Renal Blood Supply L08 Flashcards
What are the vasa recta?
- loops of blood vessels in the renal medulla (look like loops of Henle)
- stem from efferent glomerular arterioles after they become portal vessels
- absorb water and solvents from interstitial space
- secrete substances into interstitial space which can be taken up by tubules and secreted into filtrate
Explain how osmolality and plasma flow rate change through the vasa recta.
osmolality:
- on descending limb (of vasa recta), osmolality increases due to passive absorption of Na+ (and other ions) and a small amount of water movement into interstitium.
- at tip of loop = osmolality of interstitium
- on ascending limb, osmolalility decreases due to small secretion of Na+ and large absorption of water from interstitium.
- as a result, net movement of Na+ and water is into the vessels
- due to high oncotic pressure and low hydrostatic pressure in vessel
flow rate:
- decreases on descending limb due to loss of water
- increases on ascending limb due to absorption of water
Which two mechanisms are involved in the autoregulation of renal blood flow?
- myogenic response
- tubuloglomerular reflex
Describe how renal blood flow changes with changes in blood pressure?
- similar to logarithmic curve pattern
- at low pressures, as the pressure increases, flow increases
- when blood pressure reaches around 90mmHg until 200mmHg, as pressure increases, rbf remains constant (graph essentially plateaus)*
- at very high pressures (>200), flow cannot be contained so rbf increases with an increase in pressure
*blood vessels are constricting as pressure increases to maintain flow at the same rate
What is the myogenic response to renal blood flow?
when an afferent arteriole stretched due to increased pressure, a reflex is triggered which causes vasoconstriction. This increases resistance to blood flow in the vessel, thereby reducing the flow rate.
With the myogenic response, GFR can be maintained regardless of the mean arterial pressure. Also, it could help to reduce the impact of high systolic pressures on renal arterioles.
What does Pouiselle’s equation imply about the relationship between arterial diameter (or radius) and flow rate?
only a small decrease in diameter* is needed to decrease the renal flow rate and pressure in the arteriole.
*note: r^4
Describe the cellular mechanism for the myogenic response.
depolarisation of smooth muscle cells due to stretch-dependent cation-channels > opening of voltage-gated Ca2+ channels >Ca2+ influx >contraction of smooth muscle >vasoconstriction
Describe tubuloglomerular feedback mechanism which is key to reducing GFR.
high GFR > high Na+ in distal tubule > high Na+ sensed by macula densa cells of distal tubule through high activity of NKCC2 > macula densa cells release ATP > ATP broken down into adenosine > adenosine causes vasoconstriction of afferent arteriole > lower glomerular hydrostatic pressure > negative feedback > fall in GFR
What factors in the body oppose autoregulation of renal blood flow?
- renal innervation
- circulating hormones
How can nerves innervating the kidney have an effect in regulating renal blood flow?
- dense plexus of nerves innervating renal vessels
- sympathetic nerves releasing noradrenaline > vasoconstriction
Why do sympathetic nerves cause renal arterioles to vasoconstrict in the presence of hypotension?
- body wants to keep salts and water in blood plasma when there is low blood pressure
- so vasoconstriction restricts flow to the kidneys
- water retention
- increase blood pressure to normal
How do we measure renal plasma flow?
- measure using substance that is completely removed from plasma and is lost in urine
- PAH is secreted fully from the plasma and passes out of the body in urine
Similar equation to measuring GFR with creatinine:
RPF x Cpah,p = Cpah,u x V(dot)
RPF = (Cpah,u x V(dot))/Cpah,p
where Cpah,u = concentration of PAH in urine
Cpah,p = concentration of PAH in plasma
V(dot) = urine flow rate (rate of urine production)
*RPF measured in L/min
