anatomy & physiology of the urinary system
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ANATOMY & PHYSIOLOGY OF THE URINARY SYSTEM
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CHAPTER - 5
ANATOMY AND PHYSIOLOGY OF THE URINARY SYSTEM
The urinary system is one of the excretory systems of the body. It consists of the
following structures :
2 Kidneys which secrete urine.
2 ureters which convey the urine from the kidneys to the urinary bladder.
1 urinary bladder where urine collects and is temporarily stored.
1 urethra through which the urine is discharged from the urinary bladder to the
exterior.
Figure shows an overview of the urinary system.
- L V.!dnev
-Aorta
GrMIw
riie |i;irt of the uriiiaiy system (cxcludin!; (lie ui-ethrii) iind sonic nssociatcii .sfnirtitres.
A longitudinal section of the right ludney.
KIDNEYS : The kidneys lie on the posterior abdominal wall, one on each side
of the vertebral column, behind the peritoneum and below the diaphragm. They
extend from the level of the 12th thoracic vertebra to the 3rd
30
lumbar vertebra. The right kidney is usually slightly lower than the left, probably
because of the considerable space occupied by the liver. Kidneys are bean-shaped
organs, about 11 cm. long, 6 cm. wide and 3 cm. thick. They are embedded in,
and held in position by a mass offat. A sheath of fibroelasticre«<3//a5c/(7 encloses
the kidney and the renal fat. (Georatas fascia)
ORGANS ASSOCIATED WITH THE KIDNEYS : As the kidneys lie
on either side of the vertebral column each is associated with a different group
of structures.
Right Kidney : Superiorly - the right adrenal gland.
Anteriorly - the right lobe of the liver, the duodenum and the right colic
flexure.
Posteriorly - the diaphragm, and muscles of the posterior abdominal wall.
Left Kidney : Superiorly - the left adrenal gland.
Anteriorly - the spleen, stomach, pancreas, jejunum and the left colic
flexure.
Posteriorly - the diaphragm and muscles of the posterior abdomonal wall.
GROSS STRUCTURE OF THE KIDNEY : There are three areas of
tissue which can be distinguished when a longitudinal section of the kidney is
viewed with the naked eye.
1. \ fibrous capsule, surrounding the kidney.
2. The cortex is the reddish-brown layer of tissue immediately under the
capsule and between the pyramids.
3. The medulla is the innermost layer, consisting of pale conical-shaped
-31 -
striations, the renal pyramids.
The hilum is the concave medial border of the kidney where the renal
blood and lymph vessels and nerves enter.
The renal pelvis is the funnel shaped structure which acts as a receptacle
for the urine formed by the kidney. It has a number of branches called calyces
at its upper end, each of which surrounds the apex of renal pyramid. Urine formed
in the kidney passes through apapilla at the apex of a pyramid into a lesser calyx,
then into a greater calyx before passing through the pelvis into the ureter. The
walls of the pelvis contain smooth muscle and are lined with transitional
epithelium. A wave of contraction, origination in pacemaker cells in the walls
of the calyces, propels urine through the pelvis and ureters to the bladder. This
is an intrinsic function not under nerve control.
HISTOLOGY OF THE KIDNEYS: The kidney substance is
composed of about 1 million functional units, the nephrons, and a smaller
number of collecting tubules. The uriniferous tubules are supported by a
small amount of connective tissue, containing blood vessels, nerves and lymph
vessels.
THE NERPHRON : The nephron consists of a tubule closed at one
end, the other end opening into a collecting tubule. The closed or blind end is
intended to form the cup-shaped glomerular capsule (Bowman's capsule)
which almost completely encloses a network of arterial capillaries, the
g/omerM/w5.Continuingfromthegolmerularcapsuletheremainderofthenephorn
is described in three parts; thQproximal convoluted tubule, the loop ofHenle and
the distal convoluted tubule, leading into a collecting tubule.
