Lecture 33: Glomerular Filtration

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Renal Physiology

Overview of ---
-Nephrons and collecting ducts perform 3 basic processes
1) Glomerular Filtration - a portion of the blood plasma is filtered into the kidney
2) Tubular Reabsorption - water and useful substances are reabsorbed into the blood
3) Tubular Secretion

Glomerular Filtration

a portion of the blood plasma is filtered into the kidney

Tubular Reabsorption

water and useful substances are reabsorbed into the blood

Excretion

Rate of --- of any substance is its rate of filtration, plus its rate of secretion, minus its rate of reabsorption
--- = Glomerular filtration + Secretion - Reabsorption

Glomerular Filtration

-Blood pressure produces glomerular filtrate
-Filtration fraction is 20% of plasma
-48 Gallons/day filtrate reabsorbed to 1-2 qt. urine
-Filtering capacity enhanced by: thinness of membrane & large surface area of glomerular capillaries; glomerular capillary blood pressure is high due to small size of efferent arteriole

Filtration Membrane

1) Endothelial fenestration (pore) of glomerulus: stops all cells and platelets
2) Basal lamina of glomerulus: Stops large plasma proteins
3) Slit membrane between pedicels: stops medium-sized proteins, not small ones

Net Filtration Pressure

(NFP) = total pressure that promotes filtration
NFP = GBHP - (CHP + BCOP) = 10mmHg

Glomerular Filtration Rate

(GFR)
-Amount of filtrate formed in all renal corpuscles of both kidneys / minute (average adult rate is 125 mL/min)
-Homeostasis requires GFR that is constant
-Too high & useful substances are lost due to the speed of fluid passasge through nephron
-too low and sufficient waste products may not be removed from the body

Glomerular Filtration Rate

(GFR)
NFP = GHBP - CHP - BCOP
Mean Arterial Pressure > Afferent Arteriole Pressure > GHBP > NFP >GFR

Glomerular Filtration Rate

(GFR)
-Changes in net filtration pressure affect GFR
-Filtration stops if GBHP drops from 55mm Hg to 45mm Hg
NFP = GHBP is determined by MAP
-kidney functions normally with mean arterial pressures 80-180 mmHG because of GFR regulation

Glomerular Filtration Rate

(GFR) Regulation
1) Autoregulation of GFR
2) Neural Regulation of GFR
3) Hormonal Regulation of GFR

Autoregulation of GFR

Mechanisms that maintain a constant GFR despite changes in arterial blood pressure
-myogenic mechanism
-tubuloglomerular feedback

Myogenic mechanism

Autoregulation of GFR
--- (Fast)
-Systemic increases in blood pressure stretch the afferent arteriole
-Smooth muscle contraction reduces the diameter of the afferent arteriole returning the GFR to its previous level in seconds

Tubuloglomerular feedback

Autoregulation of GFR
--- (Slow)
-Elevated systemic blood pressure raises the GFR so that fluid flows too rapidly through the renal tubule -> Na+, Cl- and water are not reabsorbed
-Macula densa in ascending limb of loop of Henle detects increased Na+ and Cl- -> inhibit release of Nitric Oxide (a vasodilator) from the juxtaglomerular apparatus
-afferent arteriole constrict -> reduce GFR

Neural Regulation

--- --- of GFR:
-Blood vessels of the kidney are supplied by sympathetic fibers that cause vasoconstriction of afferent arterioles
-Norepinephrine release from sympathetic postganglionic neurons binds to alpha 1 adrenergic receptors and causes vasoconstriction of afferent arterioles
-SNS will overide renal autoregulation
-SNS will also stimulate renin release from the juxtaglomerular cells (see hormonal regulation)

Neural Regulation

--- --- of GFR:
-At rest, renal blood vessels are maximally dilated because sympathetic activity is minimal -- renal autoregulation prevails
-With moderate sympathetic stimulation, both afferent & efferent arterioles constrict equally - GFR decreases slightly
-With extreme sympathetic stimulation (exercise or hemorrhage), vasoconstriction of afferent arterioles predominates
-GFR decreases substantially
-lowers urine output to maintain blood volume
-permits greater blood flow to other tissues (i.e. muscle)

Hormonal Regulation

--- --- of GFR:
-Atrial natriuretic peptide (ANP) increases GFR
-Stretching of the atria that occrs with an increase in blood volume causes hormonal release
-relaxes glomerular mesangial cells increasing glomerular capillary surface are and increasing GFR

hormonal regulation

--- --- of GFR:
Angiotensin II reduces GFR
-Partly activated by renin from the juxtaglomerular cells
-potent vasoconstrictor that narrows both afferent & efferent arterioles reducing GFR

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