Tissue fluid is the medium in which cells are bathed. It is otherwise known as interstitial fluid. It forms about 20% of extracellular fluid (ECF).
FUNCTIONS OF TISSUE FLUID
Because of the capillary
membrane, there is no direct contact between blood and cells. And, tissue fluid
acts as a medium for exchange of various substances between the cells and blood
in the capillary loop. Oxygen and nutritive substances diffuse from the
arterial end of capillary through the tissue fluid and reach the cells. Carbon
dioxide and waste materials diffuse from the cells into the venous end of
capillary through this fluid.
FORMATION OF TISSUE FLUID
Formation of tissue
fluid involves two processes:
1. Filtration.
2. Reabsorption.
FILTRATION
Tissue fluid is formed
by the process of filtration. Normally, the blood pressure (also called hydrostatic
pressure) in
arterial end of the capillary is about 30 mm Hg. This hydrostatic pressure is
the driving force
for filtration of
water and other substances from blood into tissue spaces. Along the course of
the capillary, the pressure falls gradually and it is about 15 mm Hg at the
venous end. Capillary membrane is not permeable to the large molecules,
particularly the plasma proteins. So, these proteins remain in the blood and
exert a pressure called oncotic pressure or colloidal
osmotic pressure. It is about 25 mm Hg.
Osmotic pressure is
constant throughout the circulatory system and it is an opposing force for the filtration
of water and other materials from capillary blood into the tissue space.
However, the hydrostatic pressure
in the arterial end of
the capillary (30 mm Hg) is greater than the osmotic pressure. And, the net
filtration pressure of 5 mm Hg is responsible for continuous filtration.
Starling Hypothesis
Determination of net filtration
pressure is based on Starling hypothesis. Starling hypothesis states that the
net filtration through
capillary membrane is proportional to the hydrostatic pressure difference
across the membrane minus the oncotic pressure difference. These pressures are
called Starling forces.
REABSORPTION
Fluid filtered at the
arterial end of capillaries is reabsorbed back into the blood at the venous end
of capillaries. Here also, the pressure gradient plays an important role. At the
venous end of capillaries, the hydrostatic pressure is less (15 mm Hg) and the
oncotic pressure is more (25 mm Hg). Due to the pressure gradient of 10 mm Hg,
the fluid is reabsorbed along with waste materials from the tissue fluid into
the capillaries. About 10% of filtered fluid enters the lymphatic vessels. Thus,
the process of filtration at the arterial end of the capillaries helps in the
formation of tissue fluids and the process of reabsorption at the venous end
helps to maintain the volume of tissue fluid.
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