Essentials of Human Anatomy & Physiology
Unit 12:
The Urinary System
Organs of the Urinary system
 Kidneys (2)
 Ureters (2)
 Urinary bladder (1)
 Urethra (1)
Location of the Kidneys
 Dimensions
 Reddish-brown, bean shaped
 12cm long, 6cm wide, 3cm thick
 High on posterior abdominal wall
 at the level of T12 to L3- superior lumbar region
 Retroperitoneal & against the dorsal body wall
 The right kidney is slightly lower than the left ,convex
laterally
 Attached to ureters, renal blood vessels, and nerves
at renal hilus (medial indention)
 Atop each kidney is an adrenal gland
Coverings of the Kidneys
 Adipose capsule
 Surrounds the kidney
 Provides protection to the kidney
 Helps keep the kidney in its correct location against
muscles of posterior trunk wall
 Ptosis-kidneys drop to a lower position due to rapid
fat loss, creating problems with the ureters.
 Ptosis can lead to hydronephrosis, a condition
where urine backs up the ureters and exerts
pressure on the kidney tissue.
 Renal capsule
 Surrounds each kidney
Regions of the Kidney
 Three regions of kidneys
 Renal cortex – outer region, forms
an outer shell
 Renal columns – extensions of
cortex- material inward
 Renal medulla – inside the cortex,
contains medullary (renal) pyramids
 Medullary pyramids – triangular
regions of tissue in the
medulla, appear striated
 Renal pelvis – inner collecting tube,
divides into major and minor
calyces
 Calyces – cup-shaped
structures enclosing the tips of
the pyramids that collect and
funnel urine towards the renal
pelvis
Functions of the Urinary System
 Elimination of waste products
 filtering gallons of fluid from the bloodstream every day
creating “filtrate”
 “filtrate” includes: metabolic wastes, ionic salts, toxins, drugs
 Maintenance of blood
 Red blood cell production- by producing hormone
erythropoietin to stimulate RBC production in bone
marrow
 Blood pressure (vessel size)- by producing renin which
causes vasoconstriction
 Blood volume (water balance)- ADH released from
Anterior Pituitary targets the kidney to limit water loss
when blood pressure decreases or changes in blood
composition
 Blood composition (electrolyte balance)- water follows
salt; aldosterone reclaims sodium to the blood
 Blood pH- regulates H+ ions and HCO3- ions
Blood Flow in the Kidneys
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Rich blood supply to filter blood and adjust blood composition
~¼ of blood supply passes through the kidneys each minute
Blood enters the kidneys under extremely high pressure
Renal artery arises from abdominal aorta, divides into Segmental
artery at hilus
 Inside renal pelvis, Segmental artery divides into Lobar artery, which
branch into Interlobar artery travelling thru the renal column to reach
the renal cortex
 At the medulla-cortex junction, the Interlobar artery curves over the
medullary pyramids as the Arcuate artery.
 Small Interlobular arterioles branch off of the Arcuate artery and
move away from the renal cortex and into the Nephron of the kidney
Blood Flow in the Kidneys
 The final branches of the interlobular arteries are called afferent
arterioles.
 Afferent arterioles lead to the glomerulus, a network of capillaries
that are involved in filtration.
 Leading away from the glomerulus, blood less filtrate travels through
the efferent arterioles and into the peritubular capillaries.
 From there, blood moves through similar veins that parallel the
arteries at their respective locations.
Nephrons
 The structural and functional units of the kidneys
 Over 1 million
 Responsible for forming urine
 Consist of renal corpuscle and renal tubule
 Renal corpuscle composed of a knot of capillaries
called the Glomerulus (a.k.a. Bowman’s Capsule)
 Renal tubule- enlarged, closed, cup-shaped end
giving rise to the PCT, dLOH, aLOH, DCT, and CD.
Glomerulus
 A specialized capillary bed fed and
drained by arterioles.
 Glomerular capillaries filter fluid from
the blood into the renal tubule
 GC is attached to arterioles on both
sides in order to maintain high pressure
 Large afferent arteriole-arises from
interlobular artery (feeder vessel);
large in diameter, high resistance
vessels that force fluid & solutes
(filtrate) out of the blood into the
glomerular capsule.
 99% of the filtrate will be reclaimed
by the renal tubule cells and
returned to the blood in the
peritubular capillary beds(blood
vessels surrounding renal tubule) .
 Narrow efferent arteriole-merges to
become the interlobular vein; draining
vessel.
Glomerulus
 Glomerular capillaries are covered with
podocytes from the inner (visceral) layer of
the glomerular capsule.
