Cardiovascular Respiratory Renal/ Urinary. VOLUME 111 Nervous System. Organ ~ Systems ~

Transcription

1 Cardiovascular Respiratory Renal/ Urinary VOLUME 111 Nervous System Organ ~ Systems ~

2 Contents Section I: Cardiovascular System Chapter 1. Embryology... 3 Chapter 2. Histology Chapter 3. Anatomy Chapter 4. Physiology Chapter 5. Pathology Chapter 6. Pharmacology Section II: Respiratory System Chapter 1. Embryology Chapter 2. Histology Chapter 3. Anatomy Chapter 4. Physiology Chapter 5. Pathology Chapter 6. Pharmacology Section III: Renal/Urinary System Chapter 1. Embryology Chapter 2. Histology KAP LA!._ meulc8 I vii

3 Chapter 3. Anatomy Chapter 4. Physiology Chapter 5. Pathology Chapter 6. Pharmacology Section IV: HematologicjLymphoreticular System Chapter 1. Histology Chapter 2. Anatomy Chapter 3. Physiology Chapter 4. Pathology Chapter 5. Pharmacology Section V: Nervous System Chapter 1. Embryology Chapter 2. Histology: Nerve Tissue Chapter 3. Histology: Sensory Organs Chapter 4. Neuroanatomy: Introduction Chapter 5. Divisions of the Nervous System Chapter 6. Meninges, Ventricular System, and Cerebrospinal Fluid Chapter 7. Gross Anatomy of the Spinal Cord Chapter 8. Spinal Cord Regulation of Skeletal Muscle Activity Chapter 9. Functional Anatomy and Lesions of the Spinal Cord Chapter 10. The Autonomic Nervous System Chapter 11. The Peripheral Nervous System viii meilical

4 Chapter 12. The Brain Stem Chapter 13. Cranial Nerves Chapter 14. Lesions of the Brain Stem and Cranial Nerves Chapter 15. Reticular Formation Chapter 16. The Vestibular System Chapter 17. The Auditory System Chapter 18. The Cerebellum Chapter 19. The Visual System Chapter 20. The Diencephalon Chapter 21. The Thalamus Chapter 22. The Hypothalamus Chapter 23. The Epithalamus and Subthalamus Chapter 24. The Limbic System Chapter 25. The Motor System Chapter 26. Gross Anatomy of the Cerebral Hemispheres Chapter 27. The Cerebral Cortex Chapter 28. Blood Supply to the Brain Chapter 29. Physiology Chapter 30. Pathology Chapter 31. Pharmacology: Drugs Affecting the Autonomic Nervous System Chapter 32. Pharmacology: Drugs Affecting the Central Nervous System Chapter 33. Pharmacology: Psychoactive Drugs Index UPLA~. I meulca ix

5 SECTION I Cardiovascular System

6 Cardiovascular Embryology The cardiovascular system consists of the heart, blood vessels, and lymphatic vessels. All of these structures are derived from mesoderm; their development is outlined in this chapter. PRIMITIVE VASCULAR SYSTEM A. Blood islands. During the third week of development, mesenchymal cells associated with the yolk sac, chorion, and connecting stalk form dusters called blood islands, which acquire lumina and fuse to form endothelium-lined capillary plexuses. Peripheral cells of the islands become angioblasts that give rise to the endothelial cells of the vessels, whereas centrally located cells become embryonic hemoblasts that give rise to primitive blood cells. 1. Certain capillaries enlarge to form the major blood vessels: vitelline vessels are formed in the yolk sac wall and umbilical vessels are formed in the vascular chorion. 2. Extraembryonic blood vessels join with intraembryonic blood vessels formed from splanchnic mesoderm and the primitive vascular system is established. B. Hematopoiesis first occurs within the islands of the yolk sac. Later, blood cells are formed in the liver (1-7 months), spleen and lymphatic organs (2-4 months), and bone marrow (after 4 months). 3

