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BIOH122Human Biological Science 2

Session 17

Urinary System 2 –

Glomerular Filtration

Bioscience Department

© Endeavour College of Natural Health endeavour.edu.au 2

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


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.



o Rate of excretion of any substance= its rate of

filtration + its rate of secretion - its rate of

reabsorption



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.



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



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


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.


Net Filtration Pressure


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



Tortora, and Derrickson, 2012


Regulation of GFR


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).


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


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


Renal Autoregulation

of GFR


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



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


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


Marieb, and Hoehn, 2011

Myogenic Regulation of GFR


Tubuloglomerular Feedback

regulation of GFR


Hormonal regulation of GFR

Renin Angiotensin

Aldosterone System

(RAAS)



Sympathetic Nervous System


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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