 Podocytes have long, branching
processes called pedicels that
intertwine with one another and cling to
the glomerular capillaries.
 Filtration slits between the pedicels
form a porous membrane around the
glomerular capillaries.
 The glomerular capillaries sit within a
glomerular capsule (Bowman’s capsule)
 Expansion of renal tubule
 Receives filtered fluid
 Renal tubule coils into the PCT, then
the dLOH, aLOH, DCT and finally, the
CD.
 Along the PCT, much of the filtrate is
reclaimed
Renal Tubule
 Glomerular (Bowman’s) capsule
enlarged beginning of renal tubule
 Proximal convoluted tubule- lumen
surface (surface exposed to filtrate) is
covered with dense microvilli to increase
surface area.
 The descending limb of the nephron Loop of Henle
 The ascending limb of the nephron coils
tightly again into the distal convoluted
tubule
 Many DCT’s merge in renal cortex to
form a collecting duct
 Collecting ducts not a part of nephron
 Collecting ducts receive urine from
nephrons and deliver it to the major
calyx and renal pelvis.
 CD run downward through the
medullary pyramids, giving them their
striped appearance.
Blood Supply of a Nephron
 Peritubular capillary
 Efferent arteriole braches into a second capillary bed
 Blood under low pressure
 Capillaries adapted for reabsorption instead of filtration.
 Attached to a venule and eventually lead to the interlobular
veins to drain blood from the glomerulus
 Cling close to the renal tubule where they receive solutes and
water from the renal tubule cells as these substances from the
filtrate are reabsorbed into the blood.
 Juxtaglomerular apparatus
 At origin of the DCT it contacts afferent and efferent arterioles
 Epithelial cells of DCT narrow and densely packed, called
macula densa
 Together with smooth muscle cells, comprise the
juxtaglomerular apparatus
 Control renin secretion & indirectly, aldosterone secretion
Types of Nephrons
 Cortical nephrons
 Located entirely in the cortex
 Includes most nephrons
 Juxtamedullary nephrons
 Found at the boundary of the cortex and medulla and
their LOH dip deep into the medulla.
Urine Formation Processes
 Filtration- Water & solutes
smaller than proteins are forced
through the capillary walls and
pores (of the glomerulus) into the
renal tubule (Bowman’s capsule).
 Reabsorption- Water, glucose,
amino acids & needed ions are
transported out of the filtrate into
the peritubular capillary cells and
then enter the capillary blood.
 Secretion- Hydrogen ions,
Potassium ions, creatinine & drugs
are removed from the peritubular
capillaries (blood) and secreted by
the peritubular capillary cells into
the filtrate.
Filtration
 Beginning step of urine formation
 Occurs at the glomerulus, nonselective passive process
 Water and solutes smaller than proteins are forced through
capillary walls of the glomerulus, which act as a filter.
 Fenestrations – (openings in glomerular walls) make
glomerulus more permeable than other arterioles.
 Podocytes cover capillaries, make membrane impermeable to
plasma proteins.
 Blood cells cannot pass
out to the capillaries; filtrate
is essentially blood plasma
w/o blood proteins, blood cells.
 Filtrate is collected in the
glomerular (Bowman’s) capsule
and leaves via the renal tubule
Filtration pressure
 Hydrostatic pressure of blood forces substances through
capillary wall.
 Net filtration pressure normally always positive
 Hydrostatic pressure of blood is greater than the hydrostatic
pressure of the glomerulus capsule and the osmotic
pressure of glomerulus plasma
 If arterial blood pressure
falls dramatically, the glomerular
hydrostatic pressure falls below
level needed for filtration.
 The epithelial cells of renal
tubules lack nutrients and
cells die. Can lead to renal failure.
Filtration rate
 Rate of filtration is directly proportional to net filtration pressure.
 Regulation of filtration rate
 Rate typically constant; may need to increase or
decrease to maintain homeostasis
 1. Sympathetic nervous system reflexes
 Respond to drops in blood pressure and blood volume
As pressure drops, sympathetic nerves cause
vasoconstriction of afferent arterioles.
Decreases rate of filtration
Less urine produced, water is conserved
As pressure rises, sympathetic nerves cause
vasoconstriction of efferent arterioles.
