per located at the base of the heart (Box 14-1). minute (bpm). Just as the atria begin their contractions while the ven- tricles are relaxed, the atria also enter their relaxation phase while the ventricles are contracting. When the ventricles are contracting but the atria are relaxed, the pressure in the ventricles is much higher than the pressure in the atria, so
350 C H A P T E R 14 The Cardiovascular System the AV valves are forced shut. With the AV valves closed, the systole) while the atria are relaxed (atrial diastole). This relaxed and expanding atria can fill with blood again. At allows the ventricles to eject blood from the heart and for about the time the atria are becoming completely full, con- the atria to fill with blood again. traction comes to an end in the ventricles, and they begin to relax. This results in the pressure in the ventricles dropping NORMAL HEART SOUNDS lower than the pressure in the arteries they supply, so the aortic and pulmonary valves are forced shut. Pressure in If you’ve had a chance to listen to a heart with a stethoscope the ventricles also falls below the pressure in the full atria, so you know it makes sounds as it beats. The heart valves snap- the AV valves are pushed open. ping shut produce these sounds. One cardiac cycle produces After the AV valves open, the ventricles again fill with two distinct heart sounds (Box 14-2). The sounds are often blood from the atria. The negative pressure caused by ven- described as “lub” and “dub.” The first sound, “lub,” is pro- tricular relaxation pulling blood in from the atria generates duced when the tricuspid and mitral valves snap shut after most ventricular filling. Just as the pressure in the atria atrial systole. The pressure in the ventricles is greater than and the ventricles begins to reach equilibrium, the SA the pressure in the atria at this point so the AV valves are node “fires,” causing the atria to contract and forcibly push forced closed. Remember these are one-way valves so blood the rest of their blood into the ventricles, and the cardiac cannot backflow into the atria. cycle begins again. The second heart sound, “dub,” is produced after ven- There are two clinical terms associated with contraction tricular systole when the pulmonary and aortic valves and relaxation of cardiac muscles. Systole is the myocardium (semilunar valves) snap shut. The pressure in the pulmo- contracting, causing a chamber to empty itself of blood. This nary artery and aorta exceeds that in the ventricles at this is the working phase of the cardiac cycle when the cardiac point so the valves are forced closed. These are also muscle cells are depolarized. Diastole is the myocardium one-way valves so blood cannot backflow into the ventri- relaxing and repolarizing after a contraction, allowing the cles. Most heart sounds are best heard on the left side chambers to fill with blood again. This is the resting phase of the standing animal by placing the stethoscope on of the cardiac cycle. the chest wall at about the point of the elbow. From this Each chamber goes through systole and diastole, but not point, by moving slightly forward and backward, the all at the same time. In one cardiac cycle first the atria con- pulmonary, aortic, and mitral valve sounds are heard. tract (atrial systole) while the ventricles are relaxed (ven- The tricuspid valve sound is best heard on the left side of tricular diastole). This allows blood to flow from the atria the standing animal at approximately the same level into the ventricles. Next the ventricles contract (ventricular (Figure 14-18). BOX 14-2 The Heart Sounds of the Cardiac Cycle Ventricles full, → AV valves → Ventricles → Semilunar valves → Ventricles full, Start contraction Snap shut Empty & relax Snap shut Start contraction Start ventricular systole “Lub” Start ventricular diastole “Dub” Start ventricular systole (Atrial diastole) (Atrial systole) Right Left P AM T FIGURE 14-18 Approximate locations for auscultation of the cardiac valves on the thoracic wall. T, Tricuspid valve; P, pulmonary valve; A, aortic valve; M, mitral valve. (From Nelson RC: Small animal internal medicine, ed 4, St Louis, 2009, Mosby.)
