Answer
The microcirovasculature is comprised of terminal and metarterioles, capillaries and post capillary
venules. Capillaries and postcapillary venules are the smallest blood vessels in the body. They function to exchange substances between the blood and the interstitial fluid ( IF or ISF) and cells. The capillaries facilitate the passage of fluid, nutrients ( glucose), dissolved gases (oxygen), and electrolytes between plasma and ISF. The movement of fluid out is possible because the net filtration pressure (NFP) at the arteriolar end of the capillary beds is positive ( 8-10 mm Hg) in favor of pushing fluids/solutions out of the capillaries .
Postcapillary venules are the smallest veins. At the venous end of capillary beds the NFP is a negative 8 mm Hg pressure (8 mm Hg). This pulls fluids into the blood vessels ( absorption). Thus, water plus metabolic wastes from cells are taken up into the blood stream.
The major function of these smallest blood vessels is to facilitate the exchange of substances between the blood stream and the ISF and cells. Some of these small blood vessels like the sinusoidal capillaries have large intercellular clefts, and pores that allow the passage of cells . This emigration of lymphocytes and other white blood cells out of some capillaries allows these small blood vessels to play a roles in the inflammatory process , and n immunological responses to infection.
Work Step by Step
The NFP at the arterial end of a capillary bed =
( Capillary Hydrostatic pressure +ISF Osmotic Pressure) - (ISF Hydrostatic Presure +Capillary Osmotic Pressure)
NFP arteriolar end= (35+1)-(0.0+26) mmHg
=(36-26) = 10 mm Hg
NFP at venous end of capillary bed:
= (Capillary Hydrostatic Pressure + IF Osmotic Pressure) - (IF Hydrostatic Pressure+Capillary
Osmotic Pressure)
= (17+1)-(0.0+26)
= (18-26) = -8 mm Hg
This negative pressure results in fluid/solutions being absorbed into the blood stream at the venous end of capillary beds.However, not all the fluid that leaves the capillaries is picked up by the microvasculature. About 1.5 liters of fluid( with dissolved substances) is returned to the blood stream by the lymphatic circulation.
