The urinary system

THE URINARY SYSTEM
NIKITA SHARMA
LECTURER
BECON

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
lumbar vertebra, receiving some protection from the lower rib cage.
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, 3 cm
thick and weigh 150 gram.
They are embedded in, and held in position by, a mass of fat.
A sheath of fibrous connective tissue, the renal fascia, encloses the
kidney and the renal fat.
KIDNEYS

As the kidneys lie on either side of the vertebral column, each is
associated with different structures.
RIGHT KIDNEY
SUPERIORLY: The right adrenal gland
ANTERIORLY: The right lobe of the liver, the duodenum and the
hepatic flexure of the colon.
POSTERIORLY: The diaphragm, and muscles of the posterior
abdominal wall.
ORGANS ASSOCIATED WITH THE
KIDNEYS

LEFT KIDNEY
SUPERIORLY: The left adrenal gland
ANTERIORLY: The spleen, stomach, pancreas, jejunum and splenic
flexure of the colon.
POSTERIORLY: The diaphragm and muscles of the posterior
abdominal wall.
KIDNEYS

GROSS STRUCTURE OF THE KIDNEY
• There are three areas of tissue that can be distinguished when a
longitudinal section of the kidney is viewed with the naked eye.
• An outer fibrous capsule, surrounding the kidney
• The cortex, a reddish-brown layer of tissue immediately below the
capsule and outside the renal pyramids.
• The medulla, the innermost layer, consisting of pale conical-shaped
striations, the renal pyramids.

• The hilum, is the concave medial border of the kidney where the
renal blood and lymph vessels, the ureter and nerves enter.
• Urine formed with in the kidney passes through a renal papilla at the
apex of a pyramid into a minor calyx.
• Several minor calyces merge into a major calyx and two or three
major calyces combine forming the renal pelvis, a funnel shaped
structure that narrows when it leaves the kidney as the ureter.
• The walls of the calyces and renal pelvis are lined with transitional
epithelium and contain smooth muscles.

• Peristalsis, intrinsic contraction of smooth muscle, propels urine
through the calyces, renal pelvis and ureters to the bladder.

MICROSCOPIC STRUCTURE OF THE
KIDNEY
 The kidney contains about 1-2 million functional units, the
nephrons, and a much smaller number of collecting ducts.
The collecting ducts transport urine through the pyramids to the
calyces, giving the pyramids their striped appearance.
The collecting ducts are supported by connective tissue, containing
blood vessels, nerves and lymph vessels.

THE NEPHRON
The nephron is essentially a tubule closed at one end that joins a
collecting duct at the other end.
The closed or blind end is indented to form the cup-shaped
glomerular capsule (Bowman’s capsule), which almost completely
encloses a network of tiny arterial capillaries, the glomerulus.
These resemble a coiled tuft. Continuing from the glomerular
capsule, the remainder of the nephron is about 3 cm long and
described in 3 parts:

1. The proximal convoluted tubule
2. The medullary loop (loop of Henle)
3. The distal convoluted tubule, leading into a collecting duct.
The collecting ducts unite, forming layer ducts that empty into the minor
calyces.
The kidneys receive about 20% of the cardiac output. After entering the
kidney at the hilum, the renal artery divides into a smaller arteries &
arterioles.
THE NEPHRON

• In the cortex an arteriole, the afferent arteriole, enters each
glomerular capsule and then subdivides into a cluster of tiny arterial
capillaries, forming the glomerulus.
• Between these capillary loops are connective tissue phagocytic
mesangial cells, which are part of the monocyte-macrophage defence
system.
• The blood vessels leading away from the glomerulus is the efferent
arteriole.
THE NEPHRON

• The afferent arteriole has a greater diameter than the efferent
arteriole, which increases pressure inside the glomerulus and drives
filtration across the glomerular capillary walls.
• The efferent arteriole divides into a second peritubular capillary
network, which wraps around the remainder of the tubule, allowing
exchange between the fluid in the tubule and the blood stream.
• This maintains the local supply of oxygen and nutrients and removes
waste products.
THE NEPHRON

