Biology

The Circulatory System

The multicellular organisation in animal world has resulted in the origin and evolution of circulatary system in animals. This arrangement facilitates internal transport of various substances to all organs and organ systems. Among majority of multicellular animals this system remains as a closed type.  It has blood running inside closed blood vessels, the blood being pumped by heart. In man, as in all mammals there is a double circulation of blood. The primary circulation through pumping action of heart, supplies blood to all regions of the body. The blood later returns to the heart. It is called the systemic circulation or body circulation. A similar circulation carries blood to lungs for oxygenation and returns it back to the heart. It is called the pulmonary circulation.

Systemic and Pulmonary circulations
The most important component of this system is the heart. It is a large, muscular, valved structure having four chambers. The chambers are the right atrium, left atrium, right ventricle and left ventricle. Each atrium opens into a corresponding ventricle. The right and left chambers are separated by septa.

Systemic circulation :- The left atrium receives oxygenated blood from the lungs, through the pulmonary vein. When the atria contract, blood from the left atrium is forced into the left ventricle. Later by a contraction of the ventricle, the blood leaves the heart through the aorta. The aorta is the single systemic artery emerging from the heart. By successive branchings, the aorta gives rise to hundreds of arteries taking blood to all regions of the body. As the branchings happen, the arteries divide into numerous (4 × 106) arterioles. In the target organs they produce four times as many capillaries. A similar number of venules converge into each other forming veins of increasingly larger size. Finally, only two veins, the superior and inferior vena cavae return the blood to the right atrium. Thus the course of blood from left ventricles through the body organs and back to the atrium forms the systemic circulation

Pulmonary circulation :- The venous blood from right atrium is conducted to the right ventricle. The ventricle expels the blood via the pulmonary trunk to the lungs. The oxygenated blood later returns by the pulmonary veins to the left atrium. This circulation from right ventricle to the left atrium via the lungs  is termed the pulmonary circulation.

Portal circulation :- In the systemic circulation the venous blood passing through spleen, pancreas, stomach and interstine is not carried back directly to the heart. It passes through the hepatic portal vein to the liver. This vein begins as capilaries from the visceral organs and ends in the liver again as capillaries. These capillaries converge to form the hepatic vein which joins the inferior vena cava, conveying blood to right atrium. This route is the portal circulation.

Arterial system : The main arterial trunk carrying blood from the heart to the tissues is the aorta. At its commencement it lies in the thorax, and it is known as the thoracic aorta. Its terminal portion is in the abdomen, where it is called as the abdominal aorta. The thoracic aorta comprises the short ascending aorta, the arch of the aorta and the longer descending aorta. The descending aorta passes downward through the diaphragm. It continues up to the level of the fourth lumbar vertebra, where it terminates by dividing into two common iliac arteries. The main branches of the various portions of the aorta are as follows.

Ascending aorta:  The branches of the ascending aorta are the left and right coronary arteries. They arise close to the origin of the aorta. They supply the heart muscles with blood.

Arch of the aorta: The branches of the aortic arch are the innominate, left common carotid and left sub clavian arteries.

Innominate: (Brachiocephalic) this is the first branch of the aortic arch. It passes upward and diagonally to the right and divides into two branches, the right common carotid and the right subclavian. The common carotid artery passes up to the neck and divides into the external carotid artery and internal carotid artery. The external carotid artery supplies blood to muscles, glands, skin and other structures of the face and scalp. The internal carotid artery goes to the brain. The right subclavian artery passes laterally to the arm. In the arm, the subclavian artery forms the axillary arteries. The right axillary continues as the brachial artery, which in the region of the elbow, divides into radial and ulnar arteries. These continue in the distal portion of the arm, giving off branches to wrist and hand. The second branch of the aortic arch is the left common carotid artery. It arises close to the innominate. It passes up the left side of the neck, following a course and possessing branches similar to that of the right common carotid. The left sub clavian artery arises from the aortic arch independently of the left common carotid and supplies the left arum.

Thoracic aorta: The branches of the thoracic portion of the descending aorta supply both the internal organs and the body wall. The internal organs include bronchi, esophagus, pericardium, lungs, rib cage and intercostal muscles. Superior phrenic arteries (one pair) supply blood to the upper surface of the diaphragm.

