The arteries, arterioles, capillaries, venules and veins are organized into definite routes in order to circulate the blood throughout the body. The body takes the following basic circulatory routes in human body.
Systemic Circulation :
The flow of blood from the left ventricle to all parts of the body and back to the right atrium is called the systemic circulation. It includes all the oxygenated blood that leaves the left ventricle through the aorta and the deoxygenated blood that returns to the right atrium through the superior vena cava and inferior vena cava after travelling to all the organs including the nutrient arteries to subdivisions of the systemic circulation are the coronary (cardiac) circulation which supplies blood to the myocardium of the heart and the hepatic portal circulation, which runs from the digestive tract to the liver. Coronary circulation and hepatic portal circulation will be described separately.
The purpose of systemic circulation is to carry oxygen and nutrients to body tissues and to remove carbon dioxide and other wastes from the tissues.
All systemic arteries arise from the aorta which originates from the left ventricle of the heart. As the aorta emerges from the left ventricle it is called ascending aorta which leads to the arch of the aorta. The latter opens into descending aorta that leads to thoracic aorta. The thoracic aorta opens into abdominal aorta. The ascending aorta gives off two coronary arteries which supply blood to the heart muscle. The arch of the aorta gives rise to the branchiocephalic artery, left common carotid artery and left subclavian artery supplying blood to the head, neck and upper extremities. The thoracic aorta gives off some arteries supplying blood to the thoracic organs. The abdominal aorta gives rise to the arteries supplying blood to the abdominal organs and lower extremities.
Blood is returned to the heart through the systemic veins. All the veins of the systemic circulation open into either the superior vena cava or inferior vena cava or coronary sinus. They in turn carry blood into the right atrium.
Pulmonary Circulation :
The flow of deoxygenated blood to the right ventricle to the lungs and the return of oxygenated blood from the lungs to the left atrium is called the pulmonary circulation. The pulmonary trunk arises from the right ventricle and then divided into the right pulmonary artery and left pulmonary artery which supply deoxygenated blood to the right and left lungs respectively. Exchange of gases takes place. Two pulmonary veins from each lung transport the oxygenated blood to the left atrium.
The flow of oxygenated blood from the ascending aorta to the heart muscle and the return of deoxygenated blood from the heat muscle to the right atrium is called coronary (cardiac) circulation. The right and left coronary arteries arise from the ascending aorta which supply oxygenated blood to the heart muscle (myocardium). The coronary veins bring deoxygenated blood to the coronary sinus. The latter carries deoxygenated blood to the right atrium.
Hepatic Portal Circulation (Hepatic Portal System):
Blood enters the liver from two sources. The hepatic artery supplies oxygenated blood from the thoracic aorta and the hepatic portal vein carries deoxygenated blood from the digestive organs. The flow of deoxygenated blood from the digestive organs to the liver before returning to the heart is called hepatic portal circulation.
A vein which does not carry blood directly to the heart but forms a network of capillaries in another or intermediate organ before reaching the heart is called a portal vein. A portal vein together with small veins which it receives blood is called the portal system. A portal system is named after the organ to which it carries blood. The vertebrates possess two or three portal systems. These are hepatic portal, renal portal and hypophysial portal systems. Renal portal system is absent in human beings.
Hepatic Portal System is present in all the vertebrates including man. It brings blood from the alimentary canal, pancreas and spleen to the liver hence it is named as hepatic portal system.
The hepatic portal system includes a large hepatic portal vein and a number of other veins which collect blood from different parts of the alimentary canal and its associated structures. Some of these important veins are mentioned here. The superior mesenteric vein brings blood from the small intestine, portions of large intestine and stomach. The splenic vein collects blood from the spleen and receives tributaries from the stomach, pancreas and portions of the colon.
The tributaries from the stomach are gastric, pyloric and gastroepiploic veins. The pancreatic veins come from the pancreas, and the inferior mesenteric veins bring blood from portions of the colon. The cystic vein collects blood from the gall bladder. Ultimately, blood leaves the liver through the hepatic veins, which open into the inferior vena cava.
Importance of Hepatic Portal System, (i) The blood which comes from the alimentary canal contains absorbed 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 defficiency of food, the glycogen is converted into glucose and is transferred to the blood stream via hepatic veins. (ii) Harmful nitrogenous waste like ammonia is converted into urea which is later removed by kidneys. Thus the blood is detoxidfied (purified) of harmful nitrogenous waste. (iii) Liver produces blood proteins which are put into blood circulation.