CHAPTER 14 The Cardiovascular System 351 ABNORMAL HEART SOUNDS CLINICAL APPLICATION If the two AV valves or the two semilunar valves are not Congestive Heart Failure closing simultaneously you may hear extra heart sounds. In older dogs, congestive heart failure (CHF) is a fairly Valvular insufficiency is a heart condition where one common problem. CHF occurs when the pumping ability of or more of the cardiac valves don’t close all the way. When the heart decreases, usually due to disease of the heart muscle this happens a murmur is produced. The murmur sound or a valve malfunction that restricts the forward flow of is produced by turbulence in the blood flow and sounds blood through a valve or allows a backward flow. CHF may like a swishing or whooshing sound rather than a lub or be predominantly right-sided or left-sided. When the right side of the heart begins to fail, blood returning from the dub. In the case of valvular insufficiency the murmur is systemic circulation is no longer able to move through the caused by blood backflowing abnormally into a chamber. right heart as quickly. This causes increased blood pressure For example, a murmur caused by mitral valve insuffi- in the systemic circulation, which results in fluid accumula- ciency results when the mitral valve doesn’t close all tion in the form of ascites (fluid in the abdomen) and edema the way when the left ventricle begins systole. Instead of (fluid in the tissues). all the blood being ejected through the aortic valve into When the left side of the heart fails, venous return from the aorta, some of the blood backflows into the left the lungs is decreased, resulting in pulmonary edema, which atrium. This results in less blood entering the systemic interferes with respiratory function. The decrease in cardiac circulation. output associated with heart failure may also reduce perfu- Valvular stenosis is a heart condition where any one or sion of important organs, such as the kidneys, to dangerously more of the cardiac valves don’t open all the way. Again a low levels. Medications used to treat CHF include cardiac glycosides murmur is produced by turbulent blood flow. In the case to increase the strength of cardiac contractions, diuretics to of valvular stenosis the murmur is caused by blood promote elimination of extra fluid to relieve edema, and flowing through a partially open valve and producing vasodilators to enhance blood flow to the organs and decrease the same whooshing or swishing sound. For example, a vascular resistance to outflow from the heart. CHF in pets murmur caused by mitral valve stenosis results when the cannot be cured, but it can often be medically managed to mitral valve doesn’t open entirely during left atrial systole. improve the animal’s quality of life. Instead of all the blood from the left atrium being ejected into the left ventricle, some of the blood remains in the left atrium. This results in less blood in the left ventricle The stroke volume (SV) is the volume of blood ejected to be ejected into the systemic circulation. It also results in from the left ventricle during one contraction or systole. less blood being able to enter the left atrium from the pul- Another name for the stroke volume is systolic discharge. monary circulation because the left atrium doesn’t empty The heart rate (HR) is the number of times the ventricle completely. contracts or beats in 1 minute. For an individual animal the HR is based in part on the rate at which the SA node spon- taneously depolarizes: TEST YOURSELF 14-4 1. What is the pacemaker of the heart and where is it cardiac output (CO) = stroke volume (SV) × heart rate (HR) located? 2. List the four conductors that make up the rapid conduc- For example if we know that a dog’s heart ejects 100 millili- tion system for an impulse created by the heart’s ters (ml) of blood into the aorta with each systolic contrac- pacemaker. tion, and its heart rate is 100 beats per minute, the animal’s 3. The working phase of a cardiac cycle is ___. It involves ___ that generates an impulse that results in muscle cardiac output can be calculated using our formula: contraction. 4. What is happening in the other three heart chambers CO = 100 ml/min (SV) × 100 beats/min (HR) = 10, 000 ml/min during left atrial diastole? 5. When the mitral valve is forced closed it produces part Cardiac output for this dog would be 10,000 ml of blood per of which heart sound, the first or the second? minute. This equation can help explain why cows and horses survive with much slower heart rates than cats and dogs. A horse or cow needs more cardiac output than a dog or cat because it has much greater tissue mass, yet it normally has CARDIAC OUTPUT a slower heart rate. How can that be? Well, if The cardiac output (CO) is the volume of blood that is CO = SV × HR ejected out of the left ventricle over a unit of time, usually 1 minute. In a healthy animal the cardiac output has to be and CO goes up but HR goes down, we can write the equa- sufficient to supply oxygen and nutrients throughout the tion as: animal’s body. Two factors determine the cardiac output (1) stroke volume and (2) heart rate. CO = ? SV × ↓ HR
