Figure 1446

(a) The four factors determining fluid movement across capillaries. (b) Quantitation of forces causing filtration at the arterial end of the capillary and absorption at the venous end. Arrows in (b) denote magnitude of forces. No arrow is shown for interstitial-fluid hydrostatic pressure (PIF) in (b) because it is approximately zero.

crystalloids in the two locations do change. Thus, an increased filtration of fluid from plasma to interstitial fluid increases the volume of the interstitial fluid and decreases the volume of the plasma, even though no changes in crystalloid concentrations occur.

In summary (Figure 14-46a), opposing forces act to move fluid across the capillary wall: (1) the difference between capillary blood hydrostatic pressure and interstitial-fluid hydrostatic pressure favors filtration out of the capillary; and (2) the water-concentration difference between plasma and interstitial fluid, which results from differences in protein concentration, favors the filtration of interstitial fluid into the capillary (filtration in this direction is termed absorption). Accordingly, the net filtration pressure (NFP) depends directly upon the algebraic sum of four variables: capillary hydrostatic pressure, Pc (favoring fluid movement out of the capillary); interstitial hydrostatic pressure, PIF (favoring fluid movement into the capillary);

the osmotic force due to plasma protein concentration, 7Tp (favoring fluid movement into the capillary); and the osmotic force due to interstitial-fluid protein concentration, WIF (favoring fluid movement out of the capillary).

NFP = Pc - PIF - 7Tp + 7TIP = (Pc - Pip) - (^p - ^IF)

The four factors that determine net filtration pressure are termed the Starling forces (because Starling, the same physiologist who helped elucidate the Frank-Starling mechanism of the heart, was the first to develop the ideas).

We may now consider this movement quantitatively in the systemic circulation (Figure 14-46b). Much of the arterial blood pressure has already been dissipated as the blood flows through the arterioles, so that hydrostatic pressure at the beginning of a typical capillary is about 35 mmHg. Since the capillary also

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions offers resistance to flow, the hydrostatic pressure continuously decreases to approximately 15 mmHg at the end of the capillary. The interstitial hydrostatic pressure is very low, and we shall assume it to be zero. The plasma protein concentration would produce an osmotic flow of fluid into the capillary equivalent to that produced by a hydrostatic pressure of 28 mmHg. The interstitial protein concentration would produce a flow of fluid out of the capillary equivalent to that produced by a hydrostatic pressure of 3 mmHg. Therefore, the difference in protein concentrations would induce a flow of fluid into the capillary equivalent to that produced by a hydrostatic-pressure difference of 28 — 3 = 25 mmHg.

Thus, in the beginning of the capillary the hydrostatic-pressure difference across the capillary wall (35 mmHg) is greater than the opposing osmotic force (25 mmHg), and a net filtration of fluid out of the capillary occurs. In the end of the capillary, however, the osmotic force (25 mmHg) is greater than the hydrostatic-pressure difference (15 mmHg), and fluid filters into the capillary (absorption). The result is that the early and late capillary events tend to cancel each other out. For the aggregate of capillaries in the body, however, there is small net filtration of approximately 4 L/day (this number does not include the capillaries in the kidneys). The fate of this fluid will be described in the section on the lymphatic system.

In our example, we have assumed a "typical" capillary hydrostatic pressure of 35 mmHg. In reality, capillary hydrostatic pressures vary in different vascular beds and, as will be described in a later section, are strongly influenced by whether the person is lying down, sitting, or standing. Moreover, a very important point is that capillary hydrostatic pressure in any vascular bed is subject to physiological regulation, mediated mainly by changes in the resistance of the arteri-oles in that bed. As shown in Figure 14-47, dilating the arterioles in a particular vascular bed raises capillary hydrostatic pressure in that bed because less pressure is lost overcoming resistance between the arteries and the capillaries. Because of the increased capillary hydrostatic pressure, filtration is increased, and more protein-free fluid is lost to the interstitial fluid. In contrast, marked arteriolar constriction produces decreased capillary hydrostatic pressure and hence favors net movement of interstitial fluid into the vascular compartment. Indeed, the arterioles supplying a capillary bed may be so dilated or so constricted that the capillaries manifest only filtration or only reabsorption, respectively, along their entire length.

We have presented the story of capillary filtration entirely in terms of the Starling forces, but one other factor is involved—the capillary filtration coefficient. This is a measure of how much fluid will filter per

Starling Forces Lung

Distance along systemic blood vessels

FIGURE 14-47

Effects of arteriolar vasodilation or vasoconstriction in an organ on capillary blood pressure in that organ (under conditions of constant arterial pressure).

Distance along systemic blood vessels

FIGURE 14-47

Effects of arteriolar vasodilation or vasoconstriction in an organ on capillary blood pressure in that organ (under conditions of constant arterial pressure).

mmHg net filtration pressure. We previously ignored this factor since, in most capillary beds, it is not under physiological control. A major exception, however, are the capillaries of the kidneys; as we shall see in Chapter 16, certain of the kidney capillaries filter huge quantities of protein-free fluid because they have a very large capillary filtration coefficient, one that can be altered physiologically.

It must be stated again that capillary filtration and absorption play no significant role in the exchange of nutrients and metabolic end products between capillary and tissues. The reason is that the total quantity of a substance, such as glucose or carbon dioxide, moving into or out of a capillary as a result of net bulk flow is extremely small in comparison with the quantities moving by net diffusion.

Finally, this analysis of capillary fluid dynamics has been in terms of the systemic circulation. Precisely the same Starling forces apply to the capillaries in the pulmonary circulation, but the values of the four variables differ. In particular, because the pulmonary circulation is a low-resistance, low-pressure circuit, the normal pulmonary capillary hydrostatic pressure— the major force favoring movement of fluid out of the pulmonary capillaries into the interstitium—is only 15 mmHg. Therefore, normally, net absorption of fluid occurs along the entire length of lung capillaries.

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Essentials of Human Physiology

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  • Lelia
    What are the forces that determine fluid movement across capillary walls?
    8 years ago
    How does nfp change from the arterial end of a capillary to the venous end of a capillary?
    8 years ago
    When is net filtration pressure zero?
    8 years ago
  • hagos
    How much capillary filtrate is produced per day urine?
    8 years ago
  • cornelia
    What is the primary opposing force to capillary hydrostatic pressure?
    8 years ago

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