- 1 2 -
After entering the kidney at the hilus the renal artery divides into smaller
arteries and arterioles. In the cortex an arteriole, the afferent arteriole, enters
each glomerular capsule then subdivides into a cluster of capillaries, forming
theglomerulus.Betweenthecapillaryloopsthereareconnectivetissuephagocytic
mesangial cells. The blood vessel leading away from the golmerulus is the
efferent arteriole; it breaks up into a second capillary network to supply oxygen
and nutritional materials to the remainder of the nephron. Venous blood drained
from this capillary bed eventually leaves the kidney in the renal vein which
empties into the inferior venacava. The blood pressure in theglomerulus is higher
than in other capillaries because the calibre of the afferent arteriole is greater
than that of the efferent arteriole.
1 lutjyie
Diih^rain of tlie glomerulus and
^hmicnihir nipsnle.
Diugintn of n nephron including the aiTiin^enient of the blood vessels.
The walls of the glomerulus and the glomerular capsule consist of a
single layer oi flattened epithetial cells. The glomerular walls are more
33
permeable than those of other capillaries. The remainder of the nephron and the
collecting tubule are formed by a single layer of highly specialised cells.
The nerve supply consists of sympathetic and parasympathetic nerves.
PHYSIOLOGY OF THE KIDNEY : The kidneys form urine which
passes through the ureters to the bladder for excretion. The composition of
urine reflects the activities of the nephrons in the maintenance of homeostasis.
Waste products of protein metabolism are excreted, electrolyte balance is
maintained and the acid / base balance is influenced by the excretion of
hydrogen ions. There are three phases in the formation of urine : Simple
filtration, Selective reabsorption. Secretion.
SIMPLE FILTRATION : Filtration takes place through the
semipermeable walls of the glomerulus and glomerular capsule. Water and a
large number of small molecules pass through, some of which are reabsorbed
later. Blood cells, plasma proteins and other large molecules are unable to
filter through and remain in the capillaries.
Filtration is assisted by the difference between the blood pressure in the
golmerulus and the pressure of the filtrate in the golmerular capsule. Because
the calibre of the efferent arteriole is less than that of the afferent arteriole, a
capillary hydrostatic pressure of about 70mmHg builds up in the glomerulus.
This pressure is opposed by the filtrate hydrostatic pressure of about 5
mmHg in the golmerular capsule. The net filtration pressure is, therefore :
70 - (30 + 5) = 35 mmHg
About 180 liters of dilute filtrate are formed each day by the two
kidneys. Of these 1 to 1.5 liters are excreted as urine. The difference in
- 3 4 -
volume and concentration is due to selective reabsorption of some consituents
of the filtrate and secretion by tubular cells of others.
Blood consitiuents in
glomerular filtrate
water
minerals salts
amino acids
ketoacids
glucose
harmones
urea
uric acid
toxins
drugs
Blood consitiuents remaining in
the glomerulus
leukocytes
erythrocytes
platelets
blood proteins
crcjczoc GIOMERUUUS p.essute
7G mmHg .10 m!nHg
N«t !iIlr»tioa
;5 mmHg
aLZEDZiirnz]
n r . • » - ir- ——)|—rrnc: B I O O D I N CAPILLARY
Selective reabsorption (torn l i i tr i ; !e
Secretion f rom b lood to tubwl*
ERIM Dingrain of filtration in the nephron. Diagram of selective reabsorption and
secretion in the nephron.
SELECTIVE REABSORPTION : Selective reabsorption is the
process by which the composition and volume of the golmerular filtrate are
altered during its passage through the convoluted tubules, the loop of Henle
and the collecting tubule. The general purpose of this process is to reabsorb
- 3 5 -
those filtrate consituents needed by the body to maintain fluid and elecrtolyte
balance and blood alkalinity.
Some constituents of glomerular filtrate do not normally appear in urine
because they are completely reabsorbed unless they are present in blood in
excessive quantities. The kidneys' maximum capacity for reabsorption of a
substance is the transport maximum, e.g., normal blood glucose level is 2.5 to
5.3 mmol/1 (45 to 95 mg/100ml). If the level rises above the transport maximum
of about 9 mmol/l (160 mg/lOO ml) glucose appears in the urine because the
mechanism for active transfer out of the tubules is over loaded.
The transport maximum, or renal threshold, of some substances varies
according to the body's need for them at the time, i.e., in order to maintain
homeostasis. In some cases reabsorpation is regulated by hormones.
