5.2: Main Functions for Homeostasis
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Working in a coordinated fashion controlled mostly by the nervous system, these structures perform the two functions of the respiratory system; gas exchange and sound production. Gas exchange involves two processes; obtaining oxygen and eliminating carbon dioxide.
The respiratory system obtains oxygen by providing conditions that allow the oxygen contained in air to pass into the blood flowing through the lungs. The circulatory system then transports the oxygen throughout the body. Oxygen (O2) must be supplied to body cells because it is a raw material used by mitochondria to obtain energy from nutrients. This energy provides the power needed to perform all essential bodily activities.
The respiratory system eliminates carbon dioxide (CO2) by providing conditions that allow it to move out of the blood in pulmonary vessels and into the atmosphere. Carbon dioxide is transported from body cells to the lungs by the circulatory system.
Carbon dioxide must be eliminated because it is a waste product from the series of chemical reactions in mitochondria that release energy from nutrients. When CO2 accumulates within the body, it can interfere with body functions because CO2 combines with water to produce carbonic acid. The excess carbonic acid upsets the acid/base balance of the body. This disturbance can alter body proteins (see animation). Body structures and functions can be adversely affected, and serious illness or death can follow. Still, some acidic materials must be present for the body to achieve a normal acid/base balance, and a deficiency in acids can be as disastrous as an excess. Furthermore, some CO2 in the blood is used to make a buffer. Therefore, the respiratory system must eliminate some but not too much CO2.
Many other acidic materials in the body contribute to the acidic side of the acid/base balance. If there is an increase in acidic substances other than CO2, the respiratory system can help maintain acid/base balance by eliminating more CO2. This occurs in individuals whose kidneys do not eliminate acids adequately.
The rate of bodily activities changes from time to time. These changes cause fluctuations in the rates of O2 use, CO2 production and the amount of other acids. To maintain homeostasis, negative feedback systems employing the nervous system normally ensure that the rate of gas exchange by the respiratory system increases or decreases to meet these fluctuations. This adaptive mechanism occurs when a person begins to breathe more heavily soon after beginning vigorous physical activity.
The maximum amount that gas exchange can be increased to compensate for increases in bodily activity constitutes the reserve capacity of the respiratory system. The limited nature of respiratory capacity seems to contribute to setting a maximum limit on how vigorously a person can exercise. This limit is experienced as the sensation of feeling completely out of breath while exercising. Limitations in the maximum functional capacities of the circulatory, nervous, and muscle systems may also play a role in establishing the maximum rate of physical activity attainable.
Three operations are involved in carrying out gas exchange. Ventilation (breathing) involves moving air through the airways into and out of the lungs. Perfusion of the lungs involves the movement of blood through the pulmonary vessels. Diffusion causes the O2 in inhaled air to move into the blood while CO2 exits into the air in the lungs.
Sound production, which is the second main function of the respiratory system, is important because it helps people communicate. A short section on sound production, including the effects caused by aging, is presented at the end of this chapter.