At the same time that the diffusional exchange of nutrients, oxygen, and metabolic end products is occurring across the capillaries, another, completely distinct process is also taking place across the capillary—the bulk flow of protein-free plasma. The function of this process is not exchange of nutrients and metabolic end products but rather distribution of the extracellular fluid. As described in Chapter 1, extracellular fluid comprises the blood plasma and interstitial fluid. Normally, there is approximately three times more interstitial fluid than plasma, 10 L versus 3 L in a 70-kg person. This distribution is not fixed, however, and the interstitial fluid functions as a reservoir that can supply fluid to the plasma or receive fluid from it.
As described in the previous section, the capillary wall is highly permeable to water and to almost all plasma solutes, except plasma proteins. Therefore, in the presence of a hydrostatic pressure difference across it, the capillary wall behaves like a porous filter through which protein-free plasma moves by bulk flow from capillary plasma to interstitial fluid through the water-filled channels. (This is technically termed "ultrafiltration" but we shall refer to it simply as filtration.) The concentrations of all the plasma solutes except protein are virtually the same in the filtering fluid as in plasma.
The magnitude of the bulk flow is determined, in part, by the difference between the capillary blood pressure and the interstitial-fluid hydrostatic pressure. Normally, the former is much larger than the latter. Therefore, a considerable hydrostatic-pressure difference exists to filter protein-free plasma out of the cap illaries into the interstitial fluid, the protein remaining behind in the plasma.
Why then does all the plasma not filter out into the interstitial space? The explanation is that the hydrostatic pressure difference favoring filtration is offset by an osmotic force opposing filtration. To understand this, we must review the principle of osmosis.
In Chapter 6, we described how a net movement of water occurs across a semipermeable membrane from a solution of high water concentration to a solution of low water concentration—that is, from a region with a low concentration of solute to which the membrane is impermeable (nonpermeating solute) to a region of high nonpermeating-solute concentration. Moreover, this osmotic flow of water "drags" along with it any dissolved solutes to which the membrane is highly permeable (permeating solute). Thus, a difference in water concentration secondary to different concentrations of nonpermeating solute on the two sides of a membrane can result in the movement of a solution containing both water and permeating solutes in a manner similar to the bulk flow produced by a hydrostatic-pressure difference. Units of pressure are used in expressing this osmotic flow across a membrane just as for flow driven by a hydrostatic-pressure difference.
This analysis can now be applied to osmotically induced flow across capillaries. The plasma within the capillary and the interstitial fluid outside it contain large quantities of low-molecular-weight permeating solutes (also termed crystalloids), for example, sodium, chloride, and glucose. Since the capillary lining is highly permeable to all these crystalloids, their concentrations in the two solutions are essentially identical (as we have seen, there actually are small concentration differences for substances that are utilized or produced by the cells, but these tend to cancel each other). Accordingly, the presence of the crystalloids causes no significant difference in water concentration. In contrast, the plasma proteins (also termed colloids), being essentially nonpermeating, have a very low concentration in the interstitial fluid. The difference in protein concentration between plasma and interstitial fluid means that the water concentration of the plasma is very slightly lower (by about 0.5 percent) than that of interstitial fluid, inducing an osmotic flow of water from the interstitial compartment into the capillary. Since the crystalloids in the interstitial fluid move along with the water, osmotic flow of fluid, like flow driven by a hydrostatic-pressure difference, does not alter crystalloid concentrations in either plasma or interstitial fluid.
Akey word in this last sentence is "concentrations." The amount of water (the volume) and the amount of
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Circulation CHAPTER FOURTEEN
Circulation CHAPTER FOURTEEN
Capillary hydrostatic pressure (Pc)
Osmotic force due to plasma protein concentration (np)
Interstitial-fluid hydrostatic pressure
Osmotic force due to interstitial-fluid protein concentration
Net filtration pressure = (PC - PIF) - ( n p - TT|f)
Arterial end of capillary
Venous end of capillary
Net filtration pressure = (35 - 0) - (28 - 3) = 10 mmHg 10 mmHg favoring filtration
Net filtration pressure =
10 mmHg favoring absorption
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