THE URINARY SYSTEM
THE URINARY SYSTEM
 Urinary system
consists of:
Two kidneys
 Two Ureters
 The Bladder
 The Urethra

GENERAL FUNCTION OF THE KIDNEYS
 Kidneys are organs that:
Filters plasma and plasma constituents from the
blood – filtrate
 Then selectively reabsorb water and useful
constituents back from the filtrate.
 Ultimately excreting excesses and plasma waste
products

LOCATION OF THE KIDNEYS
 Located in the dorsal part of the abdominal
cavity on each side of the aorta and vena cava
just ventral to the first few lumbar vertebrae.
 In ruminants (filled rumen) the left kidney
maybe pushed to the right as far as the medial
plane or beyond.
 Thus, much more loosely attached to the body
wall than the right kidney, and consequently the
left renal artery and vein are longer than the
right vessels.
LOCATION OF THE KIDNEYS
 Kidneys are retro-peritoneal – located outside
the peritoneal cavity.
 They are however more closely attached to the
abdominal wall by fascia, vessels and peritoneum
than other organs
ANATOMY OF THE KIDNEYS
 Shape of the kidneys
Somewhat bean-shaped in most domestic animals
 In cow the kidneys are lobulated
 Right kidney of the horse is heart shaped

ANATOMY OF THE KIDNEYS
 Medial boarder of the kidney is concave and has
a depression – renal hilus
At the renal hilus – blood vessels and nerves enter
 Ureter and lymphatic vessels leave

 Expanded origin of the ureter within the kidney
is called the renal pelvis
 Renal pelvis receives urine from the connecting
tubules of the kidney.
 Cavity within the kidney containing the renal
pelvis is called the renal sinus
ANATOMY OF THE KIDNEY
 Medulla - portion of the kidney immediately
surrounding the renal pelvis.
It appears striated because of the radially arranged
collecting tubules.
 These tubules form the basis for the renal pyramids
which have their apices at the renal pelvis and their
bases covered by the cortex.
 In addition to the collecting tubules, the medulla also
contain the loop of henle.

ANATOMY OF THE KIDNEY
 The Cortex – located between the medulla and
the thin, connective-tissue capsule.
Cortex has a granular appearance because of the
large number of renal corpuscles
 Proximal convuluted tubules and distal
convuluted tubules are also located in the cortex in
close relation to the glomeruli and many loops of
henle

BLOOD AND NERVE SUPPLY TO THE
KIDNEY
BLOOD AND NERVE SUPPLY TO THE
KIDNEY
 Blood supply is much more extensive than the
size of the kidneys would suggest

