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The following points highlight the three types of circulations through special regions of heart. The types are: 1. Coronary Circulation 2. Hepatic Portal Circulation 3. Hypophysial Portal Circulation.
Type # 1. Coronary Circulation:
The flow of oxygenated blood from the ascending aorta to the heart muscle and the return of deoxygenated blood from the heart 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.
Type # 2. Hepatic Portal Circulation (Hepatic Portal System; Fig. 18.21):
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Blood enters the liver from two sources. The hepatic artery supplies oxygenated blood from the abdominal 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 networks of capillaries in another or intermediate organ before reaching the heart is called a portal vein. A portal vein together with small veins through 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 a characteristic feature of all vertebrates.
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The following important veins open into the hepatic portal vein:
(i) The superior mesenteric vein brings blood from the small intestine and portions of the large intestine, stomach, etc. through ilial, ileocolic, right colic, middle colic and right gastroepiploic veins.
(ii) The splenic vein drains blood from the stomach, pancreas and portions of the large intestine through left gastroepiploic, pancreatic and inferior mesenteric veins. The inferior mesenteric vein brings blood from the portions of the large intestine through the superior rectal, sigmoidal and left colic veins.
(iii) The left and right gastric veins open directly into the hepatic portal vein that brings blood from the stomach.
(iv) The cystic vein also opens directly into the hepatic portal vein and drains 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 Circulation:
(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 deficiency 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 detoxified (purified) of harmful nitrogenous waste.
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(iii) Liver produces blood proteins which are put into blood circulation.
Type # 3. Hypophysial Portal Circulation (= Hypophysial Portal System):
This minor portal system consists of two hypophysial portal veins (Fig. 18.22) which carry blood from the hypothalamus of the brain to the anterior lobe of the pituitary gland (hypophysis). This portal system enables the hormones of hypothalamus to reach the anterior lobe of the pituitary gland.
