Chapter 25 - Urinary System*
I. Introduction
A. The
urinary system consists of two kidneys that filter the blood, two ureters, a urinary bladder, and a urethra to convey waste
substances to the outside.
II. Kidney Anatomy, Location, and Function
A. External Anatomy
1.
The kidney is a reddish brown, bean-shaped organ, 12 centimeters long.
2. A renal capsule, an
adipose capsule, and renal fascia surround each kidney.
3.
The fatty adipose capsule helps hold the kidneys in position.
B. Location of the Kidneys
1.
The kidneys are positioned retroperitoneally in the superior lumbar region on either
side of the vertebral column.
2.
The kidneys are located between the twelfth
thoracic and third lumbar vertebrae.
3.
The left kidney is slightly higher than the
right.
C. Kidney Structure
1.
A kidney has a superficial cortex, a deeper medulla
consisting mainly of medullary pyramids, and a medial
pelvis.
2.
Extensions of the pelvis (calyces) surround and collect
urine draining from the apices of the medullary pyramids.
3. A medial depression in the kidney leads to a
hollow renal sinus into which blood vessels, nerves, lymphatic vessels, and the
ureter enter.
4.
Inside the renal sinus lies a renal pelvis
that is subdivided into major and minor calyces.
a.
Small renal papillae project into each
minor calyx.
5. Two distinct regions are found within the
kidney: a renal medulla and a renal cortex.
a. The
renal medulla houses tubes leading to the papillae.
b.
The renal cortex contains the nephrons, the functional units of the kidney.
*(Anatomy & Physiology, 6th
Edition; Marieb, 2004)
III. Blood and Nerve Supply
A. Blood Supply
1.
The kidneys receive 25% of the total cardiac output per
minute.
2.
The abdominal aorta gives rise to renal arteries leading to
the kidneys.
3.
As renal arteries pass into the kidneys, they branch into
successively smaller arteries:
4. The vascular pathway through a kidney is as
follows:
renal artery segmental arteries lobar arteries
interlobar arteries arcuate
arteries interlobular
arteries afferent arterioles glomeruli efferent arterioles
peritubular capillary beds interlobular veins
arcuate
veins interlobar
veins renal vein.
B. Nerve Supply
1. The nerve supply of the
kidneys is derived from the renal plexus.
IV. Nephrons
A. Structure of a Nephron
1. Nephrons
are the structural and functional units of the kidneys.
2.
A kidney contains one million nephrons
3.
Each nephron
consists of a renal corpuscle and a renal tubule.
a.
The
renal corpuscle
1)
Is
the filtering portion of the nephron.
2) The renal corpuscle is made up of a:
a) Glomerulus (a high-pressure
capillary bed); and
b) Glomerular capsule (that
receives the filtrate.)
·
The glomerular
capsule has a parietal and a visceral layer.
q The
visceral cells are called podocytes, which bear
pedicels, forming a complicated system
of slit pores.
b. The
renal tubule
1)
The renal tubule leads away from the glomerular capsule.
2)
Subdivisions of the renal tubule (from the glomerulus) are:
a.
the glomerular
capsule,
b. a
highly coiled proximal convoluted tubule
c.
loop of Henle,
and
d.
distal convoluted tubule
3)
Several distal convoluted tubules join to
become a collecting duct.
2
4)
Collecting ducts receive urine from many nephrons and help concentrate urine. They form the medullary pyramids.
5)
A second capillary bed, the
low-pressure peritubular capillary bed, is closely
associated with the renal tubule of each nephron.
B.
Cortical
and Juxtamedullary Nephrons
1.
Cortical nephrons
a.
Cortical nephrons are the more numerous type
of nephron.
b. They
are located almost entirely in the renal cortex; only a small part of their
loop of Henle penetrates into the medulla.
2. Juxtamedullary Nephrons
a.
Juxtamedullary nephrons
are located at the cortex-medulla junction, and their loop of Henle dips deeply into the medulla.
b.
Instead of directly forming peritubular
capillaries, the efferent arterioles of many of the juxtamedullary
nephrons form unique bundles of straight blood
vessels, called vasa recta that serve tubule segments
in the medulla.
c.
