Download The Urinary System

The Urinary System
Organs of the Urinary System
Ø  Kidney
Ø  Ureter
Ø  Urinary
bladder
Ø  Urethra
Functions of the Urinary System
Ø  Produce
Ø  Waste
l 
urine…
removal
urea, uric acid
Ø  Regulation
of BV, BP
of [ion]plasma
Ø  Help regulate plasma
pH
Ø  Conservation of
nutrients
Ø  Regulation
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nephrons
Ø  ~1
million in an adult human
kidney
Ø  Functional unit
Ø  Tube that processes filtered
blood, producing urine
Ø  Nephron populations
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85% cortical nephrons
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15% juxtamedullary nephrons
•  Most of kidney function
•  Capacity to concentrate urine
Glomerulus
Ø  is
~ 50 fenestrated
capillaries
Ø  functions:
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filtration of blood
formation of filtrate
filtrate is…
Ø  blood
pressure (BHP) in
capillaries forces fluid
out of plasma into
Bowman’s capsule
Juxtaglomerular complex (JGC)
Ø  2
l 
parts
macula densa
•  DCT cells that monitor filtrate
[Na]
l 
juxtaglomerular cells
•  Smooth muscle
Ø  JGC
is an endocrine
structure
l 
l 
JG cells secrete renin
EPO may come from here,
but details are not verified
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Nephron
Ø 
Ø 
modifies filtrate
urine is the product
of the modification
Renal Physiology
Ø  Maintain
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homeostasis
Volume of blood
Composition of blood
Excretion of wastes
•  Urea - most abundant, from a.a. catabolism
•  Creatinine from catabolism of creatine phosphate
•  Uric acid from RNA recycling
l 
Waste excretion requires water: this means
we have unavoidable water loss in urine
Processes of
Urine Formation
Ø 
Ø 
Ø 
Filtration
Reabsorption from filtrate
Secretion into filtrate
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Glomerular Filtration 1
Ø 
Net filtration from
glomerulus to
capsular space
Ø 
Result of 2 pressures
l 
l 
Hydrostatic pressure
Osmotic pressure
Glomerular Filtration 2:
hydrostatic pressures
Ø 
Ø 
Ø 
Ø 
2 hydrostatic pressures
glomerular hydrostatic
pressure (GHP)
l  GHP is 50 mmHg
capsular hydrostatic
pressure (CsHP)
l  CsHP is15 mmHg
Net Hydrostatic Pressure
(NHP)
l  NHP = GHP- CsHP
l  NHP = 50 mmHg – 15 mmHg
l 
NHP is 35 mmHg
Glomerular Filtration 3:
osmotic pressure
Ø  blood
colloid osmotic
pressure (BCOP) is
25 mmHg
Ø  filtrate colloid osmotic
pressure is 0 mmHg
4
Glomerular Filtration 4:
Net Filtration Pressure
Ø 
Net Filtration pressure (NFP)
NFP = NHP – BCOP
Ø 
NFP = 35 mmHg – 25 mmHg
Ø 
Ø NFP
= 10 mmHg
GFR (glomerular filtration rate)
Ø  Amount
l 
l 
of filtrate produced per minute
Averages 125 ml/min
10% of blood delivered ends up as filtrate
Ø  60
min/hour x 125 ml/min = 7500 ml/hr
ml/hr x 24hr/day = 180 liters/day
Ø  70 times the amount of total plasma volume!
