Page 1061 - Fundamentals of anatomy physiology

1048 Unit 5 Environmental Exchange Table 27–1 Water Balance Daily Input (mL)
1000
&T i p s T r i c k s Source 1200
Water movement between compartments, driven by osmotic Water content of food 300
pressure, is like water movements between compartments Water consumed as liquid
in a waterbed mattress: The total amount of fluid doesn’t Metabolic water produced during catabolism 2500
change. Fluid merely moves from one compartment to an- Daily Output (mL)
other, driven by pressure differences. Total
Method of Elimination 1200
Fluid Gains and Losses Urination 750
Evaporation at skin 400
Figure 27–3 and Table 27–1 indicate the major factors involved Evaporation at lungs 150
in fluid balance and highlight the routes of fluid exchange with Loss in feces
the environment: 2500
Total
Water Losses. You lose about 2500 mL of water each day
in mitochondria. (In Chapter 25 we described the synthesis of
through urine, feces, and insensible perspiration—the gradual water at the end of the electron transport system. p. 969)
movement of water across the epithelia of the skin and When cells break down 1 g of lipid, 1.7 mL of water is gener-
lungs. The losses due to sensible perspiration—the secretory ated. Breaking down proteins or carbohydrates yields much
activities of the sweat glands—vary with physical activity. lower amounts (0.41 mL/g and 0.55 mL/g, respectively).
Sensible perspiration can cause significant water deficits,
with maximum perspiration rates reaching 4 liters per A typical diet in the United States contains 46 percent
hour. p. 990 Fever can also increase water losses. For carbohydrates, 40 percent lipids, and 14 percent protein. Such
each degree that body temperature rises above normal, a diet produces approximately 300 mL of water per day, about
daily insensible water losses increase by 200 mL. The ad- 12 percent of your average daily requirement.
vice “Drink plenty of fluids” for anyone who is sick has a
definite physiological basis. Fluid Shifts

Water Gains. A water gain of about 2500 mL/day is re- A rapid water movement between the ECF and the ICF in re-
sponse to an osmotic gradient is called a fluid shift. Fluid shifts
quired to balance your average water losses. This value occur quickly in response to changes in the osmotic concentra-
amounts to about 40 mL/kg of body weight per day. You tion of the ECF and reach equilibrium within minutes to hours
obtain water through eating (1000 mL), drinking (1200 mL), (Figure 27–4).
and metabolic generation (300 mL).

Metabolic generation of water is the production of water
within cells, primarily as a result of oxidative phosphorylation

Figure 27–3 Fluid Gains and Losses. Fluid movements that maintain fluid balance in a normal person. The volumes are

drawn to scale; the ICF is about twice as large as the ECF.

27 Water absorbed across
digestive epithelium
(2200 mL)

Water vapor lost
at skin and lungs
(1150 mL)

ICF Metabolic ECF Water lost in
water feces (150 mL)
Water secreted
(300 mL) by sweat glands
(variable)

Plasma membranes Water lost in urine
(1200 mL)