Juxtamedullary nephrons
and the vasa recta play an important role in
establishing the medullary osmotic gradient which is important in regulating water balance.
C. Juxtaglomerular
Apparatus
1. At the point of contact between the afferent
and efferent arterioles and the the first part of the
distal convoluted tubule, the epithelial cells of the distal tubule form the macula densa.
2. Near the macula densa
on the afferent arteriole are smooth muscle cells called juxtaglomerular (JG) cells.
3. The juxtaglomerular apparatus consists of the macula densa together with the juxtaglomerular (JG) cells.
D. Blood Supply of a Nephron
1.
The glomerulus
receives blood from a fairly large afferent arteriole and passes it to a smaller
efferent arteriole.
2. The
efferent arteriole gives rise to the peritubular
capillary system, which surrounds the renal tubule.
3. Capillary
loops (vasa recta) associated with the juxtamedullary nephrons dip down
into the medulla.
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E.
Filtration
Membrane
1.
The filtration membrane consists of the:
a.
fenestrated glomerular
endothelium,
b.
intervening basement membrane, and
c.
podocyte-containing visceral
membrane of the glomerular capsule.
2.
It permits free passage of substances smaller than (most)
plasma proteins.
V. Kidney Physiology: Mechanisms of Urine Formation
A.
Functions of the Kidneys
1.
Functions
of the nephrons include:
a.
Glomerular
filtration,
b. Tubular
reabsorption, and
c.
Tubular secretion
2.
Via these functional processes, the kidneys:
a.
Eliminate nitrogenous
metabolic wastes from the blood, and
b.
Regulate the volume,
composition, and pH of body fluids.
3. The
kidneys also:
a.
Help
control the rate of red blood cell formation by secreting erythropoietin,
b. Regulate
blood pressure by secreting renin, and
c.
Regulate the absorption of calcium by
activating vitamin D.
B. Glomerular
Filtration
1. The glomeruli
function as filters.
2. Urine formation begins
when the fluid portion of the blood leaves the glomerulus
and enters the glomerular capsule as glomerular filtrate.
3.
Glomerular
filtrate contains water, glucose, urea, amino acids, creatinine,
creatine, and many ions.
4.
About one-fifth of the plasma flowing through the kidneys is filtered from the glomeruli into the renal tubules.
C. Filtration Pressure
1. 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
2. The main force responsible for moving
substances by filtration through the glomerular
capillary wall is the hydrostatic pressure of the blood inside.
3. Due to plasma proteins, osmotic pressure of
the blood resists filtration, as does hydrostatic pressure inside the glomerular capsule.
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D.
Filtration Rate
1. The factors that affect the filtration rate
are filtration pressure, glomerular plasma osmotic
pressure, and hydrostatic pressure in the glomerular
capsule.
2. When the afferent arteriole constricts in
response to sympathetic stimulation, filtration pressure, and thus filtration
rate, declines.
3. When the efferent arteriole constricts,
filtration pressure increases, increasing the rate of filtration.
4. When osmotic pressure of the glomerular plasma is high, filtration rate decreases.
5. When hydrostatic pressure inside the glomerular capsule is high, filtration rate declines.
6. The net filtration
pressure (NFP) is usually about 10 mm Hg.
a.
NFR is determined by the relationship between:
1)
Forces favoring filtration (glomerular
hydrostatic pressure) and
2)
Forces that oppose filtration (capsular hydrostatic
pressure and blood colloid osmotic pressure).
7.
On the average, glomerular
filtration rate (GFR):
a.
Is directly
proportional to the net filtration pressure and
b.
Is about 125 ml/min
or (180 liters/24 hrs.), most of which is reabsorbed.
E. Regulation of Filtration Rate
1. Glomerular
filtration rate is relatively constant because of autoregulation
of this rate, although sympathetic impulses may decrease the rate of
filtration.
a.
Renal autoregulation:
1)
Enables the kidneys to maintain a relatively constant renal
blood flow and glomerular filtration rate
2)
Involves a myogenic mechanism and a tubuloglomerular feedback mechanism mediated by the macula densa.
b.