Parathryn from the parathyroid glands and calcitonin from the thyroid
gland together regulate reabsorption of calcium and phosphate.
Antidiuretic hormone (ADH) from the posterior lobe of the pituitary gland
affects the permeability of the distal convoluted tubules and collecting tubules,
regulating water reabsorption.
Aldosterone, secreted by the cortex of the adrenal gland, influences the
reabsorption of sodium and excretion of potassium.
Waste products, such as urea and uric acid, are reabsorbed only to a slight
extent.
Substances that are not normal blood constituents are not reabsorbed.
If the blood passes through the glomerulus too quickly for filtration to clear
such substances from the blood, the tubules secrete them into the filtrate.
Substances of no physiological significance are sometimes injected into the
- 3 6 -
body to evaluate the kidney's excretory efficiency.
SECRETION : Filtration occurs as the blood flows through the
glomerulus. Substances not required and foreign materials, e.g., drugs, may
not be cleared from the blood by filtration because of the short time it
remains in the glomerulus. Such substances are cleared by secretion into the
convoluted tubules and passed from the body in the urine.
SUBSTANCES IN PLASMA AND AMOUNTS FILTERED,
REABSORBED, AND EXCRETED IN URINE :
Chemical
Water
i'roteins
Sodium
[Na^)
Chloride
'CI )
3icarbonate
[HCO3)
Glucose
Jrea
Potassium
Uric acid
Creatinine
^Urea i
''After
amount of
Plasma (Total Amount)
3000 ml
200 g
9.7 g
(420 mmol)
10.7 g
(300 mmol)
4.6 g
(75 mmol)
3 g
4.8 g
0.5 g
(12.6 mmol)
0.15 g
0.03 g
s secreted in .
being 100% r
<.* is secreted
Filtered Enters
Glomerular Capsule per Day)
180.000 ml
2 g
579.6 g
(25,200 mmol)
639.0 g
(18,000 mmol)
274.5 g
(4500 mmol)
180 g
53 g
29.6 g
(756 mmol)
8.5 g
1.6 g
Reabsorbed
(Returned to Blood per Day)
178,500 ml
1.9 g
575.0 g
(25,000 mmol)
633.7 g
(17,850 mmol)
274.5 g
(4500 mmol)
180 g
28 g
29.6 g
(756 mmol)
7.7 g
0
Urine
(Excreted per Day)
1500 ml
0.1 g
4.6 g
(200 mmol)
6.3 g
(150 mmol)
0
0
0
25 g
2.0 g
(50 mmol'')
0.8 g
1.6 g
addition to being filtered and reabsorbed.
eabsorbed in the PCX, loop and DCT, a variable
in the collecting ducts.
37-
SUMMARY OF FILTRATION REABSORPTION, AND SECRETION
REGION OF NEPHRON
Renal corpuscle
(endothelial-capsular)
membrane)
Proximal convoluted
tubule
Descending limb of
the loop of Henle
Ascending limb of thi
loop of Henle
Distal convoluted
tubule
Collecting duct
ACTIVITY
Filtration : of blood in glomerular capillaries under
hydrostatic pressure results in the formation of
filtrate that contains water, glucose, some amino
acids, Na% CI", HCO,-, K% urea, uric acid,
creatinine and other solutes in the same
concentration as in blood plasma. Plasma proteins
and cellular elements of blood normally do not
pass through the endothelial - capsular membrane
and are not found in filtrate.
Reabsorption : of solutes such as glucose, amino
acids, Na^, CI", HCO",, K^ urea, water
reabsorption along with reabsorption along with
reabsorption of sodium and glucose.
Secretion : of H , NH/^, and a little creatinine.
Reabsorpatibn : of water
Secretion of urea.
'Reabsorption of Na^, K^ and CT
Secretion of urea.
Reabsorption of Na^, K^ and CTand HC03~, water
reabsorption along with reabsorption of sodium
glucose in early portion.
Reabsorption of Na^ under influence of
aldosterone; HCO, (new), CT, and urea; water
reabsorption under influence of ADH.
Secretion of H^ and K .
