486
Chapter 14
t
his chapter deals with the homeostatic regulation of the water and inorganic ion
composition of the internal environment. The kidneys play the central role in these
processes.
Regulation of the total-body balance of any substance can be studied in terms of the balance concept
described in Chapter 1. Theoretically, a substance can appear in the body either as a result of ingestion
or as a product of metabolism. On the loss side of the balance, a substance can be excreted from the
body or can be metabolized. Therefore, if the quantity of any substance in the body is to be maintained
at a nearly constant level over a period of time, the total amounts ingested and produced must equal the
total amounts excreted and metabolized.
Refl exes that alter excretion via the urine constitute the major mechanisms that regulate the body
balances of water and many of the inorganic ions that determine the properties of the extracellular
fl uid. The extracellular concentrations of these ions appeared in Table 4–1. We will fi rst describe how
the kidneys work in general and then apply this information to how they process specifi c substances like
sodium, water, and potassium, and participate in refl exes that regulate these substances.
SECTION A
Basic Principles of Renal Physiology
Renal Functions
The adjective
renal
means “pertaining to the kidneys.” The
kidneys process the plasma portion of blood by removing sub-
stances from it and, in a few cases, by adding substances to it.
In so doing, they perform a variety of functions, as summa-
rized in
Table 14–1
.
First, the kidneys play a central role in regulating the
water concentration, inorganic ion composition, acid-base bal-
ance, and the fl uid volume of the internal environment (e.g.,
blood volume). They do so by excreting just enough water
and inorganic ions to keep the amounts of these substances
in the body relatively constant. For example, if you increase
your consumption of salt (sodium chloride), your kidneys will
increase the amount of the salt excreted to match the intake.
Alternatively, if there is not enough salt in the body, the kid-
neys will excrete very little salt.
Second, the kidneys excrete metabolic waste prod-
ucts into the urine as fast as they are produced. This keeps
waste products, which can be toxic, from accumulating in
the body. These metabolic wastes include
urea
from the
catabolism of protein,
uric acid
from nucleic acids,
creati-
nine
from muscle creatine, the end products of hemoglobin
breakdown (which give urine much of its color), and many
others.
A third function of the kidneys is the urinary excre-
tion of some foreign chemicals, such as drugs, pesticides,
and food additives, and their metabolites.
A fourth function is
gluconeogenesis.
During pro-
longed fasting, the kidneys synthesize glucose from amino
acids and other precursors and release it into the blood.
Finally, the kidneys act as secretory glands, releasing
at least two hormones: erythropoietin (described in Chapter
12), and 1,25-dihydroxyvitamin D (described in Chapter
11). The kidneys also secrete an enzyme, renin, that is
Table 14–1
Functions of the Kidneys
1.
Regulation of water, inorganic ion balance, and acid-base
balance (in cooperation with the lungs; Chapter 13)
2.
Removal of metabolic waste products from the blood and
their excretion in the urine
3.
Removal of foreign chemicals from the blood and their
excretion in the urine
4.
Gluconeogenesis
5.
Production of hormones/enzymes:
a.
Erythropoietin, which controls erythrocyte production
(Chapter 12)
b.
Renin, an enzyme that controls the formation of
angiotensin and infl uences blood pressure and sodium
balance (this chapter)
c.
1,25-dihydroxyvitamin D, which infl uences calcium
balance (Chapter 11)
previous page 514 Vander's Human Physiology The Mechanisms of Body Function read online next page 516 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off