Two renal arteries may carry as much as 25% of total
circulating blood.
 Renal arteries enters the hilus – divide into
interlobar arteries
 Interlobar arteries pass peripherally between
pyramids towards the cortex – forming arciform
or arcuate arteries
BLOOD AND NERVE SUPPLY TO THE
KIDNEY
 Each arcuate artery – divide to form
interlobular arteries which give rise to
afferent arterioles
 Each afferent arteriole branches to form a
capillary network called the glomerulus
 Efferent arterioles leaves each glomerulus –
break up into capillary network that surrounds
the rest of the nephron.
BLOOD AND NERVE SUPPLY TO THE
KIDNEY
 Arterioles leaving the glomeruli
close to the
medulla send branches directly into the medulla
as arteriae rectae – these form capillary
network around the collecting tubules and loops
of henle
 Arcuate veins drain blood from the cortex and
medulla – pass through the medulla as interlobar
veins and enter the renal vein.
 Lymph drains from the kidney to the renal lymph
nodes.
BLOOD AND NERVE SUPPLY TO THE
KIDNEY
 Kidneys are supplied with sympathetic nerves
from the renal plexus, which follow the blood
vessels and terminate largely on glomerula
arterioles.
 Branches of the vagus nerve may also supply the
kidneys.
 Both vasoconstrictor and vasodilator nerves
are found in the kidneys
URETERS
 Ureter – muscular tube that conveys urine from
the pelvis of the kidney to the bladder.
 Each ureter passes caudad to empty into the
bladder near its neck – area known as the
trigone
 Ureter passes obliquely through the wall of the
bladder – an effective valve is formed to prevent
return flow of urine to the kidney.
URINARY BLADDER
 Urinary bladder – hollow muscular organ –
varies in size and position with the amount of
urine it contains.
 Empty bladder – thick walled pear shaped
structure located on the floor of the pelvis.
 Filled bladder – as it fills with urine, the walls
become thinner and most of the bladder is
displaced craniad toward the abdominal cavity.
URINARY BLADDER
 Neck of the bladder is continuous with the
urethra caudally – muscle of the bladder wall is
arranged in a circular manner at the neck of the
bladder – forming a sphincter that controls
passage of urine into the urethra.
TRANSITIONAL EPITHELIUM
 Pelvis, ureter, bladder and urethra are all lined
with transitional epithelium.
 Important in these areas where distention of the
lumen is required.
 Empty organ – lumen small – walls thick – lining
epithelial cells are piled deeply to form a manylayered stratification.
 Distended organ – lumen enlarged – thinner
walls – transition to a much lower stratification
of the lining.
URETHRA
 Pelvic urethra extends from the bladder to the
ischial arch.
 In the male it receives the ductus deferens and
the ducts from the accessory sex glands.
 The pelvic urethra continues as the penile
urethra which transfers the urine to the outside
MICTURITION
 Micturition – expulsion of urine from the
bladder.
 Reflex activity, stimulated by distension of the
bladder from constant intake of urine from the
ureter.
 The bladder expands to gradual inflow of urine
until the pressure becomes high enough to
stimulate reflex center in the spinal cord.
 Contraction of the bladder wall occurs by way of
the sacral parasympathetic nerves.
 Voluntary control of external sphincter
surrounding the neck of the bladder prevents
reflex empting.
THE NEPHRON
 The nephron – unit of structure and function of
the kidney
 It includes:
Glomerulus
 Glomerular capsule (bowman’s capsule)
 Proximal convoluted tubule
 Loop of henle
 Distal convoluted tubule (which is continued by the
collecting tubule)

NEPHRON
 Glomerulus – tuft of capillaries interposed on
the course of an arteriole.
 Glomerular capsule (Bowman’s capsule) –
expanded blind end of the tubule which is
evaginated around the glomerulus and almost
entirely surround it.
 The visceral (inner) layer of the glomerular
capsule closely surrounds the capillaries.
 The parietal (outer) layer of the glomerular
capsule is continuous with the proximal
convoluted tubule
NEPHRON
 The complex of glomerulus and inner and outer
layer of glomerular capsule is called a renal
(malpighian) corpuscle.
 The space between the inner and outer layers of
the glomerular capsule communicates with the
lumen of the proximal tubule.
 Renal corpuscle is the major site for filtration of
fluid from the blood.
 100 times as much blood passes through the filter
as is eventually excreted in the urine.
NEPHRON
 Blood pressure is important for effective
filtration
Blood pressure in the capillaries of the glomerulus
must remain relatively high
 Ensured by capillary being on the course of an artery
rather than between artery and vein.


Both afferent and efferent arteriole (entering and
leaving the glomerulus respectively) have smooth
muscle, constriction of one or both can be used to
regulate amount of blood entering the glomerulus
and the pressure within the glomerulus.
NEPHRON
 Blood pressure
Afferent arteriole (as it approaches the glomerulus) is
surrounded by a cuff of myoepithelial cells that has
characteristics of smooth muscle and epithelium –
juxtaglomerular cells
 Juxtaglomerular cells – site of renin production
which is secreted in the blood when:

Blood pressure in the afferent arterioles falls
 Na concentration of the plasma decreases
 Distal tubular osmolarity decreases
 Sympathetic nerve fibers innervating the afferent arteriole
are stimulated.

