Rate of Blood Flow

Arteries: It is highest in aorta-400- 500 mm per second, decreasing along the arterial system.

Capillaries: It is greatly reduced in the capillaries, about 150 mm per second. The main reason for this is that cross sectional area of the capillaries is smaller than the large arteries; therefore there is decrease in the rate of blood flow. The slow rate of blood flow in the capillaries is of advantage in allowing adequate time for exchange of nutrients, hormone and metabolic wastes between the capillaries and tissues.

Veins: The larger veins have an over all larger total cross- sectional area, therefore, there is an increased rate of flow.

Pulmonary Artery: It arises from the superior surface of the right ventricle and immediately divides into right and left pulmonary arteries, going to the corresponding lung. These arteries carry deoxygenated blood.

Portal System: When a vein breaks up into capillaries it is called a portal vein. The veins coming from the alimentary canal, pancreas, spleen etc, join up to form a portal vein which enters the liver and breaks up into capillaries. The excess of glucose is filtered out through the walls of the capillaries into the hepatic cells where it is converted into glycogen and stored in them. The capillaries reunite to form the hepatic veins which open into the inferior vena cava.

The portal vein differs from other veins in that the blood it carries from the tissues pass through a second capillary network in the liver before it enters the inferior vena cava. The portal veins and its branches constitute the portal system.