THE URINARY SYSTEM
Name some by-products of cellular metabolism of nutrients.
carbon dioxide, nitrogenous wastes, excessive water, and heat
What are nitrogenous wastes?
Nitrogenous wastes, such as ammonia and urea, are formed as a result of
protein catabolism (breakdown).
What do essential ions, such as sodium, chloride, sulfate, phosphate, and hydrogen
ions, tend to do in the extracellular fluids?
accumulate in excess of the body’s needs
What is the role of the urinary system in maintaining normal body homeostasis?
The kidneys excrete water, nitrogenous wastes, some bacterial toxins, hydrogen
ions, essential ions, some heat, and some carbon dioxide from the body in the
form of urine.
Name, and then give a brief description of, three other ways in which the body rids itself
of waste products.
lungs -- The lungs excrete carbon dioxide, heat, and water into exhaled air.
sudoriferous glands -- The sudoriferous glands excrete water, ions, urea, and
heat into sweat.
gastrointestinal tract -- The gastrointestinal tract excretes feces, water, ions, and
heat.
Identify the structures of the urinary system.
The urinary system consists of:
kidneys (2)
ureters (2)
urinary bladder
urethra
What is the primary function of the urinary system? How is this accomplished?
The primary function of the urinary system is to help maintain homeostasis by
controlling the composition, volume, and pressure of blood. It does so by removing and restoring selected amounts of water and solutes from the blood.
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A.
KIDNEYS
Identify and give a brief description of the five major functions of the kidneys.
regulate blood volume and composition by removing wastes, ions, and
water to form urine
regulate blood pressure by secreting renin, which activates the
angiotensin-aldosterone pathway
secrete erythropoietin in response to decreased blood oxygen to stimulate
erythropoiesis
participate in the synthesis of calcitriol, the active form of vitamin D
participate in glucose metabolism by performing gluconeogenesis during
fasting or starvation
1.
EXTERNAL ANATOMY
Describe the kidneys as follows:
location -- The paired kidneys are located just above the waist,
between the parietal peritoneum and the posterior abdominal
wall (retroperitoneal). They lie at vertebral levels T12 - L3
and are partially protected by ribs 11 and 12.
shape and size -- The average adult kidney is about 4 - 5 inches
long, 2 - 3 inches wide, and 1 inch thick. It is shaped like a
bean, with its concave medial surface facing the vertebral
column.
hilus -- Near the center of the medial concave border is the hilus,
an indentation through which the ureter, renal artery and
vein, nerves, and lymphatics enter and exit the kidney. The
hilus serves as the entrance to a cavity in the kidney called
the renal sinus.
renal fascia -- Surrounding each kidney are three layers of
connective tissues. From outermost, these layers are the
renal fascia, the adipose capsule, and the renal capsule.
Renal fascia is a thin layer of dense connective tissue that
anchors the kidney to its surrounding structures, to the
abdominal wall, and to its partner on the opposite side.
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adipose capsule -- The adipose capsule is a mass of fatty tissue
that surrounds the two kidneys. It is used to protect the
kidneys from the trauma and help hold them in place within
the abdominal cavity.
renal capsule -- The renal capsule is a thin, transparent dense
connective tissue that forms the outer surface of the kidney
itself. It serves as a barrier against trauma and the spread of
infection to the kidney.
2.
INTERNAL ANATOMY
Describe the kidneys as follows:
appearance in coronal section -- In coronal (frontal) section, the
kidney presents an outer area, the cortex, a middle region,
the medulla, and an innermost area, the renal pelvis.
renal pyramids -- Within the medulla are 8 - 18 cone-shaped
structures called renal (medullary) pyramids that look
striated due to a high number of straight parallel tubules
(collecting ducts) and blood vessels. The bases of the renal
pyramids face the cortex, while their apices, called renal
papillae (papilla is singular) face toward the center of the
kidney, opening into the renal sinus.
renal columns -- The cortex is the smooth-textured area extending
from the renal capsule to the bases of the pyramids and into
the spaces between the pyramids, forming the renal
columns.
cortical zones -- The cortex is divided into the outer cortical zone
and the inner juxtamedullary zone. Together, the cortex and
renal pyramids form the parenchyma (functional portion) of
the kidney. The parenchyma consists of about 1 million
microscopic tubules called nephrons. The nephron is the
functional unit of the kidney.
calyces -- In the renal sinus of the kidney is a large cavity called the
renal pelvis, the edge of which contains cup-like extensions
called major (2 - 3) and minor calyces (8 - 18).