Strong sympathetic nervous system activation causes
constriction of the afferent arterioles.
This:
1)
Decreases filtrate formation, and
2)
Stimulates the juxtaglomerular
(JG) cells to release renin.
2. Another control over filtration rate is the renin-angiotensin system, which regulates sodium excretion.
a. The renin-angiotensin
mechanism, mediated by the JG cells, raises systemic blood pressure via
generation of angiotensin II, which promotes aldosterone secretion.
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b.
The renin-angiotensin
mechanism involves the following:
1)
When the sodium chloride concentration in
the tubular fluid decreases, the macula densa senses
these changes and causes the juxtaglomerular cells to
secrete renin.
2) Secretion of renin
triggers a series of reactions leading to the production of angiotensin
II, which acts as a vasoconstrictor of the afferent arteriole. This affects
filtration rate and allows more time for Na+ to be reabsorbed by the
body.
3)
Presence of angiotensin II also increases the
secretion of aldosterone, which stimulates reabsorption of sodium.
This will result in an increase in the reabsorption
of water and an increase in blood pressure.
F. Tubular Reabsorption
1.
Changes in the fluid composition from the
time glomerular filtrate is formed to when urine
arrives at the collecting duct are largely the result of tubular reabsorption of selected substances.
2. During
tubular reabsorption, needed substances are removed
from the filtrate by the tubule cells and returned to the peritubular
capillary blood.
3. Most
of the reabsorption occurs in the proximal
convoluted tubule.
a.
Most of the nutrients, 65% of the water and
sodium ions, and the bulk of actively transported ions are reabsorbed in the
proximal convoluted tubules
b.
Reabsorption of Sodium, Water, Anions and Other Substances
1)
Glucose and amino acids are reabsorbed by
active transport
2) Proteins
are reabsorbed by pinocytosis
3) Water
is reabsorbed by osmosis
4)
Sodium ions (Na+) are reabsorbed
by active transport, and negatively charged ions follow passively.
a) The
primary active transport of Na+ by a Na+-K+ ATPase pump at the basolateral
membrane accounts for Na+ reabsorption and
establishes the electrochemical gradient that drives the reabsorption
of most other solutes and H2O.
b) Na+
enters at the luminal surface of the tubule cell via facilitated diffusion
through a channel
c)
As sodium is reabsorbed, water follows by osmosis.
5)
Passive tubular reabsorption is driven by
electrochemical gradients established by active reabsorption
of sodium ions.
a)
Water, many anions, and various other substances (for
example, urea) are reabsorbed passively by diffusion via transcellular or paracellular
pathways.
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6) Secondary active tubular reabsorption occurs by cotransport
with Na+ via protein carriers.
a)
Transport of such substances is limited by the number of
carriers available.
b)
Carrier proteins have a limited transport
ability, so excessive amounts of a substance will be excreted into the urine.
c)
Substances reabsorbed actively include nutrients and some
ions.
7) Certain substances (creatinine,
drug metabolites, etc.) are not reabsorbed or are reabsorbed incompletely
because of the lack of carriers, their size, or nonlipid
solubility.
4.
Reabsorption
of additional sodium ions and water occurs in the distal tubules
and collecting ducts and is hormonally controlled.
a.
Aldosterone increases the reabsorption of sodium (and obligatory water reabsorption);
b.
Antidiuretic hormone (ADH) enhances
water reabsorption by the collecting ducts.
G. Tubular
Secretion
1.
Tubular secretion is a means of adding substances to the
filtrate.
a.
Tubular secretion transports certain substances from the plasma or tubule cells into the renal tubule
2.
It is an active process that is important in eliminating
drugs, urea, and excess ions and in maintaining the acid-base balance of the
blood.
a.
Active transport mechanisms move excess hydrogen ions into the renal tubule
along with various organic compounds.
b.
Potassium ions are secreted both actively and passively into the distal
convoluted tubule and the collecting duct.
H. Regulation of Urine Concentration and
Volume
1.
