Anatomy Reading Material (SDL-1) PDF

Heart The heart is a four-chambered, hollow muscular organ, roughly the size of a clenched fist. It is situated in the mid-thoracic cavity behind sternum between the lungs. About two-thirds of the heart is located on the left of the midline and one- third on the right of the midline. The heart is...

Heart The heart is a four-chambered, hollow muscular organ, roughly the size of a clenched fist. It is situated in the mid-thoracic cavity behind sternum between the lungs. About two-thirds of the heart is located on the left of the midline and one- third on the right of the midline. The heart is enclosed in a serous sac (serous pericardium) which in turn is enclosed in a dense fibrous connective tissue called the fibrous pericardium. The serous pericardium consists of the inner visceral and outer parietal layers with a potential cavity between the two layers called the pericardial cavity. The heart acts as a central muscular pump that pumps about 5 L of blood/min. It takes about 1 min for the blood to reach the most distal parts of the body and back to the heart. Measurements: Size: Size of one’s own fist. Weight of the heart: 280–340 g in adult males 230–280 g in adult females. Chambers of the heart (Fig 10.3) The interior of the heart is divided into four chambers: right and left atria and right and left ventricles (Fig. 10.3):  FIG. 10.3 ​ Chambers of the heart. Right and left atria are above and right and left ventricles are below. Each atrium has an ear-shaped appendage called an auricle. The atria are separated from each other by the interatrial septum while ventricles are separated from each other by the interventricular septum. The right atrium receives the deoxygenated blood from systemic circulation through the superior and inferior vena cavae. Blood from the right atrium passes through the right atrioventricular valve (tricuspid valve) into the right ventricle. The right ventricle pumps the blood into the lungs for oxygenation. The left atrium receives the oxygenated blood from pulmonary circulation through four pulmonary veins. Blood from the left atrium passes through the left atrio-ventricular valve (mitral valve or bicuspid valve) into the left ventricle. Blood vessels The blood vessels form a closed system of tubes that carry blood away from the heart to the tissues of the body and then return it back to the heart. The blood vessels include: 1. Arteries 2. Capillaries 3. Veins. Arteries Characteristic features The salient features of arteries are as follows: 1. They are thick-walled vessels and carry blood from the heart to capillaries. 2. They are often accompanied by vein/veins and nerve/nerves; three of them together form the neurovascular bundle. 3. Their lumen is smaller than that of accompanying vein/veins. 4. They do not have valves in their lumen. 5. They divide repeatedly like a branch of a tree and gradually become smaller in size. Microscopic structure The arterial wall is made up of three layers/coats (Fig. 10.7A and B). From within outwards, these are: 1. Tunica intima 2. Tunica media 3. Tunica adventitia.  FIG. 10.7 ​ Microscopic structure of a medium-sized artery: (A) cross-section showing three different layers in the wall and the lumen and (B) magnified view of the small portion of wall to show the 3 layers or coats in the wall. Tunica intima is made of the endothelium, consisting of flattened cells and basal lamina. Externally, the endothelium is supported by subendothelial loose connective tissue and a fenestrated membrane of elastic tissue called the internal elastic lamina. Tunica media is the thickest layer and is made up of layers of circularly arranged smooth muscle fibres. It also contains elastic fibres and a few collagen fibres present in between layers of muscular fibres. The tunica media is limited externally by a fenestrated membrane of elastic tissue called the external elastic lamina. Tunica adventitia is thin but strongest of all coats. The tunica adventitia is made of longitudinally arranged connective tissue fibres (both elastic and collagen) and connective tissue cells. It merges with the perivascular sheath. The structure of the artery is summarized in Table 10.1. TABLE 10.1 Structure of artery Name of the layer Composition Inner layer (Tunica intim a) Endothelium Subendothelial tissue Internal elastic lamina Intermediate layer (Tunica Circular smooth muscle m edia) fibres Elastic fibres External elastic lamina Outer layer (Tunica adventitia) Fibroelastic tissue C linica l corre la t ion Arteriosclerosis (or hardening of the arteries): It is the age-related progressive generalized degenerative disorder of arterial walls usually seen after middle age. There is generally a slow loss of elasticity and thickening of tunica intima due to an increase in collagen and accumulation of lipid. Consequently, it leads to diffuse