• Venous blood drained from this capillary bed eventually leaves the
kidney in the renal vein, which empties into the inferior vena cava.
• The walls of the glomerulus and glomerular capsule consist of a
single layer of flattened epithelial cells. The glomerular walls are
more permeable than those of other capillaries.
• The remainder of the nephron and the collecting duct are formed by a
single layer of squamous epithelium.
THE NEPHRON

• Renal blood vessels are supplied by both sympathetic and
parasympathetic nerves.
• The presence of both divisions of the autonomic nervous system
controls renal blood vessel diameter and renal blood flow
independently of autoregulation.
THE NEPHRON

FUNCTIONS OF THE KIDNEY
Formation of urine
Water balance and urine output
Electrolyte balance
pH balance

I. FORMATION OF URINE
• The kidneys form urine, which passes to the bladder for storage prior
to excretion.
• The composition of urine reflects exchange of substances between the
nephron and the blood in the renal capillaries.
• Waste products of protein metabolism are excreted, water and
electrolyte levels are controlled and pH is maintained by excretion of
hydrogen ions.
• There are 3 processes involved in the formation of urine:

FORMATION OF URINE
1. Filtration
2. Selective reabsorption
3. Secretion

1. FILTRATION
• This takes place through the semipermeable walls of the glomerulus
and glomerular capsule.
• Water and other small molecules readily pass through, although some
are reabsorbed later.
• Blood cells, plasma proteins and other large molecules are too large
to filter through and therefore remain in the capillaries.
• The filtrate in the glomerulus is very similar in composition to plasma
with the important exceptions of plasma proteins and blood cells.

FILTRATION
• Filtration takes place because there is a difference between the blood
pressure in the glomerulus and the pressure of the filtrate in the
glomerular capsule.
• Because the efferent arteriole is narrower than the afferent arteriole, a
capillary hydrostatic pressure of about 7.3 Kpa (55mmHg) builds up
in the glomerulus.
• This pressure is opposed by the osmotic pressure of the blood,
provided mainly by the plasma proteins, about 4KPa (30mmHg) and
by filtrate hydrostatic pressure of about 2KPa (15mmHg) in the
glomerular capsule.

FILTRATION
• The net filtration pressure is, therefore:
7.3- (4+2) = 1.3 Kpa
OR
55- (30+15) = 10
• The volume of filtrate formed by both kidneys each minute is called
the GFR.
• In a healthy adult the GFR is about 125mL/min, i.e. 180 liters of
filtrate are formed each day by the 2 kidneys.

FILTRATION
• Nearly all of the filtrate is later reabsorbed from the kidney tubules
with less than 1%, i.e. 1-1.5 L, excreted as urine.
• The differences in volume and concentration are due to selective
reabsorption of some filtrate constituents and tubular secretion of
others.

(A) AUTOREGULATION
Renal blood flow and therefore glomerular filtration, is protected by
a mechanism called autoregulation, whereby the renal blood flow is
maintained at a constant pressure across a wide range of systolic
pressures (from around 80-200 mmHg).
Autoregulation operates independently of nervous control, i.e. if the
nerve supply to the renal blood vessels is interrupted, autoregulation
continues to operate.
It is therefore a property inherent in renal blood vessels; it may be
stimulated by change in blood pressure in the renal arteries or by
fluctuating levels of certain metabolites, e.g. prostaglandins.

In severe shock, when the systolic blood pressure falls below 80
mmHg, autoregulation fails and renal blood flow and the hydrostatic
pressure decreases, impairing filtration with in the glomeruli.
AUTOREGULATION

2. SELECTIVE REABSORPTION
oMost reabsorption from the filtrate back into the blood takes place in
the proximal convoluted tubule, whose walls are lined with microvilli
to increase surface area for absorption.
oMany substances are reabsorbed here, including the some water,
electrolytes and organic nutrients such as glucose.
oSome reabsorption is passive, but some substances, e.g. glucose, are
actively transported.
oOnly 60-70% of filtrate reaches the medullary loop.

