Urinary
System
Anatomy and Physiology II
BIO 232
Path of Urine
•
Kidney (Anatomy)
•
Retroperitoneal position - between dorsal body wall and the parietal
peritoneum - superior lumbar position
•
From 12 thoracic vertebra to the third lumbar vertebra
•
Adult kidney weighs about 150 gram (5 ounces)
Path of Urine
•
Ureters
•
Muscular tubes that convey urine to the bladder by peristalsis
•
Urinary Bladder - hollow muscular organ
•
600ml of urine
•
Urethra
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Extends from the urinary bladder to external opening
Kidney Anatomy
Internal Anatomy of the Kidney
• Renal cortex - light in
color & granular in appearance
• Renal medulla - darker
reddish-brown color
• Contains cone-shaped - medullary or renal pyramids
• Renal columns - inward excursions of cortical tissue -
separate the pyramids
• Renal pelvis - flat,
funnel-shaped tube - continuous with the ureters
• See two or three major calyces - receive the urine
from collecting ducts
Internal Anatomy of the Kidney
Nephrons
• Over 1 million nephrons or
kidney tubules in each kidney
• Thousands of collecting
ducts, each collects urine from several nephrons
• Each nephron
• Glomerular capsule (Bowman's capsule)
•
Surrounds the glomerulus (tuft of capillaries)
•
Together - renal corpuscle
• Proximal convoluted tubule (PCT)
• Loop of Henle
• Distal convoluted tubule (DCT)
Structure of a Nephron
Capillary Beds of the Nephron
• Glomerulus - specialized for
filtration
• Fed by an afferent arteriole and drained by an
efferent arteriole
• Peritubular capillaries
• Arise from the efferent arteioles draining the
glomerulus
• Adapted for absorption - they cling closely to the
renal tubules
Vasa recta - long
straight bundles of capillaries that extend deep into the medulla paralleling
the courses of the longest loops of Henle
Function in helping
to concentrate the filtrate
Vasculature of Nephrons
Juxtaglomerular Apparatus
• DCT lies against the
afferent arteriole feeding the glomerulus
• In the arteriole wall are
juxtaglomerular cells (JG) - act as mechanoreceptors that detect blood pressure
• Macula densa - group of
tall, closely packed cells of the DCT - lie adjacent to the JG cells
• These cells are Osmoreceptors - respond to solute
content of the tubule lumen
• The above two sets of cells - regulate rate of
filtrate formation & systemic blood pressure
Filtration Membrane
•
Between the blood and the interior of the glomerular capsule
•
Three layers
1. Fenestrated
endothelium of the glomerular capillaries
2. Visceral
membrane of the glomerular capsule made of podocytes
3. Intervening
basement membrane composed of fused basal laminas of the other layers
Filtration Membrane
Glomerular Filtration
•
Passive, nonselective whereby fluids & solutes are forced through a
membrane by hydrostatic pressure - forming filtrate
•
All molecules smaller than 3nm in diameter pass from the blood into the
renal tubules
•
Water, glucose AAs & nitrogenous wastes
•
Glomerular filtration rate (GFR)
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120-125ml/min 97.5 L/hr or 180 L/day (47 gallons/day)
Renin-Angiotensin Mechanism
• Triggered by JG cells -
release renin
• Renin is an enzyme that acts
of angiotensinogen (plasma) - to get angiotensin I
• In the lungs angiotensin i
is converted into angiotensin II by an ACE
• Angiotensin II is a powerful
vasoconstrictor
• Also triggers release of aldosterone by the adrenals
• Aldosterone reclaims sodium and water fallows -
increased blood volume - increased BP
Renin Release Triggered By
• Reduced stretched of the JG cells - low BP
• Stimulation of JG cells from
the macula densa cells
• They release less
vasoconstrictor chemical
• Sympathetic input
• Angiotensin II stimulation
of JG cells
Tubular Reabsorption
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Total solute concentration of filtrate begins at 300 mOsm
•
Isotonic to plasma
•
Osmosis cannot occur unless the conc of plasma and filtrate are altered
by active transport
Reabsorption in Proximal
Convoluted Tubule
• Cell of the PCT have NA+/K+
pumps
• Pumps create a concentration
gradient that favors diffusion of Na+ across the apical membranes
into the cells of the PCT
• Na+ is extruded into the
surrounding tissue fluid by the pumps
• This creates an electrical
potential across the wall of the tubule
• CL- is passively drawn toward
the high Na+ concentration of
the tissue fluid
Reabsorption in Proximal
Convoluted Tubule
•
Now an osmotic gradient is established - osmosis occurs
•
65% of salt & water reabsorbed by PCT
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About 20% of salt & water reabsorbed by loop of Henle
Reabsorption of Glucose and Amino
Acids
•
Glucose & AAs recover by secondary active transport
•
Mediated by membrane carriers that co-transport Na+
Reabsorption By PCT
Countercurrent Multiplier System
• Ascending limb of the loop
of Henle
• Na+, K+ & Cl- passively diffuse from the
filtrate into the cells of the ascending loop ( ratio of 1:1:2)
• Na+ - actively
transported to tissue fluid by Na+/K+ pump
• CL- follows
• K+ diffuses back
into the filtrate
Countercurrent Multiplier System
• Ascending limb of the loop
of Henle
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Thin segment - nearest top of loop
•
Thick segment - varying lengths - active transport of Na+
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Walls of ascending limb are not permeable to water
Ascending Limb Structure
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Thus the filtrate - increasingly dilute as it ascends toward the cortex
•
In contrast - tissue fluid around the loop in medulla - more conc
•
Fluid leaving the loop - 100 mOsm - hypotonic
Countercurrent Multiplier System
• Descending limb of the Loop
of Henle
• Deeper regions of the
medulla - 1200 - 1400 mOsm
• To reach these conc - active
Na+ transport ascending limb
• Accumulates in the tissue
fluid of the medulla
Countercurrent Multiplier System
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Descending limb- impermeable to passive diffusion of salt
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Permeable to water
