Chapter 25Urinary system

Lecture 15

Part 3: Urine Formation, GFR, Control of GFR

Marieb’s HumanAnatomy and

PhysiologyNinth Edition

Marieb Hoehn

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Glomerular Filtration Rate (GFR)

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)

Net Filtration Pressure = force favoring filtration – forces opposing filtration (*glomerular capillary ( capsular hydrostatic pressure hydrostatic pressure) + glomerular capillary osmotic pressure )

NFP = HPg – (HPc + OPg)

Figure from: Hole’s Human A&P, 12th edition, 2010

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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

average amounts over a 24 hour period

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

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Afferent/Efferent Arterioles – Effect on GFR

• Afferent arteriole– Δ radius GFR radius GFR; radius GFR

• Efferent arteriole– Δ radius 1/GFR radius GFR; radius GFR

Innervated by sympathetic nerves

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Summary of Factors Affecting GFRFactor 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

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] )

Stabilizes systemic blood pressure and extracellular fluid volume

(ACE)

Figure from: Hole’s Human A&P, 12th edition, 2010

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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