BLOOD CIRCULATION OF CARDIO VASCULAR SYSTEM

There are two ways.

i) Pulmonary Circulation

ii) Systemic Circulation

1) Pulmonary Circulation

The pulmonary circulation is the circulation of blood from the right ventricle of the heart to the lungs and back to the left atrium. In the lungs, carbon dioxide is excreted and oxygen is absorbed. So, the gas exchange is occurred.

The pulmonary artery or trunk, carrying deoxygenated blood, leaves the upper part of the right ventricle of the heart. It passes upwards and divides into left and right pulmonary arteries at the level of the 5^th thoracic vertebra. The left pulmonary artery runs to the root of the left lung where it divides into two branches, one passing into each lobe. The right pulmonary artery divides into 2 branches. The larger branch carries blood to the middle and lower lobes, and the smaller branch to the upper lobe. Within the lung these arteries divide and subdivide into smaller arteries, arterioles and capillaries. The exchange of gases takes place between capillary blood and air in the alveoli of the lungs. In each lung the capillaries containing oxygenated blood merge into progressively larger venules, and eventually form two pulmonary veins. Two pulmonary veins leave each lung, returning oxygenated blood to the left atrium of the heart.

2) Systemic Circulation

In systemic circulation, oxygenated blood pumped out from the left ventricle is carried by the branches of the aorta around the body and deoxygenated blood returns to the right atrium of the heart by superior and inferior venae cavae. During this systemic circulation, tissue wastes enter blood for excretion and body cells extracts nutrients and O₂.

The aorta is the largest artery of the body. It begins at the upper part of the left ventricle and after passing upwards for a short way, it arches backward and to the left. The aorta can be categorized into 2 parts according to its location. They are thoracic aorta and the abdominal aorta. Thoracic aorta lies above the diaphragm. There are 3 parts of the thoracic aorta, as ascending aorta, arch of the aorta and descending aorta. Ascending aorta arise from the aorta just above the level of the aortic valve and branched into right and left coronary arteries and supply blood to heart. The continuation of ascending artery is the arch of the aorta. There are 3 branches of the arch of the aorta. They are brachiocephalic artery of trunk, left common carotid artery and left subclavian artery. The brachiocephalic artery divides into the right common carotid artery and the right subclavian artery at the level of the sternoclavicular joint.

Right and left common carotid arteries pass upwards on either side of the neck and have the same distribution on each side. At the level of the upper boarder of the thyroid cartilage, each divide into an internal carotid artery and an external carotid artery. External carotid artery supplies the superficial tissues of the head and neck via a number of branches. These branches are, superior thyroid artery which supplies  oxygenated blood for thyroid gland and adjacent muscles, lingual artery which supplies  blood to tongue, the membrane that lines the mouth, the structures in the floor of the mouth, the tonsil and the epiglottis, the facial artery which supplies blood for muscle of facial expression and structures in the mouth, the occipital artery, which supplies posterior part of the scalp, the temporal artery, which supplies frontal, temporal and parietal parts of the scalp, the maxillary artery, which supplies blood for muscles of mastication. A branch of this artery, the middle meningeal artery runs deeply to supply structures in the interior of the skull. Internal carotid artery is a major contribution to the circulus arteries which supplies the greater part of the brain and also has branches that supply the eyes, forehead and nose. Circulus arteries consists of 4 large arteries as two internal carotid arteries and two vertebral arteries. Vertebral arteries arise from the subclavian arteries, posteriorly vertebral arteries join to form the basilar artery. Anteriorly the two cerebral arteries arise from the internal carotid arteries and are joined by the anterior communicating artery. The basilar artery divides to form two posterior cerebral arteries, each of which is joined to the corresponding internal carotid artery by a posterior communicating artery, completing the circle. From this circle, the anterior cerebral arteries supply the anterior part of the brain, the middle cerebral arteries supply the sides of the brain and posterior central arteries supply the posterior part of the brain. Branches of the basilar artery supply parts of the brain system.