38-
URINE: Urineisamberincolourduetothepresenceofurobilin,abilepigment
altered in the intestine, reabsorbed then excreted by the kidneys. The specific gravity
is between 1020 and 1030, and the reaction is acid. A healthy adult passes 1000
to 1500 ml. per day. The amount of urine secreted and the specific gravity vary
according to the fluid intake and the amount of solute excreted. During sleep and
muscular exercise urine production is decreased.
Any analysis of the volume and physical, chemical and microscopic
properties of urine, called a urinalysis, tells us much about the state of the
body. The principal physical characteristics of urine are summarized below.
PHYSICAL CHARACTERISTS OF NORMAL URINE :
Volume - One to two liters in 24 hours but varies considerably.
Colour - Yellow or amber but varies with concentration and diet. Color is
due to urochrome (pigment produced from breakdown of bile). Concentrated
urine is darker in colour. Diet (reddish coloured urine from beels and green
coloured from asparagus) and certain diseases (kidney stone may produce
blood in urine) affect colour.
Turbidity - Transparent when freshly voided but becomes turbid (cloudy)
upon standing.
Odor - Aromatic but becomes ammonia - like upon standing. Some
people inherit the ability to form methylmercaptan from ingesting asparagus that
gives urine a characteristic odor. Urine of diabetics has a sweet odor due to
presence of ketone bodies.
pH - Ranges between 4.6 and 8.0; average 6.0 : varies considerably with diet.
High-protein diets increase acidity vegetarian diet increases alkalinity.
- 3 9 -
SPECIFIC GRAVITY - Specific gravity (density) is the ratio of the weight
of a volume of a substance to the weight of an equal volume of distilled water.
It ranges from 1.001 to 1.03 5. The higher the concentration of solutes, the higher
the specific gravity.
VOLUME : The volume of urine eliminated per day in the normal adult
varies between 1000 and 2000 ml. (about 1 to 2 qt.) Urine volume is influenced
by blood pressure, blood osmotic pressure, diet, temperature, diuretics, mental
state, and general health. Low blood pressure triggers the reninangiotensin
pathway, which increases reabsorption of water and salts in the renal tubules and
decreases urine volume. When blood osmotic pressure decreases, for example,
after drinking a large volume of water, secretion of ADH is inhibited and
consequently a larger volume of urine is excreted. The reverse effects occur with
high blood pressure and increased blood osmotic pressure.
CHEMICAL COMPOSITION : Water accounts for about 95% of the
total volume of urine. The remaining 5% consists of solutes derived from cellular
metabolism and outside sources such as drugs. Typical solutes present in urine
are described below. j]^ ; \\^^^
PRINCIPAL SOLUTES IN URINE OF ADULT MALE ON TYPICAL
DIET:
Constituent
ORGANIC
Urea
Amount" (g)
25.0 to 35.0
Commnets
Composes 60 to 90% of all nitrogenous material
in urine. Derived primarily from deamination of
amino acids to form ammonia (ammonia
combines with C02 to form urea).
40-
Constituent
Creatinine
Uric acid
Hippuric
acid
Indican
Ketone
bodies
Other
substances
Amount^ (g)
1.6
0.4 to 1.0
0.7
O.Ol
0.04
2.9
Commnets
Normal constituent of blood. Derived primarily
from breakdown of creatinine phosphate
(nitrogenous substance in muscle tissue).
Product of catabolism of nucleic acids (DNA and
RNA) derived from food or celullar destruction.
Because ofinsolubility, it tends to crystallize and
is a common component of kidney stones.
Form in which benzoic acid (toxic substance in
fruitsand vegetables) isbelieved to beeliminated
from body. High-vegetable diets increase
quantity of hippuric acid excreted.
Potassium salt of indole. Indole result from
bacterial breakdown of protein in large intestine
andiscarriedbybloodtoliver,whereitisprobably
changed to indican (less poisonous substance).
Also called acetone bodies. Normally found in
small amounts. In cases of diabetes mellitus and
acute starvation, ketone bodies appear in high
concentrations.
May be present in minute quantities, depending
on diet and general health. Include
carbohydrates, pigments, fatty acids, mucin,
enzymes, and hormones.