NEPHRON
 Renin
In the blood renin acts on an alpha globulin,
angiotensinogen to produce angiotensin.
 Angiotensin acts as a vasoconstrictor to increase
blood pressure
 Angiostensin also act on the adrenal gland to secrete
aldosterone.
 Aldosterone acts on kidney to conserve sodium ions

NEPHRON
 Each human kidney has approximately one
million nephrons
 Only 25% of these function at a given time under
normal conditions
 Relative constriction of afferent and efferent
arterioles determine whether or not a specific
nephron is functioning.
NEPHRON
 Proximal convoluted tubule, loop of Henle and
distal convoluted tubule are supplied by
capillaries from the efferent arterioles.
 Blood in the efferent arteriole has lost 20% of its
water, and is more concentrated (higher potential
osmotic pressure) and therefore is better able to
reabsorb water from the tubules.
NEPHRON
 Proximal convoluted tubule
Longest and most winding portion of the nephron
 Forms much of the tissue of the renal cortex
 Proximal tubule reabsorbs most of the constituents of
the glomerular filtrate that are needed by the animal
body – including 7/8 of the sodium chloride and
water.
 Facilitated by the more concentrated blood.
 In addition to selective reabsorption from the
glomerular filtrate – cells of the proximal segment
are able to secrete waste products from the blood into
the fluid passing through the lumen of the tubule.

NEPHRON
 Henle’s loop
Situated between the proximal convoluted tubule and
distal convoluted tubule
 U-shaped tube that continues from the proximal
tubule close to the glomerulus.
 Descending limb is thin – extends variable distance
into the medulla where it turns back on itself as the
thick ascending limb of the Henle’s loop.
 Thin limb – simple squamous epithelial lining
 Thick limb – cubodial epithelial lining

NEPHRON
 Henle’s loop

Contains the most concentrated fluid, with the
highest concentration at the lowest part of the loop,
close to or within the medulla – due to
countercurrent mechanism
 Distal convoluted tubule
Shorter and less twisted than the proximal
 Extends from the termination of the ascending limb
of the Henle’s loop to the collecting tubule.

NEPHRON

Collecting tubules
Initial collecting tubules – referred to as the arched
tubules – empty into the straight collecting tubules in
the cortex of the kidney.
 Straight tubules enter the medulla and unite to form
papillary ducts which empty into the pelvis of the
kidney
 Lining cells move from cuboidal epithelium in the
arched tubules to columnar epithelium in the
papillary ducts.

FUNCTION OF THE KIDNEY
 Excretion of many waste products of the body.
Excretion – removal of waste products of metabolism
– waste products made by the body itself.
 Accumulation will prevent the maintenance of a
steady state.
 Excretion is different from egestion and secretion.
 Egestion is removal of undigested food from the gut
(defaecation)
 Secretion – release of useful substances e.g.
hormones

FUNCTION OF THE KIDNEY
 Maintenance of homeostasis – relative constant
condition of the internal environment of the body.
Water balance
 pH
 Osmotic pressure
 Electrolyte levels
 Concentration of many plasma substances

 Control is achieved by – filtration through the
glomerulus – and re-absorption passively
(osmosis and diffusion) or actively (tubular cell
transport)
FUNCTION OF THE KIDNEY
 Factors affecting the action of the kidneys
include:
Composition of the blood
 Arterial blood pressure
 Hormones
 Autonomic nervous system

ULTRAFILTRATION
 Ultrafiltration – filtration under pressure.
Occurs in the renal capsule
 Pressure derived from blood pressure (pumping
pressure or hydrostatic pressure)

 Diameter of capillaries in the glomerulus is much
less than that of the arteriole - pressure rises
 Water and small solute molecules are squeezed
out of the capillary and into the renal capsule.
 Large molecules like protein, RBC, platelets are
left behind in the blood
ULTRAFILTRATION
 Epithelium of renal capsule
Made up of cells highly modified for filtration –
podocytes
 Fit together loosely leaving slits called slit pores or
filtration slits (25 nm wide) – filtered fluid pass
through these slits.

