e. Active transport mechanisms have limited
f. If the concentration of a substance in the F
exceeds its renal plasma threshold, the excess is
excreted in the urine.
g. Substances that remain in the F
concentrated as water is reabsorbed.
h. Sodium ions are reabsorbed by active transport.
(1) Negatively charged ions accompany positively
charged sodium ions out of the F
(2) Water is passively reabsorbed by osmosis as
sodium ions are actively reabsorbed.
6. Tubular secretion
a. Tubular secretion transports certain substances
from the plasma to the tubular ﬂ
b. Some substances are actively secreted.
(1) These include various organic compounds and
(2) The proximal and distal convoluted tubules
secrete hydrogen ions.
c. Potassium ions are secreted actively and passively
in the distal convoluted tubule and collecting duct.
7. Regulation of urine concentration and volume
a. Most of the sodium ions are reabsorbed before the
urine is excreted.
b. The countercurrent mechanism concentrates
sodium ions in the renal medulla.
(1) Sodium and chloride ions are actively
reabsorbed in the ascending limb.
(2) Tubular ﬂ
uid in the ascending limb becomes
hypotonic as it loses solutes.
(3) Water leaves the descending limb by osmosis,
and NaCl enters this limb by diffusion.
(4) Tubular ﬂ
uid in the descending limb becomes
hypertonic as it loses water and gains NaCl.
(5) As NaCl repeats this circuit, its concentration
in the medulla increases.
c. The vasa recta countercurrent mechanism helps
maintain NaCl concentration in the medulla.
d. The distal convoluted tubule and collecting duct
are impermeable to water, which therefore is
excreted in urine.
e. ADH from the posterior pituitary gland increases
the permeability of the distal convoluted tubule
and collecting duct, promoting water reabsorption.
8. Urea and uric acid excretion
a. Urea is a by-product of amino acid metabolism.
(1) It is passively reabsorbed by diffusion.
(2) About 50% of the urea is excreted in urine.
(3) A countercurrent mechanism involving urea
helps reabsorb water.
b. Uric acid results from the metabolism of nucleic
(1) Most is reabsorbed by active transport.
(2) Some is secreted into the renal tubule.
9. Urine composition
a. Urine is about 95% water, and it usually contains
urea, uric acid, and creatinine.
b. It may contain a trace of amino acids and
varying amounts of electrolytes, depending upon
The volume of urine varies with the ﬂ
and with certain environmental factors.
URINE FORMATION (PAGE 785)
Nephrons remove wastes from the blood and regulate
water and electrolyte concentrations. Urine is the product.
1. Glomerular F
a. Urine formation begins when water and dissolved
materials are F ltered out of the glomerular capillary.
b. The glomerular capillaries are much more
permeable than the capillaries in other tissues.
2. ±iltration pressure
a. ±iltration is mainly due to hydrostatic pressure
inside the glomerular capillaries.
b. The osmotic pressure of the blood plasma and
hydrostatic pressure in the glomerular capsule also
c. ±iltration pressure is the net force moving material
out of the glomerulus and into the glomerular
d. The composition of the F
ltrate is similar to that of
3. ±iltration rate
a. The rate of F
ltration varies with the F
b. ±iltration pressure changes with the diameters of
the afferent and efferent arterioles.
c. As the osmotic pressure in the glomerulus
d. As the hydrostatic pressure in a glomerular capsule
increases, the F
ltration rate decreases.
e. The kidneys produce about 125 milliliters of
uid per minute; most is reabsorbed.
f. The volume of F
ltrate varies with the surface area
of the glomerular capillary.
4. Control of F
a. Glomerular F
ltration rate (G±R) remains relatively
constant but may increase or decrease with need.
Increased sympathetic nerve activity can decrease
b. When tubular ﬂ
uid NaCl concentration decreases,
the macula densa causes the juxtaglomerular cells
to release renin. This triggers a series of changes
leading to vasoconstriction, which may affect G±R,
and secretion of aldosterone, which stimulates
tubular sodium reabsorption.
c. Autoregulation is the ability of an organ or tissue
to maintain a constant blood ﬂ
ow under certain
conditions when the arterial blood pressure is
5. Tubular reabsorption
a. Substances are selectively reabsorbed from the
b. The peritubular capillary is adapted for
(1) It carries low-pressure blood.
(2) It is permeable.
c. Most reabsorption is in the proximal tubule, where
epithelial cells have microvilli.
d. Different modes of transport reabsorb various
substances in particular segments of the renal
(1) Glucose and amino acids are reabsorbed by
(2) Water is reabsorbed by osmosis.
(3) Proteins are reabsorbed by endocytosis.