Abdominal aorta. This portion of the aorta has many branches. It serves the entire lower region of the body. The following branches are given off from the abdominal aorta.

1. The coeliac artery arises shortly behind the diaphragm, and sends branches to the liver, gall bladder, stomach and duodenum and digestive glands such as pancreas and liver.

2. The superior mesenteric artery, supplies the major portion of the small intestine and a part of the large intestine.

3. The middle suprarenal arteries, supply the suprarenal glands,

4. The renal arteries supply the kidneys.

5. The internal testicular arteries in the male and ovarian arteries in the female supply the testis and ovary respectively.

6. The inferior mesenteric artery supplies, the large intestine and rectum. The common iliac arteries from the dorsal aorta enter into the legs. Each iliac artery further divides into sciatic and femoral arteries supplying blood to the leg muscles.

Pulmonary circulatory structures: The pulmonary artery emerges from the superior surface of the right ventricle, passes diagonally upward to the left, and crosses the root of the aorta. It divides into right and left pulmonary arteries, branches of which enter the right and left lungs, respectively. Pulmonary veins are four in number, two from each Lang. They carry oxygenated blood from the lungs to the left atrium

Venous system:  Systemic veins are the vessels that collect blood from the tissues to the heart. The principal systemic veins are:
1). The coronary sinus.
2). The superior vena cava.
3). The inferior vena cava and their branches and
4). The portal vein which drains the abdominal viscera.

Coronary sinus: The coronary sinus is a short vein lying on the posterior side of the heart. It receives most of the blood from heart tissues.

Superior vena cava and its branches: This vessel is a large venous trunk, which empties blood to the heart from the head, neck, upper extremities and thorax. It is sounded by the union up the right and left Innominate (brachiocephalic) veins. It opens into the right atrium. The principal veins draining the head and neck are the external and internal jugular veins. The right and left subclavian veins drain the upper extremities, each terminating at its junction with the internal jugular vein to form the innominate vein. Near their termination form each subclavian vein

receives an external jugular vein. Each subclavian vein inturn is formed by the union of the cephalic and axillary vein. Each innominate vein receives the deep cervical, vertebral, internal mammary and inferior thyroid veins. In addition, the left innominate receives the left superior intercostals vein and veins from thymus, trachea, esophagus and pericardium. The subclavian veins also drain blood from the veins of the hand (brachial)


Inferior vena cava and its branches: This is the venous trunk which receives most of the blood from regions of the body below the level of the diaphragm. It is the largest vein in the body. The inferior vena cava is formed by the union of the two common iliac veins. It extends forwards to the right of the aorta, passes through the diaphragm and opens into the right atrium. The inferior vena cava receives blood from following veins, inferior phrenic (from the diaphragm), hepatic (from the liver), right suprarenal (from the adrenal gland), renal (from the kidneys), right spermatic or ovarian (from the right testis or ovary), lumbar (from the body wall) and common iliac (from the pelvic region) and the veins of the lower extremities

Pulmonary circulation :- The venous blood from right atrium is conducted to the right ventricle. The ventricle expels the blood via the pulmonary trunk to the lungs. The oxygenated blood later returns by the pulmonary veins to the left atrium. This circulation from right ventricle to the left atrium via the lungs is termed the pulmonary circulation.

Portal circulation :- In the systemic circulation the venous blood passing through spleen, pancreas, stomach and interstine is not carried back directly to the heart. It passes through the hepatic portal vein to the liver. This vein begins as capilaries from the visceral organs and ends in the liver again as capillaries. These capillaries converge to form the hepatic vein which joins the inferior vena cava, conveying blood to right atrium. This route is the portal circulation.

Components of Circulatory system
Blood vessels :  The blood vessels carrying blood away from the heart are the arteries. The Veins carry blood towards the heart. The arteries and veins are named and classified according to their anatomical position. They can also be classified according to their size and wall structure. Functionally, arteries are subdivided into conducting, distributing and resistance vessels.

1. Conducting vessels :– These are large arteries from the heart and their main branches. the walls of these vessels are elastic in nature.

2. Distributing vessels :- These are smaller arteries reaching individual organs. They branch into the organs. They have muscular walls.

