therefore, blood pressure. Because of its influence on
the reabsorption of water and NaCl from the filtrate, the
kidney plays a major role in blood pressure homeosta-
sis. Water and NaCl reabsorption is regulated by several
hormones, including antidiuretic hormone (ADH), atrial
natriuretic peptide (ANP), and the renin-angiotensin-al-
dosterone system (see Chapter 9). Let’s take a closer look
at water regulation in the body.
Water Regulation Involves
a Balance of Intake and Loss
On a daily basis, the amount of water that you take in
must be kept equal to the amount that you lose (Figure
15.11). You take in water in three ways: by drinking, by
eating foods that contain water, and through chemical re-
actions such as respiration inside cells. However, you also
lose water in four ways: through excretion by the kidneys,
through evaporation from the skin surface, through ex-
halation from the lungs, and through the gastrointestinal
tract, in feces. In women of reproductive age, additional
water is lost through menstruation. Normally, daily water
loss and gain are both equal to about 2500 mL.
However, activities such as exercise, working in a hot
environment, or not drinking enough water can upset the
water balance and change the osmolarity (or solute con-
centration) of your blood. That was the case with the ex-
ercise example we saw in the chapter opener. Water may
also be lost in vomit or diarrhea during a GI tract infec-
tion. The condition in which water loss is greater than wa-
ter gain is called dehydration.
D a i l y w a t e r b a l a n c e
F i g u r e 1 5 . 1 1
The sources and volumes of water produced and lost in a day. Normally, daily water
loss and gain are both equal to 2500 mL. The numbers are averages for adults.
Metabolic water is produced by —
chemical reactions (e.g. aerobic
respiration, dehydration reactions)
2 0 0 0
Water present in foods ■
1 0 0 0
Water from drinking -
of water gained
GI tract (100 mL)
of water lost
- Water excreted in feces
- Water evaporated from respiratory surfaces
- Water evaporated from sweat
-Water excreted in urine
The caffeine in coffee, tea, and cola sodas is a naturally occurring
substance that slows reabsorption of water by the kidneys and therefore causes
an elevated urine flow rate. What is a likely effect of drinking these beverages?
a. More substances and water will be excreted than normal.
b. Fewer substances and water will be excreted than normal.
The Kidneys Regulate the Composition of Body Fluids 455