7 cardiovascular System PRIMITIVE HEART TUBE FORMATION The pericardial cavity of the coelom lies cephalic to the buccopharyngeal membrane and neural plate in the embryonic disk. Mesenchyme clusters in this region form a pair of endothelium-lined heart tubes on either side of the midline. With transverse folding of the embryonic disk, these tubes fuse to form the single median primitive heart tube. A. Rotation. Cephalocaudal folding of the embryonic disk causes the pericardial cavity and heart tube to rotate along a transverse axis and become located ventral to the foregut and caudal to the buccopharyngeal membrane. 1. The heart tube bulges into the pericardial cavity and becomes transiently suspended from its dorsal wall by the dorsal mesocardium. 2. The mesoderm adjacent to the heart tube thickens to form the epimyocardial mantle; mantle cells differentiate into muscle cells of the myocardium and mesothelial cells of the epicardium. B. Early differentiation. The cephalic, or arterial, end of the heart tube is continuous with the aortic sac, while the caudal, or venous, end receives the vitelline veins from the yolk sac, the umbilical veins from the placenta, and the common cardinal veins from the body wall. The heart tube expands and differentiates to form, in a cephalocaudal direction, the bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus. 1. The aortic arches connect the truncus to the paired dorsal aortae, which arise from the aortic sac and lie dorsolateral to the foregut. 2. The distal portion of the bulbus, the truncus arteriosus, becomes the proximal part of the aorta and pulmonary artery. 3. The sinus venosus eventually forms a major part of the wall of the right atrium and the coronary sinus. C. Loop formation. Because the bulbus cordis and the ventricular parts of the heart grow more rapidly than the pericardial cavity, elongation of the heart tube is accomplished by the formation of a dorsoventral cardiac loop, which has its convexity directed anteriorly and to the right. 1. In the resulting S-shaped heart, the expanding atrium lies cranial to the ventricle and bulbus cordis, on either side of the truncus arteriosus, and the passage between the atrium and ventricle narrows to form the atrioventricular canal. 2. These changes in position are accompanied by a caudal migration of the pericardium and heart tube from the level of the third and fourth somites to the level of the seventeenth to the twentieth somites (Figure 1-1-1). 4

8 Embryology \ -,E' I (~""'_' ~ f ',..V't, --.:)..1, \.~ t.-, ( O~ 1'. '(! (' /\ \ (,,'\.- ~O(.,... 'y ;.....,_i -s~ 'I' \.- (.; Arteries of 10~~\Jt~ -~:JTr--- Aortic sac pharyngeal arches ~';:':J" /' '. """'1-"-'":--- Truncus arteriosus --;--.,/' Bulbus cordis,/ / l Pericardial ~ cavity \, " "'''- i- Sinus venosus -~c...,;:.;.;;':::';.:':;:'.'.-\, \.' \ y A Umbilical vein Vitelline vein -.:.;.,+---;.-- Ventricle -----,;;.t---atrium Homs of sinus venosus ~--i-.pr,~-common cardinal vein (~,..,~:.s..,-1,. r'.. Bulbus cordis :.-;. :,., \., ;', \.1;. (1";",\" I! B Sinus venosus c Horns of sinus venosus Right atrium o Figure Bending of the endocardial heart tube in the pericardium. SEPTUM FORMATION A. Primitive atrium 1. At the end of the fourth week, the septum primum grows from the roof of the primitive atrium towards two mesenchymal cushions, the endocardial (atrioventricular; AV) cushions, which appear in the ventral and dorsal walls of the AV canal. a. The transient opening between the septum primum and endocardial cushions is known as the interatrial foramen primum. b. The endocardial cushions gradually extend along the edge of the primum, thereby obliterating the foramen primum. r'. 5

