Chapter 26: Urinary System
Functions of Urinary System:
1. Excretion – excrete wastes
2. Regulate blood volume and blood pressure by:
a. adjusting water loss in urine
b. releasing erythropoietin
c. releasing renin
3. Regulate plasma concentrations of Na+, K+, Cl-, Ca++
4. Stabilize blood pH by controlling excretion of H+ and HCO3-
5. Assist liver with detoxifying poisons
Main Organs: kidneys form urine send it through urinary tract (ureter →urinary bladder →urethra)
Urinary Tract Organs
Ureters: (see page 985)
Smooth muscle tubes transport urine from renal pelvis of kidney to bladder
Retroperitoneal
Empty into posterior wall of bladder
Histology of the Ureters:
-inner lining is mucosa: transitional epithelial tissue
-middle layer is smooth muscle
peristaltic contractions roughly every 30 seconds
-outer layer visceral peritoneum – connective tissue
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Urinary Bladder:
Hollow, muscular organ ,
posterior to pubic symphysis
lies anterior to the uterus in females
held in place via peritoneum & ligaments (middle & lateral umbilical ligaments)
trigone: triangular region with openings for urethra – works as a funnel
internal urethral sphincter: located in “neck” of bladder
involuntary
innervated by both sympathetic and parasympathetic divisions of ANS
-parasympathetic: relaxes internal sphincter
Histology of Urinary Bladder:
lining is mucosa consisting of transitional epithelial tissue
muscularis layer is smooth muscle:
forms detrusor muscle: contracts to expel urine
detrusor muscle also prevents urine from flowing back into ureters
distends: muscles stretch (can hold up to 1 L)
when empty collapses into folds called
Urethra:
drains urine from urinary bladder to outside
female: urethra: anterior to vagina
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male urethra longer and subdivided into:
1.) prostatic urethra:
2.) membranous urethra: very short segment in floor of pelvic cavity
3.) penile (spongy) urethra:
external urethral meatus (orifice): opening
external urethral sphincter: (urogenital diaphragm):
the valve for urine
voluntary
lining of urethra stratified squamous epithelial tissue
Micturition Reflex: voiding or urination (page 989 figure 26-20)
1.) distension in urinary bladder activates stretch receptors ( reflex)
2.) afferent fibers in pelvic nerves carry impulses to spinal cord to thalamus to cerebral cortex
of brain
so become aware of urge.
3.) stimulates parasymp. fibers which cause contraction of bladder & relaxation of internal
sphincter.
external sphincter: voluntary can keep it closed
reflex begins when urinary bladder contains about 200 ml of urine – cycles as bladder volume
increases
at 500 ml pressure will open internal urethral sphincter
Incontinence:
May be caused by trauma to internal/external urethral sphincters
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Or damage to CNS, spinal cord or nerves to urinary bladder
stress incontinence: from stretched or damaged sphincter muscles: increased intra-abdominal
pressure causes urine to leak
Urinary Retention: occurs in males due to enlarged prostate gland (prostatic hypertrophy)
Kidneys:
bean shaped
T12 - L3:
medial indentation termed - hilus: leads into renal sinus
-renal blood vessels, ureters,
adrenal gland is found superior to each kidney
held in place via peritoneum & connective tissue
3 layers of connective tissue surround each kidney
1. fibrous renal capsule: innermost layer, directly attaches to kidney
covers outer surface of kidney (layer of collagen)
2. perinephric fat (adipose) capsule:
3. renal fascia: dense irregular connective tissue
surrounds both the kidney & adrenal gland
anchors kidney to surrounding structures – fuses with peritoneum
Sectional Anatomy of Kidney (page 956)
Cortex, Medulla, Pelvis
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1. Renal Cortex:
2. Renal Medulla:
contains renal (medullary) pyramids
papilla : rounded tip – faces renal pelvis
base: flat portion – faces cortex