 Filtered fluid in the capsule is called the
glomerular filtrate (GF)
 GF has chemical composition similar to that of
blood plasma – glucose, amino acids, vitamins,
ions, nitrogenous waste, hormones and water.
ULTRAFILTRATION
 Factors affecting the glomerular filtrate rate
(GFR)
Pressure of glomerular filtrate – lower than the
pressure in the blood
 Solute potential on either side of the filtration barrier
( water moves from less negative to more negative
solute potential i.e. less concentrated to more
concentrated solutions)
 Filtration rate can be increased by:

Raising blood pressure
 Dilating afferent arterioles (vasodilation)
 Constricting efferent arterioles (vasoconstriction)

ULTRAFILTRATION
 125 cubic cm of glomerular filtrate produced per
minute in humans = 180 cubic dm/day
 1.5 cubic dm of urine is produced daily so a great
deal of reabsorption must occur.
 80% of this reabsorption occur in the proximal
covoluted tubule.
SELECTIVE REABSORPTION IN THE
PROXIMAL CONVOLUTED TUBULES
 Proximal convoluted tubules are adapted for
reabsorption:
Large SA due to microvilli and basal channels
 Numerous mitochondria
 Closeness of blood capillaries

 80% of GF is absorbed here, including:
All glucose, amino acids, vitamins, hormones
 80% of sodium chloride and water

SELECTIVE REABSORPTION IN THE
PROXIMAL CONVOLUTED TUBULES (PCT)
 Mechanism of reabsorption
Glucose, amino acids and ions diffuse into the cells of
the PCT from the filtrate.
 They are actively transported (carrier proteins in cell
surface membranes) out of the cells into the spaces
between them and basal channels.
 Once in these spaces they enter the extremely
permeable blood capillaries by diffusion and are
transported away from the nephron.
 Constant removal of substances create a diffusion
gradient – further substances passes

SELECTIVE REABSORPTION IN THE
PROXIMAL CONVOLUTED TUBULES
 Mechanism of reabsorption
As sodium and other ions are reabsorbed (tubular
filtrate (less negative solute potential) water leave
the tubular filtrate by osmosis.
 40-50% of urea is reabsorbed by diffusion – not
needed but harmless – remainder excreted in urine.

SELECTIVE REABSORPTION IN THE
PROXIMAL CONVOLUTED TUBULES (PCT)
 Mechanism of reabsorption
Small protein – removed by pinocytosis at base of
microvilli – hydrolytic enzymes from lysosomes digest
proteins – resulting amino acids are used by the
tubules or passed to the capillaries by diffusion.
 Active secretion of some substances such as creatine
and urea occur out of capillaries – they are
eventually removed in the urine.

LOOP OF HENLE
 Function
To conserve water.
 Longer the loop of Henle, the more concentrated the
urine that could be produced.
 Useful adaptation to life on land ( beaver vs
Kangaroo rat)

 Loop of Henle together with vasa recta and
collecting ducts – create and maintain osmotic
gradient in medulla (less concentrated at the
cortex to more concentrated at the tips of the
pyramids).
LOOP OF HENLE
 Water leaves the nephron by osmosis in response
to this gradient – concentrating the urine in the
nephron.
 Descending limb – thin wall, highly permeable to
water and most solutes
 Ascending limb – impermeable to water
 Cells in thick walled ascending limb actively
reabsorb sodium, chloride, potassium and other
ions.
 Fluid in ascending limb becomes very dilute by
the time it reaches the distal convoluted tubule.
DISTAL CONVOLUTED TUBULE (DCT) AND
COLLECTING DUCT (CD)
 In these last two regions, fine tuning of the body
fluid composition is achieved
 Roles DCT
Fine control of precise amounts of water and salts important in osmoregulation
 Control of blood pH

OSMOREGULATION AND ADH
 ADH – antidiuretic hormone – allows precise
control of solute potential
 Diuresis – production of large amounts of dilute
urine. Antidiuresis is the opposite.
 ADH is antidiuretic in its effect – concentrates
urine.
 ADH – peptide also known as vasopressin
 ADH – produced in the hypothalamus
OSMOREGULATION AND ADH
 When blood becomes more concentrated (solute
potential more negative)
When too little water is consumed
 Excessive sweating has occurred
 Large amounts of salt has been eaten

