Maintenance of the Body
25 mm Hg, capsular hydrostatic pressure
Which of the pressures that determine NFP is regulated by
both intrinsic and extrinsic controls of GFR?
For answers, see Appendix H.
Urine Formation, Step 2:
Describe the mechanisms underlying water and solute
reabsorption from the renal tubules into the peritubular
Describe how sodium and water reabsorption are regulated
in the distal tubule and collecting duct.
Our total plasma volume ﬁlters into the renal tubules about
every 22 minutes, so all our plasma would drain away as urine
in less than 30 minutes were it not for
which quickly reclaims most of the tubule contents and returns
them to the blood. Tubular reabsorption is a selective
that begins as soon as the ﬁltrate enters the proxi-
To reach the blood, reabsorbed substances follow either the
. In the trans-
cellular route, transported substances move through the
, the cytosol, and the
tubule cell and then the endothelium of the peritubular cap-
illaries. Movement of substances in the paracellular route—
the tubule cells—is limited by the tight junctions
connecting these cells. In the proximal nephron, however,
these tight junctions are “leaky” and allow water and some im-
portant ions (Ca
, and some Na
) to pass through
the paracellular route.
Given healthy kidneys, virtually all organic nutrients such as
glucose and amino acids are completely reabsorbed to maintain
or restore normal plasma concentrations. On the other hand,
the reabsorption of water and many ions is continuously regu-
lated and adjusted in response to hormonal signals. Depending
on the substances transported, the reabsorption process may
Active tubular reabsorption
either directly (primary active transport) or indirectly (second-
ary active transport; see Figure 3.11, p. 75) for at least one of its
Passive tubular reabsorption
encompasses diﬀusion, fa-
cilitated diﬀusion, and osmosis—processes in which substances
move down their electrochemical gradients.
±e transport mechanisms that are so important for kidney
cell function are the same ones used by all other cells. You may
wish to review these basic transport mechanisms on pp. 67–79
to refresh your memory.
Tubular Reabsorption of Sodium
Sodium ions are the single most abundant cation in the ﬁltrate,
and about 80% of the energy used for active transport is devoted
to reabsorbing them. Sodium reabsorption is almost always ac-
tive and via the transcellular route. Let’s begin with the ATP-
constrict, increasing peripheral resistance and bringing
blood pressure back up toward normal. ±is is the barore-
ceptor reﬂex we discussed in Chapter 19. As part of this
reﬂex, the aﬀerent arterioles also constrict. Constriction of
the aﬀerent arterioles decreases GFR and so helps restore
blood volume and blood pressure to normal.
As we dis-
cussed in Chapter 19 (p. 709), the
is the body’s main mechanism
for increasing blood pressure. Without adequate blood
pressure (as might be due to hemorrhage, dehydration,
etc.), glomerular ﬁltration is not possible, so this mecha-
nism regulates GFR indirectly.
Low blood pressure causes the granular cells of the jux-
taglomerular complex to release
by one or more of
Direct stimulation of granular cells.
As part of the
baroreceptor reﬂex, renal sympathetic nerves activate
-adrenergic receptors that cause the granule cells to
Stimulation of the granular cells by input from activated
macula densa cells
. Low blood pressure or vasoconstric-
tion of the aﬀerent arterioles by the sympathetic nervous
system reduces GFR, slowing down the ﬂow of ﬁltrate
through the renal tubules. When macula densa cells
sense the low NaCl concentration of this sluggishly ﬂow-
ing ﬁltrate, they signal the granular cells to release renin.
±ey may signal by releasing less ATP (also thought to be
the tubuloglomerular feedback messenger), by releasing
of the prostaglandin PGE
, or both.
Reduced stretch of granular cells.
Granular cells act as
mechanoreceptors. A drop in mean arterial blood pres-
sure reduces the tension in the granular cells’ plasma
membranes and stimulates them to release more renin.
Other Factors Affecting GFR
Renal cells produce a battery
of chemicals, many of which act as paracrines (local signaling
molecules) aﬀecting renal arterioles. ±ese include
); adenosine can be produced ex-
tracellularly from released ATP. In addition, the kidney makes
its own locally acting
that reinforces the eﬀects of
hormonal angiotensin II described in Chapter 19 (p. 707).
Abnormally low urinary output (less than 50 ml/day), called
re-ah), may indicate that glomerular blood pres-
sure is too low to cause ﬁltration. Renal failure and anuria can
also result from situations in which the nephrons stop function-
ing, including acute nephritis, transfusion reactions, and crush
Check Your Understanding
Extrinsic and intrinsic controls of GFR serve two different
purposes. What are they?
Calculate net ﬁltration pressure given the following values:
glomerular hydrostatic pressure
50 mm Hg, blood colloid