![]() ![]() The volume of sweat normally is about 100 ml/day, but in very hot weather or during heavy exercise water loss in sweat occasionally increases to 1 to 2 L/hour. The amount of water lost by sweating is highly variable, depending on physical activity and environmental temperature. This explains the dry feeling in the respiratory passages in cold weather. 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. ![]() Because the vapor pressure of the inspired air is usually less than 47 mm Hg, water is continuously lost through the lungs with respiration. As air enters the respiratory tract, it becomes saturated with moisture, to a vapor pressure of about 47 mm Hg, before it is expelled. Insensible water loss through the respiratory tract averages about 300 to 400 ml/day. For this reason, burn victims must be given large amounts of fluid, usually intravenously, to balance fluid loss. 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. This loss is minimized by the cholesterol-filled cornified layer of the skin, which provides a barrier against excessive loss by diffusion. The 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 is termed insensible water loss because we are not consciously aware of it, even though it occurs continually in all living humans. For example, there is a continuous loss of water 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. Some of the water losses cannot be precisely regulated. Table 25-1 Daily Intake and Output of Water (ml/day) Intake of water, however, is highly variable among different people and even within the same person on different days, depending on climate, habits, and level of physical activity. This provides a total water intake of about 2300 ml/day ( Table 25-1). Water is added to the body by two major sources: (1) It is ingested in the form of liquids or water in the food, which together normally add about 2100 ml/day to the body fluids, and (2) it is synthesized in the body as a result of oxidation of carbohydrates, adding about 200 ml/day. For example, there is a highly variable fluid intake that must be carefully matched by equal output of water from the body to prevent body fluid volumes from increasing or decreasing. The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment, as well as within the different compartments of the body. 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.įluid Intake and Output Are Balanced During Steady-State Conditions Some of the most common and important problems in clinical medicine arise because of abnormalities in the control systems that maintain this constancy of the body fluids. The maintenance of a relatively constant volume and a stable composition of the body fluids is essential for homeostasis, as discussed in Chapter 1. ![]() Guyton and Hall Textbook of Medical Physiology, 12th Ed CHAPTER 25 The Body Fluid CompartmentsĮxtracellular and Intracellular Fluids Edema
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