Chapter 26
The Urinary System
Kidneys, ureters, urinary bladder
& urethra
Urine flows from each kidney, down its ureter
to the bladder and to the outside via the urethra
Filter the blood and return most of water and solutes to the
bloodstream
Overview of Kidney Functions
Regulation of blood ionic composition
Na+, K+, Ca+2, Cl- and phosphate
ions
Regulation of blood pH, osmolarity
& glucose
Regulation of blood volume
conserving or eliminating water
Regulation of blood pressure
secreting the enzyme renin
adjusting renal resistance
Release of erythropoietin & calcitriol
Excretion of wastes & foreign substances
External Anatomy of Kidney
Paired kidney-bean-shaped organ
4-5 in long, 2-3 in wide,
1 in thick
Found just above the waist between the peritoneum &
posterior wall of abdomen
retroperitoneal along with adrenal glands & ureters
Protected by 11th & 12th ribs with right kidney lower
External Anatomy of Kidney
Blood vessels & ureter enter hilus of kidney
Renal capsule = transparent membrane maintains organ shape
Adipose capsule that helps protect from trauma
Renal fascia = dense, irregular connective tissue that holds
against back body wall
Internal Anatomy of the Kidneys
Parenchyma of kidney
renal cortex = superficial layer of kidney
renal medulla
inner portion consisting of 8-18 cone-shaped renal pyramids
separated by renal columns
renal papilla point toward center of kidney
Drainage system fills renal sinus cavity
cuplike structure (minor calyces) collect urine from the
papillary ducts of the papilla
minor & major calyces empty into the renal pelvis which
empties into the ureter
Internal Anatomy of Kidney
What is the difference between renal hilus
& renal sinus?
Outline a major calyx & the border between cortex &
medulla.
Blood & Nerve Supply of Kidney
Abundantly supplied with blood vessels
receive 25% of resting cardiac output via renal arteries
Functions of different capillary beds
glomerular capillaries where
filtration of blood occurs
vasoconstriction & vasodilation
of afferent & efferent arterioles produce large changes in renal filtration
peritubular capillaries that carry
away reabsorbed substances from filtrate
Sympathetic vasomotor nerves regulate blood flow & renal
resistance by altering arterioles
Blood Vessels around the Nephron
Glomerular capillaries are formed
between the afferent & efferent arterioles
Efferent arterioles give rise to the peritubular
capillaries and vasa recta
Blood Supply to the Nephron
The Nephron
Kidney has over 1 million nephrons
composed of a corpuscle and tubule
Renal corpuscle = site of plasma filtration
glomerulus is capillaries where
filtration occurs
glomerular (Bowman’s) capsule is
double-walled epithelial cup that collects filtrate
Renal tubule
proximal convoluted tubule
loop of Henle dips down into
medulla
distal convoluted tubule
Collecting ducts and papillary ducts drain urine to the
renal pelvis and ureter
Cortical Nephron
80-85% of nephrons are cortical nephrons
Renal corpuscles are in outer cortex and loops of Henle lie mainly in cortex
Juxtamedullary Nephron
15-20% of nephrons are juxtamedullary nephrons
Renal corpuscles close to medulla and long loops of Henle extend into deepest medulla enabling excretion of
dilute or concentrated urine
Histology of the Nephron &
Collecting Duct
Single layer of epithelial cells forms walls of entire tube
Distinctive features due to function of each region
microvilli
cuboidal versus simple
hormone receptors
Structure of Renal Corpuscle
Bowman’s capsule surrounds capsular space
podocytes cover capillaries to
form visceral layer
simple squamous cells form
parietal layer of capsule
Glomerular capillaries arise from
afferent arteriole & form a ball before emptying into efferent arteriole
Juxtaglomerular Apparatus
Structure where afferent arteriole makes contact with
ascending limb of loop of Henle
macula densa is thickened part of
ascending limb
juxtaglomerular cells are modified
muscle cells in arteriole
Number of Nephrons
Remains constant from birth
any increase in size of kidney is size increase of
individual nephrons
If injured, no replacement occurs
Dysfunction is not evident until function declines by 25% of
normal (other nephrons handle the extra work)
Removal of one kidney causes enlargement of the remaining
until it can filter at 80% of normal rate of 2 kidneys
Overview of Renal Physiology
Nephrons and collecting ducts
perform 3 basic processes
glomerular filtration
a portion of the blood plasma is filtered into the kidney
tubular reabsorption
water & useful substances are reabsorbed into the blood
tubular secretion
wastes are removed from the blood & secreted into urine
Rate of excretion of any substance is its rate of
filtration, plus its rate of secretion, minus its rate of reabsorption
Overview of Renal Physiology
Glomerular filtration of plasma
Tubular reabsorption
Tubular secretion
Glomerular Filtration
Blood pressure produces glomerular
filtrate
Filtration fraction is 20% of plasma
48 Gallons/day
filtrate reabsorbed
to 1-2 qt. urine
Filtering capacity
