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CAPILLARIES
Wall of capillaries. These are blood vessels with walls that are only one cell thick. Although the blood appears confined within the capillary walls.
Permeability For Material.
Capillaries are permeable with the result, that water and dissolved substances pass in and out exchanging oxygen, carbon dioxide dissolved food and excretory products with the tissues around capillary.
Dense Network.
The capillary network is so dense that no living cell is far from a supply of oxygen and food. In the liver, every cell is in direct contact with a capillary. Diameter. The diameter of a capillary can be altered by nervous stimulation, which tends to close them, and by chemicals, such as histamine, which dilate them. The change in diameter is brought about by a change in the shape of the cells, constituting their walls.
Pre capillary sphincters.
The pre capillary sphincters also regulate the amount of blood flowing in capillaries. Thus the amount of blood flowing in a certain tissue is controlled.
Mechanisms of Exchange of material.
The capillaries are the sites where the materials are exchanged between the blood and body tissues. This exchange occurs in three ways. By Active and passive Transport.
(i) Active transport and diffusion through the cells lining the capillary wall into the interstitial or extracellular fluid, and then to the body cells, and vice versa. Through intercellular spaces.
(ii) Through the intercellular spaces of endothelial lining of wall of capillary to and from the extracellular fluid. Endocytosis and exocytosis
(iii) Materials from the cavity of capillaries are also taken up by endocytosis, and then passed to the other side by exocytosis.
Same is true for some materials enterring from the intercellular spaces (extracellular fluid) into the blood. Interstitial fluid Thus the exchange of materials takes place between blood and tissues via extracellular or interstitial fluid. Vein Formation. Capillaries join to form venules, which join to form veins.