Blood Circulation in Human

Blood circulation in human

The physiology of blood circulation was first described by Sir William Harvey in 1628. The blood circulation in our body is divisible into 3 circuits –

(1) Coronary circulation : It involves blood supply to the heart wall and also drainage of the heart wall.

(i) Coronary arteries : One pair, arising from the aortic arch just above the semilunar valves. They break up into capillaries to supply oxygenated blood to the heart wall.

(ii) Coronary veins : Numerous, collecting deoxygenated blood from the heart wall and drains it into right auricle through coronary sinus which is formed by joining of most of the coronary veins. But some very fine coronary veins, called venae cordis minimae open directly in the right auricle by small sized openings called foramina of Thebesius.

(2) Pulmonary circulation : It includes circulation between heart and lungs. The right ventricle pumps deoxygenated blood into a single, thick vessel called pulmonary aorta which ascends upward and outside heart gets divided into longer, right and shorter, left pulmonary arteries running to the respective lungs where oxygenation of blood takes place. The oxygenated blood from lungs is returned to the left auricle by four pulmonary veins. Left auricle pumps this blood into the left ventricle.

(3) Systemic circulation : In this, circulation of blood occurs between heart and body organs. The left ventricle pumps the oxygenated blood into systemic arch which supplies it to the body organs other than lungs through a number of arteries. The deoxygenated blood from these organs is returned to the right auricle through two large veins (precaval and post caval). Right auricle pumps this blood into the right ventricle. Thus, the sytemic circulation involves two circuits –

Arterial system

It involves aorta, arteries, arterioles and meta-arterioles. It supplies oxygenated blood to all parts of the body except lungs.

The left ventricle of the heart pumps the oxygenated blood into a single, question marked shaped, long vessel called left carotid-systemic aorta. It is the largest blood vessel of the body.

After ascending from the heart, the systemic aotra turns and descends down to the level of lower border of fourth lumbar vertebra. At its distal extremity, it bifurcates into right and left common iliac arteries. The sytemic aorta has following parts –

(1) Ascending aorta : It gives off left and right coronary arteries.

Brachiocephalic (innominate) : Unpaired, largest branch of the aorta divides into right subclavian towards right side and right common carotid towards left side. Right subclavian gives off vertebral artery (supplies to head and part of right shoulder) and then enters into right arm, now called axillary artery or brachial artery, which divides into ulnar and radial arteries in the region of elbow. The right common carotid, enters into head and divides into external and internal carotids which supply the right parts of head by their branches.

Left common carotid : Unpaired artery, enters into head and divides into left external and internal carotids which supply the left parts of the head by their tributaries. The external carotids of both sides provide blood to thyroid gland, tongue, throat, face, ear, scalp. The internal carotids of both sides supply to brain, eye, inner part of nose and forehead. These internal carotids go upward and enter skull through foramen magnum and unite at the base of brain along with the vertebral arteries of both sides. So, there is formation of a ring shaped artery called as 𠇌ircle of willis”. From this circle, many branches or arteries arise which go to different parts of brain.

In frog, the internal carotid has at its base, carotid labyrinth (spongy mass of non-contractile fibro-elastic tissue) which acts as a sensory organ to detect blood pressure in artery.

Left subclavian artery : Unpaired artery, it gives off a left vertebral artery (supplies to head and part of left shoulder) and then enters into left arm, now called left axillary artery or left brachial artery which divides into ulnar and radial arteries in the region of elbow. (2) Descending aorta : The aorta turns towards the back of heart and finally converts into dorsal aorta. The descending dorsal aorta is called thoracic aorta in throcic region and abdominal aorta in abdominal region.

From thoracic segment of aorta : Several pairs of small arteries arise in this region to supply various parts such as pericardium (pericardial artery) lungs and bronchi (bronchial artery) oesophagus (oesophageal artery) mediastinal organs and thymus (mediastinal artery) intercostal muscles and mammary glands (intercostals and subcostal arteries) upper surface of diaphragm (superior phrenic artery).