• Much of this, especially water, sodium and chloride, is reabsorbed in
the loop, so that only 15-20% of the original filtrate reaches the distal
convoluted tubule, and the composition of the filtrate is now very
different.
• More electrolytes are reabsorbed here, especially sodium, so the
filtrate entering the collecting ducts is actually quite dilute.
• The main function of the collecting ducts is to reabsorb as much
water as the body needs.
SELECTIVE REABSORPTION

• Active transport takes place at carrier sites in the epithelial
membrane, using chemical energy to transport substances against
their concentration gradient.
• Some ions, e.g. sodium and chloride, can be absorbed by both active
and passive mechanisms depending on the site in the nephron.
• Some constituents of glomerular filtrate (e.g. glucose, amino acids)
do not normally appear in urine because they are completely
reabsorbed unless blood levels are excessive.

• Reabsorption of nitrogenous waste products, such as urea, uric acid
and creatinine is very limited.
• The kidneys maximum capacity for reabsorption of a substance is the
transport maximum or renal threshold.
For example: the normal blood glucose level is 3.5-8mmol/L (63-
144mg/100 mL) and if this rises above the transport maximum of about
9 mmol/L (160mg/100 mL), glucose appears in the urine.
• This occurs because all the carrier sites are occupied and the
mechanism for active transport out of the tubules is overloaded.

• Other substances reabsorbed by active transport include sodium,
calcium, potassium, phosphate and chloride.
• The transport maximum, or renal threshold, of some substances
varies according to body need at a particular time, and in some cases
reabsorption is regulated by hormones:

(A) HORMONES THAT INFLUENCE
1. PARATHYROID HORMONE: this is secreted by the parathyroid
gland and together with calcitonin from the thyroid gland regulates
the reabsorption of calcium and phosphate from the distal
collecting tubules, so that normal blood levels are maintained.
Parathyroid hormone increases the blood calcium level and
calcitonin lowers it.
2. ADH: This is secreted by the posterior pituitary. It increases the
permeability of the distal convoluted tubules and collecting tubules,
increasing water reabsorption. Secretion of ADH is controlled by a
negative feedback system.

3. ALDOSTERONE: Secreted by the adrenal cortex, this hormone
increases the reabsorption of sodium and water, and the excretion of
potassium. Secretion is regulated through a negative feedback system.
HORMONES THAT INFLUENCE

4. ANP: this hormone is secreted by the atria of the heart in response to
stretching of the atrial wall when blood volume is increased. It
decreases reabsorption of sodium and water from the proximal
convoluted tubules and collecting ducts. Secretion of ANP is also
regulated by a negative feedback system.
HORMONES THAT INFLUENCE

3. TUBULAR SECRETION
Filtration occurs as blood flows through the glomerulus. Substances
not required and foreign materials, e.g. drugs including penicillin and
aspirin, may not be entirely filtered out of the blood because of the
short time it remains in the glomerulus.
Such substances are cleared by secretion from the peritubular
capillaries into the filtrate with in the convoluted tubules.
Tubular secretion of hydrogen ions is important in maintaining
normal blood pH.

COMPOSITION OF URINE
• Urine is clear and amber colour due to presence of urobilin, a bile
pigment altered in the intestine, reabsorbed then excreted by the
kidneys.
• The specific gravity is between 1020 and 1030 and the pH is around
6 (normal range = 4.5-8).
• A healthy adult passes from 1000 to 1500 mL/day.
• The volume of urine produced and the specific gravity vary according
to fluid intake and the amount of solute excreted.

