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

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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

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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

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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

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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.

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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

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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

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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.

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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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