From abdominal region of aorta : In the abdominal region, abdominal aorta gives off several pairs of arteries. Some of the major ones are as follows –

Inferior phrenic artery : Right and left to supply the lower surface of the diaphragm.

Coeliac artery : Unpaired, divides into three branches

(i) Left gastric artery : To stomach.

(ii) Common hepatic artery : To pylorus, pancreas, gall bladder, liver, cystic duct, hepatic ducts etc.

(iii) Splenic artery : To pancreas, stomach and spleen.

Superior mesenteric : Unpaired, supplies various parts of small intestine (except superior part of duodenum part of colon and caecum). Its sub branches are

(i) Pancreo duodenal artery : To pancreas and duodenum.

(ii) Jejunal artery : To jejunum.

(iii) Ilial artery : To ileum and jejunum.

(iv) Iliocolic artery : To ileum and colon.

Supra renal artery : Supplies the adrenal glands.

Renal arteries : One pair, supply to kidney.

Lumbar arteries : 4 pairs, supply the skin, muscles, joints, vertebrae, meninges, spinal cord etc. in the lumbar region.

Sacral artery : Supplies the tissues of sacral region.

Inferior mesenteric artery : Unpaired, supplies most part of colon, rectum and anal canal.

Common iliac arteries : Two, right and left, formed by bifurcation of aorta at its lower end. Each common iliac artery divides into external and internal iliac arteries. The internal iliac (hypogastric) artery supplies lies viscera and wall of pelvic region, perineum and gluteal regions. The external iliac artery enters into the leg now called femoral artery continues down the thigh, now called popliteal artery which bifurcates into anterior and posterior tibial arteries, at about the level of knee.

Venous system

It originates in tissues by union of capillaries and ends in the atrium of heart. It includes two major veins – superior and inferior vena cava which drain the deoxygenated blood into the right atrium.

(1) Superior vena cava (pre caval) : Single, formed by the union of right and left brachiocephalic (innominate) veins. It collects blood from head, neck, arms and chest region. It involves the following veins –

Brachiocephalic veins : Two, each is formed by the union of an outer subclavian vein and medial internal jugular vein. Each vein also receives blood from different thoracic parts of its sides through three main veins.

(i) Internal thoracic vein : From some muscles and mammary glands.

(ii) Inferior thyroid vein : From thyroid gland.

(iii) Left superior intercostal vein : From upper part of thorax.

Internal jugular vein : Two, right and left. Each one is formed by the union of numerous sinuses and veins of the cranial cavity, superior part of the face and some part of neck and collects blood from these regions.

Subclavian veins : Two, right and left, formed in the shoulder region by union of cephalic and axillary veins of respective sides.

(i) Axillary veins : Two, right and left, present in the respective arms and collect blood from these regions.

(ii) Cephalic veins : Two, right and left, collect blood from respective arms and shoulder region.

External jugular veins : Two, right and left, open into respective subclavian vein. They collect the blood from parotid gland, facial muscles and superficial parts of cranium.

Azygos and hemiazygos veins : Azygos vein originates in lumbar region towards right side of mediastinum and ascends upwards small veins from lumbar and thoracic parts of backbone, oesophagus, mediastinum, pericardium etc. empty into it.

Towards the left side of the body originates hemiazygos and accessory hemiazygos collects blood from oesophagus, mediastinum, intercostal muscles, mammary glands etc. and drains into Azygos which in turn opens into superior vena cava. Accessory hemiazygos drains blood into left innominate vein.

(2) Inferior vena cava : It is the largest vein, originated in inferior lumbar region by the union of right and left common iliac veins and opens into right atrium by separate opening. It collects blood from all body structures below the diaphragm. It involves following veins –

Common iliac veins : Two, right and left. Each one is formed by union of external and internal iliac veins.