9 Cardiovascular System c. Prior to its closure, the central portion of the septum primum perforates to form the interatrial foramen secundum, which insures free blood flow from the right to left primitive atrium. 2. As the sinus venosus becomes incorporated into the right atrium, the septum secundum grows from the ventral cranial wall of the atrium towards the endocardial cushions. a. The lower edge of the septum secundum encloses the foramen secundum in the septum primum but does not extend fully towards the endocardial cushions. The opening it leaves between the right and left primitive atria is known as the foramen ovale. ~', \'.,Ii, \1:'... b. The upper part of the septum prim urn disappears, but the lower part becomes the valve of the foramen ovale, which allows blood from the vena cava to pass from the right to left atrium. ',i (' "(. B. Primitive ventricles 1. The ventricle begins to dilate by the end of the fourth week. 2. The expanding walls of the apposing ventricles approach each other medially and fti~e to form the muscular interventricular septum.. 3. The interventricular foramen, which lies between the muscular interventricular septum and the endocardial cushions and permits communication between the two ventricles, is eventually closed by the membranous interventricular septum. C. Truncus arteriosus. During the fifth week, the right superior and left inferior bulbar ridges appear in the cephalic portion o{the truncus arteriosus. 1. The right superior bulbar ridge grows distally to the left, and the left inferior bulbar ridge grows distally to the right. 2. The bulbar ridges twist around each other and fuse to form the aorticopulmonary septum, which divides the truncus arteriosus into aortic and pulmonary passages. FORMATION OF CARDIAC VALVES A. Aortic and pulmonic valves 1. The semilunar valves of the aorta and pulmonary arteries develop following the formation of the aorticopulmonary septum. 2. Three swellings of endothelium-covered loose connective tissue form at the orifices of both the aorta and pulmonary artery. These swellings become hollowed at their upper surfaces to form semilunar valves. B. Atrioventricular (AV) valves 1. The AV valves form after the endocardial cushions fuse. 2. Each atrioventricular orifice becomes surrounded by endocardium-covered connective tissue swellings, which hollow on their ventricular surfaces to form valves. a. Two valve leaflets, the bicuspid (mitral) valve, are formed in the left atrioventricular canal. b. Three valve leaflets, the tricuspid valve, are formed in the right atrioventricular canal. 3. The valves remain connected to papillary muscles in the wall of the ventricle by means of chordae tendinae. 6

10 Embryology ARTERIAL SYSTEM A. Formation of the aortic arch arteries l. The aortic arch arteries arise during the fourth week from the aortic sac, the most distal part of the truncus arteriosus. 2. Each of the six pairs of arteries is embedded in the mesenchyme of its corresponding pharyngeal arch and terminates in the paired dorsal aortae. 3. The dorsal aortae fuse by the fifth week to form the descending thoracic aorta and the abdominal aorta with branches to the embryo, yolk sac (vitelline arteries), and allantois (umbilical arteries). B. Aortic arch derivatives 1. First aortic arch. The persisting portion becomes the maxillary artery, which supplies the derivatives of the first pharyngeal arch. 2. Second aortic arch. The persisting portion becomes the hyoid artery and stapedial artery, which supply derivatives of the second pharyngeal arch. 3. Third aortic arch. It gives rise to the common carotid artery and the first part of the internal carotid artery (the remainder is formed from the cranial portion of the dorsal aorta); the external carotid artery branches from this arch. 4. Fourth aortic arch. The left side forms part of the arch of the aorta between the left common carotid and left subclavian arteries. The right side forms the proximal portion of the right subclavian artery (distal portion is formed from the right dorsal aorta and the seventh intersegmental artery). 5. Fifth aortic arch. It involutes and disappears. 6. Sixth aortic arch. This is the "pulmonary arch:' a. The proximal portions become the proximal left and right pulmonary arteries. b. The right distal portion degenerates. c. The left distal portion becomes the ductus arteriosus. C. Developmental changes in the aortic arch system 1. There is obliteration of the carotid duct, the portion of the dorsal aorta between the third and fourth arches. 2. Disappearance of the right dorsal aorta occurs between the origin of the seventh intersegmental artery and the junction with the left dorsal aorta. D. Branches of the dorsal aortae 1. Dorsal intersegmental arteries (30 pairs) arise from each side of the fused dorsal aortae from the base of the skull to the sacrum. a. Dorsal ramus supplies the spinal cord, meninges, skin, and musculature of the back. Longitudinal anastomoses give rise to vertebral arteries, which fuse with the single basilar artery. The basilar artery fuses with the internal carotid artery to form the cerebral arterial circle (of Willis). b. Ventral ramus. Each fuses with its partner at the ventral body wall, giving rise to the intercostal, limb, and lumbar arteries. Flashback to General Principles Now may be a good time to review the pharyngeal arches and their derivatives in the last Embryology chapter of General Principles Book 2 (Volume II). 7\ }>' ~,','" Aortic arch derivatives: Maxillary artery = 1 st arch Hyoid artery = 2nd arch " <:0; " ; ) S_tapedial arte,ry = 2nd arch ~ Common carotid artery =,- "-, -,-, / 3rd arch /? r 1 ri. '~" ;/;-.,;.~" r,' "",, - Proximal internal carotid '. [...r\,'~ " 'J "..I artery = 3rd arch Aortic arch = 4th arch ( - ~, cj.. Sf"t_: J;..:.. ""'i," (','r"n~... '-',-" f'/ Proximal right subclavian (. /(, artery = 4th arch ;' p., i -.j -i'r., ",'," I '. Proximal pulmonary r" ~~(;,,,, arteries = 6th arch' I\' c\ Ductus arteriosus = 6th arch. t., ( ( ; I '~1""" I \.---'~ ;.- ': I,~ ",>:.~ ',r;;r 'V', '-::.J +."'t) l' :"!:,.,,(, ; (J, Q f j... 1 f'( i ),J" < (,,: r ",) (,,," u! '- ""7 '7 - _. i) J.', ~ \ II-'~ ~_ to..,... I'j ~ 7