Each minor calyx receives urine from a renal pyramid and
delivers it to a major calyx. From there, urine drains into the
renal pelvis and out through the ureter to the urinary bladder.
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3.
NEPHRON
Name, and then briefly describe, the three basic functions nephrons.
filtration -- In filtration, most substances in the blood are permitted
to pass from the blood into the lumina of the nephrons, while
others are kept out. This is called filtration, like in capillary
exchange, because it is driven by pressure across an
endothelium.
reabsorption -- As the filtered liquid, known as filtrate, flows through
nephrons, useful materials are returned to the blood by the
process of reabsorption. Reabsorption occurs by diffusion
and active transport.
secretion -- As filtrate moves through the nephrons, cells of the
nephron (tubule cells) secrete some additional materials
(particularly hydrogen ions) directly into the filtrate.
a.
b.
PARTS OF THE NEPHRON
CORTICAL AND JUXTAMEDULLARY NEPHRONS
List in order of appearance the sections of a nephron.
1--Bowman’s capsule, 2--proximal convoluted tubule,
3--proximal straight tubule, 4--loop of Henle, 5--distal straight
tubule, 6--distal convoluted tubule, and 7--collecting duct.
Describe the nephron as follows:
renal corpuscle -- A nephron consists of two portions: a
renal corpuscle, where plasma is filtered into the
nephron, and a renal tubule, into which the fluid
passes.
The renal corpuscle has two components -- a tuft of
capillaries called glomerulus, surrounded by a doublewalled cup known as Bowman’s (the glomerular)
capsule.
Describe Bowman’s capsule.
The outer wall of Bowman’s capsule is the
parietal layer and the inner wall is the visceral
layer. Between the two layers is the capsular
space, into which the blood is filtered. It is
through the visceral layer that water and most
of the solutes of plasma in the glomerulus pass
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into the capsular space. Plasma proteins and
formed elements do not cross the visceral
layer.
glomerulus -- The glomerulus is a tuft of capillary loops
sitting in the “cup” formed by Bowman’s capsule.
Blood enters a glomerulus via an afferent arteriole
and exits glomerulus via an efferent arteriole. This is
a significant departure from the usual vascular pattern
of artery, capillary, vein.
renal tubule -- From the capsular space, filtrate moves into
the renal tubule, which has three main sections: the
proximal convoluted tubule (PCT), the loop of Henle,
and the distal convoluted tubule (DCT). It is within
these sections that tubular reabsorption and tubular
secretions occur.
collecting duct -- The terminal ends of many DCTs enter a
single collecting duct. Collecting ducts are gathered
to form renal pyramids, and eventually merge to form
the large papillary ducts that open into the minor
calyces to drip newly formed urine into the renal
pelvis.
loop of Henle -- In a nephron, the loop of Henle connects the
PCT with DCT. The first portion of the loop, the
descending limb dips into the medulla, while the
ascending limb, returns to the cortex. The loop itself
is formed of proximal and distal straight segments
(simple cuboidal cells) and the loop portion, formed of
simple squamous cells.
cortical nephrons -- A cortical nephron has its glomerulus in
the outermost portions of the cortex and its loop
barely reaches the medulla.
juxtamedullary nephrons -- A juxtamedullary nephron has its
glomerulus deep in the cortex, close to the medulla.
Its loop is quite long, reaching deep into the medulla
to approach the renal papillae. Only 15 - 20% of all
nephrons are of the juxtamedullary type. These are
the nephrons that allow the formation of either very
dilute or very concentrated urine.
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c.
HISTOLOGY OF THE NEPHRON
What is the endothelial-capsular membrane?