External iliac vein : This is the continuation of femoral vein which collects blood from leg. Femoral vein in turn is formed by the union of anterior tibial vein, posterior tibial vein, popliteal vein, large saphenous vein, small saphenous vein, etc. which collect blood from different parts of leg. External iliac vein also collects blood from pubic region and parts of pelvis through number of small veins. Great saphenous vein is the longest vein of the body.

Internal iliac (Hypogastric) veins : Two, right and left. Each one is formed by union of number of small veins, which collect blood from pelvis, pelvic viscera, pelvic girdle, sacrum, rectum, ureter, urinary bladder, uterus, vagina, prostate glands, seminal vesicle, penis, scrotum etc. (i.e. number of reproductive organs).

Lumbar veins : Four pairs, which collect blood from muscles, skin and vertebrae of lumbar region and drains it into inferior vena cava.

Genital veins : In man, right testicular vein collects blood from male organs and inguinal regions and drains it into inferior vena cava. Left testicular vein drains the blood into left renal vein. In woman, the right ovarian vein drain blood from ovaries, uterus etc. and empties into inferior vena cava. The left ovarian vein opens into left renal vein.

Renal veins : Two, right and left collects blood from respective kidneys and opens into inferior vena cava. The left renal vein is about three times longer than the right one.

Suprarenal vein : Two, right and left, collects blood from adrenal glands. Right one opens into inferior vena cava whereas left one opens into left renal vein.

Inferior phrenic veins : These veins drain the blood from lower surface of diaphragm. The right one ends in post caval. The left one is often doubled with its one branch ending in left renal or suprarenal vein and the other in post caval.

Hepatic veins : They drain blood from liver into the post caval. Urea is maximum in hepatic vein while it is minimum in renal vein.

Portal system

It is a part of venous circulation which is present between two groups of capillaries i.e. starts in capillaries and ends in capillaries. The vein which drains blood into organs other than heart is called portal vein.

Types of portal system : It is of following types :

(1) Hypothalamo-hypophysial portal system : Present in higher vertebrates (amphibia, reptiles, birds and mammals). Blood from hypothalamus is collected by hypophysial portal vein which ends in anterior lobe of pituitary gland. The superior hypophysial artery which bring blood into circle of willis bifurcate outside the lobe one branch supplies the lobe itself, but the other one supplies the hypothalamus. The vein that drain the blood from hypothalamus then runs into pars distalis and divide into capillaries. Thus this is a portal vein called hypothalamo-hypophysial portal vein.

Function : This portal system enables the releasing factors and inhibiting factors from hypothalamus to reach upto anterior pituitary.

(2) Hepatic portal system : Found in all chordates. In mammals, there is a single vein called hepatic portal vein, formed by the union of six main veins, which drain venous blood from different parts of alimentary canal (digestive system) into the liver. These veins are :

(i) Posterior or Inferior mesenteric vein : Collect blood from rectal wall and anal region. This vein possess maximum diluted blood. Posterior mesenteric made up of by joining of 4 small veins that is rectal vein, sigmoid vein, left colonic vein and it opens into the splenic vein.

(ii) Anterior or Superior mesenteric vein : Collect blood from wall of colon, caecum and small intestine. This vein possesses largest concentration of nutrients (glucose, amino-acid and vitamins). This vein formed by the joining of right colonic vein, ileocolic vein and appendicular vein.

(iii) Splenic vein : Collect blood from spleen and pancreas, splenic vein possess free haemoglobin in large amount.

(iv) Right gastric vein : Receives blood from stomach.

(v) Left gastric vein : Receives blood from stomach and pancreas.

(vi) Cystic vein : Receives blood from gall bladder.

Posterior mesenteric vein open into splenic vein and splenic, anterior mesenteric, right gastric fused to form hepatic portal vein, which leads blood in to the liver.

In amphibians (example – frog), hepatic portal system is formed of single hepatic portal vein and single anterior abdominal vein. The latter collects blood from leg region and drains it into the left lobe of liver.

