Maintenance of the Body
ATPase pump at the basolateral membrane accounts for Na
reabsorption and establishes the electrochemical gradient that
drives the reabsorption of most other solutes and H
at the apical surface of the tubule cell via facilitated diﬀusion
through channels or as part of a cotransport mechanism.
Passive tubular reabsorption is driven by electrochemical
gradients established by active reabsorption of sodium ions.
Water, many ions, and various other substances (for example,
urea) are reabsorbed passively by diﬀusion via transcellular or
Secondary active tubular reabsorption occurs by cotransport
via transport proteins. Transport of such substances is
limited by the number of carriers available. Actively reabsorbed
substances include glucose, amino acids, and some ions.
±e proximal tubule cells are most active in reabsorption. Most
of the nutrients, 65% of the water and sodium ions, and the
bulk of actively transported ions are reabsorbed in the proximal
Reabsorption of additional sodium ions and water occurs in the
distal tubules and collecting ducts and is hormonally controlled.
Aldosterone increases the reabsorption of sodium; antidiuretic
hormone (ADH) enhances water reabsorption by the collecting
Urine Formation, Step 3: Tubular Secretion
Tubular secretion adds substances to the ﬁltrate (from the
blood or tubule cells). It is an active process that is important
in eliminating drugs, certain wastes, and excess ions and in
maintaining the acid-base balance of the blood.
Regulation of Urine Concentration and Volume
±e graduated hyperosmolality of the medullary ﬂuids (largely
due to the cycling of NaCl and urea) ensures that the ﬁltrate
reaching the distal convoluted tubule is dilute (hypo-osmolar).
±is allows urine with osmolalities ranging from 50 to 1200
mOsm to be formed.
±e descending limb of the nephron loop is permeable to
water, which leaves the ﬁltrate and enters the medullary
interstitial space. ±e ﬁltrate and medullary ﬂuid at the bend of
the nephron loop are hyperosmolar.
±e ascending limb is impermeable to water. Na
move out of the ﬁltrate into the interstitial space, passively in
the thin portion and actively in the thick portion. ±e ﬁltrate
becomes more dilute.
As ﬁltrate ﬂows through the collecting ducts in the inner
medulla, urea diﬀuses into the interstitial space. From here,
urea reenters the ascending thin limb and is recycled.
±e blood ﬂow in the vasa recta is sluggish, and the contained
blood equilibrates with the medullary interstitial ﬂuid. Hence,
blood exiting the medulla in the vasa recta is nearly isotonic
to blood plasma and the high solute concentration of the
medulla is maintained.
In the absence of antidiuretic hormone, dilute urine is formed
because the dilute ﬁltrate reaching the collecting duct is simply
allowed to pass from the kidneys.
When extracellular ﬂuid osmolality rises, blood levels of
antidiuretic hormone rise, and the collecting ducts become more
permeable to water. Water moves out of the ﬁltrate as it ﬂows
through the hyperosmotic medullary areas. Consequently, more
concentrated urine is produced, and in smaller amounts.
Urinary System; Topics: Early Filtrate Processing, pp. 1–22;
Late Filtrate Processing, pp. 1–13.
nephrons are located at the cortex-medulla junction, and their
nephron loops dip deeply into the medulla. Instead of directly
forming peritubular capillaries, the eﬀerent arterioles of many
juxtamedullary nephrons form unique bundles of straight vessels,
called vasa recta, that serve tubule segments in the medulla.
Juxtamedullary nephrons and the vasa recta play an important
role in establishing the medullary osmotic gradient.
Collecting ducts receive urine from many nephrons and help
concentrate urine. ±ey form the medullary pyramids.
±e juxtaglomerular complex is at the point of contact between
the aﬀerent arteriole and the most distal part of the ascending
limb of the nephron loop. It consists of the granular cells, the
macula densa, and extraglomerular mesangial cells.
Urinary System; Topic: Anatomy Review, pp. 1–20.
Kidney Physiology: Mechanisms of Urine Formation
Functions of the nephrons include glomerular ﬁltration, tubular
reabsorption, and tubular secretion. Via these functional
processes, the kidneys regulate the volume, composition, and pH
of the blood, and eliminate nitrogenous metabolic wastes.
Urine Formation, Step 1: Glomerular Filtration
±e ﬁltration membrane consists of the fenestrated glomerular
endothelium, the intervening basement membrane, and the
podocyte-containing visceral layer of the glomerular capsule. It
permits free passage of substances smaller than (most) plasma
±e glomeruli function as ﬁlters. High glomerular blood pressure
(55 mm Hg) occurs because the glomeruli are fed and drained by
arterioles, and the aﬀerent arterioles are larger in diameter than
the eﬀerent arterioles.
About one-ﬁ²h of the plasma ﬂowing through the kidneys is
ﬁltered from the glomeruli into the glomerular capsule.
Usually about 10 mm Hg, the net ﬁltration pressure (NFP) is
determined by the relationship between forces favoring ﬁltration
(glomerular hydrostatic pressure) and forces that oppose it (capsular
hydrostatic pressure and blood colloid osmotic pressure).
±e glomerular ﬁltration rate (GFR) is directly proportional to
the net ﬁltration pressure and is about 125 ml/min (180 L/day).
Intrinsic renal control, or renal autoregulation, enables the
kidneys to maintain a relatively constant renal blood ﬂow and
glomerular ﬁltration rate. Intrinsic control involves a myogenic
mechanism and a tubuloglomerular feedback mechanism
mediated by the macula densa.
Extrinsic control of GFR, via nerves and hormones, maintains
blood pressure. Strong sympathetic nervous system activation
causes constriction of the aﬀerent arterioles, which decreases
Urinary System; Topic: Glomerular Filtration, pp. 1–15.
±e renin-angiotensin-aldosterone mechanism raises systemic
blood pressure by generating angiotensin II. Renin is released
from granular cells in response to (1) direct sympathetic nervous
system stimulation, (2) paracrines released by the macula densa,
and (3) reduced stretch of granular cell membranes.
Urine Formation, Step 2: Tubular Reabsorption
During tubular reabsorption, needed substances are removed
from the ﬁltrate by the tubule cells and returned to the peritubular
capillary blood. ±e primary active transport of Na
by a Na