Water Input and Output Are Equal

A balance chart for water for an average 70-kg man is presented in Table 24.3. The person is in a stable balance (or steady state) because the total input and total output of water from the body are equal (2,500 mL/day). On the input side, water is found in the beverages we drink and in the foods we eat. Solid foods, which consist of animal or vegetable matter, are, like our own bodies, mostly water. Water of oxidation is produced during metabolism, for example, when 1 mol of glucose is oxidized, 6 mol of water are produced. In a hospital setting, the input of water as a result of intravenous infusions would also need to be considered. On the output side, losses of water occur via the skin, lungs, gastrointestinal tract, and kidneys. We always lose water by simple evaporation from the skin and lungs,- this is called insensible water loss.

Appreciable water loss from the skin, in the form of sweat, occurs at high temperatures or with heavy exercise. As much as 4 L of water per hour can be lost in sweat. Sweat, which is a hypoosmotic fluid, contains NaCl,- excessive sweating can lead to significant losses of salt. Gastrointestinal losses of water are normally small (see Table 24.3), but with diarrhea, vomiting, or drainage of gastrointestinal secretions, massive quantities of water and electrolytes may be lost from the body.

The kidneys are the sites of adjustment of water output from the body. Renal water excretion changes to maintain balance. If there is a water deficiency, the kidneys diminish the excretion of water and urine output falls. If there is water excess, the kidneys increase water excretion and urine flow to remove the extra water. The renal excretion of water is controlled by arginine vasopressin.

The water needs of an infant or young child, per kg body weight, are several times higher than that of an adult. Children have, for their body weight, a larger body surface area and higher metabolic rate. They are much more susceptible to volume depletion.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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  • juliane koenig
    How do we lose water output?
    8 years ago
  • lucien
    How can water output more than water input from body?
    8 years ago

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