Significance of hepatic portal system : The hepatic portal system has following significance.

(a) The blood which comes from the alimentary canal contains digested food like glucose and amino acids. The excess of glucose is converted into glycogen which is stored in the liver for later use. When an individual feels deficiency of food, the glycogen is converted into glucose and is transferred to the blood stream via hepatic veins.

(b) Harmful nitrogenous waste like ammonia is converted into urea which is later removed by kidneys. Thus the blood is detoxified (purified) of harmful nitrogenous waste.

(c) Liver produces blood proteins which are put into blood circulation.

(3) Renal portal system : It is well developed in fishes and amphibians it is reduced in reptiles and birds and is absent in mammals. This system carries blood from the posterior region of the body to the kidneys by renal portal veins, hence its name. The kidneys remove the waste products from the blood and then the blood is passed to the post caval by renal veins. Why renal portal system is absent in mammals ? The mammals have no renal portal system because. The heart of mammals is four chambered. Due to the four chambered heart in mammals there is total separation of oxygenated and deoxygenated blood.

Renal portal system in frog consists of one pair of renal portal vein, each one formed by the union of femoral vein and sciatic vein. It collects blood from leg region and drains it into kidney. It also collects blood from dorsal part of lumbar region through dorsolumbar vein.

Function : Renal portal system helps in blood filtration by draining it into kidney which filters the blood

Lymphatic system

It is a part of greater circulation which begins in the tissue fluid with lymphatic capillaries which are always terminally closed. This system terminates into venous system near heart. The main components of this system are :

(1) Lymph : Lymph can be defined as blood minus RBC&aposs. In addition to the blood vascular system all vertebrate possess a lymphatic system. It is colourless or yellowish fluid present in the lymph vessels. It is a mobile connective tissue like blood and is formed by the filtration of blood. This process involves the diffusion of substances from blood capillaries into the interstitial space which is, thus, the primary site of lymph formation. Two forces bring about a steady filtration of plasma fluid into the tissue spaces : capillary pressure (30-35 mm Hg) and colloid osmotic pressure in tissue fluid (8 mm Hg). After absorption by veins, a small amount of CO₂ and waste material still remains in the tissue fluid which is absorbed in the lymphatic capillaries as lymph. So, we can say that lymph is modified tissue fluid.

Hence, lymph can be represented as :

Lymph = Blood – [RBC + platelets + plasma proteins of high molecular weight]

Composition of lymph : Microscopic examination of lymph depicts that is contains a large number of leucocytes (mostly lymphocytes) ranging from 500 to 75,000 per cubic mm. No blood platelets present. The composition of the non cellular part of lymph (fasting) is as follows :

(i) Water 94%                            (ii) Solids 6%

(a) Proteins : Protein content is roughly half of the plasma and varies from 2.0 – 4.5%. It varies according to the part of the body from which is collected, i.e. in liver 6%, in limb 2% of intestinal part 4%. The varieties of proteins are found – albumin, globulin and fibrinogen. In addition to this, traces of prothrombin, fibrinogen.

(b) Fats : In fasting condition fat content is low but after a fatty diet it may be 5.0 – 15%.

(c) Carbohydrates : Sugar, 132.2 mgm per 100 ml.

(d) Other constituents : Urea, creatinine, chlorides, phosphorus, calcium, enzymes and antibodies (120 ml./hour).

$\bullet$  Normally the rate of lymph formation is equal to the rate of its return to the blood stream.

(2) Lymphatic organs : In human primary lymphatic (lymphoid) organs of the body are the Red bone marrow and Thymus gland. They are called primary lymphatic organs because they produce B and T cells the lymphocytes that carry out immune response. Haemopoietic stem cells in red bone marrow gives rise to B Cell and pre-T cells. Pre-T cells then migrate to thymus gland. Secondary lymphatic organs are the lymph nodesਊnd spleen.

