4.9: Capillaries
- Page ID
- 83991
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The capillaries, which receive blood from the smallest arteries, are often no more than 1 mm in length. They are so narrow that blood cells can pass through only in single file and often must actually fold to pass through. The capillaries run among the body cells and are so numerous and so close together that no cell is very far from a capillary (Figure 4.11).
Capillaries are porous vessels through which materials in the blood move out to the surrounding body cells and many of the materials produced by body cells (e.g., wastes, hormones) move back into the blood. Since materials are moving in both directions, this process is called capillary exchange. However, a portion of the material that moves out of the capillaries and some of the material produced by the cells do not travel back into the capillaries. This material passes instead into lymph capillaries, where it is known as lymph. The lymph then passes through lymph vessels, which deliver it into large veins near the heart.
The structure of capillaries is well suited for capillary exchange (Figure 4.12). The wall of each capillary is composed of a single layer of thin cells that are supported by a thin layer of material they secrete (basement membrane). Many small atoms and molecules pass through the capillary wall quickly and easily by the process of diffusion, which involves the movement of materials from an area of higher concentration to an area of loser concentration. Therefore, substances that are abundant in the blood diffuse outward to the cells, while other substances diffuse from the cells into the blood.
Capillary walls have pores between the cells and through the cells that constitute them, making it even easier for substances to diffuse between the blood and body cells. In addition, blood pressure pushes many atoms, ions and small molecules out of the capillaries through the pores, leaving large molecules and cells within the blood. This separation of small substances from large ones by fluid pressure is called filtration.
Age Changes in Capillaries
With increasing age, many capillaries become narrower and irregular in shape, and this retards the flow of blood. Some capillaries become so narrow that blood cells get stuck in them, further inhibiting blood flow. In some organs (e.g., heart, muscles) blood supply is further reduced because of a decrease in the number of capillaries. Finally, capillary walls become thicker and have a decrease in the number of pores; both changes inhibit capillary exchange.
Age changes reduce the ability of capillaries to meet the needs of body cells quickly. Therefore, while the cells may be able to function well at low levels of activity, both the ability to sustain vigorous activity and the maximum rate of physical activity may be lowered. This becomes evident when people tire more quickly while performing vigorous work or experience a gradual drop in the maximum speed of activity they can attain while performing vigorous activities such as running or riding a bicycle.