Histologically, the renal corpuscle consists of the visceral
layer of Bowman’s capsule and the endothelium of the
glomerular capillaries. It is this endothelial-capsular
membrane that acts as a filter for the blood.
Describe the following:
endothelium -- The endothelium of the glomerulus consists
of a single layer of fenestrated simple squamous
cells. The endothelium prevents passage of the
formed elements from the blood into the filtrate, but
allows all other blood components to pass.
basement membrane -- The basement membrane of the
glomerulus consists of the fused basement membranes of the endothelium and the visceral layer
epithelium of Bowman’s capsule. It acts to restrict
passage of the larger plasma proteins but allows all
other solutes to pass into the filtrate.
visceral layer -- The epithelial cells of the visceral layer of
Bowman’s capsule are specialized cells called
podocytes. Each cell has thousands of foot-like
processes called pedicels, between which are open
spaces called filtration slits. The pedicels function to
hold the visceral layer of Bowman’s capsule to the
glomerular capillaries. The filtration slits do not
present much of a barrier to filtration, although the
pedicels are negatively charged and help prevent
passage of anions.
As a result of these barriers to filtration, the formed
elements and plasma proteins remain in the blood of
the glomerulus while all other solutes freely pass from
glomerulus into the filtrate.
Describe the histology of the following:
proximal tubule -- The simple cuboidal cells of the proximal
tubule have an extensive brush border (microvilli) for
increased surface area for tubular reabsorption.
Thus, some 65% of water and almost 100% of some
solutes in filtrate move back into the blood via the
proximal tubule.
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loop of Henle-- The loop of Henle is composed of simple
squamous cells, indicative of an area where rapid
transport of materials occurs. The loops of Henle are
particularly important in the formation of concentrated
urine, especially those of the juxtamedullary
nephrons.
distal tubule and collecting duct -- The distal tubules and
collecting ducts are formed of simple cuboidal cells
that lack a brush border. However, most of the cells,
called principal cells, bear receptors for the hormones
ADH and aldosterone, and are involved in the control
of water excretion. On the basal surface of the cells
are extensive infoldings and mitochondria. It is in this
area that many solutes are pumped back into the
blood.
4.
BLOOD AND NERVE SUPPLY
Describe the arterial supply, capillary arrangements, and venous drainage
of the kidneys.
Since the role of the kidneys is to filter the blood, it is not surprising
that they have an abundant blood supply. The right and left renal
arteries carry 25% of resting cardiac output to the kidneys (1,250
ml/min).
Each renal artery enters its respective kidney at the hilus, and then
divides several times before it enters the parenchyma, where the
branching continues down to the level of the afferent arteriole.
There is one afferent arteriole leading into each nephron, dividing
within the cup of Bowman’s capsule to form the glomerulus. The
glomerular capillaries merge to exit Bowman’s capsule as the
efferent arteriole.
The efferent arteriole has a smaller diameter than the afferent
arteriole; this helps to raise glomerular blood pressure higher than
would be found in a normal tissue capillary bed.
The efferent arteriole leaves Bowman’s capsule and participates in
the formation of two separate capillary networks: peritubular
capillaries surround the PCT and DCT, while the vasa recta follow
the loop of Henle.
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These capillary beds merge to form venules, then veins, which
eventually merge to form the right and left renal veins that leave the
respective kidney at the hilus and join the inferior vena cava.
Describe the juxtaglomerular apparatus as follows:
location -- The juxtaglomerular apparatus is a combined structure
that is formed where the final portion of the ascending limb
of the loop of Henle (beginning of the distal convoluted
tubule) comes to lie in contact with the afferent arteriole.
macula densa -- At the point of contact, the cells of the DCT
are specialized to form macula densa cells that monitor flow
of filtrate.
juxtaglomerular cells -- Also at the point of contact, the
smooth muscle cells of the afferent arteriole are specialized
to form juxtaglomerular cells. They function in monitoring
blood pressure in the afferent arteriole and to secrete the
molecule renin when blood pressure in the afferent arteriole
falls too low.
function -- Together, the macula densa and the juxtaglomerular
cells form the juxtaglomerular apparatus. It is involved in the
regulation of blood pressure and the rate of blood filtration by
the kidneys by secreting renin when blood pressure is too
low and by decreasing its secretion of an as yet unnamed
vasoconstrictor substance when there is decreased delivery
of sodium, chloride, or water in the filtrate..
