C H A P T E R 5 0
Introduction to the kidneys and the urinary tract
797
components of the blood through the glomerulus into
the nephron tubule), tubular
secretion
(active movement
of substances from the blood into the renal tubule) and
tubular
reabsorption
(movement of substances from the
renal tubule back into the vascular system).
Glomerular filtration
The glomerulus acts as an ultrafine filter for all of the
blood that flows into it. The semipermeable membrane
keeps blood cells, proteins and lipids inside the vessel,
whereas the hydrostatic pressure from the blood pushes
water and smaller components of the plasma into the
tubule. The resulting fluid is called the filtrate. Scarring,
or swelling of, or damage to the semipermeable
membrane leads to the escape of larger plasma compo-
nents, such as blood cells or protein, into the filtrate. The
large size of these components prevents them from being
reabsorbed by the tubule, and they are lost in the urine.
Thus a clinical sign of renal damage is the presence of
blood cells or protein in the urine.
Approximately 125 mL of fluid is filtered out each
minute, or 180 L/day. About 99% of the filtered fluid is
returned to the bloodstream as the filtrate continues its
movement through the renal tubule. Approximately 1%
of the filtrate—less than 2 L of fluid—is excreted each
day in the form of urine.
Tubular secretion
The epithelial cells that line the renal tubule can secrete
substances from the blood into the tubular fluid. This
is an energy-using process that allows active transport
systems to remove electrolytes, some drugs and drug
metabolites, and uric acid from the surrounding cap-
illaries and secrete them into the filtrate. For instance,
the epithelial cells can use tubular secretion to help
maintain acid–base levels by secreting hydrogen ions
as needed.
Tubular reabsorption
The cells lining the renal tubule reabsorb water and
various essential substances from the filtrate back into
the vascular system. About 99% of the water filtered
at the glomerulus is reabsorbed. Other filtrate compo-
nents that are reabsorbed regularly include vitamins,
glucose, electrolytes, sodium bicarbonate and sodium
chloride. The reabsorption process uses a series of trans-
port systems that exchange needed ions for unwanted
ones (see Chapter 7 for a review of cellular transport
systems). Drugs that affect renal function frequently
overwhelm one of these transport systems or interfere
with its normal activity, leading to an imbalance in
acid–base or electrolyte levels. The precision of the reab-
sorption process allows the body to maintain the correct
extracellular fluid volume and composition.
Maintenance of volume and composition of
body fluids
The kidneys regulate the composition of body fluids by
balancing the levels of the key electrolytes, secreting
or absorbing these electrolytes to maintain the desired
levels. The volume of body fluids is controlled by diluting
or concentrating the urine.
Sodium regulation
Sodium is one of the body’s major cations (positively
charged ions). It filters through the glomerulus and
enters the renal tubule; then it is actively reabsorbed in
the proximal convoluted tubule to the peritubular cap-
illaries. As sodium is actively moved out of the filtrate,
it takes chloride ions and water with it. This occurs by
passive diffusion as the body maintains the osmotic and
electrical balances on both sides of the tubule.
Sodium ions are also reabsorbed via a transport
system that functions under the influence of the catalyst
carbonic anhydrase
. This enzyme speeds the combining
of carbon dioxide and water to form carbonic acid. The
carbonic acid immediately dissociates to form sodium
bicarbonate, using a sodium ion from the renal tubule
and a free hydrogen ion (an acid). The hydrogen ion
remains in the filtrate, causing the urine to be slightly
acidic. The bicarbonate is stored in the renal tubule
as the body’s alkaline reserve for use when the body
becomes too acidic and a buffer is needed.
The distal convoluted tubule acts to further adjust
the sodium levels in the filtrate under the influence of
aldosterone
(a hormone produced by the adrenal gland)
and natriuretic hormone (probably produced by the
hypothalamus). Aldosterone is released into the circu-
lation in response to high potassium levels, sympathetic
stimulation or angiotensin III. Aldosterone stimu-
lates a sodium–potassium exchange pump in the cells
of the distal tubule, causing reabsorption of sodium in
exchange for potassium (see Chapter 7 for a review of
the sodium-potassium pump). As a result of aldosterone
stimulation, sodium is reabsorbed into the system and
potassium is lost in the filtrate.
Natriuretic hormone causes a decrease in sodium
reabsorption from the distal tubules with a resultant
diluted urine or increased volume. Natriuretic hormone
is released in response to fluid overload or haemodilution.
Countercurrent mechanism
Sodium is further regulated in the medullary nephrons
in what is known as the
countercurrent mechanism
in
the loop of Henle. In the descending loop of Henle, the
cells are freely permeable to water and sodium. Sodium
is actively reabsorbed into the surrounding peritubu-
lar tissue, and water flows out of the tubule into this
sodium-rich tissue to maintain osmotic balance. The
filtrate at the end of the descending loop of Henle is con-
centrated in comparison to the rest of the filtrate.