352 C H A P T E R 14 The Cardiovascular System In order to balance the equation the stoke volume has to increase. Horses and cows have much larger hearts than dogs BLOOD VESSELS and cats; this allows them to have such a large stroke volume that they can generate sufficient cardiac output, even with a Blood vessels in the pulmonary and systemic circulations make slower heart rate than smaller animals. continuous loops to and from the heart. In the pulmonary The stroke volume represents the strength of the heart- circulation this means the blood vessels are branches from the beat. It is determined by two factors: preload and afterload. pulmonary artery and vein. In the systemic circulation the Preload is the volume of blood the ventricle receives from blood vessels are branches from the aorta and vena cava. All the atrium. Eighty percent of ventricular filling occurs pas- blood vessels are arteries, veins, or capillaries. They are hollow sively by gravity; the remaining 20% happens during atrial tubes with similar but not identical anatomy and function. systole when the atrium contracts. When the atrium doesn’t The walls of arteries and veins have three layers (Figure contract sufficiently the preload will decrease and the ven- 14-19). The inner layer that lines the lumen of the vessel is tricle receives less blood. the endothelium. It is composed of thin, smooth simple Afterload is the physical resistance presented by the squamous epithelium and is continuous with the endocar- artery the ventricle is ejecting blood into. If there is resistance dium that lines the chambers of the heart. The endothelium in the artery (e.g., partial blockage) the ventricle will not be provides a smooth surface for the vessel lumen so blood able to contract all the way and the amount of blood ejected flows easily through the vessel with little or no friction. into the artery will be decreased. The middle layer of a blood vessel wall is made up of Another factor that can affect the stroke volume is the smooth muscle, elastic fibers, or both. The smooth muscles length of the cardiac muscle cells. The longer the cells the contract and relax to change the diameter of the vessel. They more force they produce when they contract. The myocar- are controlled by the autonomic nervous system. The elastic dial cells can be stretched to a certain degree by introducing fibers provide stretchability to the vessel wall. They allow the an increased amount of blood into the ventricle during ven- blood vessel to stretch and recoil without outside control. tricular diastole. In this way myocardial cells are somewhat The outer layer of a blood vessel is composed of fibrous elastic. So if the ventricular wall is stretched the ventricle connective tissue and collagen fibers. The connective tissue chamber increases in size and holds more blood. In order to is strong and flexible which prevents vessel walls from get rid of the increased amount of blood the ventricular wall tearing. The collagen fibers extend outward from the con- has to contract with increased force. The result is an increased nective tissue and anchor the vessels so they can’t move stroke volume. around too much. They also help keep the lumen of the The normal heart rate for each species of animal is set vessel pulled open. internally by the rate of spontaneous SA node depolarization (Box 14-3). External control of the heart rate comes through ARTERIES the autonomic nervous system (discussed in Chapter 9). Arteries carry blood away from the heart. In the pulmonary circulation they carry deoxygenated blood to the lungs for oxygenation. In systemic circulation they carry oxygenated TEST YOURSELF 14-5 blood throughout the animal’s body. There are two types of 1. Stroke volume (SV) is a measurement of what? artery: elastic arteries and muscular arteries. Elastic arteries 2. If the cardiac output and stroke volume both decrease have the greatest ability to stretch when blood passes through what has to happen to the heart rate to achieve them because they have a large number of elastic fibers in equilibrium? the middle layers of their walls. These arteries are found 3. What is the difference between the preload and the closest to the heart because they have to be able to stretch afterload in reference to the stroke volume? and recoil without damage each time a surge of blood is 4. How could mitral valve stenosis affect the stroke ejected from a ventricle during ventricular systole. volume? The aorta is the largest elastic artery in the body. It has the largest layer of elastic fibers in its wall because it must be able to withstand the entire surge of blood ejected from the BOX 14-3 Normal Heart Rate (and Pulse Rate) for left ventricle. Other elastic arteries branch off the aorta. They Common Animal Species (*bpm) have smaller diameters than the aorta but still have more elastic fibers than smooth muscle fibers in their wall. Cat 120-140 Goat 70-80 Muscular arteries have more smooth muscle fibers than Dog 70-120 Pig 70-120 elastic fibers in their walls. They are found farther away from Cow 36-60 Chicken 250-300 the heart than elastic arteries and usually direct blood to Dairy cow 48-84 Guinea Pig 200-300 specific organs and tissues. Muscular arteries branch off the Horse 28-40 Rabbit 180-350 smallest elastic arteries and therefore have a smaller diame- Sheep 70-80 Rat 250-400 ter. They are located far enough away from the heart that the blood surge is not severe enough to cause damage. Muscular *Beats per minute. arteries branch into arterioles.