11 Cardiovascular System In'.,,~\~;., c\ i \: ( ~.\( '. ( I '> \,/(. (",,\ I p\..,fr...,:' i --,,,_ ~ : ( II {,~t ' I (\V t,,. (I - C\ f -. (' <: 2. Lateral splanchnic arteries arise from each side of the dorsal aorta. They supply intermediate mesoderm and derivatives and give rise to renal, suprarenal, phrenic, and testicular or ovarian arteries. 3. Ventral splanchnic arteries a. Vitelline arteries originate as a number of paired vessels running to the yolk sac. They eventually fuse to form the celiac artery to the foregut, superior mesenteric artery to the midgut, and inferior mesenteric artery to the hindgut. b. Umbilical arteries originate as paired ventral branches to the placenta in association with the allantois. ( 1) During the fourth week, each artery becomes connected with the dorsal aorta via the common iliac artery. (2) After birth, the umbilical arteries give rise to the internal iliac arteries, proximally, and the medial umbilical ligaments, distally. Bridge to Anatomy The ligamentum venosum is the fibrous remnant of the ductus venosus. It runs in a fissure on the visceral surface of the liver and is attached on one end to the left branch of the portal vein and on the other end to the inferior vena cava. The ligamentum teres is the fibrous remnant of the umbilical vein. It is located in the free margin of the falciform ligament. VENOUS SYSTEM At five weeks, the embryo has three major pairs of veins. A. Vitelline (omphalomesenteric) veins drain the yolk sac and carry blood around the duodenum and through the septum transversum to the sinus venosus. 1. The vitelline veins cephalic to the liver become the hepatocardiac channels, which enter the sinus venosus with the right channel becoming the first segment of the inferior vena cava. 2. Caudal to the liver, the vitelline veins anastomose and atrophy to form the single hepatic portal vein of the portal system that drains the viscera. B. Umbilical veins. After atrophy of the right vein and the left vein proximal to the liver, the remaining left vein carries blood from the placenta to the liver. 1. The ductus venosus, a direct connection through the liver to the right hepatocardiac channel, is formed. This prevents depletion of oxygen and nutrient-rich blood in the hepatic sinusoids. 2. After birth, the ductus venosus becomes obliterated and is fibrosed to form the ligamentum venosum. The left umbilical vein forms a similar ligament, the ligamentum teres, in the free margin of the falciform ligament. C. Cardinal veins 1. The anterior cardinal veins drain the cephalic end of the embryo, and the posterior cardinal veins drain the caudal end of the embryo. These join to form the common cardinal veins, which enter the horns of the sinus venosus. 2. By the seventh week, the subcardinal veins, which drain the kidneys, the sacrocardinal veins, which drain the lower extremities, and the supracardinal veins, which drain the body wall via intercostal veins, are formed. 3. The vena cava system is formed by anastomoses between the right and left sides of the cardinal system such that blood from the left is channeled to the right side. a. Anastomosis between the anterior cardinal veins forms the left brachiocephalic vein during the eighth week. b. The superior vena cava is formed from the proximal right anterior cardinal vein and the right common cardinal vein. 8