(i) Spleen : Spleen is mesodermal in origin. Spleen is the largest solid mass of reticulo-endothelial tissue in the body. In human measures about 12 cm (5 inch) in length and is situated in the left hypochondriac regions between the stomach and diaphragm. Like lymph nodes, spleen has hilus, where splenic artery, vein and efferent lymphatic vessels pass through. Spleen never filter lymph, because has no afferent lymphatic vessel. Histologically it is formed by following structure –

(a) Capsule : It is the outer covering of spleen formed of dense connective tissue and smooth muscles. The outer layer of the capsule is the serous coat formed of visceral peritoneum.

(b) Trabeculae : Narrow fold like septa or trabeculae extend inwards from the capsule, dividing the spleen tissue into several incomplete lobules.

They provide support and convey blood vessels in to the interior of spleen.

(c) Splenic pulp : The reticulo-endothelial tissue is called splenic pulp. It contains a denser network of blood capillaries, small sinuses and fine blood vessels. The meshes of this network are studded with numerous splenic cells, red, blood corpuscles, macrophages and lymphocytes. The splenic pulp is of two distinct types –

(i) White pulp                           (ii) Red pulp

In mammal embryos the red pulp contains myelocytes, erythroblast and also megakaryocytes. These types of cells are not present in adult spleen except in certain pathological condition.

Function : Although located close to the alimentary canal, the spleen has nothing to do with digestive system. it is, in fact, an important constituent of the reticuloendothelial system of body and performs the following functions:

(a) Its macrophages engulf (= phagocytize) and destroy wornout blood corpuscles (RBC + platelets), dead and live pathogens, cell debris, pigment granules and other useless particulate materials, thus regularly cleaning the blood of its impurities.

(b) It is active haemopoietic organ. In foetal life, the red pulp possess myeloblast, erythroblast and megakaryocytes. Hence, in foetus, it produces blood. In adults, the red pulp possess macrophages, plasma cells and lymphocytes. So, in adults, it is not producing blood rather it is screening blood.

(c) In adults, it also serves as a sort of 𠇋lood bank”. Its sinuses act as “reservoirs of blood”.

(d) White pulp of spleen functions in immunity as a site of B cell proliferation into antibody-producing plasma cells.

(e) Spleen also acts as Graveyard or Slaughter house of worn out RBCs.

(f) Haemolysin is formed in spleen (Lysolecithin).

(g) Haemoglobin is broken down into haem and globin by spleen.

Besides all these functions, the primary function of spleen is that it assists liver and helps in maintaining the composition of blood.

(ii) Thymus gland : In human thymus is located in mediastinum, between the lungs. The two thymic lobes devide into lobules by trabeculae. Each lobule consist of cortex and medulla. Cortex composed of tightly packed lymphocytes, epithelial cells, Macrophages. Pre-T cells migrats (via blood) from red bone marrow to thymus, where they proliferats and develop into mature T cells. Medulla consist of mostly of epithelial cells and more widely scattered lymphocytes. Epithelial cells produce thymin hormone for maturation of T cells. Medulla also contain characteristic thymic (Hassall’s) corpuscles, possibly, they are remanants of dying cells.

Lymphatic system in human

Lymph capillaries : Small, thin, lined by endothelium resting on a basement membrane and fine whose one end is blind and other end unites to form lymphatic ducts. These are present almost throughout the body but are absent in brain, eyeball, spinal cord, internal ear, bone marrow etc. Lymph capillaries in the region of small intestine in villi are called “lacteals” which collect chyle which is milky white in colour due to absorbed fat. Lacteals help in the absorption of digested fat.

Lymphatic ducts or vessels : Numerous, present in various parts of body. These vessels are like veins as they have all the three layers – tunica externa, tunica media and tunica interna, and are provided with watch pocket or semilunar valves but valves are more in number than veins. Valves are bicuspid.