CH APTER 14 The Cardiovascular System 353 Endothelium Valve Smooth muscle layer and elastic tissue (thicker in arteries; thinner in veins) Connective tissue (thinner than smooth muscle layer in ARTERY arteries; thickest VEIN layer in veins) FIGURE 14-19 Anatomy of arteries and veins. (From Colvile J, Oien S: Clinical veterinary language, St Louis, 2014, Mosby.) Arterioles are the smallest branches of the arterial tree. circulation the venules carry oxygenated blood; in the sys- They are in effect small muscular arteries and have the nar- temic circulation they carry deoxygenated blood and waste rowest diameter. Blood flow to areas of the animal’s body is materials. Venules have thin enough walls that some fluid controlled through contraction of the smooth muscles in exchange between interstitial fluid and plasma can take their walls under autonomic nervous system control. Their place. Their walls consist of endothelium, a thin muscle layer, smaller diameter produces blood flow resistance, which in and a few fibrous connective tissue cells. White blood cells turn helps maintain blood pressure. leave the circulation at the venule level to enter tissues at a Muscular arteries and some elastic arteries are frequently site of inflammation. named for the organ or tissue they are carrying blood Venules join together to form veins. Veins and arteries in to (e.g., the renal artery carries blood to a kidney). An a specific area run close to each other so veins are named for animal’s body is pretty much bilaterally symmetric so arter- their corresponding arteries. For example, the femoral veins ies usually come in pairs (e.g., the right and left ovarian that drain blood from the hind legs accompany the femoral arteries supply blood to the right and left ovary respectively) arteries that supply blood to the hind legs. (Figure 14-20). As veins approach the heart they become larger in diam- eter as more veins draining other areas of the body join CAPILLARIES together. The largest vein in the animal’s body is the vena Arterioles branch into many microscopic blood vessels called cava, and all other systemic veins eventually drain into it. capillaries. Capillaries do not occur singly but in groups Figure 14-21 shows the major veins in a cat’s body. called capillary beds or capillary networks. One arteriole will Many veins are working against gravity to get the blood give rise to an entire capillary bed. The wall of a capillary is back to the heart and they don’t have the force of ventricular one endothelial cell thick. It has no middle or outer layer. contraction to propel blood flow. For this reason small For this reason the exchange of gases and nutrients takes and medium veins have one-way valves in their lumens. place at this level. No cell in an animal’s body is far from a The valves allow blood to flow only in the direction of capillary bed so the waste products of cellular metabolism the heart. When blood tries to flow backward, the valves are easily exchanged for the oxygen and nutrients necessary close. These valves are similar to the semilunar valves in for cellular metabolism. the heart, but they each have only two cusps. Muscular movements in the body compress small veins and the VEINS one-way valves allow blood to move only toward the heart. In order to get the blood back to the heart the capillaries join This is the only mechanism that propels blood back to together to form tiny veins called venules. In the pulmonary the heart.
