To understand steady-state balance as it applies to the human body, the following key concepts are important.
1.
There must be a “set point” so that deviations from this baseline can be monitored (e.g., the level of the lake in the preceding example, or setting the temperature in a room by adjusting the thermostat).
2.
The sensor or sensors that monitor deviations from the set point must generate “effector signals” that can lead to changes in either input or output, or both, to maintain the desired set point (e.g., electrical signals to adjust the spillway in the dam analogy, or electrical signals sent to either the furnace or air conditioner to maintain the proper room temperature).
3.
“Effector organs” must respond in an appropriate way to the effector signals generated by the set point monitor (i.e., the spillway gates must operate, and the furnace or air conditioner must turn on).
4.
The sensitivity of the system (i.e., how much of a deviation from the set point is tolerated) depends on several factors, including the nature of the sensor (i.e., how much of a deviation from the set point is needed for the sensor to detect the deviation), the time necessary for generation of the effector signals, and how rapidly the effector organs respond to the effector signals.
1.
2.
1.
2.
3.
4.
•Fig. 2.1 Whole-Body Steady-State Water Balance. Seetextfordetails.ADH,antidiuretichormone(alsocalledargininevasopressin);CNS,centralnervoussystem;GI,gastrointestinal.
It is important to recognize that deviations from steady-state balance do occur. When input is greater than output, a state of positive balance exists. When input is less than output, a state of negative balance exists. Although transient periods of imbalance can be tolerated, prolonged states of positive or negative balance are generally incompatible with life.
Fig. 2.1 illustrates several important concepts for the maintenance of steady-state water balance (details related to the maintenance of steady-state water balance are presented in ). As depicted in
Fig. 2.1 , there are multiple inputs and outputs of water, many of which can vary but nevertheless cannot be regulated. For example, the amount of water lost through the lungs depends on the humidity of the air and the rate of respiration (e.g., low humidity and rapid breathing increase water loss from the lungs). Similarly, the amount of water lost as sweat varies according to ambient temperature and physical activity. Finally, water loss via the gastrointestinal tract can increase from a normal level of 100 to 200 mL/day to many liters with acute diarrhea. Of these inputs and outputs, the only two that can be regulated are increased ingestion of water in response to thirst and alterations in urine output by the kidneys (see
Chapter 35).
Water balance determines the osmolality of the body fluids. Cells within the hypothalamus of the brain monitor body fluid osmolality for deviations from the set point (normal range: 280-295 mOsm/kg H2O). When deviations are sensed, two effector signals are generated. One is neural and relates to the individual’s sensation of thirst. The other is hormonal (antidiuretic hormone, also called arginine vasopressin), which regulates the amount of water excreted by the kidneys. With appropriate responses to these two signals, water input, water output, or both are adjusted to maintain balance and thereby keep body fluid osmolality at the set point.
Volumes and Composition of Body Fluid Compartments