— '
41
Constituent
Na% Cr
K"
so/-
HPO/-,
4
4
Mg2
Ca -
Amount(g)
15.0
3.3
2.5
2.5
0.7
0.1
0.3
INORGANIC
Commnets
Principal inorganic salt. Amount excreted varies
with intake.
Occurs as chloride, sulfate, and phosphate
salts.
Derived from amino acids.
Occur as sodium compounds (monosodium and
disodium phosphate) that serve as buffers in
blood and urine.
Occurs as ammonium salts. Dervied from
protein catabolism and from glutamine (amino
acid) deamination in kidneys. Amount
produced by kidney may vary with need to
produce HCO, to offset acidity of blood and
tissue fluids.
Occurs as chloride, sulfate, and phosphate
salts.
Occurs as chloride, sulfate and phosphate
salts.
''Values are for a urine sample collected over 24 hours.
ABNORMAL CONSTITUENTS : If the body's chemical processes
are not operating efficiently, traces of substances not normally present may
42
appear in the urine, or normal constitutents may appear in abnormal amounts
Severalabnormalconstitutentsinurinethatmaybedetectedaspartofaurinalysis,
laboratory analysis of a urine sample are described below.
SUMMARY OF ABNORMAL CONSTITUENTS IN URINE :
Abnormal Constituents Comment
Albumin Normal constituent of plasma, but it usually
appears in only very small amounts in urine
because it is too large to pass through the pores
in capillary walls. The presence of excessive
albumin in the urine, albuminuria - indicates
an increase in the permeability of endothelial-
capsular membranes due to injury or disease,
increased blood pressure, or irritation of kidney
cells by substances such as bacterial toxins,
ether or heavy metals.
Glucose The presence of glucose in the urine
(glucosuria) usually indicates diabetes mellitus.
Occasionally, it may be caused by stress, which
can cause excessive amounts of epinephrine to
be secreted. Epinephrine stimulates the
breakdown of glycogen and liberation of
glucose from the liver.
Red blood cells
(erythrocytes)
Red blood cells in the urine is called
hematuria and general lly indicates a
- 4 3 -
Abnormal Constituents
White blood cells
(leukocvtes)
Ketone bodies
Bilirubin
Urobilinosen
Comment
pathological condition. One cause in acute
inflammation of the urinary organs as a result
of disease or irritation from kidney stones.
Other causes include tumors, and kidney
disease. One should make sure the urine
sample was not contaminated with menstrual
blood from the vagina.
The presence of white blood cells and other
components of pus in the urine, referred to as
pyuria (piyou-re-a), indicates infection in the
kidney or other urinary organs.
High quantities of ketone bodies, called
ketosis (acetonuria), may indicate diabetes
mellitus, starvation or simply too little
carbohydrate in the diet.
When red blood cells are destroyed by
reticuloendothelial cells, the globin portion of
hemoglobin is split off and the heme is convered
to biliverdin. Most of the biliverdin is
converted to bilirubin, which gives bile its
major pigmentation. An above normal level of
bilirubin in urine is called bilirubinuria.
The presennce of urobilinogen ( breakdown
-44
Abnormal Constituents
Casts
Microbes
Comment
product of hemoglobin) in urine is called
urobilinogenuria. Traces are normal, but
increased urobilinogen may be due to
hemolytic and pernicious anemia, infectious
hepatits, biliary obstruction, jaundice,
cirrhosis, congestive heart failure, or
infectious mononucleosis.
Casts are tiny masses of material that have
hardened and assumed the shape of the lumen
of a tubule in which they formed. They are
then flushed out of the tubule when filtrate
builds up behind them. Casts are named after
the cells or substances that compose them or
after their appearance. For example, there are
white blood cells casts, red blood cell casts,
and epithelial casts that contain cells from the
walls of the tubules.
The number and type of bacteira vary with
specific infections in the urinary tract. The
most common fungus to appear in urine is
Candida albicans, a cause of vaginitis. The
most frequent protozoan seen is Trichomonas
vaginalis, a cause of vaginitis in females and
urethritis in males.