Increases rate of filtration
More urine produced, water is removed
Filtration rate
 2. Renin production by JGA
 Renin is an enzyme controlling filtration rate
 Juxtaglomerular cells secrete renin in response to 3 stimuli
 Sympathetic stimulation (fast response)
 Specialized pressure receptors in afferent arterioles
sense decrease in blood pressure
 Macula densa senses decrease in chloride, potassium,
and sodium ions reaching distal tubule
 Released renin reacts with angiotensinogen in bloodstream
to form angiotensin I  which is converted into angiotensin
II by the angiotensin I converting enzyme, ACE
 Angiotensin II acts to vasoconstrict efferent arteriole
 Blood backs up into glomerulus, increasing pressure and
maintains filtration rate
 Angiotension II also stimulates secretion of aldosterone
from adrenal glands
 Stimulates tubular reabsorption of sodium & H2O follows
Reabsorption
 The composition of urine is different than the composition of
glomerular filtrate.
 Tubular reabsorption returns substances to the internal
environment of the blood by moving substances through
the renal tubule walls into the peritubular capillaries (99%)
 Some water, ions, glucose, amino acids
 Some reabsorption is passive = water  osmosis
= small ions diffusion
 Most is active using protein carriers  by active transport
 Most reabsorption occurs in the proximal convoluted
tubule, where microvilli cells act as transporters, taking up
needed substances from the filtrate and absorbing them
into the peritubular capillary blood.
 Substances that remain in the renal tubule become more
concentrated as water is reabsorbed from the filtrate.
Reabsorption – sodium and water
 The sodium potassium pump reabsorbs 70% of sodium ions
in the PCT.
 The positive sodium ions attract negative ions across the
membrane as well
 Water reabsorption occurs passively across the membrane
to areas of high solute concentration
 Therefore, more sodium reabsorption = more water
reabsorption
 Active transport of sodium
ions occurs along remainder
of nephron and collecting duct
 Almost all sodium ions
and water are reabsorbed.
Materials Not Reabsorbed
 Nitrogenous waste products
 Urea – formed by liver; end product of
protein breakdown when amino acids are
used to produce energy
 Uric acid – released when nucleic acids are
metabolized
 Creatinine – associated with creatine
metabolism in muscle tissue
 Excess water
Secretion – Reabsorption in Reverse
 Some materials move from the peritubular capillaries
into the renal tubules to be eliminated in urine.
 Example:
 Hydrogen ions; potassium ions
 Creatinine
 Drugs; penicillin; histamine
 Process is important for getting rid of substances not
already in the filtrate or for controlling pH.
 Materials left in the renal tubule move toward the
ureter
Formation of Urine
Summary:
• glomerular filtration of
materials from blood
plasma
•Reabsorption of
substances, including
glucose; water, sodium
•Secretion of substances,
including penicillin,
histamine, hydrogen and
potassium ions
Maintaining Water Balance
 Normal amount of water in the human
body
 Young adult females – 50%
 Young adult males – 60%
 Babies – 75%
 Old age – 45%
 Water is necessary for many body
functions and levels must be maintained
Distribution of Body Fluid
 Intracellular fluid
(inside cells)
 Extracellular
fluid (outside
cells)
 Interstitial fluid
 Blood plasma
The Link Between Water and Salt
 Changes in electrolyte balance causes water to move
from one compartment to another
 Alters blood volume and blood pressure (think of aldosterone)
 Can impair the activity of cells (swelling/edema)
 Water intake must equal water output
 Sources for water intake/output:
 Intake: Ingested foods and fluids, Water produced from
metabolic processes (glycolysis)
 Output: Vaporization out of the lungs, Lost in perspiration,
Leaves the body in the feces, Urine production
 Dilute vs. Concentrated Urine
 Dilute urine is produced if water intake is excessive
 Less urine (concentrated) is produced if large amounts of
water are lost
 Proper concentrations of various electrolytes must be present
Regulation of Water and Electrolyte Reabsorption
 Regulation is primarily by hormones
 Antidiuretic hormone (ADH) prevents excessive water
loss in urine
 Neurons in the hypothalamus produce ADH, which are
released by the anterior pituitary gland in response to a
decrease in blood volume or water concentration
 ADH increases the water permeability of the distal convoluted
tubule epithelium to the peritubular capillaries
Decreases volume of urine, increasing concentration
of solutes
Negative feedback control
 Aldosterone regulates sodium ion content of
extracellular fluid
 Triggered by the renin-angiotensin mechanism
 Stimulates the DCT to reabsorb sodium and excrete
potassium
 Cells in the kidneys and hypothalamus are active monitors
Maintaining Water and Electrolyte Balance
Maintaining Acid-Base Balance in
Blood
 Blood pH must remain between 7.35 and 7.45 to maintain
homeostasis
 Alkalosis – pH above 7.45
 Acidosis – pH below 7.35
 Most acid-base balance is maintained by the kidneys
 Excrete bicarbonate ions if needed
 Conserve / generate new bicarbonate ions if needed
 Excrete hydrogen ions if needed
 Conserve / generate new hydrogen ions if needed
 Regulation of these ions results in a urine pH range of 4.5 to 8.0
 Acidic urine: protein-rich diet, starvation, diabetes
 Basic urine: bacterial infections, vegetarian diet
Characteristics of Urine Used for
Medical Diagnosis
 Colored somewhat yellow due to the
pigment urochrome (from the
destruction of hemoglobin/bilirubin byproduct) and solutes
 Sterile
 Slightly aromatic
 Normal pH of around 6
 Specific gravity of 1.001 to 1.035
Urine composition
 Composition differs considerably based upon diet,
metabolic activity, urine output.