enhanced by:
thinness of membrane & large surface area of glomerular capillaries
glomerular capillary BP is high
due to small size of efferent arteriole
Filtration Membrane
#1 Stops all cells and platelets
#2 Stops large plasma proteins
#3 Stops medium-sized proteins, not small ones
Glomerular Filtration Rate
Amount of filtrate formed in all renal corpuscles of both
kidneys / minute
average adult male rate is 125 mL/min
Homeostasis requires GFR that is constant
too high & useful substances are lost due to the speed
of fluid passage through nephron
too low and sufficient waste products may not be removed
from the body
Changes in net filtration pressure affects GFR
filtration stops if GBHP drops to 45mm Hg
functions normally with mean arterial pressures 80-180
Renal Autoregulation of GFR
Mechanisms that maintain a constant GFR despite changes in
arterial BP
myogenic mechanism
systemic increases in BP, stretch the afferent arteriole
smooth muscle contraction reduces the diameter of the
arteriole returning the GFR to its previous level in seconds
tubuloglomerular feedback
elevated systemic BP raises the GFR so that fluid flows too
rapidly through the renal tubule & Na+, Cl- and water
are not reabsorbed
macula densa detects that
difference & releases a vasoconstrictor from the juxtaglomerular
apparatus
afferent arterioles constrict & reduce GFR
Neural Regulation of GFR
Blood vessels of the kidney are supplied by sympathetic
fibers that cause vasoconstriction of afferent arterioles
At rest, renal BV are maximally dilated because sympathetic
activity is minimal
renal autoregulation prevails
With moderate sympathetic stimulation, both afferent &
efferent arterioles constrict equally
decreasing GFR equally
With extreme sympathetic stimulation (exercise or
hemorrhage), vasoconstriction of afferent arterioles reduces GFR
lowers urine output & permits blood flow to other
tissues
Tubular Reabsorption &
Secretion
Normal GFR is so high that volume of filtrate in capsular
space in half an hour is greater than the total plasma volume
Nephron must reabsorb 99% of the
filtrate
PCT with their microvilli do most
of work with rest of nephron doing just the
fine-tuning
solutes reabsorbed by active & passive processes
water follows by osmosis
small proteins by pinocytosis
Important function of nephron is
tubular secretion
transfer of materials from blood into tubular fluid
helps control blood pH because of secretion of H+
helps eliminate certain substances (NH4+, creatinine, K+)
Transport Mechanisms
Water is only reabsorbed by osmosis
obligatory water reabsorption
occurs when water is “obliged” to follow the solutes being reabsorbed
facultative water reabsorption
occurs in collecting duct under the control of antidiuretic
hormone
Glucosuria
Common cause is diabetes mellitis
because insulin activity is deficient and blood sugar is too high
Reabsorption in the Loop of Henle
Tubular fluid
PCT reabsorbed 65% of the filtered water so chemical
composition of tubular fluid in the loop of Henle is
quite different from plasma
since many nutrients were reabsorbed as well, osmolarity of tubular fluid is close to that of blood
Sets the stage for independent regulation of both volume
& osmolarity of body fluids
Symporters in the Loop of Henle
Thick limb of loop of Henle has Na+
K- Cl- symporters that
reabsorb these ions
K+ leaks through K+ channels back into the tubular fluid
leaving the interstitial fluid and blood with a negative charge
Cations passively move to the vasa recta
Reabsorption & Secretion in
the Collecting Duct
By end of DCT, 95% of solutes & water have been
reabsorbed and returned to the bloodstream
Cells in the collecting duct make the final adjustments
principal cells reabsorb Na+ and secrete K+
intercalated cells reabsorb K+ & bicarbonate ions and
secrete H+
Actions of the Principal Cells
Na+ enters principal cells
through leakage channels
Na+ pumps keep the
concentration of Na+ in
the cytosol low
Cells secrete variable
amounts of K+, to adjust
for dietary changes in K+
intake
down concentration gradient due to Na+/K+ pump
Aldosterone increases Na+ and
water reabsorption & K+ secretion by principal
cells by stimulating the synthesis of new pumps and channels.
Secretion of H+ and Absorption of Bicarbonate by
Intercalated Cells
Proton pumps (H+ATPases) secrete
H+ into tubular fluid
can secrete against a concentration gradient so urine can be
1000 times more acidic than blood
Hormonal Regulation
Hormones that affect Na+, Cl-
& water reabsorption and K+ secretion in the
tubules
angiotensin II and aldosterone
decreases GFR by vasoconstricting
afferent arteriole
enhances absorption of Na+
promotes aldosterone production
which causes principal cells to reabsorb more Na+ and Cl-
and less water
increases blood volume by increasing water reabsorption
Antidiuretic Hormone
Increases water permeability of principal cells so regulates
facultative water reabsorption
When osmolarity of plasma &
interstitial fluid decreases, more ADH is secreted and facultative water reabsorption increases.