45-
WATER BALANCE AND URINE OUTPUT - Water is taken into the
body through the alimentary tract and a small amount is formed by the
metabolic processes. Water is excreted in saturated expired air, as a
constituent of the faeces, through the skin as sweat and as the main
constitutent of urine. The amount lost in expired air and in the faeces is
fairly constant and the amount of sweat produced is associated with the
maintenance of normal body temperature.
The balance between fluid intake and output is controlled by the kidneys.
The minimum urinary output, consistent with the essential removal of waste
material, is about 500 ml. per day. The amount produced in excess of this is
controlled mainly by the antidiuretic hormone (ADH) released into the blood
by the posterior lobe of the pituitary gland. There is a close link between the
posterior pituitary and the hypothalamus in the brain.
There are cells in the hypothalamus {osmoreceptors) sensitive to
changes in the osmotic pressure of the blood. Nerve impulses from the
osmoreceptors stimulate the posterior lobe of the pituitary gland to release
ADH. When the osmotic pressure is raised, ADH output is increased and as
a result, water reabsorption is increased, reducing the blood osmotic
pressure and ADH output. This feedback mechanism maintains the blood
concentration within normal limits.
46
Blood osmotic pressure raised
i
Inhibition of
^ Osmoreceptors in hypothalamus
Posterior pituitary stimulated
Antidiuretic hormone secretion increased
i Water reabsorption by kidneys
i ._^ Blood asmotic pressure lowered
Feedback mechanism for the control of antidiuretic harmone (ADH)
secretion.
The feedback mechanism may be overridden when there is an excessive
amount of a dissolved substance in the blood so that it can be removed from
the body, e.g. in diabetes mellitus when the blood glucose level is above the
transport maximum of the renal tubules, excess water is excreted with the
excess glucose. This polyuria may lead to dehydration in spite of increased
production of ADH but it is usually accompanied by acute thirst and
increased water intake.
ELECTROLYTE BALANCE - Changes in the concentration of
electrolytes in the body fluids may be due to changes in the amounts of
water or of electrolytes. There are several methods of maintaining the
balance between water and electrolyte concentration.
47-
SODIUM AND POTASSIUM CONCENTRATION - Sodium is the
mostcommoncation(positively charged ion) inextracellularfluidandpotassium
is the most common intracellular cation.
Sodium is a constituent of almost all foods and it is often added to food
during cooking. This means that the intake is usually in excess of the body's needs.
It is excreted mainly in urine and sweat.
Sodium is a normal constituent of urine and the amount excreted is
regulated by the hormone aldosterone, secreted by the cortex of the adrenal gland
(suprarenal gland). Cells in the afferent arteriole of the nephron are stimulated
to produce the enzyme renin by sympathetic nerves or by low arterial blood
pressure. Renin converts angiotensinogen, produced by the liver, to angiotensin
which stimulates the adrenal gland to secrete aldosterone. Water is reabsorbed
with sodium and together they increase the blood volume, leading to reduced
renin secretion. When sodium reabsorption is mcvQdiSedpotassium excretion is
increased, indirectly reducing intracellular potassium. The amount of sodium
excreted in sweat is insignificant except when sweating is excessive. This may
occur when there is a high environmetal temperature or during sustained
physical exercise. Normally the renal mechanism described above maintains the
cation concentration within physiological limits. When excessive sweating is
sustained, e.g., living in a hot climate or working in a hot environment,
acclimatisation occurs in about 7 to 10 days and the amount of electrolyte lost
in sweat is reduced.
Sodium and potassium occur in high concentrations in digestive juices -
sodium in gastric juice and potassium in pancreatic and intestinal juice.
Normally these ions are reabsorbed by the colon but following acute and
- 4 8 -
prolonged diarrhoea they may be excreted in large quantities with resultant
electrolyte imbalance.
In order to maintain the normal blood pH, hydrogen ions are secreted
by the cells of the convoluted tubules and are excreted in urine. They are
secreted in combination with bicarbonate as carbonic acid, with ammonia as
ammonium chloride and with hydrogen phosphate as dihydrogen phosphate.
The normal pH of urine varies from 4.5 to 7.8 depending on diet, time of
day and a number of other factors.
Summary of the relationship between renal blood flow and selective
reabsorption by the nephron.