• The constituents of urine are:
- Water = 96%
- Urea = 2%
- Creatinine, uric acid, ammonia, sodium, potassium = 2%
- Chlorides, phosphates, sulphates, oxalates = 2%
COMPOSITION OF URINE

II. WATER BALANCE AND URINE
OUTPUT
• The source of most body water is dietary food and fluid although a
small amount is formed by cellular metabolism.
• Water is excreted as the main constituent of urine, in expired air,
faeces and through the skin as sweat.
• The amount lost in expired air and faeces is fairly constant; the
amount of sweat produced is associated with environmental and body
temperatures.

OUTPUT
• The balance between fluid intake and
output is controlled by the kidneys.
• The minimum urinary output, i.e. the
smallest volume to excrete body waste
products, is about 500 mL/day.
• Urinary volume in excess of this is
controlled mainly by ADH released into
the blood by the posterior pituitary
gland.

• This negative feedback mechanism may be over-riden even though
there may be an excessive amount of a dissolved substance in the
blood.
• E.g. in diabetes mellitus when blood glucose levels exceed the
transport capacity of the renal tubules the excess glucose remains in
the filtrate, drawing water with it.
• Large volumes of urine are passed, which may lead to dehydration
despite increased ADH secretion.
OUTPUT

• However, acute thirst and increased water intake usually compensate
for the polyuria, at least to some extent.
• When blood volume is increased, stretch receptors in the atria of the
heart are stimulated and cardiac muscle cells release ANP.
• This reduces reabsorption of sodium and water by the proximal
convoluted tubules and collecting ducts, meaning that more sodium
and water are excreted.
• In turn, this lowers blood volume and reduces atrial stretching and
through the –ve feedback mechanism ANP secretion is switched off.
OUTPUT

• Raised ANP levels also inhibit secretion of ADH and aldosterone,
further promoting loss of sodium and water.
OUTPUT

III. ELECTROLYTE BALANCE
Changes in the concentration of electrolytes in the body fluids may be
due to changes in:
• The body water content or,
• Electrolyte levels
Several mechanisms maintain the balance between water and
electrolyte concentration.

(A)SODIUM AND POTASSIUM BALANCE
• Sodium is the most common cation in extracellular fluid and
potassium is the most common intracellular cation.
• Sodium is the constituent of almost all foods and furthermore, salt is
often added to food during cooking.
• This means that intake is usually in excess of the body’s needs.
• It is excreted mainly in urine and sweat.
• The amount of sodium excreted in sweat is usually insignificant
unless sweating is excessive.

SODIUM AND POTASSIUM BALANCE
• This may occur when there is pyrexia, a higher environmental
temperature or during sustained physical exercise.
• Normally the renin-angiotensin-aldosterone mechanism maintains the
concentration of sodium and potassium with in physiological limits.
• When excessive sweating is sustained e.g. living in a hot climate or
working in a hot environment , acclimatization occurs in
approximately 7-10 days and electrolyte secretion lost in sweat is
reduced.

SODIUM AND POTASSIUM BALANCE
• 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 prolonged diarrhea they may be excreted in large quantities
causing electrolyte imbalance.

(B) RENIN-ANGIOTENSIN-
ALDOSTERONE SYSTEM
• Sodium is normally constituent of urine and its excretion is regulated
by the hormone aldosterone, secreted by the adrenal cortex.
• Cells in the afferent arteriole of the nephron release the enzyme renin
in response to sympathetic stimulation, low blood volume or by low
arterial blood pressure.
• Renin converts the plasma protein angiotensinogen, produced by the
liver, to angiotensin 1.
• ACE, formed in small quantities in the lungs, proximal convulated

RENIN-ANGIOTENSIN-ALDOSTERONE
SYSTEM
• Tubules and other tissues, converts angiotensin 1 in to angiotensin 2.,
which is very potent vasoconstrictor and increases blood pressure.
• Renin and raised blood potassium levels also stimulate the adrenal
gland to secrete aldosterone.
• Water is reabsorbed with sodium and together they increase the blood
volume, which reduces renin secretion through the negative feedback
mechanism.
• When sodium reabsorption is increased potassium excretion is
increased, indirectly reducing intracellular potassium.