The Right and left axillary arteries are continuation of the right and left subclavian arteries. The first part of axillary arteries lies deeply, then they run more superficially in the right and left side of the body, to become the right and left brachial arteries. About 1cm below from the elbow joint, brachial arteries divide into the radial and ulna arteries in right and left  sides. The right and left radial arteries pass down the radial side of the forearm to the wrist in right and left side of the body. Then artery passes between the first and second metacarpal bones and enters the palm of the hand. The right and left ulnar arteries run downwards on the ulnar aspect of the forearm to cross the wrist and pass into the hand in each side. There are anastomoses between the radial and ulnar arteries in left and right side of the body and from that anastomoses palmar metacarpal and palmar digital arteries arise to supply the structures in the hand and fingers. In this way, the right subclavian artery supplies oxygenated blood for right arm and the left subclavian artery supplies oxygenated blood for the left arm.

The descending aorta in the thorax is continuous with the arch of the aorta and gives many paired branches which supply the walls and organs of the thoracic cavity. Paired bronchial arteries supply blood for lung tissues. Paired esophageal arteries supply blood for esophagus. Paired intercostal arteries supply the intercostal muscles, some muscles of the thorax, the ribs, skin and its underlying connective tissues.

Abdominal aorta is the 2^nd part of the aorta according to the categorization of their locations. This continuation of the thoracic aorta and passes behind the diaphragm at the level of 12^th thoracic vertebra. It descends Infront of the vertebral column to the level of the 4^th lumbar vertebra, where it divides into the right and left iliac arteries. Many branches are arisen from the abdominal aorta, some are paired and some are unpaired, suppling the abdominal structures and organs. Paired phrenic arteries supply blood for diaphragm. Paired renal arteries supply blood for kidneys. Paired suprarenal arteries supply the adrenal glands. Paired gonadal arteries supply the ovaries in female and the testes in male. unpaired coeliac artery arises immediately below the diaphragm and divides into 3 branches. They are left gastric artery which supplies blood for stomach, the splenic artery, which supplies blood for spleen and pancreas, the hepatic artery which supplies blood for liver, gall bladder and parts of the stomach, duodenum and pancreas. Superior mesenteric artery, which branches from the aorta between the coeliac artery and the renal arteries. It supplies the entire small intestine and about half the proximal large intestine. Inferior mesenteric artery, which arises from the aorta, divides into the common iliac arteries. It supplies the distal half of the large intestine and part of the rectum.  

At the level of 4^th lumbar vertebra, the abdominal aorta divides in to the right iliac artery and the left iliac artery. Infront of the sacroiliac joint, each left and right iliac artery divide into internal and the external iliac arteries. Right and left Internal iliac arteries supply blood for pelvis and pelvis organs in right and left sides of the body. Right and left external iliac arteries supply blood for right and left legs. The external iliac arteries become femoral arteries in right and left sides of thigh. At the popliteal space, femoral arteries become popliteal arteries. They supply blood to the structures of the thigh and some superficial pelvic and inguinal structures include the knee joint. At the lower border of the popliteal fossa, it divides into the anterior and posterior tibial arteries in each side. Left and right anterior tibial arteries supply the structures in the front of the left and right legs. The dorsalis pedis artery is a continuation of the anterior tibial artery and passes over the dorsum of the foot. The posterior tibial artery runs downwards and medially on the back of the leg. Near its origin it gives off a large branch called peroneal artery which supplies the lateral aspect of the leg. Peroneal artery continuous as the planter artery which supplies the structures in the sole of the foot. This artery its branches and the dorsalis pedis artery form the planter arch from which the digital branches arise to supply the toes.

The superior and inferior venae cava are the largest veins in the body and empty blood directly into the right atrium of the heart. The superior venae cava drains all body structures lying above the diaphragm and the inferior vena cava drains blood from all structures below the diaphragm. The superior vena cava formed by the union of the left and right brachiocephalic veins. The inferior vena cava formed by the union of left and right iliac veins.

Venous blood from the head and neck is returned by deep and superficial veins. Superficial veins with the same names as the branches of the external carotid artery return venous blood from the superficial structures of the face and scalp in left and right sides and unite to form the left and right external jugular veins. External jugular veins enter the left and right subclavian veins which carry venous blood from the left and right upper limbs. Venous blood from the deep area of the brain is collected into channels called the Dural venous sinuses. These sinuses are, superior sagittal sinus, which continuous as the right transverse sinus, the inferior sagittal sinus which form the straight sinus, the straight sinus which forms the left transverse sinus, the left and right sigmoid sinuses which continuous with the left and right transverse sinuses.  The left and right internal jugular veins formed by the continuation of the left and right sigmoid sinuses. So, the left and right internal jugular veins drain venous blood from the left and right sides of the head and neck. Behind the clavicle left and right internal jugular veins unite with the left and right subclavian veins to form left and right brachiocephalic veins. The left brachiocephalic vein joins the right brachiocephalic vein to form the superior vena cava.

