Transports extracellular fluid and mixes blood in the circulatory system
The walls of the capillaries are permeable to most of the plasma molecules of blood, except for plasma proteins, possibly because of their size too large to pass through the capillaries.
Extracellular fluid is transported through the body in two stages. The first stage is a movement within the blood vessels of the body, and the second is the reciprocal movement between the blood capillaries and the intercellular space.
The figure shows the entire circulation of blood in the body.
All blood in the circulatory system passes through the entire circulation on average once a minute when the body is at rest and 6 times a minute when the person is active. As blood passes through the blood capillaries, a continuous exchange of extracellular fluid occurs between the plasma portion of the blood and the intracellular fluid. This process is illustrated in the figure.
Figure. Diffusion of liquids and dissolution of components across capillary walls and intercellular spaces
The walls of the capillaries are permeable to most of the plasma molecules of the blood, except for plasma proteins, possibly because they are too large to pass through the capillaries. Thus, most of the fluid and its soluble components diffuse back and forth between the blood and the interstitial and are shown by arrows in the figure. This diffusion process is the kinetic movement of molecules in both plasma and intracellular fluid. That is, the liquid molecules and solutes are continuously moving chaotically in all directions in both plasma and intracellular fluid-like through capillaries. The numerous particles larger than 50 micrometres in the capillaries ensure the diffusion of most substances from the capillaries into the cells within seconds. Thus, extracellular fluid is present everywhere in the body,