Ø  7500
Controlling GFR
Ø  heavily
dependent on MAP
decrease in MAP leads to a decrease in GFR
Ø  if GHP drops to 40 mmHg, filtration stops…
Ø  causes of drop in GHP:
Ø  a
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Hemorrhage, shock, dehydration…
Ø  result:
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accumulation of metabolic wastes
control of pH jeopardized
ability to regulate blood volume lost
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Control of GFR
Ø  Important
Ø  Autoregulation
Ø  Hormonal
Ø  Autonomic
Autoregulation of GFR
Ø  accomplished
diameters of
l 
by changing
efferent and afferent arterioles
Ø  when
blood pressure in
afferent arteriole decreases,
l 
l 
l 
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afferent arteriole dilates
glomerular capillaries dilate
efferent arteriole constricts
effect on GFR is : ________
Ø  If
blood pressure in afferent
arteriole increases, afferent
arteriole constricts…
Hormonal
regulation of GFR
Ø  Renin-angiotensin
Ø  Natriuretic
system
peptides
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Hormonal
regulation of GFR
Ø  If
autoregulation is insufficient
to maintain GFR, renin is
secreted by this sequence:
l 
l 
l 
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a decrease in BP at glomerulus
causes
decreased [Na] of tubular fluid
at macula densa
Result: renin secretion…reninangiotensin system activation
Note: sympathetic stimulation
of JG cells also causes renin
secretions
renin
angiotensin
system
ANP/BNP
Ø  atrial
natriuretic peptide
•  atria of heart
Ø  brain
natriuretic peptide
•  ventricles of heart
l 
l 
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secreted if BV increases
dilation of afferent arterioles
constriction of efferent arterioles
Ø  increases
GFR
natriuresis
Ø  Result: increased urine production &
decreased blood volume and pressure
Ø  Induces
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Autonomic
regulation of GFR
Ø  sympathetic
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l 
effect:
powerful
vasoconstriction in
afferent arteriole
decreases GFR,
slowing filtrate
production
Processes of Urine
Formation
Ø 
Filtration
Ø  Reabsorption
l 
l 
l 
from filtrate
diffusion
osmosis
carrier-mediated transport
•  membrane proteins necessary
Ø  Secretion
understanding
reabsorption and
secretion
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Regions of nephron & function
Ø  PCT
l 
reabsorption
•  ions, organic molecules,
vitamins, water
Ø  Nephron
l 
loop
reabsorption
•  water, NaCl
Ø  DCT
l 
l 
reabsorption/secretion
drugs, toxins
Ø  Collecting
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system
water reabsorption
Osmolarity of body fluids
Ø  Osmolarity
l 
l 
refers to number of solute particles/liter
expressed in milliosmoles/L (mOsm/L)
Ø  Osmolarity
of body fluids = 300 mOsm/L
Reabsorption is
based on carriermediated transport
Ø  160
memory: carriers are substrate specific
Ø  many types of transport proteins exist in the
tubule cells
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Renal Threshold
Ø  carrier
proteins can be “saturated”; this means a
“transport maximum” (Tm) exists for solutes
Ø  Tm determines renal threshold
l 
This is the plasma concentration at which a given solute
will appear in the urine
Ø  For
example, hyperglycemia results in glucosuria
PCT
Ø  reabsorption
of 60-70%
of filtrate volume
Ø  reabsorbed solutes &
water enter peritubular
capillaries
Ø  secreted solutes enter
the filtrate in the tubule
lumen
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PCT functions
Ø  reabsorption
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organic nutrients
•  List some…
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ions
HCO3- (CO2)
water
osmolarity…
Ø  secretion
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H+
Ø  consider
Na+ role
nephron loop
(loop of Henle)
Ø 
Ø 
Ø 
reabsorbs
l  water
+
l  Na and Cl ions
maintains high [osmotic]
of medulla
provides the means of
concentrating the urine
nephron loop
Ø  significant
l 
regions
descending limb
•  thin segment
•  permeable to water
•  Impermeable to solutes
l 
ascending limb
•  thick segment
•  impermeable to H2O
•  pumps out
l 
Na+ & Cl- ions
Ø  removes
~2/3 of the Na
and Cl that enters the loop
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nephron loop function
Ø 
Ø 
Ø 
pumping of Na and Cl ions
out of ascending limb
elevates osmotic
concentration of peritubular
fluid
results in