Production of Dilute or Concentrated Urine
Homeostasis of body fluids despite variable fluid intake
Kidneys regulate water loss in urine
ADH controls whether dilute or concentrated urine is formed
if lacking, urine contains high ratio of water to solutes
Formation of Dilute Urine
Dilute = having fewer solutes than plasma (300 mOsm/liter).
diabetes insipidus
Filtrate and blood have equal osmolarity
in PCT
Principal cells do not reabsorb water if ADH is low
Formation of Concentrated Urine
Compensation for low water intake or heavy perspiration
Urine can be up to 4 times greater osmolarity
than plasma
Cells in the collecting ducts reabsorb more water & urea
when ADH is increased
Summary
H2O Reabsorption
PCT---65%
loop---15%
DCT----10-15%
collecting duct---
5-10% with ADH
Dilute urine has not had enough water removed, although
sufficient ions have been reabsorbed.
Reabsorption within Loop of Henle
Diuretics
Substances that slow renal reabsorption
of water & cause diuresis (increased urine flow
rate)
caffeine which inhibits Na+ reabsorption
alcohol which inhibits secretion of ADH
prescription medicines can act on the PCT, loop of Henle or DCT
Evaluation of Kidney Function
Urinalysis
analysis of the volume and properties of urine
normal urine is protein free, but includes filtered &
secreted electrolytes
urea, creatinine, uric acid, urobilinogen, fatty acids, enzymes & hormones
Blood tests
blood urea nitrogen test (BUN) measures urea in blood
rises steeply if GFR decreases severely
plasma creatinine--from skeletal
muscle breakdown
renal plasma clearance of substance from the blood in
ml/minute (important in drug dosages)
Dialysis Therapy
Kidney function is so impaired the blood must be cleansed
artificially
separation of large solutes from smaller ones by a
selectively permeable membrane
Artificial kidney machine performs hemodialysis
directly filters blood because blood flows through tubing
surrounded by dialysis solution
cleansed blood flows back into the body
Anatomy of Ureters
10 to 12 in long
Varies in diameter from 1-10 mm
Extends from renal pelvis to bladder
Retroperitoneal
Enters posterior wall of bladder
Physiological valve only
bladder wall compresses arterial opening as it expands
during filling
flow results from peristalsis, gravity & hydrostatic
pressure
Histology of Ureters
3 layers in wall
mucosa is transitional epithelium & lamina propria
since organ must inflate & deflate
mucus prevents the cells from being contacted by urine
muscularis
inner longitudinal & outer circular smooth muscle layer
distal 1/3 has additional longitudinal layer
peristalsis contributes to urine flow
Location of Urinary Bladder
Posterior to pubic symphysis
In females is anterior to vagina & inferior to uterus
In males lies anterior to rectum
Anatomy of Urinary Bladder
Hollow, distensible muscular organ with capacity of 700 -
800 mL
Trigone is smooth flat area
bordered by 2 ureteral openings and one urethral
opening
Histology of Urinary Bladder
3 layers in wall
mucosa is transitional epithelium & lamina propria
since organ must inflate & deflate
mucus prevents the cells from being contacted by urine
muscularis (known as detrusor muscle)
3 layers of smooth muscle
inner longitudinal, middle circular & outer longitudinal
circular smooth muscle fibers form internal urethral
sphincter
circular skeletal muscle forms external urethral sphincter
adventitia layer of loose connective tissue anchors in place
superior surface has serosal layer
(visceral peritoneum)
Micturition Reflex
Micturition or urination (voiding)
Stretch receptors signal spinal cord and brain
when volume exceeds 200-400 mL
Impulses sent to micturition
center in sacral spinal cord (S2 and S3) & reflex is triggered
parasympathetic fibers cause detrusor
muscle to contract, external & internal sphincter muscles to relax
Filling causes a sensation of fullness that initiates a
desire to urinate before the reflex actually occurs
conscious control of external sphincter
cerebral cortex can initiate micturition
or delay its occurrence for a limited period of time
Anatomy of the Urethra
Females
length of 1.5 in., orifice between clitoris & vagina
histology
transitional changing to nonkeratinized
stratified squamous epithelium, lamina propria with elastic fibers & circular smooth muscle
Males
tube passes through prostate, UG diaphragm & penis
3 regions of urethra
prostatic urethra, membranous
urethra & spongy urethra
circular smooth muscle forms internal urethral sphincter
& UG diaphragm forms external urethral sphincter
Urinary Incontinence
Lack of voluntary control over micturition
normal in 2 or 3 year olds because neurons to sphincter
muscle is not developed
Stress incontinence in adults
caused by increases in abdominal pressure that result in
leaking of urine from the bladder
coughing, sneezing, laughing, exercising, walking
injury to the nerves, loss of bladder flexibility, or damage
to the sphincter
Aging and the Urinary System
Anatomical changes
kidneys shrink in size from 260 g to 200 g
Functional changes
lowered blood flow & filter less blood (50%)
diminished sensation of thirst increases susceptibility to
dehydration
Diseases common with age
acute and chronic inflammations & canaliculi
infections, nocturia, polyuria, dysuria, retention or
incontinence and hematuria
Cancer of prostate is common in elderly men
Disorders of Urinary System
Renal calculi
Urinary tract infections
Glomerular disease
Renal failure
Polycystic kidney disease