Describe the nerve supply of the kidney.
The nerves supplying the kidneys are derived from the renal plexus
of the sympathetic nervous system.
These nerves are associated with the blood supply, particularly the
afferent and efferent arterioles, and thus regulate blood flow to the
glomerulus (and therefore the rate of filtrate formation) by regulating the diameters of the arterioles.
B.
PHYSIOLOGY OF URINE FORMATION
Identify the three important functions carried out by the nephrons.
1.
2.
3.
control blood concentration and volume by removing selected
amounts of water and solutes.
regulate blood pH
remove toxic wastes from the blood
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What is the net effect of these functions?
As the nephrons accomplish these functions, they remove materials from
the blood, return the ones that the body requires, and excrete (eliminate)
from the body the remainder as urine.
Urine formation requires three processes. Name them in order of when they
occur in the nephron.
1.
2.
3.
1.
glomerular filtration by the renal corpuscle
tubular reabsorption by the renal tubule
tubule secretion by the renal tubule
GLOMERULAR FILTRATION
a.
NET FILTRATION PRESSURE
Define the principle of filtration.
Filtration is the forcing of fluids and solutes through a
membrane by mechanical pressure (all capillaries move
fluids in this manner).
Name the membrane across which filtration occurs in the kidney.
Filtration occurs in the renal corpuscle across the
endothelial-capsular membrane.
What is the driving force for glomerular filtration?
Blood hydrostatic pressure forces water and solutes from
plasma through the endothelial fenestrations, the fused
basement membrane, and the filtration slits of the visceral
layer of Bowman’s capsule, into the capsular space.
What is the resulting fluid in the capsular space called?
The fluid found in the capsular space is known as filtrate.
How much is formed per day?
Approximately 180 liters of filtrate are formed per day. This
is some 60 times the entire blood plasma volume.
What does it contain?
In a healthy person filtrate contains everything in blood
except formed elements and plasma proteins.
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Name and give a brief description of the three structural
adaptations that enhance glomerular filtration.
glomerular capillaries are long -- The total length of the
capillaries within the glomerulus is very high, yielding
a very large surface area for filtration.
filter is porous and thin -- The endothelial-capsular membrane, with its fenestrae, fused basement membrane,
and filtration slits, is about 50 times more porous than
regular capillaries.
afferent vs efferent arteriole diameter -- Since the efferent
arteriole has a smaller diameter than the afferent
arteriole there is a damming effect in the glomerular
capillaries causing glomerular blood hydrostatic
pressure to be high.
Describe the following pressures:
glomerular blood hydrostatic -- Glomerular blood hydrostatic
pressure (GBHP) (60 mm Hg) is the chief pressure of
glomerular filtration. It is the pressure of blood within
the glomerular capillaries pressing against the inside
of the vessels. It is an outward force, trying to move
fluid into the capsular space.
capsular hydrostatic -- Capsular hydrostatic pressure (CHP)
(15 mm Hg) is the pressure of filtrate in the capsular
space pressing against the visceral layer of
Bowman’s capsule. It is, therefore, an inward force,
trying to move fluid into the glomerular blood.
blood colloid osmotic -- Blood colloid osmotic pressure
(BCOP) (27 mm Hg) is the osmotic pressure exerted
by the trapped plasma proteins in the glomerular
blood. It tends to cause water to move back into the
glomerular blood. It is, therefore, an inward force.
net filtration pressure -- The net filtration pressure (NFP) is
determined as:
NFP
=
=
=
=
=
GBHP- (CHP+BCOP)
outward - inward
60 - (15+ 27)
60- 42
+18 mm Hg
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The positive number means that the flow of fluid is
into the capsular space. Normal capillaries have an
NFP of +10 mm Hg on the arterial end. This higher
NFP helps ensure that renal filtration occurs.
filtration fraction -- The filtration fraction is the percentage of
plasma entering the nephrons to become filtrate. It is
normally about 10% per pass through the kidneys.
cardiac output = 5,250 ml/min
25% goes to the kidneys = 1,250 ml/min
filtration fraction = 10%
Therefore, 125 ml/min filtrate formed.
b.
