Capillary filtrate: hydrostatic pressure, plasma colloidal pressure and capillary filtration coefficient
Hydrostatic pressure tends to push fluid and solutes through the capillary pores into the interstitial space. In contrast, osmotic pressure tends to induce osmosis from the interstitial spaces into the blood.
The main function of microcirculation is to transport nutrients to tissues and remove cellular waste products. Small arterioles control blood flow to individual tissues and their local condition, by controlling the diameter of the arterioles. Thus, in most cases, each tissue's regulation of flow is related to its own needs.
The walls of the capillaries are very thin and are composed of a layer of highly permeable endothelial cells. Thus, water, cellular nutrients and cellular secretion products can be exchanged quickly and easily between tissues and circulating blood.
The human body's peripheral circulatory system has about 10 billion capillaries with an estimated total surface area of 500 to 700 square meters (about one-eighth of the surface area of a football field). Thus any functioning cell has a capillary that feeds it no more than 20-30 micrometres away.
The hydrostatic pressure in the capillaries tends to push the fluid and its solutes through the capillary pores into the interstitial space. In contrast, the osmotic pressure induced by plasma proteins (called colloidal osmotic pressure) tends to induce fluid movement by osmosis from the interstitial spaces into the blood. This osmotic pressure is generated by plasma proteins, which normally prevent significant loss of fluid from the blood into the interstitial space.
The lymphatic system is also important in returning to the circulation small amounts of excess protein and fluid that leak from the blood into the interstitial.
Figure. Fluid pressure and colloidal osmotic pressure exert forces at the capillary membrane, which tend to push fluid out and in through the capillary pores.
The remainder of this chapter discusses the mechanisms that regulate capillary filtration along with the lymphatic fluid function to regulate the respective volumes of plasma and interstitial fluid.
Hydrostatic and colloidal pressure determine the movement of fluid across the capillary membrane:
The figure shows four main forces that will determine the movement of fluid out of the blood into the interstitial fluid or in the opposite direction. These forces, known as the "Starling forces," were the first to demonstrate their importance by physiologist Ernest Starling.
1. Capillary pressure (Pc), tends to push fluid out through the capillary membrane.
2. Interstitial fluid pressure (Pif), tends to keep fluid inside the capillary membranes when Pif is positive but pushes out when Pif is negative.
3. The osmotic pressure (Πp) of the plasma in the capillary lumen, tends to cause fluid permeation through the capillary membrane.
4. Osmotic pressure (Πif) of the interstitial fluid, tends to create an osmotic pressure of the fluid to the outside through the capillary membrane.
Sum of the forces, if the combined filtration pressure is positive, the filtrate will pass through the capillaries. If the sum of the Starling forces is negative, there will be an absorption of liquid from the interstitial spaces into the capillaries. The net filter pressure (NFP) is calculated as follows:
NFP = Pc - Pif - Πp - Πif
As discussed, NFP is slightly positive under normal conditions, resulting in a net filtration pressure that filters fluid through the capillaries into the interstitial in most organs. The rate of fluid filtration in a tissue is also determined by the number and size of the pores in each capillary, as well as the number of capillaries in which blood is flowing. These factors are often expressed together in the capillary filtration coefficient (Kf).
Kf is therefore a measure of the capillary membrane's ability to filter water for a given NFP and is usually expressed in ml/min per mmHg of NFP.
Therefore, the rate of fluid filtration in the capillaries is determined as follows:
Filter rate = Kf x NFP
The following sections discuss each force that determines the rate of filtration of fluid in the capillary.