each renal pyramid forms renal lobe
each renal lobe contains nephrons which make urine
3. Renal Pelvis:
formed from the minor and major calyces (minor calyx, major calyx)
funnel shaped area
collects
pyelitis: infection of renal pelvis (usually from untreated UTI)
pyelonephritis: infection of kidney
● Signs/symptoms: high fever, intense pain on affected side, vomiting, diarrhea, blood and pus
in urine
● Tx: intense antibiotic therapy
Blood and Nerve Supply: (p. 958) kidneys receive 20-25% of cardiac output
blood leaves heart to aorta – aortic arch – thoracic aorta – abdominal aorta:
renal arteries enter thru hilus branch into
↓
segmental (lobar) arteries
↓
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interlobar arteries ( pyramids to cortex)
↓
arcuate arteries(arch along boundary of cortex & medulla
↓
interlobular (cortical radiate) arteries (supply cortex)
↓
afferent arteriole (takes blood into glomerulus)
↓
glomerulus (capillary network)
↓
efferent arteriole (drains blood from glomerulus)
↓
peritubular capillaries (surround nephron tubules)
↓
venules
↓
interlobular veins
↓
arcuate veins
↓
Interlobar (cortical radiate) veins
↓
renal vein
Note: Glomerulus is special capillary network:
Receives blood from afferent arteriole
Drained by the efferent arteriole
Fenestrated capillaries
Has higher pressure than systemic capillary beds
Renal plexus: ANS:
-sympathetic stimulation controls blood vessels: vasoconstriction/vasodilation
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Nephrons:
each kidney contains
consists of:
1. glomerular (Bowman's ) capsule (surrounds glomerulus)
note Renal Corpuscle is Bowman’s capsule & glomerulus
2. proximal convoluted tubule (PCT)
3. Loop of Henle – descending & ascending limbs
4. distal convoluted tubule (DCT)
5. collecting duct
Adaptations for each region of the nephron: (p. 960 Summary Table 26-1)
1. Bowman’s (Glomerular) Capsule: Site of Filtration
Two layers:
parietal epithelium: outer wall of capsule
visceral epithelium: covers glomerulus
podocytes:
filtration slits:
Lamina densa (basal lamina)
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2. Proximal Convoluted Tubule: Main site of reabsorption
- simple cuboidal epithelium
-have microvilli
3. Loop of Henle: descending & ascending limbs
-lower part of descend limb (thin segment) contains simple squamous epithelial tissue
-permeable to water
- impermeable to
-ascending limb thick segment:
-permeable to salts
-impermeable to
4. Distal Convoluted Tubule:
- simple cuboidal epithelial cells
-lack
-better suited for
Two types of Nephrons:
1. Cortical Nephrons: 85% of all nephrons
Located mostly within
Loop of Henle is short
Efferent arteriole delivers blood into
2. Juxtamedullary Nephrons: 15% of all nephrons
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Loop of Henle extends deep into medulla
Peritubular capillaries connect to vasa recta
Juxtaglomerular Apparatus:
regulates BP & rate of filtrate formation
located at junction of DCT & afferent arteriole
3 Types of Cells:
1.) Mesangial Cells: support capillaries
2.) Granular (JG) cells in afferent arteriole:
-smooth muscle cells
-contain granules of renin
-mechanoreceptors:
3.) macula densa cells
-contains chemoreceptors (osmoreceptors) respond
Principles of Renal Physiology
47 gallons fluid enter each day
only 1% becomes urine and eliminated
uses 20-25% of cardiac output
nephrons form the urine by filtering blood
- maintain homeostasis by regulating volume & composition of blood
-excrete wastes (urea, creatinine, uric acidfrom blood,
nephron filters them out of blood to eliminate them in urine
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Basic Processes of Urine Formation
1.) Filtration: hydrostatic pressure forces water/solutes out of blood into nephron (Bowman’s capsule)
2.) Reabsorption: removal of water/solutes from filtrate back into blood
3.) Secretion: transport of solutes from peritubular capillaries back into filtrate
Key Terms:
Osmolarity: Is the osmotic concentration of a solution

