Fluid Movement at the Capillary Level and the Starling Hypothesis

Up until now, our discussion has dealt with fluid movement across the cell membrane. In this case, osmosis is the driving force. Capillaries rely on different forces to move fluid back and forth between the capillaries and interstitial space.

Remember that blood contains an ICF component (fluid inside the cells) and an ECF component (plasma). Capillaries contain tiny pores that allow various molecules to pass through the capillary wall. Blood cells and platelets are too large to pass through these pores, so they stay in the circulatory system. Substances carried in the plasma are able to pass through the pores into the interstitial fluid to supply nutrients to the body’s cells. Likewise, substances (i.e., waste) can pass through the pores in the capillary walls from the interstitial space.

Really, there is not much difference between plasma and interstitial fluid. They are pretty much continuous with one another across the capillary pores. There is one difference that needs to be mentioned, because it plays an important role: protein has a tendency to stay in the intravascular space (due to its large size).

Because various molecules (in addition to water) can pass through the capillary pores, osmosis is ineffective. Fluid movement is controlled partially by hydrostatic pressure. Blood is always pressing against vessel walls. Since capillaries have pores, a portion of plasma is pushed out of the capillaries into the interstitial space. An opposing force that pulls fluid back into the capillaries can be compared to osmosis. As mentioned earlier, proteins are very large and tend to stay in the intravascular space. Water is drawn through the capillary pore toward an area of higher protein concentration, much like it is drawn across cell membranes to an area of higher solute concentration.

Albumin is the most plentiful of the plasma proteins and globulins account for the majority of the remainder. All plasma proteins are colloids. A colloid is a heavy molecule that is too large to pass through capillary pores. Therefore, colloid density is much higher in the intravascular space than in the interstitial space. Water is drawn toward the higher colloid density. This force is known as colloid osmotic pressure (COP).

As you can see, hydrostatic pressure and COP are opposing forces working together to move water into and out of the capillary. This is referred to as the Starling Hypothesis. Hydrostatic pressure is the dominant force at the arteriolar end of the capillary, but as the pressure drops toward the venous end, COP becomes the more dominant force. Overall, more fluid is pushed out of the capillary by hydrostatic pressure than is pulled in by COP.

Exercises

1. What would happen to extravascular fluid if a colloid was infused to a patient?
2. If normal saline were infused, what would happen to the interstitial fluid?