Histology Cardiovascular System PDF - AQA September 2024

HISTOLOGY CARDIOVASCULAR SYSTEM Prepared by: Zen, Mhyr, Julius, Isah, Rasdin Lecturer: DR. FERDAUZIA E. BASTERO Date: September 20, 2024...

HISTOLOGY CARDIOVASCULAR SYSTEM Prepared by: Zen, Mhyr, Julius, Isah, Rasdin Lecturer: DR. FERDAUZIA E. BASTERO Date: September 20, 2024 CARDIOVASCULAR SYSTEM The cardiovascular system consists of a muscular pump, The orifice between the right atrium and right ventricle is closed the heart, and two systems of blood vessels. by the tricuspid valve In both, the blood pumped from the heart passes successively through arteries of diminishing diameter, to Between the left atrium and ventricle is the mitral valve. networks of minute capillaries, and then back to the heart through veins of increasing caliber. Aortic and pulmonary semilunar valves prevent reflux of blood from these vessels during relaxation of the heart. TWO SYSTEM OF BLOOD VESSELS 1.The pulmonary circulation- carries blood to and from the The four cardiac valves—tricuspid, pulmonary, mitral, and lungs. aortic—maintain unidirectional blood flow. Valve function 2. The systemic circulation (peripheral depends on the mobility, pliability, and structural integrity of the circulation)-distributes blood to all the other tissues and leaflets of the atrioventricular valves (tricuspid and mitral) or organs of the body. cusps of the semilunar valves (aortic and pulmonary). THE HEART The human heart is a remarkably efficient, durable, and reliable pump, propelling more than 7500 L of blood through the body CARDIAC VALVES each day, and beating more than 40 million times a year—the Are lined by endothelium and share a similar, trilayered wellspring for tissue oxygenation, nutrition, and waste removal. architecture: 1. Fibrosa layer- A dense collagenous layer at the In utero, the heart and vasculature are the first fully functional outflow surface, connected to the valvular supporting organ system (at roughly 8 weeks of gestation). structures and providing mechanical integrity. 2. Spongiosa layer- A central core of loose connective Heart weight varies with body habitus, averaging approximately tissue. 0.4% to 0.5% of body weight (250 to 320 g in the average adult 3. Ventricularis or Atrialis layer- (depending on which female and 300 to 360 g in the average adult male). chamber it faces). A layer rich in elastin on the inflow surface, providing leaflet recoil The heart is about 12 cm long, 9 cm wide, and 6 cm in anteroposterior diameter. WALL OF THE HEART Consists of three layers endocardium, myocardium, and Blood returning to the heart in the superior and inferior venae epicardium which are homologues (having the same relation, cavae enters the right atrium and passes from it to the right relative position, or structure) of the tunica intima, tunica ventricle. From there, it is pumped, via the pulmonary artery, to the media, and tunica adventitia, of the blood vessels. lungs, where it is aerated and then returned to the left atrium via the pulmonary vein. 1. Endocardium- is lined by an endothelium of polygonal squamous cells that is continuous with the Contraction of the left atrium expels blood into the left ventricle, endothelium of the vessels entering and leaving the from where it is pumped into the aorta and through its branches heart. throughout the body. Directly underlying the endothelium is a thin layer of collagenous and elastic fibers containing occasional fibroblasts External to this, there is a layer of denser connective tissue that makes up the bulk of the thickness of the endocardium. This Figure 1. Systemic Circulation layer is rich in elastic fibers and contains varying numbers of smooth muscle cells, especially in the region of the ventricular septum. SUBJECT 1 Prepared by: Date: HISTOLOGY CARDIOVASCULAR SYSTEM Prepared by: Zen, Mhyr, Julius, Isah, Rasdin Lecturer: DR. FERDAUZIA E. BASTERO Date: September 20, 2024 Therefore, these peptides have an important role in the A subendocardial layer of loose connective tissue binds the regulation of fluid and electrolyte balance, blood volume, endocardium to the myocardium and is continuous with its and blood pressure. endomysium. This layer contains small blood vessels, nerves, and bundles of fibers of the conduction system of the heart. CONDUCTION SYSTEM In the thin wall of the atria, the connective tissue of the The heart normally beats about 70 times a minute. This endocardium may extend through narrow spaces between rate is largely determined by pacemaker cells in sinoatrial muscle fiber bundles to become continuous with