pressure in the glomerular capillaries. When blood pres-
sure increases or decreases slightly, changes in the diam-
eters of the afferent and efferent arterioles can actually
keep net filtration pressure steady and maintain normal
glomerular filtration.
The rate at which fluid is filtered in the renal corpus-
cle is called the glomerular filtration rate (GFR). GFR
is about 105 mL/minute in females and 125 mL/minute in
males. As mentioned earlier, reabsorption of substances
from the filtrate depends on the concentrations and rates
of flow. If GFR is too high, substances are not adequate-
ly reabsorbed and leave the body in the urine (because
it’s difficult to collect materials that are flowing by too
quickly). If GFR is too low, substances get reabsorbed, and
waste products are not adequately excreted (because it’s
easier to collect materials that are moving slowly through
the tubing).
Remember what you read about the afferent arterioles
being more constricted than the efferent arterioles during
exercise? Now you can follow the steps more closely: as a re-
sult of the constriction, blood flow into the glomerular capil-
laries is greatly decreased, net filtration pressure decreases,
and GFR drops. These changes reduce urine output, which
helps conserve blood volume and permits greater blood flow
to other body tissues needing it for exercise.
Because the filtration membrane normally prevents
the passage of medium- to large-sized proteins, excessive
amounts of the blood protein albumin in the urine (albu-
minuria, al'-bu-mi-NOO-re-a) indicate that the filtration
membrane is damaged due to injury, disease, high blood
pressure, or kidney cell damage.
Now let’s take a closer look at what happens during
tubular reabsorption and tubular secretion.
Reabsorption and Secretion Occur
Along the Length of the Renal Tubule
The filtrate gets modified as it passes through the renal
tubule. Most of the water and dissolved solutes get reab-
sorbed. The reabsorbed solutes include large quantities of
ions (such as Na+, K+, Cl-, Ca2+, Mg2+, HCO3-, PO42-, and
SO42-), all of the glucose, and all of the amino acids. Among
the many waste substances secreted into the filtrate are
ammonia and urea (by-products of protein catabolism),
creatinine (a waste product from muscle creatine—see
Chapter 5), drugs (such as penicillin), and some ions (in-
cluding H+ and K+).
As a result of tubular secretion, some drugs pass from
blood into the urine and can be detected by urine tests. For
example, urine tests can detect the presence of performance-
enhancing drugs such as anabolic steroids, human growth
hormone, and amphetamines in the urine of athletes. Urine
tests can also be used to detect the presence of alcohol or il-
legal drugs such as marijuana, cocaine, and heroin.
How does the kidney know which substances to absorb
and which to secrete? The presence of specific transport-
ers on the lumen side of the tubular membrane cells and
others on the interstitial fluid side of the tubular cells de-
termines which substances are absorbed and which are
secreted. For example, as a sodium ion (Na+) in the fil-
trate passes a proximal tubule cell with an Na+ channel,
the Na+ enters the cell because its concentration is higher
in the filtrate than inside the cell. The Na+ subsequently
gets pumped out the other side of the cell by the sodium-
potassium pump and gets absorbed from the interstitial
fluid into the blood.
Because it proceeds from one side of a cell to the oth-
er, this pathway for sodium reabsorption through a cell is
a transcellular route of reabsorption
(trans
= across);
think of the ion as moving across the cell. Transcellular
routes involve many different active and passive trans-
port mechanisms, including
sym porters
(cotransporters that
move substances in the same direction;
sym
= same),
channels,
antip o rters
(cotransporters that move substanc-
es in opposite direction;
a n ti
= against), and pumps (see
Chapter 3).
It is also possible for the Na+ to pass between the
cells through tight junctions (paracellular route;
p a ra
= beside); think of the ion as moving between or beside
cells. Paracellular routes usually involve passive transport
in response to osmotic and/or electrochemical gradients.
Most transcellular reabsorption is linked to the reabsorp-
tion of Na+.
The movement of some ions (such as H+ and HCO3-)
depends on the activity of enzymes, such as carbonic an-
hydrase, found on the surfaces and inside renal tubule
cells. This enzyme assists the kidney in the regulation of
the blood pH—a topic that is discussed further at the end
of this chapter.
The rate and amount of reabsorption of any substance
depends on the concentration of the substance in the fil-
trate and the flow rate of the filtrate. Generally, high con-
centrations of any substance in the filtrate lead to greater
reabsorption, but there are limits. If the concentration of
the substance exceeds the capacity of the transporters, the
excess amount is excreted in the urine. Likewise, if the flow
rate of the filtrate past the transporters is too fast, the trans-
porters cannot “snag” the substance, and it is excreted.
The Urinary System Plays a Vital Role in Maintaining Homeostasis
447
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