Figure 165

Fluid pressure and colloid osmotic pressure forces operate at the capillary membrane, tending to move fluid either outward or inward through the membrane pores.

of fluid volume from the blood into the interstitial spaces.

Also important is the lymphatic system, which returns to the circulation the small amounts of excess protein and fluid that leak from the blood into the interstitial spaces. In the remainder of this chapter, we discuss the mechanisms that control capillary filtration and lymph flow function together to regulate the respective volumes of the plasma and the interstitial fluid.

Four Primary Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane. Figure 16-5 shows the four primary forces that determine whether fluid will move out of the blood into the interstitial fluid or in the opposite direction. These forces, called "Starling forces" in honor of the physiologist who first demonstrated their importance, are:

1. The capillary pressure (Pc), which tends to force fluid outward through the capillary membrane.

2. The interstitial fluid pressure (Pif), which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative.

3. The capillary plasma colloid osmotic pressure (np), which tends to cause osmosis of fluid inward through the capillary membrane.

4. The interstitial fluid colloid osmotic pressure (nif), which tends to cause osmosis of fluid outward through the capillary membrane.

If the sum of these forces, the net filtration pressure, is positive, there will be a net fluid filtration across the capillaries. If the sum of the Starling forces is negative, there will be a net fluid absorption from the interstitial spaces into the capillaries. The net filtration pressure (NFP) is calculated as:

As discussed later, the NFP is slightly positive under normal conditions, resulting in a net filtration of fluid across the capillaries into the interstitial space 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

Figure 16-6

Isogravimetric method for measuring capillary pressure.

Figure 16-6

Isogravimetric method for measuring capillary pressure.

which blood is flowing. These factors are usually expressed together as the capillary filtration coefficient (Kf). The Kf is therefore a measure of the capacity of the capillary membranes to filter water for a given NFP and is usually expressed as ml/min per mm Hg net filtration pressure.

The rate of capillary fluid filtration is therefore determined as:

Filtration = Kf x NFP

In the following sections we discuss in detail each of the forces that determine the rate of capillary fluid filtration.

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Essentials of Human Physiology

Essentials of Human Physiology

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