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unit four

Artery

Vein

Artery

Vein

Veins Diagram

Figure 15.24

(a) The wall of an artery and (b) the wall of a vein.

Figure 15.24

(a) The wall of an artery and (b) the wall of a vein.

.Arteriole

Smooth muscle cell

.Arteriole

Smooth muscle cell

Figure 15.25

Small arterioles have smooth muscle fibers in their walls; capillaries lack these fibers.

Figure 15.25

Small arterioles have smooth muscle fibers in their walls; capillaries lack these fibers.

squamous epithelial cells (fig. 15.28a). These thin walls form the semipermeable layer through which substances in the blood are exchanged for substances in the tissue fluid surrounding body cells.

Capillary Permeability

The openings or intercellular channels in the capillary walls are thin slits where endothelial cells overlap. The sizes of these openings, and consequently the permeability

Figure 15.26

Scanning electron micrograph of an arteriole cross section (2,200x).

Figure 15.26

Scanning electron micrograph of an arteriole cross section (2,200x).

© R.G. Kessel and R.H. Kardon. Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979.

Metarteriole (forming arteriovenous

Precapillary sphincter

Arteriole

Metarteriole (forming arteriovenous

Precapillary sphincter

Arteriole

Hypovolemia Cardiac

Blood flow

Figure 15.27

Some metarterioles provide arteriovenous shunts by connecting arterioles directly to venules.

Blood flow

Blood flow

Figure 15.27

Some metarterioles provide arteriovenous shunts by connecting arterioles directly to venules.

Blood flow of the capillary wall, vary from tissue to tissue. For example, the openings are relatively small in the capillaries of smooth, skeletal, and cardiac muscle, whereas those in capillaries associated with endocrine glands, the kidneys, and the lining of the small intestine are larger.

Capillaries with the largest openings include those of the liver, spleen, and red bone marrow. These capillaries are discontinuous, and the distance between their cells appears as little cavities (sinusoids) in the organ. Discontinuous capillaries allow large proteins and even intact cells to pass through as they enter or leave the circulation (fig. 15.28fo and c). Clinical Application 3.2 discusses the blood-brain barrier, the protective tight capillaries in the brain. The barrier is not present in the pituitary and pineal glands and parts of the hypothalamus.

Capillary Arrangement

The higher a tissue's rate of metabolism, the denser its capillary networks. Muscle and nerve tissues, which use abundant oxygen and nutrients, are richly supplied with capillaries; cartilaginous tissues, the epidermis, and the cornea, where metabolism is slow, lack capillaries.

If the capillaries of an adult were unwound and spread end to end, they would cover from 25,000 to 60,000 miles.

The patterns of capillary arrangement also differ in various body parts. For example, some capillaries pass directly from arterioles to venules, but others lead to highly branched networks (fig. 15.29). Such physical arrangements make it possible for the blood to follow different pathways through a tissue that are attuned to cellular requirements.

Blood flow can vary among tissues as well. During exercise, for example, blood is directed into the capillary networks of the skeletal muscles, where the cells require more oxygen and nutrients. At the same time, the blood bypasses some of the capillary networks in the tissues of the digestive tract, where demand for blood is less critical. Conversely, when a person is relaxing after a meal, blood can be shunted from the inactive skeletal muscles into the capillary networks of the digestive organs.

Regulation of Capillary Blood Flow

The distribution of blood in the various capillary pathways is mainly regulated by the smooth muscles that encircle the capillary entrances. As figure 15.27 shows, these muscles form precapillary sphincters, which may close a capillary by contracting or open it by relaxing. The precapillary sphincters respond to the needs of the cells. When the cells have low concentrations of oxygen and nutrients, the sphincter relaxes and blood flow increases; when cellular requirements have been met, the sphincter may contract again.

Tissue fluid

Endothelial cell

Tissue fluid

Endothelial cell

Tissue fluid

Capillary

Tissue fluid

Capillary

Nucleus of endothelial cell

Endothelial cell cytoplasm

Lumen of capillary

Cell junction

Nucleus of endothelial cell

Endothelial cell cytoplasm

Lumen of capillary

Cell junction

ure 15.28

Figure

(a) Substances are exchanged between the blood and tissue fluid through openings (slits) separating endothelial cells.

(b) Transmission electron micrograph of a capillary cross section (11,500x). (c) Note the narrow slitlike openings at the cell junctions (arrow) (micrograph b enlarged to 62,500x).

Arteriole

Capillary

Venule

Figure 15.29

Light micrograph of a capillary network (100x).

Arteriole

Capillary

Venule

Figure

Figure 15.29

Light micrograph of a capillary network (100x).

U Describe a capillary wall.

^9 What is the function of a capillary?

^9 What controls blood flow into capillaries?

Exchanges in the Capillaries

The vital function of exchanging gases, nutrients, and metabolic by-products between the blood and the tissue fluid surrounding body cells occurs in the capillaries. The biochemicals exchanged move through the capillary walls by diffusion, filtration, and osmosis.

O^1 Reconnect to chapter 3, Movements into and out of the Cell, pages 82-87. Diffusion is the most important means of transfer. Because blood entering certain capillaries carries high concentrations of oxygen and nutrients, these substances diffuse through the capillary walls and enter the tissue fluid. Conversely, the concentrations of carbon dioxide and other wastes are generally greater in the tissues, and such wastes tend to diffuse into the capillary blood.

The paths these substances follow depend primarily on their solubilities in lipids. Substances that are soluble in lipid, such as oxygen, carbon dioxide, and fatty acids, can diffuse through most areas of the cell membranes that make up the capillary wall because the membranes are largely lipid. Lipid-insoluble substances, such as water, sodium ions, and chloride ions, diffuse through pores in the cell membranes and through the slitlike openings between the endothelial cells that form the capillary wall (see fig. 15.28). Plasma proteins generally remain in the blood because they are not soluble in the lipid portions of

Blood Vessel Medical

Tissue cells

Blood flow from arteriole

* Capillary

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.

Tissue cells

Blood flow from arteriole

* Capillary

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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