that of the node, a small area about 10 mm in length and 3 to 5 mm in epicardium. width, located at the junction of the superior vena cava with the right atrium. A subendocardial layer is absent on the papillary muscles and The nodal myocytes are much more slender than those of chordae tendineae that tether the free edges of the mitral and the rest of the atrium and are endowed with the special tricupid valves. property of undergoing spontaneous depolarization at a frequency of about 70 times a minute, resulting in the generation of impulses that spread to the surrounding 2. Myocardium- The pumping function of the heart atrial myocardium and over special bundles of myocytes, occurs through coordinated contraction (during called the anterior, middle and posterior internodal systole) and pathways, to the atrioventricular node, which is located beneath the endocardium of the interatrial septum, just Left ventricular myocytes are arranged in a spiral above the attachment of the septal leaf of the tricuspid circumferential orientation to generate vigorous valve. coordinated waves of contraction spreading from the cardiac apex to the base of the heart. The components of the conduction system includes the following: In contrast, right ventricular myocytes have a less structured organization, generating overall less robust SA node pacemaker, at the junction of the right atrial contractile forces. appendage and superior vena cava AV node, located in the right atrium along the interatrial Contraction is achieved by shortening of serial contractile septum elements (sarcomeres) within parallel myofibrils. Although Bundle of His (AV bundle), connecting the right atrium to the heart is primarily a pump, it is worth remembering that the ventricular septum it also has other functions (e.g., endocrine). For example, Right and left bundle branch divisions that stimulate their atrial cardiomyocytes have cytoplasmic storage granules respective ventricles via further arborization into the that contain atrial natriuretic peptide, and ventricular Purkinje network myocytes contain B-type natriuretic peptide. Both of these are protein hormones that are released in response to The atrioventricular node is about 6 mm long and 2-3 mm increased stretch; they both promote arterial vasodilation wide and consists of a complex meshwork of specialized and stimulate renal salt and water elimination (natriuresis myocytes embedded in connective tissue. The terminals of and diuresis). the internodal conduction path are in contact with special impulse- conducting Purkinje fibers which continue from Myoendocrine cells of the myocardium are located mainly the node as components of the atrioventricular bundle in the atrial appendages and ventricular septum. (bundle of His). The bundle penetrates the annulus of dense Their most conspicuous feature is an accumulation of small connective tissue that separates the atrial and secretory granules in the central column of sarcoplasm ventricular myocardium and courses down the that contains the nucleus and Golgi complex. interventricular septum for about 1 cm and then divides into right and left branches. Smaller numbers of granules are found between the The right branch, a cylindrical bundle 1-2 mm thick, myofibrils throughout the cell. These cells synthesize a courses down the septum beneath the endocardium prohormone of 126 amino acids which is broken down to a and gives rise to many branches that fan out to the family of smaller peptides that have been variously papillary muscles and all parts of the right ventricle. designated cardiona trin, cardiodilatin, atnopeptin, and The left bundle branch is a flat band that proceeds atrial natriuretic polypeptide. down the left side of the septum and breaks up into branches that radiate throughout the wall of the left Collectively, they have a potent diuretic action and ventricle. increase sodium excretion. The conduction system of the heart enables impulses generated in the pacemaker cells of the They suppress aldosterone secretion by the adrenal and sinoatrialnode to activate the atrium and to travel induce relaxation of vascular smooth muscle. rapidly to the atrioventricular node and then tospread from there over both ventricles. SUBJECT 2 Prepared by: Date: HISTOLOGY CARDIOVASCULAR SYSTEM Prepared by: Zen, Mhyr, Julius, Isah, Rasdin Lecturer: DR. FERDAUZIA E. BASTERO Date: September 20, 2024 More additional information A comparable thickening of the scallop associated with the posterior cusp of the aortic valve provides attachment for The cells of the cardiac conduction system