Flow of lymph in lymphatics : Pulsations of lymph hearts in frog create sufficient force to maintain a steady flow of lymph in the lymphatic system. In mammals, the credit for maintaining onwards flow of lymph goes to (i) the “squeezing force” created by the skeletal muscles known as milking reaction (ii) the breathing movements of diaphragm and thoracic cage, (iii) mild peristalsis created by smooth muscles of the wall, of lymphatics themselves, and (iv) the pressure created by increasing amount of lymph in the lymphatics. Certain compounds like fats increase the rate of lymph flow and are called lymphata gogue. Blocking of lymph flow causes oedema.

Types of lymphatic ducts : Two main types :

(1) Right lymphatic duct : It is the smallest lymphatic duct with the length of approximately 1.25 cm. Its one end is blind and other one opens into right subclavian vein at the junction of right internal jugular vein. It collects lymph from one-fourth of the body (right part of head, neck, thoracic cavity and right arm).

(2) Left lymphatic duct/thoracic duct : It is the longest lymphatic duct with the length of approximately 38-45 cm. It originates from cisterna chyli and empties into left subclavian vein. It collects lymph from three-fourth part of the body i.e. complete posterior part through cisterna chyli, left part of head, neck, thoracic cavity and left arms.

Cisterna chyli/Receptaculum chyli : It is a dilated sac like structure present below the diaphragm in lumbar region at the level of second lumbar vertebra. It collects lymph from posterior part of body i.e. abdomen, pelvic region and hind limbs and drains it in the left lymphatic duct.

It shows inflation and deflation due to the movement of diaphragm which is a passive movement. Hence, it is also called as passive lymphatic artery. It is also called as second heart.

Lymph nodes or lymph glands : These are the masses of lymphatic tissue and connective tissue (reticular tissue) and are located on the capillaries either solitary or in cluster. Where they are present solitary and in few number, such tissues are called diffused lymphatic tissues and where they are in clusters, they are called tonsils.

$\bullet$ Lymphadenitis : During infection, central part of follicle shows rapid division and formation of plasma cells. hence, this part is also called reaction centre. The inflammation of lymph nodes in such condition is called Lymphadenitis.

Some of the common lymph nodes are – Axillary nodes (in armpits), genital (Inguinal) nodes (in pubic region), cervical nodes (in neck region), intercostal nodes (in chest region), lumbar nodes (in lumbar region), iliac nodes (in pelvic region) and payer’s patches (in small intestine). Besides these lymphatic nodes, a number of them are also present near major blood vessels (arteries), specially dorsal aorta.

$\bullet$ Tonsils : Clusters of lymph nodes. They are very often called as policemen. Various tonsils are – Normal tonsils (in pharynx), adenoid tonsils (in nasopharynx), abdominal tonsils (in vermiform appendix) and policeman of intestine (in lamina propria of ileum). Adenoid tonsils are present upto 7 years of age, then they are degenerated. Their swelling is called adenoid. Inflammation of tonsils is called Tonsilitis.

$\bullet$ Haemal lymph node : In many animals some lymph nodes are found to possess red colour, due to the presence of blood in them. In man they are found in the retroperitoneal tissues and also in the mediastinum. Spleen may be regarded as the modified haemal lymph (haemolymph) node. Lymph nodes are located at intervals along its course.

Function of lymph nodes

(i) They produce and supply lymphocytes to the blood and as a supportive function the trabeculae carry blood vessels which supply the node.

(ii) They make screening of the lymph by means of phagocytic activity.

(iii) They serve a great defensive role against bacterial infections.

(iv) They temporarily stop the spread of cancer cells as those cells have to penetrate through the lymph vessels to the lymph nodes from where they spread in the body.

(v) They act as mechanical filters to resist the entrance of poisonous substances into circulation.

(vi) They carry out immunological responses. They help in elaboration of antibodies and in the development of immunity.

(vii) Lymph nodes produce $\gamma$-globulin.