Category : 11th Class
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
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