Balance in the Homeostasis of Chemicals

Many homeostatic systems are concerned with the balance between the addition to and removal from the body of a chemical substance. Figure 7-8 is a generalized schema of the possible pathways involved in such balance. The pool occupies a position of central importance in the balance sheet. It is the body's readily available quantity of the particular substance and is frequently identical to the amount present in the extracellular fluid. The pool receives substances from and contributes them to all the pathways.

The pathways on the left of the figure are sources of net gain to the body. A substance may enter the body through the gastrointestinal (GI) tract or the lungs. Alternatively, a substance may be synthesized within the body from other materials.

The pathways on the right of the figure are causes of net loss from the body. A substance may be lost in the urine, feces, expired air, or menstrual fluid, as well as from the surface of the body as skin, hair, nails, sweat, and tears. The substance may also be chemically altered and thus removed by metabolism.

The central portion of the figure illustrates the distribution of the substance within the body. The substance may be taken from the pool and accumulated in storage depots (for example, the accumulation of fat in adipose tissue). Conversely, it may leave the storage depots to reenter the pool. Finally, the substance may be incorporated reversibly into some other molecular structure, such as fatty acids into membranes or iodine into thyroxine. Incorporation is reversible in that the substance is liberated again whenever the more complex structure is broken down. This pathway is distinguished from storage in that the incorporation of the substance into the other molecules produces new molecules with specific functions.

NET GAIN TO BODY DISTRIBUTION WITHIN NET LOSS FROM

BODY BODY

NET GAIN TO BODY DISTRIBUTION WITHIN NET LOSS FROM

BODY BODY

Balance Diagram For Chemical Substance

FIGURE 7-8

Balance diagram for a chemical substance.

FIGURE 7-8

Balance diagram for a chemical substance.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

It should be recognized that not every pathway of this generalized schema is applicable to every substance. For example, mineral electrolytes such as sodium cannot be synthesized, do not normally enter through the lungs, and cannot be removed by metabolism.

The orientation of Figure 7-8 illustrates two important generalizations concerning the balance concept: (1) During any period of time, total-body balance depends upon the relative rates of net gain and net loss to the body; and (2) the pool concentration depends not only upon the total amount of the substance in the body, but also upon exchanges of the substance within the body.

For any chemical, three states of total-body balance are possible: (1) Loss exceeds gain, so that the total amount of the substance in the body is decreasing, and the person is said to be in negative balance; (2) gain exceeds loss, so that the total amount of the substance in the body is increasing, and the person is said to be in positive balance; and (3) gain equals loss, and the person is in stable balance.

Clearly a stable balance can be upset by alteration of the amount being gained or lost in any single pathway in the schema; for example, severe negative water balance can be caused by increased sweating. Conversely, stable balance can be restored by homeostatic control of water intake and output.

Let us take sodium balance as another example. The control systems for sodium balance have as their targets the kidneys, and the systems operate by inducing the kidneys to excrete into the urine an amount of sodium approximately equal to the amount ingested daily. In this example, we assume for simplicity that all sodium loss from the body occurs via the urine. Now imagine a person with a daily intake and excretion of 7 g of sodium—a moderate intake for most Americans—and a stable amount of sodium in her body (Figure 7-9). On day 2 of our experiment, the subject changes her diet so that her daily sodium consumption rises to 15 g—a fairly large but commonly observed intake—and remains there indefinitely. On this same day, the kidneys excrete into the urine somewhat more than 7 g of sodium, but not all the ingested 15 g. The result is that some excess sodium is retained in the body on that day—that is, the person is in positive sodium balance. The kidneys do somewhat better on day 3, but it is probably not until day 4 or 5 that they are excreting 15 g. From this time on, output from the body once again equals input, and sodium balance is once again stable. (The delay of several days before stability is reached is quite typical for the kidneys' handling of sodium, but should not be assumed to apply to other homeostatic responses, most of which are much more rapid.)

Homeostatic Balance

FIGURE 7-9

Effects of a continued change in the amount of sodium ingested on sodium excretion and total-body sodium balance. Stable sodium balance is reattained by day 4 but with some gain of total-body sodium.

FIGURE 7-9

Effects of a continued change in the amount of sodium ingested on sodium excretion and total-body sodium balance. Stable sodium balance is reattained by day 4 but with some gain of total-body sodium.

But , and this is an important point, although again in stable balance, the woman has perhaps 2 percent more sodium in her body than was the case when she was in stable balance ingesting 7 g. It is this 2 percent extra body sodium that constitutes the continuous error signal to the control systems driving the kidneys to excrete 15g/day rather than 7g/day. [Recall the generalization (Table 7-1, no. 3) that homeostatic control systems cannot maintain complete constancy of the internal environment in the face of continued change in the perturbing event since some change in the regulated variable (body sodium content in our example) must persist to serve as a signal to maintain the compensating responses.] An increase of 2 percent does not seem large, but it has been hypothesized that this small gain might facilitate the development of high blood pressure (hypertension) in some persons.

SECTION A SUMMARY

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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Responses

  • daniela
    What are the three possible states of a total body balance of any chemical?
    7 years ago
  • Kalle
    What is balance as a body function?
    7 years ago
  • tobias
    What are three possiable states of the total body balance of any chemical?
    7 years ago
  • patrick ness
    What are three possible states of the total bodybalance of?
    7 years ago
  • toivo
    Why is a balance sheet important for body functions?
    7 years ago
  • William Turpin
    What are the eight blood homeostatic functions?
    7 years ago
  • silvia
    What are the 3 possible states of total body balance of any chemicals?
    6 years ago
  • ave sal
    Does iodine balance homeostasis?
    6 years ago
  • Fern Wilson
    Which substance balence the body?
    4 months ago
  • sandra
    What is net gain and net loss physiology?
    2 months ago

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