The maintenance of a relatively constant volume and stable composition of the body fluids is essential for homeostasis. Some of the most common and important problems in clinical medicine arise because of abnormalities in the control systems that maintain this relative constancy of the body fluids.
In this chapter and in the following chapters on the kidneys, we discuss the overall regulation of body fluid volume, constituents of the extracellular fluid, acid-base balance, and control of fluid exchange between extracellular and intracellular compartments.
FLUID INTAKE AND OUTPUT ARE BALANCED DURING STEADY-STATE CONDITIONS
The relative constancy of the body fluids is remarkable because there is a continuous exchange of fluid and solutes with the external environment, as well as within the different body compartments.
For example, fluid added to the body is highly variable and must be carefully matched by an equal output of water from the body to prevent body fluid volumes from increasing or decreasing.
DAILY INTAKE OF WATER
Water is added to the body by two major sources: (1) it is ingested in the form of liquids or water in food, which together normally add about 2100 ml/day to the body fluids; and (2) it is synthesized in the body by oxidation of carbohydrates, adding about 200 ml/day.
These mechanisms provide a total water intake of about 2300 ml/day (Table 25-1). However, intake of water is highly variable among different people and even within the same person on different days, depending on climate, habits, and level of physical activity.
DAILY LOSS OF BODY WATER
Insensible Water Loss:Some water losses cannot be pre-precisely regulated. For example, humans experience continuous water loss by evaporation from the respiratory tract and diffusion through the skin, which together account for about 700 ml/day of water loss under normal conditions.
This loss is termed insensible water loss because we are not consciously aware of it, even though it occurs continually in all living people. Insensible water loss through the skin occurs independently of sweating and is present even in people who are born without sweat glands; the average water loss by diffusion through the skin is about 300 to 400 ml/day.
This loss is minimized by the cholesterol-filled, cornified layer of the skin, which provides a barrier against excessive loss by diffusion. When the cornified layer becomes denuded, as occurs with extensive burns, the rate of evaporation can increase as much as 10-fold, to 3 to 5 L/ day. For this reason, persons with burns must be given large amounts of fluid, usually intravenously, to balance fluid loss.
Insensible water loss through the respiratory tract normally averages about 300 to 400 ml/day. As air enters the respiratory tract, it becomes saturated with moisture to a vapor pressure of about 47 mm Hg before it is expelled.
Because the vapor pressure of the inspired air is usually less than 47 mm Hg, water is continuously lost through the lungs with respiration. In cold weather, the atmospheric vapor pressure decreases to nearly 0, causing an even greater loss of water from the lungs as the temperature decreases. This process explains the dry feeling in the respiratory passages in cold weather.
Fluid Loss in Sweat: The amount of water lost by sweating is highly variable, depending on physical activity and environmental temperature. The volume of sweat normal- ly is about 100 ml/day, but in very hot weather or during heavy exercise, fluid loss in sweat occasionally increases to 1 to 2 L/hour. This fluid loss would rapidly deplete the body fluids if intake were not also increased by activating the thirst mechanism.
Water Loss in Feces: Only a small amount of water (100 ml/day) normally is lost in the feces. This loss can increase to several liters a day in people with severe diarrhea. Therefore, severe diarrhea can be life-threatening if not corrected within a few days.
Water Loss by the Kidneys:The remaining water loss from the body occurs in the urine excreted by the kidneys. Multiple mechanisms control the rate of urine excretion.
The most important means whereby the body maintains a balance between water intake and output, as well as a balance between intake and output of most electrolytes in the body, is by controlling the rate at which the kidneys excrete these substances.
For example, urine volume can be as low as 0.5 L/day in a dehydrated person or as high as 20 L/day in a person who has been drinking tremendous amounts of water.
This variability of intake is also true for most of the electrolytes of the body, such as sodium, chloride, and potassium. In some people, sodium intake may be as low as 20 mEq/day, whereas in others, sodium intake may be as high as 300 to 500 mEq/day.
The kidneys have the task of adjusting the excretion rate of water and electrolytes to match the intake of these substances precisely, as well as compensating for excessive losses of fluids and electrolytes that occur in certain disease states. we discuss the mechanisms that allow the kidneys to perform these remarkable tasks.