narrowing of the lumen of blood vessels. Ischaemia of the tissue supplied by affected arteries occurs. In the lower limb, it is seen in the form of intermittent claudication, characterized by pain and weakness in muscle at work, especially during walking that is almost immediately relieved by rest. In the brain, it is seen in the form of mental function. Thrombosis/atherosclerosis: It is a blood clot that forms on a vessel wall when platelets, proteins and cells stick together. It may block the flow of blood. The details are as under: AN 5.7 It is arteriosclerosis in which plaques (patchy accumulation) of lipid, fibrous tissue and macrophages called atheroma accumulate in the tunica intima, which make the luminal surface of the artery uneven that can initiate the formation of a blood clot called a thrombus which may occlude the lumen, e.g. if it occurs in coronary artery it may lead to myocardial infarction (heart attack). Infarction: It is a tissue death (necrosis) due to an inadequate blood supply to the tissues in the affected area, due to blockages and rupture of arteries etc. AN 5.7 Aneurysm: It is a ballooning of a segment of an artery due to the weakening or thinning of its wall. Sometimes arterial wall becomes thin enough to rupture. Ruptured aneurysm can cause internal bleeding or stroke. It can sometimes be fatal. AN 5.7 N.B. An embolus is a thrombus that has dislodged from the wall of the vessel and moves into the bloodstream. Both thrombus and embolus can occlude flow. An embolus lodged in the coronary artery (coronary embolism) is called coronary thrombus in a vessel of the lung, it is called a pulmonary thrombus and if in a vessel of the brain, it is called cerebral thrombus.  FIG. 10.9 ​ Seven sites where arterial pulses are routinely palpated. C linica l corre la t ion Sites where the arterial pulses are routinely palpated: A pulse is a palpable impulse of pressure wave of blood flow initiated by ventricular systole. The student must know the sites where the arterial pulses are routinely palpated by the clinicians. These seven sites are listed in Table 10.3 and illustrated in Figure 10.9.  TABLE 10.3 Seven routinely palpated sites of arterial pulse Pulse Site Assessment criteria Carotid Along the anterior border Most reliable pulse in pulse of sternocleidomastoid physiological shock or muscle in neck at the level cardiac arrest, when pulses of the cricoid cartilage at other sites are not palpable Brachial Medial to the tendon of To auscultate the blood pulse biceps brachii in front of pressure elbow Radial Radial side of front of Most commonly felt pulse to pulse forearm at wrist lateral to assess the character and rate the tendon of flexor carpi of the pulse radialis Femoral Below the inguinal To locate femoral vein for pulse ligament midway between venipuncture, which lies pubic symphysis and just medial to it anterior superior iliac spine Popliteal Behind the knee in the To assess the status of pulse popliteal fossa circulation in the lower limb Posterior Medial side of ankle To assess the status of tibial midway between medial circulation in the foot pulse malleolus and tendocalcaneous Dorsalis Over the dorsum of the To assess the status of pedis foot just medial to the circulation in the foot pulse tendon of extensor hallucis longus Veins Characteristic features The salient features of veins are as follows: 1. They are thin-walled vessels that carry blood from capillaries to the heart. 2. The large veins are formed by the union of smaller veins like tributaries of a river. 3. They have a larger lumen as compared to arteries and less amount of muscular and elastic tissue in their walls. 4. Their lumen is often provided with valves, which prevent the reflux of the blood and thus, maintain a unidirectional flow of blood even against gravity. 5. Large veins have dead space around them for their dilation during increased venous return. General structure The general structural features of the veins are similar to those of arteries, i.e. their wall is also made up of three layers but they are ill-defined and seen clearly only in large veins (Fig. 10.10). The tunica adventitia is the thickest and the best developed layer. It contains collagen, elastic and muscle fibres. However, the bundles of smooth muscle fibres in this layer are arranged longitudinally.  