12 Embryology c. Anastomosis between the sacrocardinal veins forms the left common iliac vein. The right sacrocardinal vein becomes the sacrocardinal segment of the inferior vena cava. 4. The fourth to eleventh right intercostal veins empty into the right supracardinal vein, which joins the posterior cardinal vein to form the azygos vein. The fourth to seventh left intercostal veins enter the left supracardinal vein (hemiazygos vein), which empties into the azygos vein. LYMPHATICS Development of this system parallels that of the veins. It begins with six primary lymph sacs, which later form a network of lymphatic vessels. Aggregates of lymphatic tissue, or lymph nodes, form in this network shortly before or after birth. CIRCULATION OF BLOOD A. Fetal pattern (Figure 1-1-2) 1. In the fetus, blood is oxygenated at the placenta and travels via the umbilical vein, most of it bypassing the liver through the ductus venosus, to the inferior vena cava. a. There, it mixes with deoxygenated blood from the lower body and hepatic portal system and, subsequently, enters the right atrium. b. The valve at the orifice of the inferior vena cava directs most of the flow out of the right atrium, through the foramen ovale, into the left atrium. c. From the left atrium, the oxygenated blood, plus some deoxygenated blood from the lungs, passes into the left ventricle and, hence, into the ascending aorta toward the brain, heart, and upper extremities. 2. Deoxygenated blood from the head and upper extremities returns to the right atrium via the superior vena cava, and passes through the tricuspid valve to the right ventricle. From there, most of it is short -circuited away from the inactive lungs by the ductus arteriosus into the descending aorta to supply the trunk and lower extremities. 3. Since the right ventricle must pump blood against the relatively high resistance of the unexpanded lungs and through the ductus to the general circulation, pressure on the right side of the heart is greater than on the left. Thus, prior to birth, the thickness of the right ventricle wall is similar to that of the left. 4. Blood in the aorta travels to the placenta via the umbilical arteries, which now arise from the internal iliac arteries. B. Changes at birth. Cessation of placental blood flow at birth and an increase in flow through the lungs cause the pressure in the right atrium to decrease, while that in the left rises. As a result, the septum primum is apposed to the septum secundum, and the foramen ovale is closed functionally. Fusion occurs in about one year. 9

13 Cardiovascular System vena cava Foramen ovale Inferior vena cava Portal vein t---~!+-+-- Umbilical vein i0 2 cord--~ I Shunts in bold I Right and left umbilical arteries.10 2 Figure Fetal circulation. In a Nutshell Tetralogy of Fallot Pulmonary stenosis Right ventricular hypertrophy (RVH) Interventricular septal defect Overriding aorta CONGENITAL MALFORMATIONS A. Tetralogy of Fallot is a common abnormality, which consists of pulmonary stenosis, right ventricular hypertrophy (secondary to pulmonary stenosis), interventricular septal defect, and an overriding aorta. 1. It is a result of asymmetrical development of the truncus arteriosus septum to form a very small pulmonary artery and a very large aorta. Unoxygenated blood is shunted to the left side and blood flows from both right and left ventricles into the enlarged aorta with little reaching the lungs. 2. This abnormality, compatible with life, is the most important cause of neonatal cyanosis. B. Transposition of the great vessels is an anomaly in which the aorta emerges from the right ventricle and the pulmonary artery from the left ventricle. It is thought to be due to failure of the bulbar septum to spiral. C. Patent ductus arteriosus is a failure of anatomic obliteration that allows oxygenated blood from the aorta to be shunted back into the pulmonary artery. D. Atrial septal defects (ASD) are the failure of the septa primum and secundum to form. The most common form is a patent foramen ovale, resulting from incomplete adhesion between the septum primum and septum secundum. 10

14 Embryology E. Ventricular septal defects (VSD) are usually due to malformation of the membranous interventricular septum. F. Persistent truncus arteriosus is a failure of the partitioning of the truncus arteriosus into the aorta and pulmonary artery. G. Pulmonary valvular atresia is an unequal division of the truncus arteriosus such that the pulmonary trunk has no lumen or orifice at the level of the pulmonary valve. H. Aortic coarctation is a narrowing of the aorta just above or below the ductus arteriosus. Bridge to Pathology These congenital abnormalities and more are discussed in detail in the Cardiovascular Pathology chapter. 11