Total milliosmoles per liter (mOsm/L)

number of solute particles per liter

Body fluids have osmotic concentration of about 300 mOsm/L

Ion concentrations


In milliequivalents per liter (mEq/L)
Concentrations of large organic molecules

Grams or milligrams per unit volume of solution (mg/dL or g/dL)
Reabsorption and Secretion use the following methods of transport:
Osmosis
Diffusion
Carrier-mediated transport: requires carrier protein – if saturate carrier protein cannot transport
molecule (termed transport maximum or Tm) which indicates renal threshold
if renal threshold reached for reabsorption cannot reabsorb the substance and it is lost in
urine
example: Tm for glucose is 180 mg/dL
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If plasma glucose is greater than 180 mg/dL

Tm of tubular cells is exceeded

Glucose appears in urine:
– glycosuria
facilitated diffusion:
active transport
cotransport (symport)
countertransport
1. Glomerular Filtration
filtration is passive
driven by hydrostatic pressure
note: glomerular blood pressure is 50 mmHg vs 35 at arterial end of capillary bed
nonselective: water, ions, glucose, AA, wastes all are filtered
filtration membrane: barrier
materials pass through membrane into capsule space and is termed filtrate
Filtration Membrane:
-filters b/w blood (in glomerulus) and glomerular (Bowman’s) capsule
-porous w/ 3 layers:
1. Fenestrated capillaries:
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2. Visceral membrane of podocytes
3. Basement membrane (lamina densa): keeps most proteins out
Net Filtration Pressure (NFP): The net force that causes filtration
The balance between HP and OP of the glomerulus
Pressures (forces) that push materials out of glomerulus – pressures that pull back into
glomerulus
NFP = G.H.P. - (G.O.P.+ Cs.H.P.)
G.H.P. = glomeruluar hydrostatic pressure: (50 mmHg) – dependent on
G.O.P.= glomerular osmotic pressure: (25 mmHg) – dependent on
Cs.H.P.= capsule hydrostatic pressure: (15 mmHg)
No Cs.O.P. why not?
What type of factors would increase NFP?
NFP = 50 - (25 + 15)
NFP = 10 mmHg
What type of factors would decrease NFP?
Glomerular Filtration Rate (GFR)
-amount of fluid filtered from blood into kidney per minute – 125 ml/minute
Depends on:
1. total surface area for filtration
2. permeability of filtration membrane
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3. NFP
GFR is directly proportional to NFP therefore anything that alters NFP effects GFR
Regulation of Glomerular Filtration
A. Intrinsic: kidneys autoregulate
B. Hormonal: initiated by kidneys
C. Extrinsic: Sympathetic division of ANS
A. Intrinsic: kidneys (renal) autoregulation
Maintains GFR despite changes in
kidney regulate glomerular filtration thru monitoring flow speed & contents (osmolarity)
if flow rapid (large amounts of filtrate produced) might not have time for reabsorption
or if there is decreased filtrate and flow it will move too slowly and might absorb
1. Myogenic Mech.: responds to pressure changes:
if systemic BP increases = rise in renal BP
stretches walls of afferent arteriole which then causes the smooth muscle to contract
which constricts the
so it keeps glomerular pressure
if reduced blood flow and/or systemic BP falls, causes dilation of the
Kidneys can autoregulate at SBPs b/w 80 – 180 mmHg
2. Tubuloglomerular Feedback Mechanism