Kidney blood flow decreased
i
Inhibition of
-> Renin secretion by kidneys
i Renin + angiotensinogen = angiotensin
i
Adrenal cortex stimulated
Aldoserone secretion increased
i Sodium and water reabsorption increased
Potassiun excretion increased
i Blood volume increased
i ~> Blood flow to kidneys increased
49-
URETERS - Once urine is formed by the nephrons and passed into
collecting ducts, it drains through papillary ducts into the calyces surrounding
the renal papillae. The minor calyces join to become major calyces that unite to
form the renal pelvis. Form the pelvis, the urine drains into the ureters. Peristaltic
contractions carry urine to the urinary bladder. From the urinary bladder, the urine
is discharged from the body through the single urethra.
Urine that flows into a calyx changes little or not at all during the rest of
its passage in the body. The few bacteria that are present may multiply and some
dead epithelial cells may slough from the walls of the rest of the urinary system.
Structure - There are two ureters one for each kidney. Each ureter is an
extension of the pelvis of the kidney and stretches 25 to 30 cm. (10 to 12 in.)
to the urinary bladder. As the ureters descend, their
thick walls increase in diameter, but at their widest
point they measure less than 1.7 cm. (0.7 in) in
diameter. Like the kidneys the ureters are
retroperitoneal in placement. The ureters enter the
urinary bladder medially from the posterior aspect.
Although there is no anatomical valve at the
opening of each ureter into the urinary bladder, there
is a functional one that is quite effective. The ureters
pass obliquely through the wall of the urinary bladder.
As the urinary blader fills with urine, pressure inside
&P \URnEBO-pELVIC ' * JUHCTION
CnOSSING "WE ILIAC ARTEBV
JUnAPOSITKIK WVASCmiKliS
ORBROAO SAMENT
ENTERING BUDDEftWAll-'
URETERIC ORIFICE
Normal anatomical narrowings of the ureter
compresses the ureteral openings and prevents backup of urine into the
50-
ureters. When this physiological valve is not operating, it is possible for cystitis
(urinary bladder inflammation) to develop into kidney infection.
Histology - Three coats of tissue form the wall of the ureters. The inner
coat, or mucosa, is a mucous membrane with transitional epithelium. The solute
concentration and pH of urine differ drastically from the internal environment
of cells that form the walls of the ureters. Mucus secreted by the mucosa prevents
the cells from coming in contact with urine. Throughout most of the length of
the ureters, the second or middle coat, the muscularis, is composed of inner
longitudinal and outer circular layers of smooth muscle. The muscularis of the
distal third of the ureters also contains an outer layer of longitudinal muscle.
Peristalsis is the major function of the muscularis. The third, or external, coat
of the ureters is the serosa, a layer of fibrous connective tissue. Extensions of
the serosa anchor the ureters in place.
Physiology - The principal function of the ureters is to transport urine from
the renal pelvis intothe urinary bladder. Peristaltic contractions of the muscular
walls of the ureters push urine toward the bladder, but hydrostatic pressure and
gravity also contribute. Peristaltic waves pass from the kidney to the urinary
bladder, varying in rate from one to five per minute, depending on the rate of
urine formation.
URINARY BLADDER - The urinary bladder is a hollow muscular organ
situated retroperitoneally in the pelvic cavity posterior to the pubic symphysis.
In the male, it is directly anterior to the rectum. In the female, it is anterior to
the vagina and inferior to the uterus. It is a freely movable organ held
in position by folds of the peritoneum. The shape of the urinary bladder
-51 -
depends on how much urine
it contains. Empty, it is
collapsed. It becomes
spherical when slightly
distended. As urine volume
increases, it becomes pear
shaped and rises into the
abdominal cavity. In general
Z>»v\/mi^ lnv*t.t
vf«trw»lor«c«
Cnrftffii/ suction, vietved frttrn anterior
Female urinary binddcr and urethra.
bladder capacity is smaller in females because the uterus occupies space just
above the bladder.
Structure - In the floor of the urinary bladder is a small triangular area,
the trigone. The two posterior corners of the trigone contain the two ureteral
openings and the opening into the urethra (internal urethral orifice) lies in
the anterior corner. Because its mucosa is firmly bound to the muscularis,
the trigone has a smooth appearance.
