Materials are exchanged in capillary beds by filtration osmosis and diffusion

The walls of capillaries are made of a single layer of thin en-dothelial cells. In most tissues of the body other than the brain, capillaries have tiny holes called fenestrations (Latin,

49.10 Anatomy of Blood Vessels The anatomical characteristics of blood vessels match their functions.

Red blood cells must pass through capillaries in single file.

Red blood cells must pass through capillaries in single file.

Red Cells Squeezing Through Capillary

49.11 A Narrow Lane Capillaries have a very small diameter, and blood flows through them slowly.

"windows"). Capillaries are permeable to water, to some ions, and to some small molecules, but not to large molecules such as proteins. Blood pressure therefore squeezes water and some small solutes out of the capillaries and into the surrounding intercellular spaces. Why don't water and small-molecular-weight solutes collect in the intercellular spaces? How is the blood volume maintained if fluid is continuously leaking out of the capillaries?

An answer to this question was put forth more than a hundred years ago by the physiologist E. H. Starling. Starling suggested that water balance in capillary beds is a result of two opposing forces, which have come to be known as Starling's forces. One force is blood pressure, which squeezes water and small solutes out of the capillaries, and the other is osmotic pressure created by the large protein molecules that cannot leave the capillaries. Starling called this second force colloidal osmotic pressure. He hypothesized that blood pressure is high at the arterial end of a capillary bed and drops steadily as blood flows to the venous end (Figure 49.12). The colloidal osmotic pressure, however, is constant along the capillary. As long as the blood pressure is above the osmotic pressure, water leaves the capillary, but when blood pressure falls below the osmotic pressure, water returns to the capillary. The actual numbers for a normal capillary bed in a resting person suggest that there would be a slight net loss of water to the intercellular spaces.

Several observations supported Starling's model. In people with severe liver disease or protein starvation, there is a fall in blood protein concentration that leads to an accumulation of water in the extracellular spaces, which results in tissue swelling, or edema. Edema is also a characteristic of the inflammation reaction that accompanies tissue damage or allergic responses (see Figure 18.4). Histamine, a mediator of inflammation that is released by certain white blood cells, increases the permeability of capillaries and also relaxes the smooth muscles of the arterioles, resulting in higher blood pressure in the capillaries. The hypothesis modeled in Figure 49.12 predicts that edema should occur in all of these cases.

Only recently have Starling's forces come into question as the complete explanation for fluid exchange in capillary beds. There are situations that are not explained by Starling's hypothesis. During strenuous exercise, the blood pressure in the arterioles serving the muscles rises substantially, yet edema does not occur. In birds, the blood pressure in arteri-oles is much higher than in mammals, and the colloidal osmotic pressure is lower. If edema is not a chronic problem in exercising muscles and in birds, what is missing from Starling's model?

Recent research suggests that bicarbonate ions (HCO3-) in the blood plasma are an important contributor to the osmotic attraction of water back into the capillary. As the blood flows through the capillary, CO2 diffuses into the plasma and is converted into bicarbonate ions (see Figure 48.14); therefore, there is a substantial rise in HCO- concentration as blood

Fluid is squeezed out of the capillary by blood pressure.

Fluid is pulled back into the capillary by osmotic pressure.

Fluid is squeezed out of the capillary by blood pressure.

Fluid is pulled back into the capillary by osmotic pressure.

Blood Colloidal pressure osmotic (40) pressure (25)

Blood Colloidal pressure osmotic (16) pressure (25)

Blood Colloidal pressure osmotic (40) pressure (25)

Arteriole end

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Responses

  • patrick
    What materials are exchanged in capillary beds?
    8 years ago
  • xander
    Do solutes leave capillary bed through osmosis?
    8 years ago
  • gail abbas
    What is the fluid squeezed by the capillaries known as?
    8 years ago
  • BELLISIMA
    Why is diffusion possible in capillary beds?
    8 years ago
  • LAILA
    How are substances exchanged in capillary bed?
    8 years ago
  • JOSHUA RICHARDSON
    Why is osmotic pressure constant along capillary?
    8 years ago
  • wolfgang schmidt
    How does starling's forces maintain water balance in capillary beds?
    8 years ago
  • gerda
    How do blood and osmotic pressures maintain water balance in capillary beds?
    8 years ago
  • halfred
    What are the three ways of capillary exchange (diffusion, filtration, and osmosis)?
    8 years ago
  • christopher
    How are materials exchanged through capillaries?
    8 years ago
  • Medhanie
    How is fluid pulled into capillaries?
    6 years ago
  • aleisha
    What is bleeding from capillary beds called?
    2 years ago

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