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Surrounding interstitial fluid - hypertonic
•
Water drawn out by osmosis
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As tubular fluid descends to the tip of the loop the concentration of
the fluid is increased & the volume decreased
Countercurrent Multiplier System
• Countercurrent
multiplication
• Countercurrent flow (flow in
opposite direction) in the ascending and descending limbs
• The close proximity of the
limbs allows interaction
Countercurrent Multiplier System
• Conc of filtrate in
descending limb reflects conc of the surrounding tissue fluid
• Conc of the tissue fluid -
raised - active transport - ascending limb
• This is a positive feedback
system
• More salt excreted by the
ascending limb - more concentrated will be the fluid delivered to it by the
descending limb
• Multiplies the conc of the
tissue fluid & the descending limb fluid
Countercurrent Multiplier System
Vasa Recta
•
For the CCMS to work - extruded salt must remain in the tissue fluid
while most of the water leaves
•
Vasa recta accomplishes this
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Peritubular capillaries which run with the Loops of Henle
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Maintains the hypertonicity of the renal medulla
Countercurrent Exchange
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Salt & urea - high conc in medullary tissue fluid diffuses into
blood as it descends into the loops of the vasa recta
•
Then the solutes diffuses out of the ascending vessels & back into
the descending vessels (conc is lower here)
Countercurrent Exchange
•
Salt & urea - high conc in medullary tissue fluid diffuses into
blood as it descends into the loops of the vasa recta
•
Then the solutes diffuses out of the ascending vessels & back into
the descending vessels (conc is lower here)
Countercurrent Exchange
•
Thus solutes are cycled &
trapped in the medulla
•
Solute conc are equal on the inside &outside since the walls of the
vasa recta are freely permeable
•
Colloid osmotic pressure in the vasa recta is higher than in the
interstitial fluid
•
Get osmotic movement of water at both ends of the vasa recta
Collecting Duct & ADH
• Filtrate entering collecting
duct is hypotonic
• Surrounding medulla is
hypertonic
• Some reabsorption of Na+
and secretion of K+ - cortical region
• Wall of duct - water
channels
• Water channels produced as
proteins within the membranes of vesicles that bud from the Golgi
• In the absence of ADH -
vesicles in cytoplasm of the collecting duct cells
Collecting Duct & ADH
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When ADH binds to receptors
•
Then C-AMP mediated fusion of vesicles - exocytosis
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New water channels
•
Collecting duct now more permeable to water
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When ADH drops - water channels removed by endocytosis
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Thus water channels are recycled
Control of ADH Secretion
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ADH controlled by the hypothalamus
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Osmoreceptors in hypothalamus detect inc blood osmotic pressure
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ADH secreted
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During dehydration
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ADH secreted & collecting duct reabsorbs more water
Role of Aldosterone in Na+/K+
Balance
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90% of filtered Na+ & K+ reabsorbed
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Na+ & K+ reabsorption controlled by
aldosterone
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No aldosterone - 80% of remainder of Na+ still absorbed - DCT
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Thus only 2% Na+ excreted (30 g /day)
•
High Aldosterone - all Na+ reabsorbed
Role of Aldosterone in Na+/K+
Balance
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Potassium Secretion
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90% of filtered K+ - reabsorbed early
•
No aldosterone - all of filtered K+ that remains is
reabsorbed
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Aldosterone - stim secretion of K+ from peritubular
capillaries to DCT & collecting duct
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Aldosterone high - 50 time more K+ excreted
Control of Aldosterone Secretion
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A rise in blood K+ levels - directly stimulates secretion of
aldosterone
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Renin-Angiotensin system
Nervous Control of Urination
(Micturition)
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When urinary bladder full of urine
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Stretch receptors from bladder send impulses to the spinal cord
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Spinal cord sends impulses back to bladder - reflex contraction
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Each contraction further stimulates stretch receptors
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After several contraction - refractory - stops for minutes to 1 hr
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Then cycle is repeated
Nervous Control of Urination
(Micturition)
• At some point - internal
sphincter is stretched - urine forced into urethra
• Now 2nd stretch reflex
• Inhibits spinal MNs that
maintain tonic external sphincter contraction
• Once urination begun -
positive feedback maintains flow
Voluntary Control of Urination
•
11/2 - 3 yrs of age
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First a series of bladder contractions - signal brain
•
Voluntary initiation - descending pathway that inhibits MN of the
external sphincter & surrounding muscle of the pelvic floor
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Urine enters the urethra - positive feedback
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Reflex emptying of bladder assisted by muscles of the lower abdomen