3. Resistance vessels :- These are mostly arterioles. While these vessels are smaller, their walls are highly muscular. Hence these vessels can reduce  pressure of blood due to peripheral resistance.

4. Exchange vessels :- These are the capillaries. The walls of these vessels allow exchanges between blood and the tissue fluid surrounding the cells. The substances commonly exchanged are oxygen, carbon-di-oxide, nutrients, water, inorganic ions, vitamins, hormones, metabolic products and antibodies.

5. Capacitance or reservoir vessels :- These are the larger vessels and veins. These are of varying sizes. They collect and convey blood back to the heart. The higher capacitance of these vessels is due to their distensibility. Hence their blood content is more, even at low pressure. The number of such veins is also enormous.. Thus the veins are called as the “blood reservoirs”

Structure of blood vessels   The blood vessels show a vast range of structural modifications.
However a few basic patterns can be studied. A blood vessel consists of a wall and a lumen or cavity. The wall of the blood vessels is made up of 3 distinct layers or tunica. They are the tunica intima, tunica media and tunica externa or tunica adventitia.

The tunica intima is formed of an endothelium, a delicate connective tissue and elastic fibres. The tunica media contains smooth muscle cells. It causes vasoconstriction and vasodilation. The tunica externa is composed of connective tissue. The composition and thickness of layers varies with the diameter of the blood vessels and the type.

Types of blood vessels
1. Large elastic arteries :- The walls of these arteries contain elastic fibres. The smooth wall measures about 1micron in thickness. It gets stretched under the effect of pulse and recoils elastically

2. Muscular arteries :- There are larger and smaller muscular arteries. The larger muscular arteries are inelastic and they have thick walls. The wall has 30-40microns in diameter in the layers of smooth muscles. Since they regulate blood supply, they are called distributing arteries. The small muscular arteries are capable of vasodilation and vasoconstriction.

3. Arterioles :- They conduct blood from the arteries to the capillary bed. These are small vessels capable of vasodilation and vasoconstriction.

4. Capillaries :- These are fine vessels found between arterioles and venules. They measure 5-8micron in diameter.

5. Venules :- These are tubes of flat, oval or polygonal endothelial cells. Each venule is formed by the convergence of two or more capillaries. Its diameter ranges upto 30micron.

6. Veins :- Veins seen in anatomy are medium veins. They run in between venules and large veins. Large veins transport blood to the heart. Veins with diameter above 2 mm have valves. They are of semilunar type. They allow movement of blood towards the heart. There are several valves in the medium veins.

Branching of blood vessels :– When an artery divides into two equal branches, the original artery ceases to exist. Hence the branches are called terminal branches. The smaller branching vessels formed on the sides are called the collateral branches. When arteries are joined to each other it is named as anastomosis.

Blood supply to blood vessels :- As any other region, the cells and tissue on the wall of the blood vessel require nourishment. Some amount can diffuse from blood in the lumen. For vessels having diameter greater than 1 mm, diffusion of nutrients may not be possible. Such vessels have very minute vessels called vasa vasorum spread over them. They penetrate into the wall of the blood vessels.

Innervation of blood vessels :– The walls of the blood vessels are innervated by sympathetic nerve fibres. They regulate the contraction of the musculature. They effect vasoconstriction.

The Heart
The heart is a hollow, fibromuscular organ. It is somewhat conical or pyramidal in form. It is roughly the size of a closed fist. An average heart measures 12 cm from base to the apex. Transverse diameter at its broadest region is 8-9 cm. It is 6 cm thick antero-posteriorly. While in adult male the heart weighs 280-340 g, in female it weighs 230-280 g.

The thoracic organs such as heart, trachea and oesophagus form a midline partition called the mediastinum. The heart lies obliquely in the mediastinum. The heart is surrounded by a double layered membrane called the pericardium. The outer layer is called the fibrous pericardium. The inner membrane is called the serous pericardium. In between heart and pericardium, there is a pericardial space. This space is filled with a fluid called the pericardial fluid.

The wall of the heart is made up of three tissue layers. They are the epicardium, myocardium and endocardium. The epicardium forms the smooth outer surface of the heart. The middle myocardium is composed of cardiac muscle. This layer plays an important role in the functioning of the heart. The endocardium forms the smooth inner surface. It is formed of squamous epithelium. . 

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Mallikarjuna

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