GLOMERULAR FILTRATION RATE
Define glomerular filtration rate (GFR)?
The glomerular filtration rate (GFR) is the amount of filtrate
formed in all renal corpuscles in both kidneys per minute
(125 ml/min).
What factors determine the glomerular filtration rate?
The GFR is directly related to the pressures that determine
the net filtration pressure (NFP). Anything that alters the
NFP will ultimately alter the GFR.
c.
REGULATION OF GFR
Homeostasis of body fluids requires that the kidneys maintain a
relatively constant GFR. What happens if the GFR is too high?
Needed substances pass through the kidneys so quickly
they are not reabsorbed and are lost into the urine.
What if the GFR is too low?
Nearly all filtrate is reabsorbed and waste products may not
be properly excreted from the body.
Glomerular blood flow depends on two factors. Name them.
Glomerular blood flow depends on two factors:
1.
systemic blood pressure
2.
diameter of the afferent and efferent arterioles
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List the three principle mechanisms that regulate these two
factors.
1.
2.
3.
renal autoregulation of GFR
hormonal regulation of GFR
neural regulation of GFR
Describe the mechanisms by which renal autoregulation
protects the GFR.
Under normal conditions, macula densa cells secrete
a vasoconstrictor substance that maintains a “normal”
level of afferent arteriole vasoconstriction. When
filtrate flow rate is low, macula densa cells secrete an
inhibitor to the secretion of the vasoconstrictor.
What is the result?
As a result of removal of the vasoconstrictor, the
afferent arterioles are allowed to vasodilate.
Increased blood flow into the glomerular capillaries
means increased GBHP, increased NFP, and
increased GFR..
Name and describe the two hormonal mechanisms used to regulate
the glomerular filtration rate.
Hormonally, the renin-angiotensin-aldosterone system and
atrial natriuretic peptide are used to regulate GFR.
Renin, secreted by the juxtaglomerular cells of the juxtaglomerular apparatus in response to decreased blood
pressure in the afferent arteriole, results in the formation of
angiotensin II, which has four important functions.
(1) It is a potent vasoconstrictor body-wide, thus increasing
BP. In the efferent arteriole, this would cause increased
GBHP and therefore increased NFP.
(2) Angiotensin II stimulates thirst and thereby increases
blood volume. This in turn increases systemic BHP.
(3) Angiotensin II stimulates aldosterone secretion.
Aldosterone stimulates increased reabsorption of Na+ and
therefore increased reabsorption of water. This causes
increased blood volume and therefore BHP.
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(4) Angiotensin II stimulates anti-diuretic hormone (ADH)
secretion. ADH stimulates increased water reabsorption,
again raising blood volume and therefore BHP.
The net result of these combined effects is increased
systemic blood pressure, so that NFP is increased, and
therefore, GFR is increased.
From where is atrial natriuretic peptide secreted and why? How
does ANP help regulate the glomerular filtration rate?
ANP is secreted from the atrium of the heart in response to
stretch caused by increased blood volume.
ANP stimulates excretion of Na+, and therefore water,
resulting in decreased blood volume. Decreased blood
volume causes decreased BHP and therefore decreased
NFP and decreased GFR.
Describe the neural regulation of glomerular filtration rate.
Based on blood pressure changes, sympathetic innervation
to the afferent and efferent arterioles can be increased or
decreased to change the GHBP and therefore alter the NFP
and GFR.
2.
TUBULAR REABSORPTION
Define the concept of tubular reabsorption. How is it accomplished?
Tubular reabsorption is the movement of water and selected
solutes back into the blood of the peritubular capillaries and the
vasa recta.
Renal tubular epithelial cells use very discriminating processes that
are dependent upon the body’s needs at the moment. In general,
most of the body’s nutrients are retained while wastes are eliminated.
What is tubular maximum (Tm)?
