(iii) VEINS :

Definition.

These blood vessels transport blood from body cells towards heart. Capillaries join to form venules, which join to form veins.

Wall of veins.

The wall of veins has same three layers as are present in arteries. But middle layer is relatively thin and only slightly muscular, with few elastic fibres.

Semilunar valve.

The semilunar valves are present in the veins. These valves prevent the back flow of blood, as it is moving towards the heart.

Blood flow.

The pressure of surrounding muscles, when they contract, tends to squash the veins and assist the return of blood towards heart.

Vena cava.

Veins join to form larger veins, and ultimately form venae cavae (Inferior vena cava and superior vena cava) which pour the blood into the right atrium of the heart. Pulmonary Veins.

The oxygenated blood from the lungs is brought to the left atrium by pulmonary veins.

Interstitial Fluid.

Definition.

The pressure within capillaries causes a continuous leakage of fluid from the blood plasma into the spaces that surround the capillaries and tissues. This fluid, known as interstitial fluid Composition. It consists primarily of water, in which the dissolved nutrients, hormones gases, wastes, and small proteins from the blood are present. Larger substances. Large proteins red blood cells and platelets cannot cross the intercellular spaces of capillary wall, so they remain within capillaries. But some white blood cells can squeeze out through the intercellular spaces of capillary wall. Function. Interstitial fluid is the medium through which the exchange of materials between the blood and nearby cell occurs.