Steady-state balance in the human body can be understood by considering the following important concepts. Firstly, there needs to be a "set point" that serves as a baseline for monitoring deviations. This can be compared to setting the temperature in a room using a thermostat or monitoring the level of a lake. Secondly, sensors are required to detect deviations from the set point and generate "effector signals" that can lead to adjustments in input or output to maintain the desired set point. These signals can be compared to electrical signals used to adjust the spillway in a dam or regulate the temperature in a room using a furnace or air conditioner. Thirdly, effector organs must respond appropriately to the effector signals generated by the set point monitor. For example, in the dam analogy, the spillway gates must operate, and in the room temperature analogy, the furnace or air conditioner must turn on. Lastly, the sensitivity of the system, which refers to the degree of deviation from the set point that can be tolerated, depends on factors such as the nature of the sensor, the time required for generating effector signals, and the speed of response from effector organs.

It is important to note that deviations from steady-state balance can occur. When input exceeds output, a state of positive balance is present, and when output exceeds input, a state of negative balance is present. While temporary periods of imbalance can be tolerated, prolonged states of positive or negative balance are generally incompatible with life.

Figure 2.1, titled "Whole-Body Steady-State Water Balance," illustrates key concepts related to maintaining steady-state water balance. Various inputs and outputs of water are depicted, some of which can vary but cannot be regulated. Examples include water lost through the lungs, which depends on air humidity and respiration rate, and water lost as sweat, which varies with ambient temperature and physical activity. The gastrointestinal tract can also exhibit increased water loss due to factors such as acute diarrhea. However, the only two inputs and outputs that can be regulated are increased water intake in response to thirst and alterations in urine output by the kidneys (see Chapter 35 for further details).

Water balance plays a crucial role in determining the osmolality of body fluids. Cells within the hypothalamus of the brain monitor the osmolality of body fluids for deviations from the set point (normal range: 280-295 mOsm/kg H2O). When deviations are detected, two types of effector signals are generated. One is neural and relates to the individual's sensation of thirst. The other is hormonal, specifically antidiuretic hormone (also known as arginine vasopressin), which regulates the amount of water excreted by the kidneys. Through appropriate responses to these signals, adjustments can be made to water input, water output, or both in order to maintain balance and keep body fluid osmolality at the set point.

The volumes and composition of body fluid compartments are important considerations in understanding steady-state balance.
To understand steady-state balance as it applies to the human body, the following key concepts are important.
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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).
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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).
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“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).
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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.
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•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