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Figure

Water and other substances leave the capillaries because of a net outward filtration pressure at the arteriolar end of the capillaries. Water enters at the venular end of the capillaries because of a net inward force of colloid osmotic pressure.

the endothelial cell membranes, and they are too large to diffuse through the membrane pores or slitlike openings between the endothelial cells of most capillaries.

In filtration, hydrostatic pressure forces molecules through a membrane. In the capillaries, the blood pressure generated when ventricle walls contract provides the force for filtration.

Blood pressure also moves blood through the arteries and arterioles. This pressure decreases as distance from the heart increases because of friction (peripheral resistance) between the blood and the vessel walls. For this reason, blood pressure is greater in the arteries than in the arterioles and greater in the arterioles than in the capillaries. It is similarly greater at the arteriolar end of a capillary than at the venular end.

The walls of arteries and arterioles are too thick to allow blood components to pass through. However, the hydrostatic pressure of the blood pushes small molecules through capillary walls by filtration. This effect primarily occurs at the arteriolar ends of capillaries, whereas diffusion takes place along their entire lengths.

The presence of an impermeable solute on one side of a cell membrane creates an osmotic pressure. Because plasma proteins are trapped within the capillaries, they create an osmotic pressure that draws water into the capillaries. The term colloid osmotic pressure is often used to describe this osmotic effect due solely to the plasma proteins.

The effect of capillary blood pressure, favoring filtration, and the plasma colloid osmotic pressure, favoring reabsorption, have opposite actions. At the arteriolar end of capillaries, the blood pressure is higher (35 mm Hg outward) than the colloid osmotic pressure (25 mm Hg inward), so at the arteriolar end of the capillary, filtration predominates. At the venular end, the colloid osmotic pressure is essentially unchanged (25 mm Hg inward), but the blood pressure has decreased due to resistance through the capillary (18 mm Hg outward). Thus, at the venular end, reabsorption predominates (fig. 15.30). (The interstitial fluid also has hydrostatic pressure and osmotic pressure, but the values are quite low and tend to cancel each other out; as such, they can be omitted from this discussion.)

Normally, more fluid leaves the capillaries than returns to them. Lymphatic vessels collect the excess fluid and return it to the venous circulation. This mechanism is discussed in chapter 16 (p. 654).

Sometimes unusual events increase blood flow to capillaries, and excess fluid enters spaces between tissue cells (interstitial spaces). This may occur, for instance, in response to certain chemicals such as histamine that va-sodilate the metarterioles and increase capillary permeability. Enough fluid may leak out of the capillaries to overwhelm lymphatic drainage, and affected tissues become swollen (edematous) and painful.

Endothelium Middle Lumen layer

Outer layer

Figure 15.31

Note the structural differences in these cross sections of (a) an artery (100x) and (b) a vein (160x).

Endothelium Middle Lumen layer

Outer layer

Endothelium Middle Lumen Outer (b) layer layer

Figure 15.31

Note the structural differences in these cross sections of (a) an artery (100x) and (b) a vein (160x).

What forces are responsible for the exchange of substances between blood and the tissue fluid?

Why is the fluid movement out of a capillary greater at its arteriolar end than at its venular end?

Since more fluid leaves the capillary than returns to it, how is the remainder returned to the vascular system?

If the right ventricle of the heart is unable to pump blood out as rapidly as it enters, other parts of the body may develop edema, because the blood backs up into the veins, venules, and capillaries, increasing blood pressure in these vessels. As a result of this increased back pressure, osmotic pressure of the blood in the venular ends of the capillaries is less effective in attracting water from tissue fluid, and the tissues swell. This is true particularly in the lower extremities if the person is upright, or in the back if the person is supine. In the terminal stages of heart failure, edema is widespread, and fluid accumulates in the peritoneal cavity of the abdomen. This condition is called ascites.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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