l  water flow out of
descending limb
l  increasing solute
concentration in
ascending limb
high concentration in
ascending limb facilitates
more ion pumping
concentration gradient of
medulla
Ø  due
to Na and Cl ion
pumping by
ascending limb
Ø  also…papillary duct is
permeable to urea
Ø  urea adds to the
concentration
gradient of the deep
medulla
benefits of countercurrent
multiplication
Ø  efficient
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l 
reabsorption of
water
solutes
Ø  Later
on…permits
passive reabsorption of
water in collecting system
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DCT
Ø  receives
15-20% of
initial volume filtered
Ø  reabsorption
Ø  secretion
Ø  sensitive
to hormones,
allows for the
adjustment of urine
concentration and
volume
DCT: reabsorption (this slide)
& secretion (next 2 slides)
Ø  reabsorption
of
remaining ions
l 
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Na+ and Clmore Na+ if
aldosterone is
present
Ca++ if PTH is
present
DCT: reabsorption (prev. slide)
& secretion (this slide and next)
Ø  secretion
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of K+
If aldosterone is
present
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DCT: reabsorption (2 slides back)
& secretion (this slide and previous)
Ø  secretion
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of H+
increases as
blood pH falls
two routes
•  plasma pH
decline
•  tubular cell amino
acid deamination
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both generate
HCO3- ions that
are returned to
blood
result: addition of
buffers in blood
Collecting
System
Ø 
Ø 
Includes DCT and collecting
duct
Sensitive to hormones
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Ø 
Reabsorption
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Ø 
Aldosterone
ADH
Na+
HCO3Urea
water
Secretion
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H+ if pHperitubular fluid is 6
HCO3- if pHperitubular fluid is 5
Control of Urine Volume &
Osmotic Concentration
Ø  Controlling
water reabsorption
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water reabsorption
Ø  result
of osmosis along the
nephron
Ø  water permeability cannot be
adjusted
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in PCT
descending loop of Henle
Ø  85%
of filtered water is
reabsorbed – can’t be altered
Ø  “obligatory water reabsorption”
the amount of water in urine
Ø  depends
on how much of the
remaining 15% is reabsorbed in
DCT & collecting system
Ø  “how much” is reabsorbed is
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variable
precisely controlled
called “facultative water reabsorption”
facultative water reabsorption
Ø  controlled
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by ADH
via the insertion of water
channels (aquaporins) in
collecting system
Ø  #
of water channels α to
[ADH]
l 
if [ADH] increases, then
number of water channels
increases…and vice versa
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Absence of ADH
Absence of
ADH
ADH
Overview of renal function
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Normal urine composition
Ø  pH
4.5- 8 (average is 6)
855-1335
Ø  water content 93-97%
Ø  volume 700-2000 ml/day
Ø  color clear yellow (urobilin)
Ø  bacteria none*
Ø  osmolarity
l 
*unless a urinary tract infection
is present
Urine Transport, Elimination,
and Storage
Ø  calyces
Ø  ureter
Ø  bladder
Ø  urethra
urethral sphincters
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Micturition reflex controls
urination
Ø 
Ø 
Ø 
Ø 
Ø 
Ø 
Ø 
sensations of urine in
bladder occur when ~200
ml is in bladder
Bladder contracts and
increases pressure…
again ~1 hour later
repeats, repeats …
sensations increase in
intensity
voluntary relaxation of the
external sphincter relaxes
internal sphincter
urine is released
Micturition reflex may cause
unplanned urination
Ø 
Ø 
Ø 
Ø 
500 ml may result in
strong enough
contractions to force
open internal urethral
sphincter
followed by a reflexive
relaxation of the
external sphincter
leading to unplanned
urination
Capacity: up to 1000 ml
10 ml remaining after
urination
Incontinence: inability to control
urination
Ø  causes
l 
l 
damaged sphincters
decreased muscle tone
Ø  damage
to nervous
system and effects
l 
damaged CNS nervous
supply to external urethral
sphincter
•  loss of voluntary control
l 
•  “automatic bladder”
damaged pelvic nerves
•  distended bladder
•  continual leak of urine
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