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The term capillary exchange refers to all exchanges at microcirculatory level, most of which occurs in the capillaries. Sites where material exchange occurs between the blood and tissues are the capillaries, which branch out to increase the swap area, minimize the diffusion distance as well as maximize the surface area and the exchange time. [4]
There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and sinusoids, a type of open-pore capillary found in the liver, bone marrow, anterior pituitary gland, and brain circumventricular organs. Capillaries and sinusoids are short vessels that directly ...
Diffusion occurs through the walls of the vessels due to a concentration gradient, allowing the necessary exchange of ions, molecules, or blood cells. The permeability of a capillary wall is determined by the type of capillary and the surface of the endothelial cells. A continuous, tightly spaced endothelial cell lining only permits the ...
A typical pair of human lungs contains about 480 million alveoli, [11] providing a total surface area for gas exchange of between 70 and 80 square metres. [10] Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. [12] The diameter of an alveolus is between 200 and 500 μm. [12]
The majority of exchange through the peritubular capillaries occurs because of chemical gradients osmosis and hydrostatic pressure. Movement of water into the peritubular capillaries is due to the loss of water from the glomerulus during filtration, which increases the colloid osmotic pressure of the blood.
In intercellular clefts of capillaries, it has been calculated that the fractional area of the capillary wall occupied by the intercellular cleft is 20m/cm 2 x 20 nm (length x width)= 0.004 (0.4%). This is the fractional area of the capillary wall exposed for free diffusion of small hydrophilic solutes and fluids 5 .
There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart.
As all blood vessels allow a degree of protein leak , true equilibrium across the membrane cannot occur and there is a continuous flow of water with small solutes. The molecular sieving properties of the capillary wall reside in a recently-discovered endocapillary layer rather than in the dimensions of pores through or between the endothelial ...