p r o c e ss
diagram
R e a b s o r p t i o n a n d s e c r e t i o n in t h e n e p h r o n • F i g u r e 1 5 . 6
The reabsorption of water and solutes, along with the secretion
of solutes, are shown as they proceed through the various
segments of the renal tubule.
| In the PCT, Na+, K+, and water are
absorbed equally. So the osmolarity of
the filtrate remains the same as that of
blood plasma. H+, NH4+, urea, and
creatinine are secreted.
) In the nephron loop, the following occurs:
• The absorption of water, Na+, and K+ are separated.
Water is absorbed only in the descending limb. But
more Na+ and K+ are absorbed in the thick ascending
limb.
• The differences in reabsorption and the countercurrent
flow set up a standing osmotic gradient within the
medulla. (Deep layers have higher osmolarities than
superficial ones, which is important for the reabsorption
of water from the collecting ducts.)
• When the filtrate leaves the nephron loop, it is
hypotonic compared to the blood plasma.
• Urea is secreted here as well as in the PCT.
Urine
In the early DCT, Na+, Cl- , and water are
absorbed. Also, Ca2+ is reabsorbed here,
under regulation by parathyroid hormone
(PTH).
In the late DCT and collecting ducts, the
following occurs :
• Principal cells reabsorb Na+ and secrete K+.
This reabsorption is influenced by the
hormone aldosterone.
• Intercalated cells reabsorb K+ and HCO3-
while secreting H+.
• The permeability of the cells to water is
regulated by the hormone ADH. When
stimulated by ADH, water channels
(aquaporin
- 2
channels) are inserted into the
apical membranes, water is reabsorbed
according to the osmotic gradient set up in
the medulla by the nephron loop, and urine
is concentrated. In the absence of ADH, the
remaining water does not get reabsorbed,
resulting in dilute urine.
The Nephron Loop Contributes to
Water Conservation in the Body
The amounts of solutes and water reabsorbed by the renal
tubule are not equal in all areas of the tubule. Approxi-
mately two-thirds of the water and solutes are reabsorbed
in the proximal convoluted tubule (PCT), and the tubular
fluid is isosmotic (that is, has the same osmolarity) com-
pared to the blood plasma (Figure 15.6). Secretion of cer-
tain materials (for example, urea, creatinine, ammonium
ion, and hydrogen ion) can also occur in the PCT.
However, as the fluid moves into the nephron loop, it
passes through an interstitial fluid gradient composed of
sodium chloride. This gradient is more and more concen-
trated deeper in the medulla of the kidney and creates an
osmotic gradient to promote water movement. In the de-
scending limb of the nephron loop, which dips deep into the
medulla, the medullary gradient promotes reabsorption of
water, returning it to the blood supply of the vasa recta.
As the filtrate moves back toward the cortex through
the ascending limb of the nephron loop, the interstitial
fluid gradient becomes more dilute. To prevent water from
reentering the tube, the ascending limb is not permeable
to water. Sodium chloride is pulled back out of the filtrate
in the ascending limb via an active transport process to
maintain the medullary gradient. This process, known as
the countercurrent multiplier, concentrates the urine
to minimize water loss from the body and helps to main-
tain proper blood volume.
As the NaCl is removed from the filtrate in the ascend-
ing limb of the nephron loop, the tubular fluid becomes hy-
potonic (that is, has less osmolarity). In the distal convoluted
tubule (DCT) and collecting ducts, much of the remaining
sodium is reabsorbed, and some potassium is secreted, but
this process depends on stimulation by the hormone aldo-
sterone. The amount of calcium ions reabsorbed in the
DCT depends on stimulation by parathyroid hormone
(PTH). Finally, the amount of water reabsorbed in these
latter segments depends on stimulation by antidiuretic
hormone (ADH), which we discuss later in the chapter. Ad-
ditional materials (such as calcium, urea, bicarbonate ion,
and hydrogen ion) are also reabsorbed and secreted in the
distal convoluted tubule and collecting ducts.
448 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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