Transcript NVCC Bio 212

Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 25
Urinary system
Lecture 15
Part 3:
Urine Formation, GFR, Control of GFR
Glomerular Filtration Rate (GFR)
NFP
=
HPg
–
(HPc + OPg)
Net Filtration Pressure = force favoring filtration – forces opposing filtration
(*glomerular capillary
( capsular hydrostatic pressure
hydrostatic pressure)
+ glomerular capillary
osmotic pressure )
Figure from:
Hole’s Human
A&P, 12th
edition, 2010
Net filtration
pressure is normally
positive, i.e., favors
the movement of
fluid out of the
glomerular
capillaries
GFR = amount of
filtrate produced each
minute (~125 ml/min)
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Glomerular Filtration Rate (GFR)
Net filtration pressure, although normally positive, is
relatively low ( 10 mm Hg)
Glomerular hydrostatic pressure is the blood pressure in the
glomerular capillaries, and is usually higher than the other
systemic capillary pressures
Capsular hydrostatic pressure tends to push water and
filtrate BACK into the capillaries
Anything that alters the filtration pressures will alter GFR
* Blood pressure is the most important factor altering the
glomerular hydrostatic pressure (and NFP).
A MAP fall of 10% at the glomerulus will severely impair
glomerular filtration; a fall of 15-20% will stop it.
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Amounts of Glomerular Filtrate and Urine
Figure from: Hole’s Human A&P, 12th edition, 2010
Glomerular Filtration Rate
(GFR) is directly proportional
to the net filtration pressure
GFR  125 ml/min (180 L/day)
This means that all of the
plasma is filtered ~ 60x every
day (How did we get this
number?)
Notice that urine output is only
0.6 – 2.5 L per day (an average
of about 1.8 L, or about 1% of
glomerular filtrate); 99% of
filtrate is reabsorbed!!
average amounts over a 24 hour period
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Afferent/Efferent Arterioles – Effect on GFR
Innervated by sympathetic nerves
• Afferent arteriole
– Δ radius  GFR
–  radius  GFR;  radius  GFR
• Efferent arteriole
– Δ radius  1/GFR
–  radius  GFR;  radius  GFR
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Summary of Factors Affecting GFR
Factor
Effect
Vasoconstriction (↑ Sympathetic stimulation)
Afferent arteriole
 GFR
Efferent arteriole
↑ GFR
Vasodilation ( Sympathetic stimulation)
Afferent arteriole
↑ GFR
Efferent arteriole
 GFR
Increased capillary hydrostatic pressure
↑ GFR
Increased colloid osmotic pressure
 GFR
Increased capsular hydrostatic pressure
 GFR
Know this table – it’s important!
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Regulation of GFR
• Autoregulation
– Maintains GFR despite changes in local blood pressure and
blood flow (between 90 – 180 mm Hg mean systemic
pressure)
– Myogenic mechanism – contraction of afferent arteriolar
vascular smooth muscle when stretched (increased BP);
relaxation occurs when BP declines
– Tubuloglomerular mechanism – MD cells detect  flow rate
and/or  osmolarity of filtrate in DCT -> JG cells contract ->
afferent arteriole constricts ->  GFR
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Regulation of GFR
• Neural (Autonomic) Regulation
– Mostly sympathetic postganglionic fibers =
vasoconstriction of afferent arterioles  GFR
(conserves water, redirects blood to other organs)
– Stimulates juxtaglomerular apparatus to secrete renin
– May override autoregulatory mechanism at afferent
arteriole
• Hormonal Regulation
– Renin-angiotensin system – stabilizes BP and ECF
volume
– Atrial Natriuretic Peptide (ANP) - ↑ GFR, ↑ fluid loss
(dilates afferent arteriole, constricts efferent arteriole)
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Renin-Angiotensin System
Figure from:
Hole’s Human
A&P, 12th
edition, 2010
Renin is released by the
juxtaglomerular
apparatus due to:
1) Decline of BP
(Renin  1/Pressure)
(ACE)
2) Juxtaglomerular
stimulation by
sympathethic NS
3) Decline in osmotic
concentration of
tubular fluid at
macula densa
( Renin  1/[NaCl] )
Stabilizes systemic blood pressure and extracellular fluid volume
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Review
• Glomerular filtration
– Glomerular filtration rate (GFR)
•
•
•
•
Amount of filtrate produced each minute
Directly proportional to net filtration pressure
May be determined with creatinine or inulin tests
Approximately 125 ml/min (180 L/day)
– Factors affecting GFR
•
•
•
•
Vasoconstriction / vasodilation
Capillary hydrostatic pressure
Capsular hydrostatic pressure
Capillary osmotic pressure
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Review
• Glomerular filtration (cont’d)
– Factors controlling GFR
• Autoregulation
– Myogenic
– Tubuloglomerular
• Hormonal
– Renin-Angiotensin System
– ANP
• Autonomic nervous system
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