Controlled by Macula Densa cells of juxtaglomerular apparatus
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in distal tubules respond to slow flowing filtrate and/or osmotic signals:
if the flow is too slow need to speed it up by dilating the afferent arteriole
but if flow is fast or high osmolarity, will constrict
B. Hormonal Mechanisms (pg. 971)
1.) Hormones of the rennin-angiotensin system
2.) ANP ( ANF)
1.) Juxtaglomerular (JG) cells release renin cause anigiotensinogen →angtiotensin I
which is converted to angiotensin II (with A.C.E.)
Angtiotensin II:
1.) Triggers release of aldosterone
2.) Constricts efferent arteriole to increase glomerular blood pressure & GFR
3.) Powerful vasoconstrictor (systemic)
4.) Triggers release of ADH
5.) Stimulates thirst
3 factors trigger release of rennin from the JG cells
1. Decreased stretch of JG cells from decreased BP and/or decreased BV
2. Macula Densa cells cause vasodilation due to decrease is osmolarity of filtrate
3. Sympathetic nervous system
2.) ANP ( ANF)
Released from the atria of the heart in response to stretching walls due to
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Increases Na+ excretion in the urine
C. Extrinsic (Autonomic):sympathetic nervous system
stress or emergency want to shunt blood to muscles
Sympathetic activation: (NE & Epi)
1. constricts afferent arterioles
2. decreases GFR - slows filtrate
3. triggers the release of renin by JG cells
2. Tubular Reabsorption
Reabsorbing substances from filtrate that were reabsorbed earlier
filtrate similar to blood plasma except
urine: mainly water, wastes
need to get materials from filtrate back into blood
so it doesn't become urine and is lost
begins when filtrate enters the PCT
Reabsorbed substances must pass through 3 cell layers to enter peritubular capillaries
1.) into tubule cell
2.) out of tubule cell into IF
3.) into peritubular capillary
Glucose, AA reabsorbed via secondary active transport: cotransport with Na
active transport: ion pumps for Na+, K+, HCO3-, Mg++, PO43
passive reabsorption of:
water
urea, Cl- and lipid soluble molecules
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Different Regions of Tubules and What They Absorb:
Proximal Convoluted Tubule: most involved in reabsorption
Absorbs: 100% of the glucose & amino acids
75-80% of the Na+, ClLoop of Henle:
Decending limb: permeable to water: water leaves via osmosis
Ascending limb: permeable to salts: Na+, Cl- pumped out
Distal convoluted tubule:
Under hormonal control
-Aldosterone: controls
-ADH: controls
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3. Tubular Secretion
blood secretes molecules back into tubule (filtrate)
last chance for
Blood entering peritubular capillaries contains
substances that did not cross filtration memrane
H+ & K+ are both secreted in exchange
for Na+
-
Countertransport:
compete for secretion:
secretion depends on their amounts in
peritubular capillary blood
Important for:
1. Disposing of undesirable substances
2. Eliminating excess K+
3. Getting rid of substances that were
4. Controlling blood pH
when pH decreases:
when pH increases:
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note: aldosterone controls secretion of K+
Regulation of Urine Concentration and Volume: Loop of Henle
Refers to exchange between tubular fluids moving in opposite directions