 Osmoreceptors in hypothalamus detect a fall in
blood solute potential – signal is sent to posterior
pituitary to release ADH
 ADH is sent to kidney where it increases
permeability of distal convoluted tubule and
collecting duct to water.
OSMOREGULATION AND ADH
 Increased permeability is achieved by increasing
the number of water channels in the membrane
lining the tubules.
 When ADH secretion is stopped the process goes
into reverse – reduction of water channels.
OSMOREGULATION AND ADH
 In the presence of ADH
Increased water channels
 Water moves from GF into cortex and medulla by
osmosis
 Water is carried away in the blood
 Volume of urine is reduced and concentrated

OSMOREGULATION AND ADH
 ADH also increases permeability of the collecting
duct to urea
Urea diffuses out of the urine into the tissue fluid of
the medulla
 Increases the osmotic concentration
 Resulting in the removal of increased volume of
water from the descending limb

OSMOREGULATION AND ADH
 Opposite occurs with high intake of water
Less negative solute potential of the blood
 ADH release is inhibited
 Walls of DCT and CD become impermeable to water
 Less water is reabsorbed from the filtrate
 Large volume of dilute urine is excreted.

CONTROL OF BLOOD SODIUM LEVEL
 Maintaining blood plasma level at a steady state
is controlled by aldosterone (steroid hormone)
 It also influences water absorption
 Secreted by the cortex (outer portion of adrenal
gland)
 Decrease in blood sodium leads to a decrease in
blood volume because less water enters the blood
by osmosis.
 Results in a decrease in blood pressure
CONTROL OF BLOOD SODIUM LEVEL
 Decrease in pressure and volume stimulates the
juxtaglomerular complex - release renin
 Renin activates angiotensin – releases
aldosterone from the adrenal cortex.
 Aldosterone acts on the DCT – stimulating
sodium-potassium pumps – more sodium is
pumped out of the filtrate in the DCT into the
blood capillaries around the DCT
 K+ moves in the opposite direction – active
transport
CONTROL OF BLOOD SODIUM LEVEL
 Aldosterone also
Stimulate sodium absorption in the gut
 Decrease loss of sodium in sweat
 Both effects assist in raising blood sodium level

 All together more water enters the blood by
osmosis, increasing its volume and hence its
pressure.
CONTROL OF BLOOD PH
 Normal pH of blood plasma is 7.4 – must be kept
in narrow limits as enzymes could become
denatured – fatal
 Body produces more acids than bases – need to
reduce acidity.
 Production of carbon dioxide increases acidity –
carbonic acid (H₂CO₃)
 Carbonic acid – dissociate into hydrogen ions
and hydrogen-carbonate ions
CONTROL OF BLOOD PH
 Buffers – chemicals that resist pH changes
 Hydrogencarbonate ions can act as buffer.
 Hydrogencarbonaate and phosphate buffers help
to prevent acidity.
 If the blood begins to become too acid
Hydrogen ions are actively transported from the
blood to the tubules of the collecting ducts.
 If source of the hydrogen ions is carbon dioxide, then
hydrogen carbonate will move from tubule to blood by
diffusion

CONTROL OF BLOOD PH
 Fall in pH also stimulates the kidney cells to
produce ammonium ions (NH₄⁺), which combines
with acids brought to the kidney – excreted as
ammonium salts.
PATHOLOGY OF URINARY SYSTEM
 Nephritis – general term for inflammation of the
kidneys – all or part of the nephrons, connective
tissues or renal vessels may be affected.
 Nephrosis – non-inflammatory kidney disease

Degeneration of tubules resulting in lowered albumin
in the blood, albumen in the urine and edema –
caused by toxins such as salts and heavy metals.
 Uremia – urine in the blood. Term used in
conjunction with kidney failure.
PATHOLOGY OF THE URINARY SYSTEM
 Urinary calculi – also called uroliths – stones in
the urinary system.
 Kidney failure
Result in the need for kidney transplant
 Or – haemodialysis – use of artificial membrane in
a kidney machine
 Or – peritoneal dialysis – use of a natural
membrane in the patients own body – peritoneum.