Tm is the maximum amount of a solute that can be reabsorbed
under any condition.
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Discuss the active and passive transport mechanisms by which tubular
reabsorption is accomplished.
Tubular reabsorption is carried out through both active and passive
transport mechanisms, primarily in the proximal tubule.
Active transport involves the use of specific receptors (carrier
molecules) found on the luminal surfaces of the tubular epithelial
cells.
When a substance in the filtrate binds to its specific receptor on the
luminal surface of the cell, it is brought into the cell with the
expenditure of energy, against its concentration gradient.
Glucose and amino acids are 100% reabsorbed under normal
conditions, since their active transport carriers are activated at all
times.
Other substances are actively transported only when their carrier
systems have been activated by the endocrine system (ex: Na and
K+ -- aldosterone, Ca -- parathyroid hormone).
In passive transport, no energy is used when solutes simply diffuse
into the tubular epithelial cells, either following their concentration
gradient or their electrical gradient (ex: Cl, phosphate).
Distinguish between obligatory and facultative reabsorption of water.
In obligatory reabsorption, water follows reabsorbed solutes,
primarily glucose and Na+, due to the osmotic gradients that are
created between the filtrate and the intracellular fluid of the renal
tubular cells.
How is this type of reabsorption controlled?
Since Na+ is the major extracellular cation, it has enormous
effects on water movement. Its control by aldosterone,
therefore, is a major control of water balance as well.
In facultative reabsorption of water, the permeability of distal tubule
and collecting duct cells to water is controlled directly by antidiuretic hormone (ADH). ADH is secreted from the hypothalamus
during times of dehydration.
What is the effect of ADH?
ADH increases the number of water channels in the tubular
cells, thus allowing water molecules to leave the filtrate and
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enter the cells. This process is used to control the remaining
10% of water reabsorption and is the major controller of
moment-to-moment needs in water balance.
3.
TUBULAR SECRETION
Define tubular secretion.
The third process in urine formation is tubular secretion. In this
process, renal tubular cells remove solutes from the blood and
directly secrete them into the filtrate so that they will be excreted
with the urine.
What are its major functions?
Tubular secretion has two principle effects:
1.
to rid the body of substances that tend to accumulate in
the body fluids (H+, NH3, K+, creatinine, some drugs); and
2.
help control body pH
How does it control body fluid pH?
The body must tightly control pH at 7.35 - 7.45, despite the fact that
a normal diet and normal metabolism promote the accumulation of
hydrogen ions and therefore acidic conditions. To help raise body
pH, renal tubule cells secrete H+ and ammonium (NH4) ions directly
into the filtrate, so that they are lost in the urine. The ions are
exchanged for Na+, so that Na+, and water, are reabsorbed.
What is the relationship between tubular secretion and potassium?
Potassium (K+) ions tend to accumulate in the body. In response to
aldosterone, K+ ions are in exchange for Na+ ions, with water
following.
4.
SUMMARY OF NEPHRON FUNCTIONS
a.
Proximal tubule
reabsorbs by osmosis -active transport --
simple diffusion --
water
sodium ions
glucose
amino acids
potassium ions
chloride ions
bicarbonate ions
urea
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secretes --
b.
Loop of Henle
reabsorbs by osmosis -active transport --
secretes -c.
C.
water
sodium ions
chloride ions
urea
potassium ions
Collecting duct
reabsorbs by osmosis -simple diffusion --
secretes --
water
sodium ions
potassium ions
chloride ions
urea
Distal tubule
reabsorbs by osmosis -active transport --
secretes -d.
hydrogen ions
ammonium ions
urea
creatinine
water
bicarbonate ions
urea
hydrogen ions
URETERS
1.
STRUCTURE
2.
FUNCTION
3.
PHYSIOLOGY
Trace urine flow from the nephron to the outside of the body.
From the collecting ducts urine is drained through papillary ducts
into the minor calyces that drain into the major calyces that unite to
form the renal pelvis. Urine then drains into the ureters. Using
peristaltic movements, the ureters move urine to the urinary
bladder, which stores the urine until it is expelled from the body via
the urethra.