Most of the sodium ions are reabsorbed
before the urine is excreted, and sodium is concentrated in the renal medulla
by the countercurrent mechanism.
2.
TERMS to Know:
a. Osmolality: the number of solute particles dissolved in
one liter (1,000g) of water; reflects the solution's ability to cause osmosis.
b. Osmolarity: the total concentration of all solute
particles in a solution.
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3.
The 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).
a.
This allows urine with osmolalities
ranging from 65 to 1200 mOsm to be formed.
4.The
descending
limb of the loop of Henle is permeable to water, which leaves the
filtrate and enters the medullary interstitium
(interstitial space).
a.
The filtrate and medullary fluid at the tip of the loop of Henle are hyperosmolar.
5.
The thick ascending limb is impermeable to water, but Na+ and
Cl- are actively transported out of the
filtrate into the interstitial space.
a.
The filtrate becomes more dilute as it continues to lose
salt
b.
Na+ and Cl-
move passively out of the thin portion of the ascending limb.
6.
Normally the distal convoluted tubule and collecting duct are
impermeable to water unless the hormone ADH is present.
7.
As filtrate flows through the collecting ducts in the inner
medulla, urea diffuses into the interstitial space.
a.
Some urea enters the ascending limb and is recycled.
8.
The blood flow in the vasa recta
is sluggish, and the contained blood equilibrates with the medullary
interstitial fluid.
a.
Thus, blood entering and exiting the medulla in the vasa recta is isotonic to blood plasma, and
b.
The high solute concentration of the medulla is maintained.
9.
In the
absence of antidiuretic hormone (ADH):
a.
Dilute urine is formed because the dilute filtrate reaching
the collecting duct is simply allowed to pass from the kidneys.
10. When ADH is present, the countercurrent
multiplier creates a large concentration gradient for water reabsorption
in the interstitial fluid surrounding the distal convoluted tubule and
collecting duct and can reabsorb even more water from the filtrate.
a.
As blood levels of antidiuretic
hormone rise, the collecting ducts become more permeable to water, and water
moves out of the filtrate as it flows through the hyperosmotic
medullary areas.
b.
Consequently, more concentrated urine is produced, and in
smaller amounts.
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I. Renal Clearance
1.
Renal clearance is the volume flow rate (ml/min) at which
the kidneys clear the plasma of a particular solute.
2.
Studies of renal clearance provide information about renal
function or the course of renal disease.
VI.
Urea and Uric Acid Excretion
A.
Urea
1. Urea is a by-product of amino acid
metabolism.
2.
Urea is passively reabsorbed by diffusion but about 50% of urea is excreted in
the urine.
a.
A countercurrent mechanism involving urea
aids in the reabsorption of water.
B.
Uric
acid
1.
Uric acid is a by-product of nucleic acid metabolism.
2.
Most uric acid is reabsorbed by active transport and up to 10% is apparently
secreted into the renal tubule.
VII. Urine
Characteristics and Composition
A. Characteristics
1.
Urine is typically clear, yellow, aromatic, and slightly
acidic.
2.
Its specific gravity ranges from 1.001 to 1.035.
3.
Daily urinary volume is typically 1.5-1.8 L, but this
depends on the state of hydration of the body.
B. Composition of Urine
1.
Urine composition varies from time to time and reflects the
amounts of water and solutes that the kidneys eliminate to maintain homeostasis.
2.
Urine is 95% water
3.
Solutes in urine include:
a.
Nitrogenous wastes (urea, uric acid, and creatinine),
b.
Trace amounts of amino acids, and
c.
Various electrolytes or ions
(always sodium, potassium, sulfate, and phosphate).
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4.
Substances not normally found in urine include:
a.
glucose,
b.
proteins,
c.
erythrocytes,
d.
pus,
e.
hemoglobin, and
f.
bile pigments.
VII.
Elimination
of Urine
A. Urine Elimination Pathway
1.
After forming in the nephrons,
urine passes from the collecting ducts to the renal papillae;
2. Urine
passes to the minor and major calyces, then out the renal pelvis to the ureters;
3. From
the ureters, it passes to the urinary bladder (where
it is stored).