15 Cardiovascular Histology The cardiovascular system is a closed, double pathway for the separate circulation of blood to the lungs and the peripheral tissue. It consists of a four-chambered heart and a system of vessels for delivery (arteries and capillaries) and subsequent return (veins) of the blood to the heart. This chapter will review the different tissue types found in each of these components, and how these tissues are specialized for their specific functions. CARDIAC MUSCLE CELLS A. Cardiac muscle cells are elongated, branching fibers that exhibit transverse banding patterns like that of skeletal muscle. 1. They possess one or two centrally placed nuclei. 2. Individual cells are surrounded by a delicate connective tissue containing an abundant capillary network. 3. Cardiac myofibers are joined together by extensive junctional structures called intercalated disks. 4. Unlike skeletal muscle, cardiac muscle cells are electrically coupled to each other through gap junctions located in the disks. B. Cardiac muscle sarcomeres are similar to those found in skeletal muscle. Their filaments, however, are not segregated into discrete myofibrils but are found in continuous fields that are partially divided by portions of sarcoplasm. Actin filaments attach to the intercalated disks at the ends of the cells. e. Control of cardiac muscle. Cardiac muscle does not require neural input for activation, but its activity is modulated by the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS). 1. Specialized muscle cells in the right atrium, the sinoatrial (SA) node, spontaneously depolarize rhythmically to initiate each heart beat. 2. Electrical coupling via gap junctions at the intercalated disks causes depolarization, which is initiated at the SA node and spreads through the atria. a. The impulse reaches the atrioventricular (AV) node and is delayed there for approximately 0.1 seconds. b. From the AV node, specialized conduction fibers in the bundle of His and its branches transmit the impulse to the ventricular muscle. 13

16 Cardiovascular System 3. The large-diameter conducting fibers are modified muscle cells called Purkinje fibers, which have few myofibrils and abundant sarcoplasm containing mitochondria and glycogen. In a Nutshell Inside -7 outside Endocardium -7 myocardium -7 epicardium In a Nutshell Vena cavae -7 RA -7 tricuspid valve -7 RV -7 pulmonic valve -7 pulmonary artery -7 lungs -7 pulmonary vein -7 LA -7 mitral valve-7 LV -7 aortic valve -7 aorta HEART The heart is a muscular organ, composed primarily of cardiac muscle tissue, which contracts rhythmically to pump blood throughout the body. A. Structure of the heart wall. The walls of the heart are constructed in layers that are similar to those of the major blood vessels. 1. Endocardium is the innermost layer of the heart and is lined with endothelium. Veins, nerves, and components of the impulse conducting system are present in the subendocardial connective tissue layer. 2. Myocardium is composed of branching, anastomotic cardiac myocytes attached to one another by intercalated disks. Most of these cells are involved in the pumping function of the heart; others are specialized for the control of rhythmicity (impulse conducting system) or secretion (myocardial endocrine cells). 3. Epicardium is a serous membrane that forms the visceral lining of the pericardium. Its external mesothelium is supported by a loose connective tissue subepicardial layer. B. Cardiac skeleton is composed mainly of dense connective tissue and consists of the annuli fibrosi, the trigonum fibrosum, and the septum membranaceum. C. Cardiac valves are composed of dense fibrous tissue covered by endothelium. 1. Unidirectional flow is maintained from the: a. Right atrium to the right ventricle (tricuspid valve) b. Right ventricle to the pulmonary artery (pulmonic semilunar valve) c. Left atrium to the left ventricle (mitral/bicuspid valve) d. Left ventricle to the aorta (aortic semilunar valve) 2. Tricuspid and mitral valves are attached to papillary muscles by cords of fibrous connective tissue (chordae tendineae) and prevent reflux of blood into the atria during ventricular contraction (systole). 3. Semilunar valves (aortic and pulmonic) prevent reflux of blood back into the ventricles during ventricular relaxation (diastole). D. Impulse conducting system of the heart consists of specialized cardiac myocytes that are characterized by automaticity and rhythmicity (i.e., they are independent of nervous stimulation and possess the ability to initiate heart beats). These specialized cells are located in the sinoatrial (SA) node (pacemaker), internodal tracts, atrioventricular (AV) node,avbundle (of His), left and right bundle branches, and numerous smaller branches to the left and right ventricular walls (Figure 1-2-1). Impulse conducting myocytes are in electrical contact with each other and with normal contractile myocytes via communicating (gap) junctions. Specialized wide-diameter impulse conducting cells (Purkinje myocytes), with greatly reduced myofilament components, are well adapted to increase conduction velocity. They rapidly deliver the wave of depolarization to ventricular myocytes. 14