FIG. 10.10 ​ Microscopic structure of a large-sized vein: (A) cross-section showing different wall layers and the lumen and (B) magnified view of the wall layers. V = vasa vasorum, L = longitudinal bundles of muscle fibres. The tunica media is poorly developed. The lumen of veins, being thin- walled is often collapsed and contains some blood. The proper internal elastic lamina in the tunica intima is absent. The differences between arteries and veins are enumerated in Table 10.5. TABLE 10.5 Differences between arteries and veins AN 5.3 Arteries Veins Thick-walled Thin-walled More muscular Less muscular More elastic Less elastic Smaller lumen (always remain Larger lumen (may be collapsed) patent) Tunica media thicker than Tunica media thinner than tunica adventitia tunica adventitia No valves in the lumen Valves mostly present in the lumen N.B. Thickest coat in arteries is tunica media, whereas the thickest coat in veins is tunica adventitia. Classification of veins The veins are classified into the following three types: 1. Large 2. Medium-sized 3. Small (also called venules). Large veins: All three layers of wall, i.e. tunica intima, tunica media and tunica adventitia are well differentiated. The tunica adventitia is always thicker than tunica media. The walls of large veins have some elastic tissues that resist the pressure of the right atrial systole, e.g. inferior vena cava (IVC) and superior vena cava. Medium-sized veins: These veins also have all three layers in their wall but it becomes gradually difficult to distinguish the three layers with decreasing size of medium-sized veins, splenic vein, testicular vein, etc. Small veins or venules: Venous valves The inner lining of most medium- and small-sized veins are thrown at intervals into delicate semilunar folds called cusps. With the wall of the vein, each cusp forms a bulging pocket or sinus. The cusps are arranged in pairs facing each other to form valves (Fig. 10.11). The valves open only in one direction, i.e. towards the heart. FIG. 10.11 ​ Opening and closing of venous valves: (A) the valve opens by forward pressure towards the heart and (B) the valve closes by backward pressure. N.B. Venous valves are bicuspid and consist of two valvule. The valves present near the termination of the internal jugular, subclavian and femoral veins prevent the venous blood from being forced back into the head, neck and limbs during increased intra-thoracic pressure (e.g. during deep inspiration) and during increased intra-abdominal pressure (e.g. during defaecation; Fig. 10.12). FIG. 10.12 ​ Venous valves near the heart. N.B. The valves are most numerous in the veins of the limbs, and they are commonly placed just before the mouth of a tributary. The muscle fibres in the venous wall tend to arrange in a loop near the point of drainage of a tributary to act as a sluice gate.  C linica l corre la t ion Sites of venepuncture: The superficial veins are commonly used for obtaining blood samples or giving intravenous injections, transfusions and infusions. The common sites of venepuncture are: (a) Cubital fossa (into median cubital, cephalic, and basilic veins) (b) Dorsal aspect of hand (into a dorsal venous arch) (c) In front of the ankle joint (into the great saphenous vein) Circulation of blood In adult individuals, the blood circulates through an estimated 60,000 miles of vessels throughout the body to provide food and oxygen to trillions of living cells in the body. At the same time, it removes the waste products of their metabolism. Types of blood circulation AN 5.2. AN 5.5 Blood circulation in the body can be categorized into the following three types (Fig. 10.15): 1. Pulmonary circulation: The deoxygenated blood entering the right atrium passes to the right ventricle which pumps it through the pulmonary trunk and pulmonary arteries to the capillaries of the lungs (where it is oxygenated), then through the pulmonary veins it returns to the left atrium. 2. Systemic circulation: The oxygenated blood from left atrium passes to the left ventricle which pumps it through aorta and its branches to the capillaries of the rest of the body. The capillaries join to form veins that become increasingly larger till finally the superior and inferior vena cavae and cardiac veins return the deoxygenated blood to the right atrium. The right atrium receives same amount of deoxygenated blood that was pumped by the left ventricle, and at the same rate. N.B. Students must note that blood appears red when oxygen content is high and appears blue when oxygen content is low. Therefore, blood is ‘red’ in pulmonary veins, on the left side of the heart, and in systemic arteries, whereas the blood is ‘blue’ in systemic veins, right side of the heart, and in pulmonary arteries. These structures, therefore, should be coloured red and blue accordingly in anatomical illustrations. Difference between pulmonary and systemic circulation Pulmonary circulation Systemic circulation Sends deoxygenated blood from Sends oxygenated blood from the heart to lungs through heart to rest of the body, through pulmonary arteries for aorta and its branches to whole oxygenation and receives body and returns deoxygenated oxygenated blood from the blood from body to the heart lungs to the heart through through superior and inferior vena pulmonary veins cava FIG. 10.15 ​ Types of circulation: pulmonary, systemic and portal. TABLE 10.6