Venous System & Portal System | Biology for JAMB PDF Download

VENOUS SYSTEM IN MAN

PORTAL SYSTEM

When the vein of any organ of the body does not open in the caval vein or heart but it divides into capillaries in any other organ and its blood is transported by vein of that other organs to the heart, then this type of system is termed as portal system.

It is of following types:-

(i) Renal portal system

(ii) Hepatic portal system

(iii) Hypophyseal portal system

(I) Renal portal system:-Veins which collect blood from posterior parts of the body and legs combine to form a renal portal vein. This vein goes into kidney and divides into capillaries kidneys separate nitrogenous wastes from this blood. This partly purified blood is now transported to the heart. It is present in frog

•  In mammals, renal portal system is absent.

In Frog both the portal systems ; renal portal system and hepatic portal system are present.

(II) Hepatic Portal System.

It is a portal system which brings venous blood directly from digestive tract, spleen, pancreas and gall bladder to liver for extraction of nutrients and other metabolites by breaking up into single celled thick capillaries and sinusoids. Portal vein is about 8 cm long. It is formed by following veins.

(i) Splenic from spleen and gastric from stomach which join to form lienogastric vein.

(ii) Superior mesenteric from small intestine, caecum, ascending and transverse parts of colon.

(iii) Inferior mesenteric from rectum, sigmoid and descending part of colon.

(iv) Cystic vein from gall bladder.

(v) Paraumbilical vein from abdominal wall.

(vi) Duodenal vein from Duodenum and Pancreas.

Hepatic portal vein branches the branches enter into different liver lobes. They divide and redivide.

Ultimate branches.of hepatic portal veins open into sinusoids for exchange of materials between portal blood and liver cells. Along with deoxygenated blood of liver, portal blood is recollected by venules which form hepatic veins.

(III) Hypophyseal Portal System: It is a portal system formed by a vein from hypothalamus which breaks up into capillaries in hypophysis or pituitary gland. The Vein is called hypophyseal portal vein. It is formed by union of capillaries and venules in hypothalamus. The portal vein enters anterior lobe of pituitary gland or adenohypophysis where it breaks up into capillaries. Hypophysial portal system is a short circuit arrangement. Hypothalamus produces a number or hormones for controlling endocrine activity of adenohypophysis. They are pored into blood in hypothalamus. The same is directly drained by hypophysial portal vein into adenohypophysis.