depolarize the septal leaf of thetricuspid valve and continues spontaneously, potentially enabling all of them to function downward as the septum membranaceum, the thin upper as cardiac pacemakers. portion of the interventricular septum. Because the normal rate of spontaneous depolarization in In addition to providing attachment for myocytes and for the SA node (60 to 100 beats/min) is faster than the other the valve leaflets, the annuli around the atrioventricular components, it normally sets the pace. orifices ensure discontinuity between the myocardium of However, if nodal tissues become dysfunctional, other cells the atria and ventricles so that the only in the conduction system can take over, generating, for electro-physiological connection between them is via the example, a junctional escape rhythm (usually at a much specialized conduction tissue of the atrioventricular slower rhythm). bundle. This is necessary to maintain the orderly sequence The AV node has a gatekeeper function; by delaying the of events in the cardiac cycle. transmission of signals from the atria to the ventricles, it ensures that atrial contraction precedes ventricular BLOOD SUPPLY OF THE HEART systole. The blood supply to the heart is carried by the coronary arteries EPICARDIUM which arise from the anterior and left posterior aortic sinuses just above the valve cusps. The epicardium consists of an inner layer of fibroelastic connective The right coronary artery descends between the right atrial tissue and an outer layer of squamous mesothelial cells. The appendage and the pulmonary trunk, then along the connective tissue of the inner layer is continuous with the atrioventricular sulcus to supply the right atrium and endomysium of the underlying myocardium. The major coronary ventricle. blood vessels course through the connective tissue layer of the The larger left coronary artery courses downward between epicardium, enveloped by varying amounts of adipose tissue. the pulmonary trunk and the left atrial appendage and turns to the left in atrioventricular sulcus. it gives rise to The epicardium also forms the visceral layer of the two or three branches running toward the apex of the pericardium, a serous cavity that surrounds the heart. heart. Around the roots of the aorta and pulmonary artery, the Anastomosis have been described between the branches epicardium is continuous with the parietal layer of the of the right and left coronary arteries. pericardium, a similar mesothelium-coated layer of fibroelastic tissue. The visceral and parietal layers of the pericardium bound a narrow space, called the pericardial cavity. This contains a small amount of fluid that permits the smooth mesothelial surfaces of the epicardium and parietal pericardium to glide freely over one another during contraction and relaxation of the heart. If the pericardial cavity becomes infected (pericarditis), these layers may become adherent, obliterating the space between them. They then impose considerable restraint on the beating of the heart. CARDIAC SKELETON The heart has a continuous framework of dense connective tissue components that are referred to collectively as the cardiac skeleton. They provide attachment for many of the working muscle fibers. Its principal components are the annuli fibrosi, encircling the base of the aorta, pulmonary artery, and the Normal cardiac muscle in longitudinal section shows a syncytium of atrioventricular orifices. myocardial fibers with central nuclei. Faint pink intercalated discs The annuli associated with the great vessels consist of cross some of the fibers. The same thich myosin and thin actin three scallops conforming to the bases of the valve cusps. filaments are present in cardiac muscle fibers as are in skeletal The scallop associated with the left posterior cusp of the muscle fibers. Both are “striated” muscle because if tge cross aortic valve has a roughly triangular thickening called the striations that are present. trigonum fibrosum which is continuous with the mitral valve annulus and provides attachment for the anterior leaf of this valve SUBJECT 3 Prepared by: Date: HISTOLOGY CARDIOVASCULAR SYSTEM Prepared by: Zen, Mhyr, Julius, Isah, Rasdin Lecturer: DR. FERDAUZIA E. BASTERO Date: September 20, 2024 Seen at high magnification with a trichrome stain are myocardial fibers forming a syncytium. There are intercalated disk in the fibers of the syncytium. The cross striations and central nuclei are visible. The light brown-staining pigment next to the nuclei is lipochrome, seen in older persons. SUBJECT 4 Prepared by: Date: