Urine Formation Involves Three
Processes and Helps Maintain the
Blood's Volume and Composition
To produce urine, nephrons and collecting ducts perform
three basic processes, summarized in Figure 15.4:
Glomerular filtration
Tubular reabsorption
Tubular secretion
These processes are much like the children’s “fish pond”
game that you find in many amusement parks. We will use
that game as an analogy as we look at each of the three
processes.
The game starts by letting water and differently col-
ored plastic fish flow into a stream; this is analogous to
filtration, which is defined as the forcing of fluids and dis-
solved substances smaller than a certain size through a
membrane by pressure. Glomerular filtration is the first
step of urine production. Blood pressure forces blood plas-
ma (consisting of water and dissolved substances) across
the wall of glomerular capillaries into the renal corpuscle.
Along the length of the stream, little children use
poles to remove the plastic fish from the stream (usually
getting a little wet in the process). In tubular reabsorp-
tion, specific transport mechanisms allow cell membranes
along the renal tubule to absorb substances and water
from the filtrate. Tubule and duct cells return about 99%
of the filtered water and many useful solutes to the blood
flowing through peritubular capillaries. Only 1%
of the fil-
tered water actually leaves the body in urine.
Occasionally, children lean too far over the stream and
drop items that they are carrying (loose change, tickets,
toys, and so on) into the stream. Tubular secretion also
takes place as fluid flows along the tubules and through
the collecting duct. Specific transport mechanisms secrete
substances into the fluid along the length of the renal tu-
bule. They remove substances such as wastes, drugs, and
excess ions from blood in the peritubular capillaries and
transport them into the fluid in the renal tubules.
What remains in the fishing stream flows out of the
game area; similarly, urine gets excreted from the body. It
is the balance of these processes that determine what the
children keep (composition of the blood) and what gets
thrown away (composition of the urine).
Figure 15.4 also compares the substances that are
filtered, reabsorbed, and excreted in urine per day in an
adult male. Although the values shown are typical, they
vary considerably according to diet and gender. For exam-
ple, the kidneys filter about 180 liters (about 48 gallons)
daily in adult males and about 150 liters daily (about 40
gallons) in adult females.
The volume of urine eliminated per day in a normal
adult is 1
to 2 liters per day (about 1
to 2 quarts). Water
accounts for 95% of the total volume of urine. In addition
to urea, creatinine, potassium, and ammonia, typical sol-
utes normally present in urine include uric acid as well as
sodium ions, chloride ions, and other ions.
Filtration is driven by net pressures within the renal
corpuscle, and tubular reabsorption and tubular secretion
are driven by membrane transport processes across the
cells of the renal tubule, such as diffusion, osmosis, pas-
sive transport, and active transport. (For a review of these
processes, see Chapter 3.) Tubular reabsorption and secre-
tion are also influenced by the rate of filtrate flow through
the tubule; slow flow rates yield greater absorption and
secretion, while fast flow rates yield less absorption and
secretion.
As nephrons perform their functions, they help main-
tain homeostasis of the blood’s volume and composition.
They also adapt to changing physiological conditions.
Think back to the exercise example at the beginning of
this chapter. As you sweat from exercise, evaporation
causes you to lose body fluids. If you do not increase your
fluid intake, the kidneys compensate by reabsorbing more
water from the fluid. We will discuss how that happens
when we discuss water balance later in this chapter. The
urine may appear to be more concentrated and less wa-
tery. Even so, you will most likely need to drink more fluids
to replace the water you have lost.
Like most blood vessels of the body, those of the kid-
neys are supplied by the sympathetic neurons of the auto-
nomic nervous system. At rest, sympathetic stimulation is
low and the afferent and efferent arterioles are relatively
dilated. With greater sympathetic stimulation, as occurs
during exercise or hemorrhage, the afferent arterioles are
constricted more than the efferent arterioles. As you will
see shortly, this constriction leads to changes that help
conserve blood volume and permit greater blood flow to
other body tissues.
Let’s take a closer look at glomerular filtration.
444 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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