17 Histology Superior vena cava ~ Sinoatrial node (SA node) Right atrium -+\-:r Inferior ~ vena cava Atrioventricular node (AV node) Left atrium ~ 7 Pulmonary veins \ 11 \t '\ ' His bundle --\-+-ii~,-- Left ventricle Bundle branches In a Nutshell Conduction Pathway SA node (located near SVC in RA) ~ AV node R bundle branch + Bundle of His / ~ L bundle branch t + Purkinje system Purkinje system Figure Impulse-conducting system of the heart. E. Myocardial endocrine cells are also specialized myocytes found mainly in the atria. These cells contain numerous secretory granules, which contain atrial natriuretic peptides (ANP). When released into the blood, these hormones play roles in the regulation of blood pressure and blood volume. ARTERIES Arteries are classified according to their size, the appearance of their tunica media, or their major function. Note There are three types of heart cells: Contractile Impulse conducting (Purkinje) Endocrine (ANP) A. Large elastic conducting arteries include the aorta and its large branches. Unstained, they appear yellow due to their high content of elastin. 1. The tunica intima is composed of endothelium and a thin subjacent connective tissue layer. An internal elastic membrane marks the boundary between the intima and media. 2. The tunica media is extremely thick in large arteries and consists of circularly organized, fenestrated sheets of elastic tissue with interspersed smooth muscle cells. These cells are responsible for producing elastin and other extracellular matrix components. The outermost elastin sheet is considered as the external elastic membrane, which marks the boundary between the media and the tunica adventitia. 3. The tunica adventitia is a longitudinally oriented collection of collagenous bundles and delicate elastic fibers with associated fibroblasts. Large blood vessels have their own blood In a Nutshell Inside ~ outside Intima ~ media ~ adventitia Note Vasa vasorum is Latin for "vessels of the vesse!." 15

18 cardiovascular System supply (vasa vasorum), which consists of small vessels that branch profusely in the walls of larger arteries and veins. B. Muscular distributing arteries are medium-sized vessels that are characterized by their predominance of circularly arranged smooth muscle cells in the media interspersed with a few elastin components. Up to 40 layers of smooth muscle may occur. Both internal and external elastic limiting membranes are clearly demonstrated. The intima is thinner than that of the large arteries. C. Arterioles are the smallest components of the arterial tree. Generally, any artery less than 0.5 mm in diameter is considered to be a small artery or arteriole. A subendothelial layer and the internal elastic membrane may be present in the largest of these vessels but are absent in the smaller ones. The media is composed of several smooth muscle cell layers, and the adventitia is poorly developed. An external elastic membrane is absent. Note More than 70% of total blood volume is found in the venous system at any one time. Return blood flow to the heart is aided by the action of smooth muscle and special unidirectional valves in the walls of veins themselves, as well as by the action of adjacent skeletal muscles. CAPILLARIES Capillaries are thin-walled, narrow-diameter, low-pressure vessels that generally permit easy diffusion across their walls. Most capillaries have a cross-sectional diameter of 7-12 ~m. They are composed of a simple layer of endothelium, which is the lining of the entire vascular system, and an underlying basal lamina. They are attached to the surrounding tissues by a delicate reticulum of collagen. Associated with these vessels at various points along their length are specialized cells called pericytes. These cells, enclosed within their own basal lamina, which is continuous with that of the endothelium, contain contractile proteins and thus may be involved in the control of capillary dynamics. They may also serve as stem cells at times of vascular repair. Capillaries are generally divided into three types, according to the structure of their endothelial cell walls. A. Continuous (muscular, somatic) capillaries are formed by a single uninterrupted layer of endothelial cells rolled up into the shape of a tube and can be found in locations such as connective tissue, muscle, and nerve. B. Fenestrated (visceral) capillaries are characterized by the presence of pores in the endothelial cell wall. The pores are covered by a thin diaphragm (except in the glomeruli of the kidney) and are usually encountered in tissues where rapid substance interchange occurs (e.g., kidney, intestine, endocrine glands). C. Sinusoidal capillaries can be found in the liver, hematopoietic and lymphopoietic organs, and in certain endocrine glands. These tubes with discontinuous endothelial walls have a larger diameter than other capillaries (up to 40 ~m), exhibit irregular cross-sectional profiles, have more tortuous paths, and often lack a continuous basal lamina. Cells with phagocytic activity (macrophages) are present within, or just subjacent to, the endothelium. VEINS Veins are low-pressure vessels that have larger lumina and thinner walls than arteries. In general, veins have more collagenous connective tissue and less muscle and elastic tissue than their arterial counterparts. Although the walls of veins usually exhibit the three layers described above, they are much less distinct than those of the arteries. Unlike arteries, veins contain one-way valves composed of extensions of the intima that prevent reflux of blood away from the heart. Veins can be divided into small veins or venules, medium veins, and large veins. A. Venules are the smallest veins, ranging in diameter from approximately ~m (postcapillary venules) up to 1-2 mm (small veins). The walls of the smaller of these are structurally and functionally like those of the capillaries; they consist of an endothelium 16

