Exchange of Fluid Between Capillaries and Tissues

The distribution of extracellular fluid between the plasma and interstitial compartments is in a state of dynamic equilibrium. Tissue fluid is not normally a "stagnant pond"; rather, it is a continuously circulating medium, formed from and returning to the vascular system. In this way, the tissue cells receive a continuously fresh supply of glucose and other plasma solutes that are filtered through tiny endothelial channels in the capillary walls.

Filtration results from blood pressure within the capillaries. This hydrostatic pressure, which is exerted against the inner capillary wall, is equal to about 37 mmHg at the arteriolar end of systemic capillaries and drops to about 17 mmHg at the venu-lar end of the capillaries. The net filtration pressure is equal to the hydrostatic pressure of the blood in the capillaries minus the hydrostatic pressure of tissue fluid outside the capillaries, which opposes filtration. If, as an extreme example, these two values were equal, there would be no filtration. The magnitude of the tissue hydrostatic pressure varies from organ to organ. With a hydrostatic pressure in the tissue fluid of 1 mmHg, as it is outside the capillaries of skeletal muscles, the net filtration pressure would be 37 - 1 = 36 mmHg at the arteriolar end of the capillary and 17 - 1 = 16 mmHg at the venular end.

Glucose, comparably sized organic molecules, inorganic salts, and ions are filtered along with water through the capillary channels. The concentrations of these substances in tissue fluid are thus the same as in plasma. The protein concentration of tissue fluid (2 g/100 ml), however, is less than the protein concentration of plasma (6 to 8 g/100 ml). This difference is due to the restricted filtration of proteins through the capillary pores. The osmotic pres sure exerted by plasma proteins—called the colloid osmotic pressure of the plasma (because proteins are present as a colloidal sus-pension)—is therefore much greater than the colloid osmotic pressure of tissue fluid. The difference between these two osmotic pressures is called the oncotic pressure. Since the colloid osmotic pressure of the tissue fluid is sufficiently low to be neglected, the oncotic pressure is essentially equal to the colloid osmotic pressure of the plasma. This value has been estimated to be 25 mmHg. Since water will move by osmosis from the solution of lower to the solution of higher osmotic pressure (chapter 6), this oncotic pressure favors the movement of water into the capillaries.

Whether fluid will move out of or into the capillary depends on the magnitude of the net filtration pressure, which varies from the arteriolar to the venular end of the capillary, and on the on-cotic pressure. These opposing forces that affect the distribution of fluid across the capillary are known as Starling forces, and their effects can be calculated according to this equation:

Fluid movement is proportional to:


Pc = hydrostatic pressure in the capillary ni = colloid osmotic pressure of the interstitial (tissue) fluid

Pi = hydrostatic pressure of interstitial fluid np = colloid osmotic pressure of the blood plasma

The expression to the left of the minus sign represents the sum of forces acting to move fluid out of the capillary. The expression to the right represents the sum of forces acting to move

Tissue fluid

Tissue fluid


Arterial end of capillary

Venous end of capillary

(Fluid in)


Arterial end of capillary

Venous end of capillary

(Fluid in)

(37 + 0) - (1 + 25)

(17 + 0) - (1 + 25)

= 11 mmHg

= -9 mmHg

Net filtration

Net absorption

Where Pc = hydrostatic pressure in the capillary ni = colliod osmotic pressure of interstitial fluid P = hydrostatic pressure of interstitial fluid np = colloid osmotic pressure of blood plasma

Where Pc = hydrostatic pressure in the capillary ni = colliod osmotic pressure of interstitial fluid P = hydrostatic pressure of interstitial fluid np = colloid osmotic pressure of blood plasma

■ Figure 14.9 The distribution of fluid across the walls of a capillary. Tissue, or interstitial, fluid is formed by filtration (orange arrows) as a result of blood pressures at the arteriolar ends of capillaries; it is returned to the venular ends of capillaries by the colloid osmotic pressure of plasma proteins (yellow arrows).

fluid into the capillary. Figure 14.9 provides typical values for blood capillaries in skeletal muscles. Notice that the sum of the forces acting on the capillary is a positive number at the arterio-lar end and a negative number at the venular end of the capillary. The positive value at the arteriolar end indicates that the Starling forces that favor the extrusion of fluid from the capillary predominates. The negative value at the venular end indicates that the net Starling forces favor the return of fluid to the capillary. Fluid thus leaves the capillaries at the arteriolar end and returns to the capillaries at the venular end (fig. 14.9, top).

