Renal Blood Circulation and Glomerular Filtration Flashcards
What is glomerular filtration?
Process whereby a clear fluid from which blood cells and macromolecules (e.g. Proteins) are excluded, is produced from the blood perfusing the glomerulus at the beginning of each nephron
Of the renal plasma flow how much becomes glomerular filtrate?
20%
What is the filtration fraction?
glomerular filtration rate/renal plasma flow
- represents the proportion of the fluid reaching the kidneys that passes into the renal tubules
What is a normal filtration fraction?
Fraction of plasma filtered is 20% of plasma flowing through the kidney
What are the 3 layers of the glomerular capillary wall?
- endothelium
- basement membrane
3, epithelium - podocytes
Describe the endothelium of the glomerular capillary wall?
has fenestrations - 70nm in diameter
- allow solutes and proteins through but not red blood cells
Describe the basement membrane of the glomerular capillary wall?
- has strong negative electrical charges
- Charges associated with proteoglycans
- negative charge repels plasma proteins - has tiny pores
- prevent plasma proteins from going through
Describe the epithelium of the glomerular capillaries?
interdigitated podocyte cells that encircle the outer surface of glomerulus
- filtration slits are found between these cells
Filtration depends on which molecular properties?
- size
- charge - positive ions are filtered more
- molecular weight
Filterability of substances by glomerular capillaries decreases with?
molecular weight
e.g. albumin
What is Dextran?
Polysaccharide that can be manufactured as neutral, -ve or +ve charge
- +ve charged molecules are filtered much more readily than –ve charged molecules
What is minimal change nephropathy?
a condition whereby there is loss of -ve charge in the basement membrane
- happens mostly in children
What is glomerular filtration rate?
The volume of fluid filtered in the glomerulus per unit time
- The sum of the filtration rates of all the
functioning nephrons
What is considered a normal filtration rate?
- 90-140 ml/min for males
- 80-125 ml/min for females
- GFR is ≈ 180 L/day in average human (125
ml/min)
Glomerular filtration rate is proportional to?
body surface area
Glomerular filtration rate can be calculated by?
measuring the excretion and plasma concentration of a substance that is freely filtered
e.g. inulin and creatinine
GFR declines with?
- age
2. renal disease
What are the determinants of GFR?
Starling forces
- Glomerular capillary hydrostatic pressure (Pgc)
- Colloid osmotic pressure (Pπ) in glomerular capillaries
- Bowman’s capsule hydrostatic pressure (Pbs)
What is glomerular capillary hydrostatic pressure?
the primary means for physiological regulation of GFR
- promotes filtration : pushes solute + water in blood plasma through glomerular filter
- Increase in glomerular hydrostatic pressure increases GFR, decrease in hydrostatic pressure reduces GFR
Hydrostatic pressure is determined by?
- arterial pressure
- afferent arteriolar resistance
- efferent arteriolar resistance
Glomerular capillary hydrostatic pressure is estimated at?
55 mm Hg
Describe how the resistance of afferent arteriole can affect GFR?
- Increased resistance of afferent arteriole reduces GFR
- renal blood flow is reduced - Dilation of afferent arteriole increases GFR
- renal blood flow is increased
Describe how the vasoconstriction of efferent arterioles affects the GFR?
- there is a decrease in renal flow out of the glomerulus
- there is more blood in the glomerulus which increases the hydrostatic pressure in the capillaries
- the increase in hydrostatic pressure increases the GFR
Describe how dilation of the efferent arterioles affects the GFR?
- there is an increase in renal blood flow
- there is a decrease in resistance of blood leaving the glomerular capillaries and therefore a decrease in hydrostatic pressure
- a decrease in hydrostatic pressure leads to a decrease in GFR
What is colloid osmotic pressure in glomerular capillaries?
Generated as a result of increased plasma protein concentration after filtration
Describe plasma protein concentration in the efferent arteriole?
increases by 20% after filtration
- depends on arterial pressure and filtration fraction
How does colloid osmotic pressure affect GFR?
- increase in solute concentration increases osmotic pressure and decrease GFR
- decrease in solute concentration decreases osmotic pressure and increases GFR
- water follows solutes
What is considered a normal colloid osmotic pressure?
Pπ ≈ 30 mm Hg
What is Bowmans capsule hydrostatic pressure?
pressure exerted by fluid on the walls of the glomerulus
How does Bowmans capsule hydrostatic pressure affect GFR?
increase in hydrostatic pressure in Bowmans space decreases filtration rate
- Can increase markedly in pathological states (e.g. obstruction of urinary tract)
- water and solute cant move up a pressure gradient
What is considered a normal Bowmans capsule hydrostatic pressure?
≈ 15 mm Hg
How does Bowmans capsule hydrostatic pressure affect GFR?
increase in hydrostatic pressure in Bowmans space decreases filtration rate
- water and solute cant move up a pressure gradient
What is net filtration pressure?
PGC – Pπ – PBS
= 55 - 30 - 15
= 10 mm Hg
Afferent and efferent arterioles are innervated by?
sympathetic nervous system
- Strong activation of SNS causes constriction of renal arterioles
What is the myogenic mechanism in autoregulation of renal blood flow?
Intrinsic capacity of afferent arteriole smooth muscles to increase state of contraction in response to increase in renal perfusion pressure
The myogenic mechanism is mediated by?
vasoactive agents produced by endothelial cells acting on smooth muscle cells of afferent arteriole
How does the myogenic mechanism work?
Increase in vessel diameter opens stretch-activated, non selective cation channels in vascular smooth muscles, depolarising the cell leading to influx of Ca2+ that stimulates contraction
GFR in individual nephrons is regulated according to?
the rate of solute flow in that nephron
Fluid ionic composition is sensed by?
macula densa
Describe the tubuloglomerular feedback?
- macula densa senses increase in NaCl concentration
- macula densa secretes adenosine
- Adenosine acts on adenosine 1 receptor on afferent arteriole to mediate vasoconstriction
Describe the macula densa feedback mechanism for autoregulation during decreased arterial pressure?
Name the 3 hormones that constrict arterioles to decrease RBF and GFR?
Norepinephrine, epinephrine and endothelin
When are the vasoconstricting hormones particularly critical?
during severe haemorrhage
When is endothelin released in particular?
is released by damaged vascular endothelial cells of kidney and other tissues
- Endothelin levels increased during damage to blood vessel
Angiotensin II acts as a?
vasoconstrictor
Angiotensin II preferentially constricts?
efferent arterioles
How does Angiotensin II work?
- Increases glomerular hydrostatic pressure (PGC), while reducing RBF
- more blood enters the glomerulus and less blood leaves it - Maintains excretion of waste substances (e.g. urea, creatinine)
When are Angiotensin II levels increased?
in circumstances of reduced arterial pressure, or volume depletion, preventing decrease in GFR
Name hormones that act as vasodilators?
- Endothelial-derived NO - decreases renal vascular resistance
- Prostaglandins (e.g. PGE2 and PGI2) and bradykinin - vasodilators that oppose vasoconstriction of afferent arteriole