4. The
urethra then conveys urine from the urinary bladder to the outside.
4.
B.
Ureters
1. The ureters are
slender muscular tubes running retroperitoneally from each kidney to the base of the urinary bladder.
2. The wall of the ureter is composed of three layers:
a.
mucous coat,
b. muscular
coat, and
c.
outer fibrous coat.
3. Muscular peristaltic waves convey urine from
the renal pelvis to the urinary
bladder where it passes through a flaplike valve in
the mucous membrane of the urinary bladder.
C. Urinary Bladder
1. The urinary bladder, which functions to store
urine, is a hollow, distensible, muscular sac that lies in the pelvic cavity posterior to the pubic symphysis.
2. The internal floor of
the bladder includes the trigone, which is composed
of:
a. Two inlets from the two ureters
and
b.
One outlet to the urethra.
1) In males, the
prostate gland surrounds its outlet.
3. The
wall of the urinary bladder consists of three layers:
a.
A thick mucosa containing transitional
epithelium,
b. A
thick muscular layer made up of a
three-layered detrusor muscle, and
c.
A thick
adventitia.
4.
The portion of the detrusor muscle that surrounds the
neck of the bladder forms an internal sphincter muscle
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D. Urethra
1.
The urethra is a muscular tube that conveys
urine from the urinary bladder to the outside.
2. Where
the urethra leaves the bladder, it is surrounded by an internal urethral
sphincter (an involuntary smooth muscle sphincter.)
3. Where
it passes through the urogenital diaphragm, the
voluntary external urethral sphincter is formed by skeletal muscle
4. In the female, the urethra is 3-4 centimeters
long, opens at the external urethral orifice, and conducts only urine.
5.
In the male, the urethra is 20 cm long, conveys
both urine and semen, and has three portions:
a.
The prostatic
urethra,
b. The
membranous urethra, and
c.
The penile urethra.
D.
Micturition
1. Micturition is
emptying of the bladder
2. Urine
leaves the bladder by the micturation reflex.
3. Stretching of the urinary bladder by
accumulating urine triggers the micturation reflex center located in the sacral portion of
the spinal cord.
4. The micturition reflex results in the following:
a.
Return Parasympathetic fibers, in response to signals from
the micturition center of the pons, cause the
following:
1)
The detrusor muscle contracts in waves, and an urge
to urinate is sensed.
2)
When these contractions become strong enough, the internal
urethral sphincter is forced open.
3) The
external urethral sphincter (in the urogenital
diaphragm) is composed of skeletal muscle and is under conscious control
a) The external urethral sphincter must relax
for micturition (urination) to occur.
b)
Because the external sphincter is voluntarily controlled, micturition
can usually be delayed temporarily.
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VIII. Developmental Aspects of the Urinary System
A.
Three sets of kidneys (pronephric, mesonephric, and metanephric)
develop from the intermediate mesoderm. The metanephros
is excreting urine by the third month of development.
B. Common congenital abnormalities are:
1.
Horseshoe kidney,
2.
Polycystic kidney (composed of many cysts), and
3.
Hypospadias (In males, a defect on
the ventrum of the penis so that the urethral meatus is more proximal than its normal glandular location;
in females, the urethra opens into the vagina.)
C.
The kidneys of newborns cannot concentrate urine.
1.
Their bladder is small and voiding is frequent.
2.
Neuromuscular maturation generally allows toilet training
for micturition to begin by 18 months of age.
D.
The most common urinary system problems in children and young to middle-aged
adults are bacterial infections.
E. Renal failure is
uncommon but serious.
1.The
kidneys are unable to concentrate urine, nitrogenous wastes accumulate in the
blood, and acid-base and electrolyte imbalances occur.
F.
With age:
1.
Nephrons are lost,
2.
Filtration rate decreases, and
3.
Tubule cells become less efficient at concentrating urine,
G.
Bladder capacity and tone decrease with age, leading to frequent micturition and (often) incontinence.
1.
Urinary retention is a common problem of elderly men.
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