Clinical Investigation Clues

Remember that Charlie was given intravenous fluid containing albumin.

Why was Charlie given albumin?

What advantage does intravenous albumin have over intravenous saline or dextrose (glucose)?

Cardiac Output, Blood Flow, and Blood Pressure

This "classic" view of capillary dynamics has been modified in recent years by the realization that the balance of filtration and reabsorption varies in different tissues and under different conditions in a particular capillary. For example, a capillary may be open or closed off by precapillary muscles that function as sphincters. When the capillary is open, blood flow is high and the net filtration force exceeds the force for the osmotic return of water throughout the length of the capillary. The opposite is true if the precapillary sphincter closes and the blood flow through the capillary is reduced.

Through the action of the Starling forces, plasma and tissue fluid are continuously interchanged. The return of fluid to the vascular system at the venular ends of the capillaries, however, does not exactly equal the amount filtered at the arteriolar ends. According to some estimates, approximately 85% of the capillary filtrate is returned directly to the capillaries; the remaining 15% (amounting to at least 2 L per day) is returned to the vascular system by way of the lymphatic system. Lymphatic capillaries, it may be recalled from chapter 13 (see fig. 13.34), drain excess tissue fluid and proteins and, by way of lymphatic vessels, ultimately return this fluid to the venous system.

In the tropical disease filariasis, mosquitoes transmit a nematode worm parasite to humans. The larvae of these worms invade lymphatic vessels and block lymphatic drainage. The edema that results can be so severe that the tissues swell to produce an elephant-like appearance, with thickening and cracking of the skin. This condition is thus aptly named elephantiasis (fig. 14.10). The World Health Organization estimates that this disease currently affects at least 120 million people, primarily in India and Africa. A new drug regimen has been found to be 99% effective against the filariasis parasite, and a worldwide effort to treat this disease is now underway.

Causes of Edema

Excessive accumulation of tissue fluid is known as edema. This condition is normally prevented by a proper balance between capillary filtration and osmotic uptake of water and by proper lymphatic drainage. Edema may thus result from

1. high arterial blood pressure, which increases capillary pressure and causes excessive filtration;

2. venous obstruction—as in phlebitis (where a thrombus forms in a vein) or mechanical compression of veins (during pregnancy, for example)—which produces a congestive increase in capillary pressure;

3. leakage of plasma proteins into interstitial fluid, which causes reduced osmotic flow of water into the capillaries (this occurs during inflammation and allergic reactions as a result of increased capillary permeability);

4. myxedema—the excessive production of particular glycoproteins (mucin) in the extracellular matrix caused by hypothyroidism;

■ Figure 14.10 The severe edema of elephantiasis. Parasitic larvae that block lymphatic drainage produce tissue edema and the tremendous enlargement of the limbs and external genitalia in elephantiasis.

Table 14.2 Causes of Edema



Increased blood pressure or venous obstruction

Increases capillary filtration pressure so that more tissue fluid is formed at the arteriolar ends of capillaries.

Increased tissue protein concentration

Decreases osmosis of water into the venular ends of capillaries. Usually a localized tissue edema due to leakage of plasma proteins through capillaries during inflammation and allergic reactions. Myxedema due to hypothyroidism is also in this category.

Decreased plasma protein concentration

Obstruction of lymphatic vessels

Decreases osmosis of water into the venular ends of capillaries. May be caused by liver disease (which can be associated with insufficient plasma protein production), kidney disease (due to leakage of plasma protein into urine), or protein malnutrition.

Infections by filaria roundworms (nematodes) transmitted by a certain species of mosquito block lymphatic drainage, causing edema and tremendous swelling of the affected areas.

5. decreased plasma protein concentration, as a result of liver disease (the liver makes most of the plasma proteins) or kidney disease where plasma proteins are excreted in the urine;

6. obstruction of the lymphatic drainage (table 14.2).

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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