Describe the ureters as follows:
gross anatomy -- Each ureter is an extension of the renal pelvis of
its respective kidney. It extends 10 - 12 inches, retroperitoneally, to enter the urinary bladder medially from its
posterior aspect.
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union with bladder -- There is no anatomical valve located at the
junction of the ureter with the bladder. Instead, as the
bladder fills with urine, pressure in the bladder wall compresses the ureteral openings and prevents reflux of urine
into the ureters.
histology -- Histologically, the ureters are lined with transitional
epithelium, have longitudinal and circular layers of smooth
muscle, and an outer adventitia that attaches the ureters to
the overlying peritoneum.
function -- The principle function of the ureters is to transport urine
to the bladder. In addition to hydrostatic pressure, 1 - 5
peristaltic waves/minute pass down the ureters, moving the
urine forward.
D.
URINARY BLADDER
1.
STRUCTURE
2.
FUNCTION
3.
PHYSIOLOGY
Describe the bladder as follows:
gross anatomy -- The urinary bladder is a hollow muscular organ
situated retroperitoneally in the pelvic cavity, posterior to the
pubic symphysis. It is a freely movable organ held in place
by folds of the peritoneum. When completely empty, it is
collapsed. As it fills, it moves superiorly, rising into the
abdominal cavity.
male vs female -- In the male, the bladder is directed anterior to the
rectum. In the female, it is anterior to the vagina and inferior
to the uterus.
epithelium -- Histologically, the urinary bladder is formed of the
same three layers as the ureters (transitional epithelium,
muscularis, and adventitia). The difference between the two
is in the muscularis.
detrusor muscle -- The muscularis of the bladder is collectively
known as the detrusor muscle. It consists of three layers of
smooth muscle (longitudinal, circular, and longitudinal) and
works as a unit during the micturition reflex.
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sphincters -- Around the opening of the urethra, the middle circular
layer of smooth muscle is thickened to form the internal
urethral sphincter. It is under unconscious (reflexive)
control.
The muscle of the pelvic diaphragm forms the external
urethral sphincter where the urethra pierces through it. This
muscle is consciously controlled.
trigone -- In the floor of the bladder is a triangular area, the trigone,
formed by the two urethral openings, and anteriorly by the
opening into the urethra.
What is micturition?
Urine is expelled from the bladder by an act known as micturition
(urination or voiding). This process occurs in response to a
combination of involuntary and voluntary nerve impulses.
Describe the micturition reflex.
The average capacity of the bladder is 700 - 800ml. When urine
volume is 200 - 400ml, stretch receptors in the bladder wall transmit
impulses to the sacral spinal cord. By way of ascending tracts,
these impulses initiate the conscious desire (cerebral cortex) to
expel urine.
In the sacral spinal cord itself, the micturition reflex is integrated.
Parasympathetic outflow from the sacral spinal cord causes
contraction of the detrusor muscle and relaxation of the internal
urethral sphincter. At the same time, the cerebral cortex permits
the voluntary relaxation of the external sphincter (this is a learned
response), and urine is voided from the body.
E.
URETHRA
1.
HISTOLOGY
2.
PHYSIOLOGY
What is the urethra?
The urethra is a small tube leading from the floor of the bladder to
the body exterior.
Describe it as follows:
epithelium -- The epithelium of the urethra is a blend of types. It
begins as a continuation of the transitional epithelium of the
bladder and ends as stratified squamous at its termination,
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where it blends into the keratinized stratified squamous of
the skin.
female urethra -- In the female, the urethra lies directly posterior to
the pubic symphysis, in front of the anterior wall of the
vagina. It is about 1.5 cm long and opens to the exterior
(external urethral orifice), between the clitoris and vaginal
introitus.
male urethra -- In the male, the urethra is about 8 inches long.
Upon leaving the bladder it passes immediately into and
through the prostate gland (prostatic urethra), then the pelvic
diaphragm (membranous urethra), and finally the penis
(penile urethra), ending at the external urethral orifice.
function(s) -- Functionally, the urethra serves as the final passageway for urine from the body. In the male, it also serves as
the duct through which semen is discharged to the exterior.
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