When arterial blood reaches the capillary bed, the fluid from the capillary moves into the interstitial spaces

Question

After arterial blood has passed through the capillaries into the venous network, fluid will flow back again into the blood from the interstitial fluid. Give the Starling principle of fluid exchange, and use this principle to explain the movement of fluids at the capillary bed.

Answer 1

Answer: The Starling principle of fluid exchange is given as: NFP = (Pcap-Pif) - (πcap- πif), where NFP is the net filtration pressure, Pcap is the hydrostatic pressure in the capillary, Pif is the hydrostatic pressure in the interstitial fluid, πcap is the osmostic pressure in the capillary, and πif is the osmotic pressure in the interstitial fluid. Arterial blood has high pressure as it approaches the capillary bed, and this pressure is the major force pushing fluids from the blood into the interstitial spaces. Because continuous capillaries are permeable only to small molecules, plasma proteins and blood cells remain behind in the blood as fluids are pushed out, resulting in the blood having a higher osmotic pressure than the interstitial fluid. After the blood has moved through the capillaries, the osmotic pressure due to proteins (termed "oncotic pressure") has increased in the capillaries, and tends to suck fluids back into the blood. Moreover, because the hydrostatic pressure of the blood declines as it moves from the arterial to venous end of the capillary bed, the osmotic pressure exceeds hydrostatic pressure, and this also tends to allow fluids to reenter the blood.