Transcript Document

Glomerular Filtration
• As blood flows through the glomerulus
protein-free plasma filters through the
glomerular capillaries into Bowman’s capsule
• Normally about 20% of the plasma that enters
the glomerulus is filtered
• This process is known as glomerular
filtration which is the first step in urine
formation
Glomerular Filtration Rate
• The rate at which glomerular filtrate is formed
• GFR is determined by
(1) the balance of hydrostatic and colloid osmotic forces
acting across the capillary membrane
(2) the capillary filtration coefficient (Kf)( the product of
the permeability and filtering surface area of the
capillaries)
• The glomerular capillaries have a much higher rate of
filtration than most other capillaries because of high
glomerular hydrostatic pressure and a large Kf
• In the average adult human the GFR is about 125
ml/min or 180 L/day
•
The fraction of the renal plasma flow that is filtered
(the filtration fraction) averages about 0.2
• This means that about 20 percent of the plasma
flowing through the kidney is filtered through the
glomerular capillaries
Filtration Fraction = GFR/Renal plasma flow
Determinants of the GFR
• The net filtration pressure represents the sum
of the hydrostatic and colloid osmotic forces
that either favor or oppose filtration across the
glomerular capillaries
Forces Favoring Filtration (mm Hg)
Glomerular hydrostatic pressure
Bowman's capsule colloid osmotic pressure
Forces Opposing Filtration (mm Hg)
Bowman's capsule hydrostatic pressure
Glomerular capillary colloid osmotic pressure
60
0
18
32
• Increased Glomerular Capillary Filtration
Coefficient Increases GFR and Decreased
Glomerular Capillary Filtration Coefficient
Decreases GFR
The Kf is a measure of the product of the hydraulic
conductivity and surface area of the glomerular
capillaries
• Increased Bowman's Capsule Hydrostatic
Pressure Decreases GFR
(obstruction caused by stones)
• Increased Glomerular Capillary Colloid
Osmotic Pressure Decreases GFR
• Two factors that influence the glomerular
capillary colloid osmotic pressure are
(1) the arterial plasma colloid osmotic pressure
(2) the fraction of plasma filtered by the
glomerular capillaries (filtration fraction)
• Increased Glomerular Capillary Hydrostatic Pressure
Increases GFR
• Changes in glomerular hydrostatic pressure serve as the
primary means for physiologic regulation of GFR
• Increased resistance of afferent arterioles reduces
glomerular hydrostatic pressure and decreases GFR and
dilation of the afferent arterioles increases both
glomerular hydrostatic pressure and GFR
• Efferent arteriolar constriction has a biphasic effect on
GFR
Renal Blood Flow
• Renal blood flow is 1100ml/min (22% of the
cardiac out put)
Determinants of Renal Blood Flow
• Changes in arterial pressure influence renal
blood flow but the kidneys have effective
mechanisms for maintaining renal blood flow
and GFR relatively constant over an arterial
pressure range between 75 and 160 mm Hg, a
process called autoregulation
• Blood Flow in the Vasa Recta of the renal medulla
is very low as compared with flow in the renal
cortex
• The outer part of the kidney the renal cortex
receives most of the kidney's blood flow
• Blood flow in the renal medulla accounts for only
1 to 2 percent of the total renal blood flow
• Flow to the renal medulla is supplied by a
specialized portion of the peritubular capillary
system called the vasa recta
Physiologic Control of Glomerular
Filtration and Renal Blood Flow
• Sympathetic nervous system activation decreases
GFR and the renal blood flow due to constriction
of the renal arterioles
• The renal sympathetic nerves are important in
reducing GFR during severe acute disturbances
lasting for a few minutes to a few hours such as
severe hemorrhage
Hormonal and Autacoid Control of
Renal Circulation
• Norepinephrine, Epinephrine, and Endothelin
constrict renal blood vessels and decrease GFR
• Angiotensin II preferentially constricts efferent
arterioles
• In most physiologic conditions increased
angiotensin II levels raise glomerular hydrostatic
pressure while reducing renal blood flow
• Increased angiotensin II formation usually occurs
in circumstances associated with decreased
arterial pressure or volume depletion which tend
to decrease GFR
• In these circumstances the increased level of
angiotensin II by constricting efferent arterioles
helps to prevent decrease in glomerular
hydrostatic pressure and GFR
• Endothelial-derived Nitric Oxide decreases
renal vascular resistance and increases GFR
• Prostaglandins and Bradykinin tend to Increase
GFR
Autoregulation of GFR and Renal
Blood Flow
• Feedback mechanisms intrinsic to the kidneys
normally keep the renal blood flow and GFR
relatively constant despite marked changes in
arterial blood pressure
• This relative constancy of GFR and renal blood
flow is referred to as autoregulation
• The major function of autoregulation in the
kidneys is to maintain a relatively constant GFR
and to allow precise control of renal excretion of
water and solutes
Importance of GFR Autoregulation in Preventing
Extreme Changes in Renal Excretion
• Normally GFR is about 180 L/day and tubular
reabsorption is 178.5 L/day leaving 1.5 L/day of fluid
to be excreted in the urine
• In the absence of autoregulation relatively small
increase in blood pressure (from 100 to 125 mm Hg)
would cause a 25 percent increase in GFR (from about
180 to 225 L/day)
• If tubular reabsorption remained constant at 178.5
L/day this would increase the urine flow to 46.5 L/day
• Because the total plasma volume is only about 3 liters,
such a change would quickly deplete the blood volume
Tubuloglomerular Feedback and
Autoregulation of GFR
• The kidneys have a feedback mechanism that
links changes in sodium chloride concentration
at the macula densa with the control of renal
arteriolar resistance
• This feedback helps ensure a relatively
constant delivery of sodium chloride to the
distal tubule and helps prevent fluctuations in
renal excretion
• The tubuloglomerular feedback mechanism
has two components that act together to
control GFR:
(1)an afferent arteriolar feedback mechanism
(2) an efferent arteriolar feedback mechanism
• Decreased GFR slows the flow rate in the loop of Henle
causing increased reabsorption of sodium and chloride ions
and reducing the concentration of sodium chloride at the
macula densa cells
• This decrease in sodium chloride concentration initiates a
signal from the macula densa that has two effects
(1) It decreases resistance to blood flow in the afferent
arterioles which raises glomerular hydrostatic pressure and
helps return GFR toward normal
(2) It increases renin release from the juxtaglomerular cells
of the afferent and efferent arterioles
• Renin acts as an enzyme to convert
angiotensinogen into angiotensin I
• Angiotensin I is converted to angiotensin II by
the angiotensin converting enzyme
• The angiotensin II constricts the efferent
arterioles, increasing glomerular hydrostatic
pressure and helping to return GFR toward
normal
Myogenic Autoregulation of Renal
Blood Flow and GFR
• The myogenic constrictor response in afferent
arterioles occurs within seconds and prevents
transmission of increased arterial pressure to
the glomerular capillaries
• High protein intake and increased blood
glucose increase renal blood flow and GFR