 ~95% water, contains urea and uric acid,
electrolytes and amino acids (trace amount)
 Volume produced ranges from 0.6-2.5 liters per day
(1.8L average).
 Depends on fluid intake, body and ambient air
temperature, humidity, respiratory rate, emotional
state
 Output of 50-60ml per hour normal, less than 30ml
per hour may indicate kidney failure
Ureters
 Slender tubes attaching the kidney to the bladder 10-12” long & ¼”
diameter
 Superior end is continuous with the renal pelvis of the kidney
 Mucosal lining is continuous with that lining the renal pelvis and
the bladder below.
 Enter the posterior aspect of the bladder at a slight angle
 Runs behind the peritoneum
 Peristalsis aids gravity in urine transport from the kidneys to the
bladder.
 Smooth muscle layers in the ureter walls contract to propel urine.
 There is a valve-like fold of bladder mucosa that flap over the ureter
openings to prevent backflow.
 Renal calculi= calculus means little stone; result of precipitated uric
acid salts created by bacterial infections, urinary retention, and
alkaline urine. Lithotripsy or surgery are common treatments.
Urinary Bladder
 Smooth, collapsible, muscular sac
 Temporarily stores urine
 Located retroperitoneally in the pelvis
posterior to the pubic symphysis.
Urinary Bladder
 Trigone – three openings
 Two from the ureters (ureteral orifices)
 One to the urethra (internal urethral orifice) which drains the
bladder.
 Common site for bacterial infections
 In males, prostate gland surrounds the neck of the bladder where it
empties into the urethra.
Urinary Bladder Wall
 Three layers of smooth muscle (detrusor muscle)
 Mucosa made of transitional epithelium
 Walls are thick and folded in an empty bladder 2-3” long
 Bladder can expand significantly without increasing
internal pressure
 As it fills, the bladder rises superiorly in the abdominal
cavity becoming firm and pear shaped.
 A moderately full bladder can hold ~500mL (1 pint) of
urine.
 A full bladder can stretch to hold more than twice that
amount.
Urethra
 Thin-walled tube that carries urine from the bladder to
the outside of the body by peristalsis
 Release of urine is controlled by two sphincters
 Internal urethral sphincter (involuntary) – a thickening
of smooth muscle at the bladder-urethra jxn. keeps
urethra closed when urine is not being passed.
 External urethral sphincter (voluntary) -skeletal muscle that controls urine as the
urethra passes through the pelvic floor.
Urethra Gender Differences
 Length
 Females – 3–4 cm (1-1.5 inches)
 Males – 20 cm (7-8 inches)
 Location
 Females – along wall of the vagina
 Males – through the prostate and penis
 Function
 Females – only carries urine
 Males – carries urine and is a passageway for
sperm cells
Urethra Gender Differences
Females:
•Feces can enter urethral opening causing
•Uretritis-inflammation of the urethra
•Pyelitis or pyelonephritis-inflammation of the kidneys
•Urinary tract infections-bacterial infection
•Dysuria
•Urgency
•Frequency
•Fever
•Cloudy urine
•Bloody urine
Males:
•Prostatic, membranous and spongy (penile) urethrae
•Enlargement of the prostate gland causes urinary retention
•can be corrected with a catheter
Micturition (Voiding)
 Both sphincter muscles must open to allow
voiding
 The internal urethral sphincter is relaxed after
stretching of the bladder ~200mL
 Activation is from an impulse sent to the spinal
cord and then back via the pelvic nerves
 The external urethral sphincter must be
voluntarily relaxed
 Incontinence-inability to control micturition
 Retention-inability to micturate