Formation of tissue fluid across capillary wall

Cells are the building blocks of our body. The cells exist in an internal sea or “Milieu interior” and this is called extracellular fluid (ECF). From this fluid, cells take up O₂ and nutrients and cells they discharge waste products into it. ECF consists of two components, they are interstitial fluid and circulating blood plasma. The interstitial fluid is also known as tissue fluid and it is in outside the vascular system, bathing the cells.

            Tissue fluid consists of water, ion and dissolved gasses and food substances that is formed at the capillary membrane. The capillary membrane is a thin membrane made up of endothelial cells. Substances pass through the junctions between endothelial cells and through fenestration when they are present. Some also pass through the cells by vesicular transport. The factors other than vesicular transport that are responsible for transport across the capillary wall are diffusion and filtration. Diffusion is quantitatively much more important. O₂ and glucose are in higher concentration in the blood stream than the tissue fluid and diffuse into the tissue fluid, whereas CO₂ diffuse in the opposite direction.

The rate of filtration along the capillary membrane depends on 2 factors. These factors are known as starling forces. These starling forces are the hydrostatic pressure gradient across the capillary membrane and the osmotic pressure gradient across the capillary membrane. The hydrostatic pressure is a force generated by the pressure of fluid on the capillary wall either by the blood plasma or tissue fluid, which tends to push fluid out of the blood stream, and the osmotic pressure is exert by proteins in the blood plasma or tissue fluid, which tends to pull fluids  from tissue fluid back to the capillary and is due mainly to the presence of plasma proteins, specially albumin.

Hydrostatic pressure gradient is measured as, the hydrostatic pressure in the capillary, minus the hydrostatic pressure of the tissue fluid. The osmotic pressure gradient across the capillary wall is measured as the colloid osmotic pressure of the plasma, minus colloid osmotic pressure of the tissue fluid. So,

Fluid movement across the

 capillary endothelium                 =     K [(P_c – P_i) - (π_c - π_i)]

 

            K = capillary filtration coefficient

            P_c = capillary hydrostatic pressure

            P_i = interstitial hydrostatic pressure

            π_{c =} capillary colloid osmotic pressure

            π_i = interstitial colloid osmotic pressure

 

            The hydrostatic pressure at the arteriole end is 37mmHg and the hydrostatic pressure at the venule end is 17mmHg. The hydrostatic pressure of the interstitial fluid is 1mmHg. Colloid osmotic pressure of interstitial fluid usually negligible (as capillary walls impermeable to proteins). So, the osmotic pressure gradient usually equals the oncotic pressure. Oncotic pressure is a form of osmotic pressure and the osmotic pressure gradient across the capillary is 25mmHg. Therefore, the pressure differential at the arteriolar end of the capillary is 11mmHg. ([37 – 1] -25). The arteriole pressure is greater than the tissue fluid. So, the fluid is filtered from capillary to the interstitial fluid. It is an outward movement. The pressure differential at the venule end of the capillary is 9mmHg [25 – (17 -1)]. So, the fluid is reabsorbed from the interstitial fluid to the capillary. It is an inward movement. From this way fluid moves into the interstitial space at the arteriolar end of the capillary and into the capillary at the venular end as the diagram in below.

 In other capillaries the balance of starling forces maybe different, for example fluid moves out of almost the entire length of the capillaries in the renal glomeruli on the other hand, fluid moves into the capillaries through almost their entire length in the intestines.

            The capillary filtration coefficient is proportional to the permeability of the capillary wall and the surface area available for filtration. About 24L of fluid is filtered through the capillaries per day. This is about 0.3% of the cardiac output. About 85% of the filtered fluid is reabsorbed into the capillaries and the remainder 15% returns to the circulation via lymphatics. The interstitial fluid volume is therefore constant.

In other capillaries the balance of starling forces maybe different, for example fluid moves out of almost the entire length of the capillaries in the renal glomeruli on the other hand, fluid moves into the capillaries through almost their entire length in the intestines.

            The capillary filtration coefficient is proportional to the permeability of the capillary wall and the surface area available for filtration. About 24L of fluid is filtered through the capillaries per day. This is about 0.3% of the cardiac output. About 85% of the filtered fluid is reabsorbed into the capillaries and the remainder 15% returns to the circulation via lymphatics. The interstitial fluid volume is therefore constant.