Post on 06-Mar-2018
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BIOH122Human Biological Science 2
Session 17
Urinary System 2 –
Glomerular Filtration
Bioscience Department
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Session Plan
o Overview of Renal Physiology • Glomerular filtration
• Tubular reabsorption
• Tubular secretion
o Glomerular Filtration • Filtration Membrane
• Net Filtration Pressure
• Glomerular Filtration Rate
o Regulation of GFR• Renal Autoregulation
• Neural Regulation
• Hormonal Regulation
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Overview of Renal Physiology
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Overview of Renal Physiology
o Nephrons and collecting ducts perform 3 basic
processes
• Glomerular filtration: Filtration of waste-laden blood in
the glomerulus.
• Tubular reabsorption: The process of returning
important substances from the filtrate back to the
body.
• Tubular secretion: The movement of waste materials
from the body to the filtrate.
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Overview of Renal Physiology
o Rate of excretion of any substance= its rate of
filtration + its rate of secretion - its rate of
reabsorption
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Glomerular Filtration
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Glomerular Filtration
o Glomerular filtration: The formation of a protein-free filtrate (ultra filtrate) of plasma across the glomerular membrane.
o Glomerular filtrate: Fluid that enters capsular space
• Daily volume 150-180 liters – more than 99% returned to blood plasma via tubular reabsorption
• In the average adult, the body’s entire extracellular fluid volume is filtered about 12 times per day.
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Glomerular Filtration
o Filtration membrane: A leaky barrier formed by the glomerular capillaries and the podocytes, which completely encircle the capillaries
o Significance:• Permits filtration of water and small solutes
• Prevents filtration of most plasma proteins, blood cells and platelets
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Glomerular Filtration
o Three filtration barriers:
• Glomerular endothelial cells fenestrations
• Basal lamina between endothelium
• A filtration slit formed by a podocyte
o Volume of fluid filtered is large because of large surface
area, thin and porous membrane, and high glomerular
capillary blood pressure
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Glomerular Filtration
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Net Filtration Pressure
o Net Filtration Pressure (NFP): The total pressures that drive
glomerular filtration
o Pressures that determine NFP:
• Glomerular Blood Hydrostatic Pressure (GBHP): Promotes filtration.
• Capsular Hydrostatic Pressure (CHP): Opposes filtration.
• Blood Colloid Osmotic Pressure (BCOP): Opposes filtration.
o NFP = GBHP - CHP - BCOP
• The NFP of only about 10 mm Hg causes a normal
amount of blood plasma (minus plasma proteins) to filter
from the glomerulus into the capsular space.
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Net Filtration Pressure
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Glomerular Filtration Rate
o Glomerular Filtration Rate (GFR): The amount of filtrate
formed in all the renal corpuscles of both kidneys each
minute.
• average adult male rate is 125 mL / min
o Homeostasis of body fluids: requires constant GFR
• Too high: Useful substances are lost due to the speed
of fluid passage through the nephron
• Too low: Sufficient waste products may not be
removed from the body
o Directly related to NFP
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Glomerular Filtration
Tortora, and Derrickson, 2012
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Regulation of GFR
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Regulation of GFR
o Three principal mechanisms:
• Renal autoregulation occurs when the kidneys
themselves regulate GFR.
• Neural regulation occurs when the ANS regulates
renal blood flow and GFR.
• Hormonal regulation involves angiotensin II and atrial
natriuretic peptide (ANP).
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Renal Autoregulation of GFR
o Renal autoregulation: Mechanisms that maintain a
constant GFR despite changes in arterial BP
• Myogenic mechanism
– Increases in Systemic BP, stretch the afferent
arteriole
– Smooth muscle contraction reduces the diameter
of the arteriole returning the GFR to its previous
level in seconds
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Renal Autoregulation of GFR
• Tubuloglomerular feedback
– Elevated systemic BP raises the GFR so that fluid
flows too rapidly through the renal tubule and Na+,
Cl- and water are not reabsorbed
– Macula densa detects that difference and releases
a vasoconstrictor from the juxtaglomerular
apparatus
– Afferent arterioles constrict and reduce GFR
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Renal Autoregulation
of GFR
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Neural Regulation of GFR
o Sympathetic input: Blood vessels of the kidney are
supplied by sympathetic fibers that release
norepinephrine cause vasoconstriction of afferent
arterioles
Tortora, and Derrickson, 2012
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Neural Regulation of GFR
o At rest, renal blood vessels are maximally dilated
because sympathetic activity is minimal
• renal autoregulation prevails
o With moderate sympathetic stimulation, both afferent
and efferent arterioles constrict equally
• decreasing GFR equally
o With extreme sympathetic stimulation (exercise or
haemorrhage), vasoconstriction of afferent arterioles
reduces GFR & reduced urine blood flow
• lowers urine output and permits blood flow to other
tissues
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Hormonal Regulation of GFR
o Atrial natriuretic peptide (ANP) increases GFR
• stretching of the atria that occurs with an increase in
blood volume causes hormonal release
– relaxes glomerular mesangial cells increasing
capillary surface area and increasing GFR
o Angiotensin II reduces GFR
• potent vasoconstrictor that narrows both afferent and
efferent arterioles reducing GFR
Marieb, and Hoehn, 2011
Myogenic
Tubuloglomerular
Feedback
Hormonal
Neural
Regulation of GFR: Summary
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Marieb, and Hoehn, 2011
Myogenic Regulation of GFR
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Tubuloglomerular Feedback
regulation of GFR
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Hormonal regulation of GFR
Renin Angiotensin
Aldosterone System
(RAAS)
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Neural Regulation of GFR
Sympathetic Nervous System
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Readings and Resources
o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy and Physiology, 14th edn, Wiley.
o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby’s Dictionary of Medicine, Nursing and Health Professions, 2nd edn, Mosby Elsevier.
o Guyton, AC & Hall, JE 2011, Textbook of Medical Physiology, 12th edn, Saunders Elsevier.
o Marieb, EN & Hoehn, K 2011, Human Anatomy and Physiology, 9th edn, Benjamin Cummings Pearson.
o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically Orientated Anatomy, 6th edn, Lippincott, Williams & Wilkins.
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