RENIN-ANGIOTENSIN-ALDOSTERONE
SYSTEM
• Profound diuresis may lead to hypokalaemia.
(C) ANP: This hormone is also involved in the regulation of the
sodium levels.

(D) CALCIUM BALANCE
• Regulation of the calcium levels is maintained by the secretion of
parathyroid hormone and calcitonin.

IV. PH BALANCE
• In order to maintain normal pH, the proximal convoluted tubules
secrete hydrogen ions into the filtrate where they combine with
buffers.
• Bicarbonate, forming carbonic acid
• Ammonia, forming ammonium ions
• Hydrogen phosphate, forming dihydrogen phosphate
Carbonic acid is converted to CO2 and H2O and the CO2 is
reabsorbed, maintaining the buffering capacity of the blood.

PH BALANCE
• Hydrogen ions are excreted in the urine as ammonium salts and
hydrogen phosphate.
• The normal pH of urine varies from 4.5 to 8 depending on diet, time
of day and other factors.
• Individuals whose diet contains a large amount of animal proteins
tend to produce more acidic urine (lower pH) than vegetarians.

URETERS
• The ureters carry urine from the kidneys to the urinary bladder.
• They are about 25-30 cm long with a diameter of approximately 3
mm.
• The ureter is continuous with the funnel-shaped renal pelvis.
• It passes downwards through the abdominal cavity, behind the
peritoneum in front of the psoas muscle into the pelvic cavity, and
passes obliquely through the posterior wall of the bladder.

POSITION OF THE URETERS WHERE IT
PASSES THROUGH THE BLADDER WALL

• Because of this arrangement, as urine accumulates and the pressure in
the bladder rises, the ureters are compressed and the openings into the
bladder are occluded.
• This prevents backflow of urine into the ureters as the bladder fills
and also during micturition, when pressure increases as the muscular
bladder will contracts.
URETERS

STRUCTURE OF URETERS
• The wall of the ureters consist of 3 layers of tissues:

1. An outer covering of fibrous tissue, continuous with the fibrous
capsule of the kidney.
2. A middle muscular layer containing a interlacing smooth muscle
fibres that form a functional unit round the ureter and an additional
outer longitudinal layer in the lower third.
3. An inner layer, the mucosa, composed of transitional epithelium.
STRUCTURE OF URETERS

FUNCTIONS
Peristalsis is an intrinsic property of the smooth muscle layer that
propels urine along the ureter.
Peristaltic waves occur several times per minute, increasing in
frequency with the volume f urine produced, sending little spurts of
urine along the ureter towards the bladder.

URINARY BLADDER
• The urinary bladder is a reservoir for urine.
• It lies in the pelvic cavity and its size and position vary, depending on
the volume of urine it contains.
• When distended, the bladder rises into the abdominal cavity.

BLADDER

STRUCTURE OF THE BLADDER
• The bladder is roughly pear shaped, but becomes more balloon
shaped as it fills with urine.
• The posterior surface is the base.
• The bladder opens into the urethra at its lowest point, the neck.
• The peritoneum covers only the superior surface before it turns
upwards as the parietal peritoneum, lining the anterior abdominal
wall.

• Posteriorly it surrounds the uterus in the female and the rectum in the
male.
• The bladder wall is composed of 3 layers:
1. The outer layer of loose connective tissues, containing blood and
lymphatic vessels and nerves, covered on the upper surface by the
peritoneum.

