Porth's Essentials of Pathophysiology, 4e

168

Integrative Body Functions

U N I T 2

Sources of BodyWater Gains and Losses in the Adult

TABLE 8-2

Gains

(approximate) Losses (approximate)

Oral intake

Urine

1500 mL

As water

1000 mL

Insensible losses Lungs

In food

1300 mL 200 mL

300 mL 500 mL 200 mL 2500 mL

Water of

Skin

oxidation Total

Feces

2500 mL

Total

Total body water = 60%

Regulation of Sodium Balance Sodium is the most plentiful electrolyte in the ECF compartment, with a concentration ranging from 135 to 145 mEq/L (135 to 145 mmol/L). 3 Sodium does not readily cross the cell membrane; as a result, only a small amount (10 to 15 mEq/L [10 to 15 mmol/L]) is located in the ICF compartment. 1 As the major cat- ion in the ECF compartment, Na + and its attendant Cl – and HCO 3 – anions account for approximately 90% to 95% of the osmotic activity in the ECF. Thus, serum osmolality usually varies with changes in serum sodium concentration. Sodium normally enters the body through the gas- trointestinal tract, being derived from dietary sources. Although body needs for sodium usually can be met by as little as 500 mg/day, dietary intake frequently exceeds that amount. 3 As package labels indicate, many com- mercially prepared foods and soft drinks contain con- siderable amounts of sodium. Most sodium losses occur through the kidney. The kidneys are extremely efficient in regulating sodium output. When sodium intake is limited or conservation of sodium is needed, the kidneys are able to reabsorb almost all the Na + that has been filtered in the glomeru- lus, resulting in essentially sodium-free urine. Usually, less than 10% of sodium intake is lost through the gastrointestinal tract and skin. Sodium losses increase with conditions such as vomiting, diar- rhea, and gastrointestinal suction, all of which can remove sodium from the stomach or small intestine. Sodium leaves the skin by way of the sweat glands, which secrete a hypotonic solution containing both sodium and chloride. Although sodium losses due to sweating are usually negligible, they can increase greatly during heavy exercise and periods of exposure to a hot environment. 2 Mechanisms of Water and Sodium Regulation There are two major physiologic mechanisms for regu- lating body levels of water: thirst, which is primarily a regulator of water intake, and the antidiuretic hormone (ADH), which controls the output of water by the kid- ney. Thirst and ADH function in the maintenance of

Total body water = 30%

to adults who have only about a third in their ECF compartment. 1,3 The greater ECF water content of an infant can be explained in terms of a higher metabolic rate, larger surface area in relation to its body mass, and inability to concentrate urine because of imma- ture kidney structures. Because ECF is more readily lost from the body, infants are more vulnerable to fluid deficit than are older children and adults. As an infant grows older, TBW decreases, and by the second year of life, the percentages and distribution of body water approach those of an adult. The main source of water gain is through oral intake and metabolic processes. Oral intake, including that obtained from liquids and solid foods, is absorbed from the gastrointestinal tract. The amount of water gained from metabolic processes is much less than from oral intake, varying from 150 to 300 mL/day, depending on the metabolic rate. Normally, the largest loss of water occurs through the kidneys, with lesser amounts being lost through the skin, lungs, and gastrointestinal tract. Even when oral or parenteral fluids are withheld, the kidneys continue to produce urine as a means of ridding the body of met- abolic wastes. The urine output that is required to elimi- nate these wastes is called the obligatory urine output . Water losses that occur through evaporative losses from skin and to moisten the air in the respiratory system are referred to as insensible water losses because they occur without a person’s awareness. The amount of water lost from the skin through sweating varies depending on physical activity and environmental temperature. The sources of body water gains and losses are summarized in Table 8-2. 220 lbs FIGURE 8-6. Body composition of a lean and an obese individual. (Adapted with permission from Statland H. Fluids and Electrolytes in Practice. 3rd ed. Philadelphia, PA: J.B. Lippincott; 1963.) 220 lbs