The upper limb is drained by both deep and superficial veins. The deep veins follow the course of the arteries and have the same names. They are, left and right palmer metacarpal veins, left and right deep palmar venous arch, left and right ulnar and radial veins, left and right brachial veins, left and right axillary veins left and right subclavian veins. The superficial veins are the left and right cephalic veins, left and right basilic veins, left and right median veins, left and right median cubital veins. The left and right cephalic veins collect blood from a complex of superficial veins. Infront of the elbow, cephalic vein gives a large branch, the median cubital vein, which join the basilic vein in left and right sides of the body. At the Infront of the shoulder joint, cephalic vein end, joining the axillary vein. Throughout the length of the cephalic vein it receives blood from the superficial tissues on the lateral aspects of the hand, forearm and arm. The basilic vein begins at the back of the hand on the ulnar aspect and joins the axillary vein in left and right sides. It receives blood from the medial aspect of the hand, forearm and arm. There are many small veins which link the cephalic and basilic veins. The median vein begins at the palmar surface of the hand, ascends on the front of the forearm and ends in the basilic vein or the median cubital vein. The right and left axillary veins join to the right and left subclavian veins in each sides of the body. So, the left and right subclavian veins drain venous blood from left and right upper limb. The union of the subclavian vein and the internal jugular vein form the brachiocephalic vein in each left and right sides of the body. Left and right brachiocephalic veins join to form superior vena cava. So superior vena cava drains all venous blood from head, neck, and upper limbs and terminates in the right atrium.

Most of the venous blood from the organs in the thoracic cavity is drained into the azygos vein and the hemiazygos vein. Some of the main veins that join them are the bronchial, esophageal and intercostal veins. The azygos vein joins the superior vena cava and the hemiazygos vein join the left brachiocephalic vein. At the distal end of the esophagus, some esophageal veins join the azygos vein and other the left gastric vein.

Venous blood drains from some abdominal organs directly into the inferior vena cava via veins named as the corresponding arteries. Hepatic veins drain the liver. The right and left renal veins drain right and left kidneys. Right and left suprarenal veins drain the right and left adrenal glands. Lumbar veins drain lower abdominal structures and right and left gonadal veins drain the ovaries in female and testes in male. The most blood from the digestive organs in the abdomen is drained into the hepatic portal vein and passes through the liver before being emptied into the inferior vena cava. The portal vein formed by the union of several veins. The unpaired  splenic vein  drains blood from the spleen, pancreas and part of the stomach, The unpaired inferior mesenteric vein, which returns venous blood from the small intestine and the proximal parts of the large intestine, the caecum, ascending and transverse colon, it unites with the splenic vein to form the portal vein. The unpaired gastric vein drains blood from the stomach and the distal end of the esophagus, then join the portal vein. The cystic vein drains blood from the gall bladder and joins the portal vein. In this way, blood with a high concentration of nutrients, absorbed from the stomach and intestines, goes to the liver first.

There are both deep and superficial veins in the lower limb. The deep veins accompany the arteries and their branches and have the same names. Right and left femoral veins become the right and left external iliac veins. Right and left external iliac veins receive venous blood from the right and left legs. Right and left external iliac veins join the right and left internal iliac veins. Right and left internal iliac veins drain venous blood from the organs of the pelvic cavity in right and left sides of the body. Internal iliac vein and the external iliac vein join together to form common iliac vein in left and right side of the body as the right common iliac vein and the left common iliac vein. At the site of 5^th lumber vertebra, the right common iliac vein and left common iliac vein join together to form the inferior vena cava. The small saphenous vein and the great saphenous vein are the superficial veins which collect blood from the lower limb. The small saphenous vein joins to the popliteal vein and the great saphenous vein joins to the femoral vein.