2. The middle layer, consisting of interlacing smooth muscle fibres
and elastic tissues loosely arranged in the 3 layers. This is called the
detrusor muscle and when it contacts, it empties the bladder.
3. The inner mucosa, composed of transitional epithelium that
readily permits distension of the bladder as it fills.
When the bladder is empty the inner lining is arranged in folds, or
rugae, which gradually disappear as it fills.

• The bladder is distensible but as it fills, awareness of the need to pass
urine is felt.
• The total capacity is rarely more than 600 mL.
• The 3 orifices in the bladder wall form a triangle or trigone.
• The upper 2 orifices on the posterior wall are the openings of the
ureters; the lower orifice is the opening into the urethra.
• The internal urethral sphincter, a thickening of the urethral smooth

• Muscle layer in the upper part of the urethra, controls outflow of
urine from the bladder.
• This sphincter is not under voluntary control.

URETHRA
 The urethra is a canal extending from the neck of the bladder to the
exterior, at the external urethral orifice.
It is longer in the male than in the female.
The male urethra is associated with both the urinary and reproductive
systems.
The female urethra is approximately 4 cm long and 6 mm in
diameter.
It run downwards and forwards behind the symphysis pubis and
opens at the external urethral orifice just in front of the vagina.

URETHRA
The external urethral orifice is guarded by the external urethral
sphincter, which is under voluntary control.
The wall of the female urethra has 2 main layers: an outer muscle
layer and an inner lining of mucosa, which is continuous with that of
the bladder.
The muscle layer has 2 parts, an inner layer of smooth muscle that is
under autonomic nerve control, and an outer layer of striated
(voluntary) muscle surrounding it.
The striated muscle forms the external urethral sphincter and is
under voluntary control.

URETHRA
The mucosa is supported by loose fibro-elastic connective tissue
containing blood vessels and nerves.
Proximally it consists of transitional epithelium while distally it is
composed of stratified epithelium.

MICTURITION
In infants, accumulation of urine in the bladder activates stretch
receptors in the bladder wall generating sensory (afferent) impulses
that are transmitted to the spinal cord, where a spinal reflex is
initiated.
This stimulates involuntary contraction of the detrusor muscle and
relaxation of the internal urethral sphincter and expel urine from the
bladder- this is micturition or voiding of urine.

MICTURITION
• When bladder control is established, the micturition reflex is still
stimulated but sensory impulses also pass upwards to the brain and
there is awareness of the need to pass urine as the bladder fills
(around 300-400mL in adults).
• By learned and conscious effort, contraction of the external urethral
sphincter and muscles of the pelvic floor can inhibit micturition until
it is convenient to pass urine.

MICTURITION
Urination can be assisted by increasing the pressure with in the
pelvic cavity, achieved by lowering the diaphragm and contracting
the abdominal muscles.
Over-distension of the bladder is extremely painful, and when this
occurs there is a tendency for involuntary relaxation of the external
sphincter to occur allowing a small amount of urine to escape,
provided there is no mechanical obstruction.
Incontinence is the voluntary loss of urine after bladder control has
been established.

THE EFFECTS OF AGEING OF THE
URINARY SYSTEM
The kidneys have a substantial function reserve; the loss of one
kidney does not cause problems in an otherwise healthy individual.
The number of nephrons declines with age, GFR falls and the renal
tubules function les efficiently; the kidneys become less able to
concentrate urine.
These changes means the older adults become more sensitive to
alterations in fluid balance and problems associated with fluid
overload or dehydration are more prevalent.

URINARY SYSTEM
 Elimination of drugs also becomes less efficient with declining
kidney function which may lead to accumulation and toxicity.
The ability to inhibit contraction of the detrusor muscle declines and
may result in the urgent need to pass urine and urinary frequency.
Nocturia becomes increasingly common in older adults.
Incontinence is more prevalent in older adults affecting 15% of
women and 10% of men over 65, figures which double by 85 years of

URINARY SYSTEM
Enlargement of the prostate gland is common in older men and may
cause retention of urine and problems with micturition.

The urinary system

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