4.2: Main Functions for Homeostasis and need for Adaptability
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Transportation
One main function of the circulatory system is transportation of materials within the body. Transportation helps maintain homeostasis by ensuring that the concentrations of substances surrounding body cells are kept at proper and fairly steady levels. Materials consumed by the cells are immediately replenished, and materials produced by the cells are swept away before their concentrations become too high.
The flowing blood also transports useful materials from their point of entry into the body to the organs that need them. For example, oxygen from the lungs and nutrients from the digestive system are delivered to muscles. Furthermore, some cells manufacture substances (e.g., hormones) needed by cells in other organs, and the circulatory system provides the delivery service for them.
As a person's rate of activity changes, the rate of activity of that person's body cells also changes. This causes the rate of consumption of nutrients and the production of wastes and hormones to fluctuate. The circulatory system helps the body adapt to these changes by altering the rate of blood flow through each part of the body.
Defense
The circulatory system also makes an important contribution to the defense of the body. Lymph nodes trap and destroy dangerous chemicals and microbes before they can spread throughout the body. For example, toxins and bacteria that enter the lymph from an infected wound are inactivated as they pass through the lymph vessels. Blood and lymph contain several types of white blood cells (WBCs). Some WBCs eliminate dangerous materials contained in blood and lymph, while others leave the blood in the capillaries and travel among the cells of the body to seek out and destroy noxious materials and microbes. Other defense cells, located on the inner walls of blood vessels, monitor the contents of the flowing blood and remove undesirable materials.
Like transportation, defense is increased or decreased to meet changing needs. The number and speed of movement of WBCs and their rates of producing defensive chemicals increase temporarily whenever harmful microbes or foreign materials are detected within the body.
Temperature Control
Another homeostatic function of the circulatory system is temperature control. Temperature regulation and the role of dermal blood vessels in this process were discussed in Chapter 3.
Another way in which the circulatory system contributes to thermal regulation is by distributing heat from heat‑producing sites to areas that cannot keep themselves warm. For example, muscles produce much heat, and blood carries some of it to smaller structures such as the spinal cord, slower‑acting organs such as bones, and cooler areas such as the skin. In this way heat distribution helps prevent overheating in any single area of the body while sustaining activities in all its regions.
Acid/Base Balance
Besides being sensitive to heat, many substances in the body are altered by the balance between acidic and basic (alkaline) materials in their surroundings. The relative amounts of acids and bases are usually indicated by a numerical value called pH. The normal range for the body is about pH 7.35 to pH 7.45. An acid/base balance resulting in a pH within this range preserves the proper shape and activities of molecules in the body. Deviations from this range adversely alter these molecules, leading to malfunction, damage, and even the death of cells.
Maintaining proper acid/base balance requires the constant action of negative feedback systems because the ongoing activities of most cells result in the formation of acids. Foods and beverages can also add acids or bases to the body. Excess amounts of acid or base must be neutralized or eliminated to maintain a proper pH. This is where the circulatory system makes a major contribution. Certain minerals and protein molecules in blood plasma and red blood cells – buffers – act as reservoirs for acids These buffers absorb and store excess acids. When bases become too abundant, some stored acid is released to balance them, preserving the acid/base balance. These buffers have a limited capacity for balancing pH, and acid/base balance also depends on the activities of the respiratory and urinary systems.
We will now examine components of the circulatory system in greater detail, beginning with the heart.