354 C H A P T E R 14 The Cardiovascular System Right common carotid Left common carotid Left subclavian Right axillary Left axillary Right brachial Left brachial Right subclavian Brachiocephalic Pulmonary Aorta Celiac Diaphragm Cranial mesenteric Aorta Right ovarian Left renal Left testicular Caudal mesenteric Left external iliac Left internal iliac Left femoral Coccygeal FIGURE 14-20 Major arteries of the cat. (Modified from McBride DF: Learning Veterinary terminology, ed 2, St Louis, 2002, Mosby.) dioxide exchange, they need only enough blood to keep the BLOOD CIRCULATION IN THE FETUS growing lung tissues alive. Consequently, in the fetus there Now that you are familiar with how blood moves through are bypasses that allow most of the blood in the fetal circula- an animal, we’ll look at some differences in blood flow in a tion to go around the lungs instead of through them. developing fetus. The major difference between a fetus and The fetus receives oxygen through the placenta, an organ a newborn is that the newborn receives oxygen through its containing a network of tiny blood vessels that allows oxygen own lungs, and a fetus receives oxygen from the blood of its exchange between fetal and maternal circulation (Figure mother. Because fetal lungs are not used for oxygen/carbon 14-22). The oxygenated blood from the mother flows from
CHAPTER 14 The Cardiovascular System 355 Right external jugular vein Left internal jugular vein Left subclavian vein Right brachial vein Left axillary vein Left brachial vein Cranial vena cava Right brachiocephalic vein Azygous vein Diaphragm Hepatic vein Caudal vena cava Ovarian vein Renal vein Testicular vein Caudal vena cava Common iliac vein Internal iliac vein External iliac vein Caudal vein Femoral vein Medial saphenous vein FIGURE 14-21 Major veins of the cat. (Modified from McBride DF: Learning Veterinary terminology, ed 2, St Louis, 2002, Mosby.) the placenta into the fetus through the umbilical vein. The most of the fetal blood to bypass the lung tissue, because the vessel that carries oxygenated blood to the fetus is called a blood in the right atrium has already been oxygenated from vein because it flows toward the heart of the fetus. the maternal blood, and the lungs of the fetus do not perform The oxygenated blood in the umbilical vein flows through oxygen exchange. The first bypass is the foramen ovale the fetal liver and into the caudal vena cava, where it mixes between the right and left atria (foramen means “opening,” with deoxygenated blood from the fetal systemic circulation. and ovale means “oval”). Much of the blood from the right Just as in the newborn animal, blood from the vena cava fills atrium flows directly into the left atrium, but some does flow the right atrium. However, in the fetus two structures allow through the tricuspid valve into the right ventricle and then
356 C H A P T E R 14 The Cardiovascular System Pulmonary Pulmonary artery Aorta artery Aorta Pulmonary Pulmonary trunk trunk Ductus Ductus arteriosus Cranial arteriosus closes and vena cava is open becomes (patent) ligamentun Cranial arteriosus vena cava LA LA RA Descending RA aorta Oval foramen Oval foramen Caudal closes and Caudal vena cava becomes the vena cava LV fossa ovalis RV LV Liver Liver RV B Umbilical artery Umbilical vein A FIGURE 14-22 Circulation in the fetus and the newborn. A, Fetal circulation, B, newborn circulation. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. into the pulmonary artery. Blood from the pulmonary artery CLINICAL APPLICATION may flow into the lungs or through another bypass, the ductus arteriosus, directly into the aorta. Remember that Patent Ductus Arteriosus this blood was oxygenated when it passed through the During gestation, the fetal circulation does not carry much placenta. Blood travels through the fetal aorta to the fetal blood through the lungs; instead, there is an opening between systemic circulation, where it supplies oxygen and collects the left pulmonary artery and the aorta that allows much of waste products from the tissues. The deoxygenated blood is the blood that leaves the right ventricle to bypass the lungs then sent back to the placenta for oxygenation through the on the way to the aorta and systemic circulation. Because the fetus does not breathe air—oxygen is provided to the fetus umbilical arteries. from the blood of the dam—there is only enough pulmo- With the first breath after birth, the lungs inflate, and the nary circulation to nourish the tissues of the growing lungs. newborn begins to oxygenate its own blood. In a normal Following the rupture of the umbilical cord at birth, the fetus newborn, the foramen ovale and ductus arteriosus close at must generate its own oxygenated blood, so circulation that time so that blood can no longer bypass the lungs. through the lungs must be increased to include all of the blood that leaves the right ventricle. Normally, the shortcut opening between the pulmonary TEST YOURSELF 14-6 artery and aorta, known as the ductus arteriosus, closes soon 1. From the aortic valve in the left ventricle to the right after birth. Occasionally, the opening fails to close in the atrium, list, in order, the types of blood vessel a drop newborn, a condition called patent ductus arteriosus of blood will pass through. (PDA). Young animals with PDA suffer from inadequate 2. The coronary artery is the first branch off the aorta. Is oxygenation of their blood; in the long term, the condition it a muscular or elastic artery? Why? is incompatible with life. PDA may be treated with drug 3. In a pregnant ewe, which are the only two veins that therapy to promote closure, or surgical closure of the ductus are carrying oxygenated blood? arteriosus may be an option. 4. What two bypass structures are found in the fetus that allow most of its blood to bypass the pulmonary circulation?
