1. Anatomy and Physiology Part 3: Urine Formation, GFR, Control of GFR
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
Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1]
Slides 16 – 38; 50 min [Lecture 2]
118 min (38 slides plus review of course Web sites and syllabus)

2. 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 )
Net filtration pressure is normally positive, i.e., favors the movement of fluid out of the glomerular capillaries
Figure from: Hole’s Human A&P, 12th edition, 2010
GFR = amount of filtrate produced each minute (~125 ml/min)

3. 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.

4. Amounts of Glomerular Filtrate and Urine
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!!
Figure from: Hole’s Human A&P, 12th edition, 2010
For every 1mm Hg filtration pressure, the kidneys produce about 12.5 ml of filtrate. This is called the filtration coefficient. (Men, ~ 125ml/min, women, ~ 105 ml/min.)
average amounts over a 24 hour period

5. 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

6. Summary of Factors Affecting GFR
Effect
Vasoconstriction (↑ Sympathetic stimulation)
Afferent arteriole
 GFR
Efferent arteriole
↑ GFR
Vasodilation ( Sympathetic stimulation)
Increased capillary hydrostatic pressure
Increased colloid osmotic pressure
Increased capsular hydrostatic pressure
Know this table – it’s important!

7. 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

8. Neural (Autonomic) Regulation
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)

9. Renin-Angiotensin System
Renin is released by the juxtaglomerular apparatus due to:
1) Decline of BP (Renin  1/Pressure)
2) Juxtaglomerular stimulation by sympathethic NS
3) Decline in osmotic concentration of tubular fluid at macula densa ( Renin  1/[NaCl] )
Figure from: Hole’s Human A&P, 12th edition, 2010
(ACE)
Renin secretion is inversely proportional to 1) arteriolar pressure and 2) amount of Na+ and Cl- entering the distal tubules.
Stabilizes systemic blood pressure and extracellular fluid volume

10. 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

11. Review Glomerular filtration (cont’d) Factors controlling GFR
Autoregulation
Myogenic
Tubuloglomerular
Hormonal
Renin-Angiotensin System
ANP
Autonomic nervous system