Histology - Three coats make up the wall of the urinary bladder. The
mucosa, the innermost coat, is a mucous membrane composed of
transitional epithelium and an underlying lamina propria (connective
tissue). Transitional epithelium is able to stretch - a marked advantage for
an organ that must continually inflate and deflate. Rugae (fold in the
mucosa) are also present. Surrounding the mucosa is a muscular layer called
the detrusor muscle. It consists of three layers of smooth muscle : inner
longitudinal, middle, circular and outer lingitudinal. Around the opening to
-52
the urethra, the circular fibers form an internal urethral sphincter muscle. Below
the internal sphincter is the external urethral sphincter, which is composed of
skeletal muscle and is a modification of the urogenital diaphragm muscle. The
outermost coat is formed by the peritoneum on the superior surface of the bladder.
The rest of the urinary bladder has a fibrous connective tissue coat (serosa) that
is continuous with the same coat of the ureters.
Physiology - Urine is expelled from the urinary bladder by an act called
micturition commonly known as urination or voiding. This response is brought
about by a combination of involuntary and voluntary nerve impulses, the average
capacity of the urinary bladder is 700 to 800 ml. When the amount of urine in
the urinary bladder exceeds 200 to 400 ml. stretch receptors in the wall transmit
nerve impulses to the lower portion of the spinal cord. These impulses, by way
of sensory tracts to the cortex, initiate a conscious desire to expel urine and, by
way of a center in the sacral spinal cord, a reflex called the micturition reflex.
Parasympathetic impulses from the micturition reflex center of the spinal cord
conduct to the urinary bladder wall and internal urethral sphincter. They cause
contractionofthedetrusormuscleandrelaxationofthe internal urethral sphincter.
Then the cerebral cortex of the brain permits voluntary relaxation of the external
urethral sphinter, and urination takes place. Although emptying the urinary
bladder is a reflex, it may be initiated volunatarily and stopped at will because
of cerebral cortical control of the external urethral sphincter and certain muscles
of the urogenital (pelvic) diaphragm.
53-
URETHRA - The urethra is a small tube leading from the floor of the
urinary bladder to the exterior of the body. In females, it lies directly posterior
to the pubic symphysis and is in front of the anterior wall of the vagina. Its
undilated diameter is about 6 mm (0.25 in.), and its length is approximately
3.8cm.(1.5in.).Thefemaleurethraisdirectedobliquely,inferiorly,andanteriorly.
The opening of the urethra to the exterior, the external urethral orifice, is located
between the clitoris and vaginal opening.
In males, the urethra is about 20 cm. (8 in.) long. Immediately below the
urinary bladder it passes vertically through the prostate gland (prostatic urethra),
then pierces the urogenital diaphragm (membranous urethra), and finally pierces
the penis (spongy urethra) and takes a curved course through its body.
Histology - The wall of the female urethra consists of three coats : an
inner mucouscoat, an intermediate thin layerof spongy tissue containingaplexus
of veins, and an outer muscular coat that is continus with that of the urinary
bladder and consists of circularly arranged smooth muscle fibers. The mucosa
is usually lined with transitional epithelium near the urinary bladder. The
remainder consists of stratified squamous epithelium with areas of stratified
columnar or pseudostratified epithelium.
The male urethra is composed of two coats; an inner mucous membrane
and an outer submucous tissue that connects the urethra with the structures
through which it passes. The mucosa varies in different regions. The mucosa
of the prostatic urethra is continuous with that of the urinary bladder and is
lined by transitional epithelium. The mucosa of the membranous urethra is
54
lined by pseudostratified epithelium. The spongy urethra is lined mostly by
pseudostratified epithelium.Near itsopeningtotheexterior it islined by stratified
squamous epithelium. In the spongy urethra, especially, there are glands, called
urethral (Littre) glands, that produce mucus for lubrication during sexual
intercourse.
Physiology - The urethra is the terminal portion of the urinary system. It
serves as the passageway for discharging urine from the body. The male urethra
also serves as the duct through which reproductive fluid (semen) is discharged
from the body.
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