19 Histology surrounded by delicate collagen fibers and some pericytes. In those vessels of increased diameter, circularly arranged smooth muscle cells occur surrounding the intima layer, but unlike in the small arteries, these cells are loosely woven and widely spaced. Venules are important in inflammation because their endothelial cells are sensitive to histamine released by local mast cells. This causes endothelial cells to contract and separate from each other, exposing a naked basement membrane. Neutrophils stick to the exposed collagen and extravasate (i.e., move out into the connective tissue). Histamine also causes local arterioles to relax, affecting a rise in venous pressure and increased leaking of fluid. This produces the classic signs of inflammation: redness, heat, and swelling. B. Medium veins in the range of 1-9 mm in diameter have a well-developed intima, a media consisting of connective tissue and loosely organized smooth muscle, and an adventitia (usually the thickest layer) composed of collagen bundles, elastic fibers, and smooth muscle cells oriented along the longitudinal axis of the vessel Venous valves are sheet-like outfoldings of endothelium and underlying connective tissue that form flaps to permit unidirectional flow of blood. C. Large veins, such as the external iliac, hepatic portal, and vena cavae, are the major conduits of return toward the heart. The intima is similar to that of medium veins. Although a network of elastic fibers may occur at the boundary between the intima and media, a typical internal elastic membrane as seen in arteries is not present. A tunica media mayor may not be present. If present, smooth muscle cells are most often circularly arranged. The adventitia is the thickest layer of the wall and consists of elastic fibers and longitudinal bundles of collagen. In the vena cava, this layer also contains well-developed bundles of longitudinally oriented smooth muscle. Note The majority of veins (with the exception of the main trunks) are small or medium-sized. Note Artery-media is thickest layer Vein-adventitia is thickest layer LYMPHATIC VESSELS Lymphatic vessels are discussed in the Histology chapter of the Heme/Lymph System section in this book. 17

20 Cardiovascular Anatomy The heart is located in the middle portion of the thoracic cavity known as the mediastinum. This chapter will review the structures located in the mediastinum as well as the gross anatomy of the heart itself and the branches of the abdominal aorta, inferior vena cava, and azygos vein. MEDIASTINUM Located between the pleural cavities, the mediastinum is divided into inferior and superior parts by a plane passing from the sternal angle anteriorly, to the intervertebral disc between T 4 and TS posteriorly. The inferior mediastinum is classically subdivided into middle, anterior, and posterior parts (Figures and 1-3-2). A. Middle mediastinum. This section contains the pericardium, phrenic nerves, and heart. 1. Pericardium is the outer fibrous sac; it is lined by a double-layered serous membrane that encloses the pericardial cavity between its parietal and visceral layers. a. Transverse pericardia! sinus is a space posterior to the ascending aorta and pulmonary trunk and anterior to the superior vena cava. b. Oblique pericardia! sinus is a blind, inverted, U-shaped space posterior to the heart and bounded by reflection of serous pericardium around the eight vessels entering and leaving the heart. 2. Phrenic nerves a. Phrenic nerves arise from the ventral primary rami of cervical nerves 3, 4, and 5. b. They are the sole motor supply of the diaphragm and convey sensory information from the central portion of both the superior and inferior portions of the diaphragm. c. Both phrenic nerves pass through the middle mediastinum lateral to fibrous pericardium and anterior to the root of the lung. 3. The heart (discussed separately below) Clinical Correlate Cardiac tamponade is a lifethreatening condition resulting in compression of the heart due to an accumulation of excess fluid in the pericardial cavity. The fluid must be removed by pericardiocentesis to relieve the pressure. Pericardiocentesis is performed by inserting a needle through the skin and subcutaneous tissues along the underside of the xiphoid process, directed inward and upward, toward the pericardial sac (alternatively, the pericardial sac can be approached from the xiphocostal angles, or through the fifth left intercostal space). Withdrawal of even a small amount of fluid can restore cardiac function in many cases. Care must be taken to access the pericardium through the "bare area" to avoid violation of the costal pleura. 19