Lecture 3 Flashcards
During Glomerular Filtration, Fluid filtered from the glomerulus into Bowman’s capsule must pass through the three layers that make up the glomerular membrane:
- The glomerular capillary wall
- The basement membrane
- The inner layer of Bowman’s capsule
- These layers function as a fine molecular sieve
The glomerular capillary wall is perforated by many large pores
- 100 times more permeable to H2O and solutes
The endothelial cells of the glomerular membrane themselves are also
perforated by large holes or fenestrations
The basement membrane of the glomerular membrane is an
acellular (lacking cells) gelatinous layer composed of collagen and glycoproteins that is sandwiched between the glomerulus and Bowman’s capsule
Plasma proteins are almost completely excluded from the filtrate
Urine is normally protein free
The inner layer of Bowman’s capsule consists of
podocytes
- octopus like epithelial cells that encircle the glomerulus.
The glomerular membrane also has
Filtration slits
To accomplish glomerular filtration, a force must drive a part of the plasma in the glomerulus through the openings in the glomerular membrane
Passive physical forces similar to those acting across capillaries elsewhere accomplish glomerular filtration.
[Glomerular capillary blood pressure]
Glomerular capillary blood pressure
The same principles of fluid dynamics apply here that cause ultrafiltration across other capillaries, except for two important differences:
- The glomerular capillaries are more permeable than capillaries elsewhere, so more fluid is filtered for a given filtration pressure
- The balance of forces across the glomerular membrane is such that filtration occurs the entire length of the capillaries.
Three physical forces are involved in glomerular filtration
- Glomerular capillary blood pressure
- Plasma-colloid osmotic pressure
- Bowman’s capsule hydrostatic pressure.
Glomerular Filtration Rate
The net filtration pressure of 10 mm Hg forces large volumes of fluid from the blood through the highly permeable glomerular membrane.
The actual rate of filtration
= the glomerular filtration rate (GFR)
the glomerular filtration rate (GFR) depends on
The net filtration pressure
How much glomerular surface area is available for penetration
How permeable the glomerular membrane is
[These properties of the glomerular membrane are collectively referred to as the filtration coefficient (Kf)]
GFR = Kf x net filtration pressure
Average GFR of 125 mL/min in males
Unregulated Influences on the GFR
Plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic pressure normally do not vary much and cannot be regulated.
Controlled Adjustments in the GFR
- Glomerular capillary blood pressure can be controlled to adjust the GFR to suit the body’s needs
Two major control mechanisms regulate the GFR:
- Autoregulation
- Extrinsic sympathetic control
Mechanisms Responsible for Autoregulation of the GFR
GFR would increase in direct proportion to an increase in arterial pressure
A fall in arterial blood pressure would cause a decline in GFR
Autoregulation by the kidneys
The net filtration pressure and GFR can be reduced to normal by constriction of the afferent arteriole
Glomerular pressure can be increased to normal by vasodilation of the afferent arteriole
Two mechanisms contribute to autoregulation of the GFR:
- A myogenic mechanism
- A tubuloglomerular feedback mechanism
A myogenic mechanism
- which responds to changes in pressure within the nephron’s vascular component
A tubuloglomerular feedback mechanism
- which senses changes in salt level in the fluid flowing through the nephron’s tubular component.
> Granular cells
> Macula densa
Macula densa - function in the Autoregulation of the GFR
chief cells within the kidney, playing key sensory and regulatory functions in the maintenance of body fluid, electrolyte homeostasis, and blood pressure.
- responds to the change in the Na concentration in the distal tubules, signals the juxtaglomerular cells to release renin.
- this triggers contraction of the afferent arteriole, reducing flow of blood to the glomerulus and the glomerular filtration rate.
> Release ATP and adenosine, both of which act locally as a paracrine
- Nitric oxide in macula densa can dilate afferent arterioles.
Importance of Autoregulation of the GFR
- The myogenic and tubuloglomerular feedback mechanisms work in unison to autoregulate the GFR within the mean arterial blood pressure range of 80 to 180 mm Hg
- Autoregulation is important because unintentional shifts in GFR could lead to dangerous imbalances of fluid, electrolytes, and wastes.
- When changes in mean arterial pressure fall outside the autoregulatory range, these mechanisms cannot compensate.
Importance of Autoregulation of the GFR
- The myogenic and tubuloglomerular feedback mechanisms work in unison to autoregulate the GFR within the mean arterial blood pressure range of 80 to 180 mm Hg
- Autoregulation is important because unintentional shifts in GFR could lead to dangerous imbalances of fluid, electrolytes, and wastes.
- When changes in mean arterial pressure fall outside the autoregulatory range, these mechanisms cannot compensate.
Importance of Extrinsic Sympathetic Control of the GFR
Extrinsic control mechanisms override the autoregulatory responses
Mediated by sympathetic nervous system input to the afferent arterioles
The parasympathetic nervous system
does not exert any influence on the kidneys
Baroreceptor Reflex in Extrinsic Control of the GFR
- The afferent arterioles are innervated with sympathetic vasoconstrictor fibers to a far greater extent than the efferent arterioles are
- Increase/decrease in GFR
- Increased/decreased tubular reabsorption of H2O and salt regulate plasma volume
GFR depends on
the filtration coefficient (Kf) as well as on the net filtration pressure
Kf is subject to change under physiologic control
The surface area and the permeability of the glomerular membrane can be modified by
contractile activity within the membrane.
The surface area available for filtration - The inner surface of the glomerular capillaries that comes into contact with blood
mesangial cells
Podocytes
The kidneys and the cardiac output
20% of the plasma that enters the kidneys is converted into glomerular filtrate
An average GFR of 125 mL/min - the total renal plasma flow must average about 625 mL/min.
55% of whole blood consists of plasma - the total flow of blood through the kidneys averages 1140 mL/min.
This quantity is about 22% of the total cardiac output of 5 L/min
Most of the blood goes to the kidneys not to supply the renal tissue but to be adjusted and purified by the kidneys