CH APTER 14 The Cardiovascular System 357 with the animal’s pulse. As a general rule, larger animals PULSE have slower pulse rates and smaller animals have faster pulse The pulse is the rate of alternating stretching and recoiling rates. The pulse cannot be taken over a vein because the pulse of the elastic fibers in an artery as blood passes through wave dissipates in the capillaries and none of it is passed into it with each heartbeat. The artery has to stretch and the veins. recoil because the left ventricle doesn’t eject blood in a continuous flow. Every time the left ventricle contracts BLOOD PRESSURE (systole) it ejects a bolus of blood into the aorta. When the left ventricle relaxes (diastole) blood flow into the As the name implies blood pressure is a measure of the aorta stops. This sets up a pulse wave of stretching amount of pressure flowing blood exerts on arterial walls. It and recoiling that travels through all the arteries and arteri- is dependent on the interaction between the heart rate, oles and dissipates in the capillaries. In most animals the stroke volume, the diameter and elasticity of the artery, and pulse is felt on superficial arteries lying against firm surfaces the total blood volume. Any condition or medication that such as bones. affects any one or more of these factors will also affect the Clinically the pulse is used to evaluate the regularity of blood pressure. the pulsations and the strength of the pulsations. The pulse and the heartbeat in an animal are not the same thing. The pulse is felt on a superficial artery; the heartbeat is counted using a stethoscope to listen to the animal’s chest (ausculta- tion) to hear the heart sounds. In some animals the heart- beat can be felt through the chest wall. This is still not a true pulse because it is not felt over an artery. PULSE POINTS The pulse is best felt on different arteries in different species of animal (Box 14-4). It is most commonly evaluated over a medium-sized artery. In a healthy animal the pulse rate and the heart rate should be equal. When feeling for a pulse, use A the tips of your index and middle fingers, not your thumb because the thumb has its own pulse (Figure 14-23). The pulse wave that passes through your thumb may be confused BOX 14-4 Common Pulse Points in Common Animal Species Cat Femoral artery—medial surface of the thigh near the belly Dog Femoral artery—medial surface of the thigh near B the belly Cow Coccygeal artery—ventral midline of the tail near the base Facial artery—passes over the mandible near the angle of the jaw Horse Mandibular artery—passes over the mandible near the angle of the jaw Posterior digital artery—on the pastern between the coronary band and the fetlock Sheep Femoral artery—medial surface of the thigh near the belly Goat Femoral artery—medial surface of the thigh near the belly C Piglet Femoral artery—medial surface of the thigh near FIGURE 14-23 Finding the pulse on A, a dog or cat, B, a cow, the belly C, a horse. (A, From Thomas J, Lerche, P: Anesthesia and analgesia for Pig Coccygeal artery—ventral midline of the tail veterinary technicians, ed 4, St Louis, 2011, Mosby; B and C, From near the base Bassert J, McCurnin D: McCurnin’s clinical textbook for veterinary techni- cians, ed 8, St Louis, 2014, Saunders.)