Fluid in descending limb flows toward renal pelvis

Fluid in ascending limb flows toward cortex
The countercurrent mechanisms establish and maintain an osmotic gradient extending from the cortex through the depths of
the medulla that allows the kidneys to vary urine concentration dramatically.
Loop of Henle:
(thin) Descending Limb: permeable to water
osmolarity increases as filtrate moves from cortex to medulla
concentrated filtrate in ascending limb accelerates Na+
Cl- transport into peritubular fluid
(thick) Ascending Limb: impermeable to water
Contains active transport proteins to pump
Salts elevate osmotic concentration in peritubular fluid
Higher osmolarity of peritubular fluid creates osmotic pressure
to pull water out of descending limb
due to countercurrent mechanism
This creates a hypertonic medulla which facilitated the
removal of water from the descending limb
The concentration of the interstitial fluid of the kidney
increases as it moves from the cortex to the medulla.
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Increases solute concentration of filtrate: peaks 1200 mosm
Na+ and Cl- account for aprx. 750 ml of the total 1200 ml
of the IF of the medulla
The rest is from urea
Collecting tubules in deep medullary regions:
permeable to urea
as urine passes thru deep medullary regions urea
leaves filtrate & enters IF of medulla to concentrate it
diet deficient in protein would effect ability of medulla to be concentrated
Vasa recta: special capillary loop
found near Loop of Henle in juxtamedullary nephrons
acts as countercurrent exchanger:
maintains osmotic gradient
Formation of Dilute Urine:
filtrate diluted as passes thru:
when ADH not released by
posterior pituitary:
Formation of Concentrated Urine:
ADH: decreases urine, by
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opening aquaporins
hypertonic medulla creates
OP to pull water from filtrate
facultative water reabsorption:
obligatory water reabsorption:
release of ADH: continuously (when plasma osmolarity rises above 300 mosm)
Urine production: typical urine has osmotic concentration of 800 – 1000 mOsm/L
Diuresis:
The elimination of urine
Typically indicates production of large volumes of urine
Diuretics:
Drugs that promote water loss in urine:
(Loop diuretics: Lasix: block Na+ pumps in ascending limb)
Given to reduce:
Osmotic diuretic: substance that is in the filtrate in collecting tubule in excessively high amounts and shouldn’t be there
(glucose).
Acts as an osmotic diuretic b/c ↑ tonicity of filtrate which creates OP
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Alcohol: inhibits ADH
Caffeine: increases GFR – acts as diuretic
Renal Clearance:
creatinine clearance test used to estimate GFR
monitor creatinine in blood and amount in urine over 24 hr. period
inulin is gold standard b/c its not secreted or reabsorbed – what goes in comes out
RC of inulin is 125 ml/min
RC< 125 means: some was absorbed
RC = 0 indicates: completely reabsorbed
RC > 125 means: some was secreted
RC of urea= 70 ml/min
of sodium= .9 ml/min
creatinine= glucose=0 ml/min
Aging and the Urinary System:
●decrease in number of functional nephrons (30-40% loss between ages 25 – 85)
● reduction in GFR
● reduced sensitivity to ADH
● problems with micturition reflex
Characteristics of Urine:
Urine sample depends on osmotic movement of water across walls of tubules & collecting ducts
Clear, sterile solution
Water content:
Color:
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Urinalysis: the analysis of a urine sample is an important diagnostic tool
Normal components of urine: urea, creatinine, ammonia, uric acid, bilirubin, urobilin
Na+, Cl-, K+
Volume:
Oliguria
Polyuria
pH:
Specific gravity: the density of urine compared to water
Specific gravity of water is 1.00
Clinical Applications
Urinalysis:
Hematuria
Glycosuria
Proteinuria (albuminuria)
Ketonuria
Pyuria
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Blood Tests done to assess Kidney function:
BUN:
Creatinine:
Potassium:
Clinical Applications:
1. Kidney stones (renal calculi)
Urine crystallizes to form hard stones
S/S: pain, nausea, vomiting, hematuria
Usually calcium phosphate or calcium oxalate or struvite
Associated with higher intake of calcium & uric acid
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Seen on x-ray or CT scan
medication, surgery or lithotropy
Maintain hydration – less caffeine & dark sodas
2. Urinary Tract Infections (UTIs)
bacterial or fungal pathogens
more common in females (shorter, close to anus, sexual intercourse can push bacteria into
urethra)
● urethritis: inflammation of urethra
● cystitis: inflammation of urinary bladder
● dysuria:
3. Bladder cancer
4X more prevalent in males
most patiens are b/w 60-70 yrs. of age
highest among cigarette smokers & employees in chemical & rubber companies
S/S: hematuria, dysuria, change in bladder habits (usually no symptoms until cancer
has progressed)
spreads through adjacent lymph nodes/tissue quickly – prognosis for metastasized
bladder cancer poor
4. Glomerulonephritis:
Acute:
usually associated with streptococcal infection (7-21 days after infect)
∙immune complexes becomes lodged in glomerulus, damages basement
membrane
Also can occur from bacterial endocarditis, viral infections, Lupus,
signs/symptoms: hematuria, proteinuria (foamy urine), edema, hypertension,
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fatigue, decrease in urination, elevated BUN and creatinine levels
Tx: depends on cause: antibiotics OR corticosteroids, plasmapheresis, dialysis
Chronic: many causes: hypercholesterolemia, diabetes mellitus and lupus erythmatosus
occurs from untreated acute glomerulonephritis
irreversible – results in decrease GFR and accumulation of toxins
often progresses to chronic kidney disease & end-stage renal disease
5. Renal Failure:
kidney unable to perform functions
Affects fluid balance, pH, muscle contraction, metabolism & digestion
S/S: elevated BUN, plasma creatinine
Oliguria
Often hypertension occurs
Decreased EPO causes decreased Hct
Affects CNS: sleeplessness, seizures, delirium and even coma
Acute renal failure:
Occurs when renal ischemia, urinary obstruction, trauma or exposure of nephrotoxic
drugs causes filtration to slow or stop.
Decreased kidney function occurs over a few days and might persist for weeks.
Require dialysis
Kidneys may regain partial or complete function once underlying problems is treated.
Chronic renal failure
Kidney function deteriorates gradually and problems accumulate over years
Irreversible – progression only slowed
Go into “end-stage renal failure”: when kidneys function at <10% of normal capacity
requires dialysis and/or kidney transplant
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