Renal Blood Supply Flashcards
Describe the course of the vessels that precede the vasa recta.
- The efferent arterioles from the glomeruli run into portal vessels in the cortex.
- The portal vessels plunge into the medulla, forming a hairpin loop (the vasa recta).
List 2 functions of the vasa recta.
1 - To absorb water and solvents from the interstitial space after processing by the tubules.
2 - To supply substances into the interstitial spaces. Since the capillaries are permeable (excluding proteins), the vasa recta dilutes the interstitium at the descending limb but concentrates it on the ascending limb.
Describe the changes in osmolarity through the course of the vasa recta.
- There is a countercurrent system within the vasa recta.
- As the vasa recta descends, sodium enters the vasa recta and water moves out as the vasa recta is hypoosmolar here. These changes increase osmolarity.
- As the vasa recta ascends, sodium leaves the vasa recta and water moves in as the vasa recta is hyperosmolar here. These changes decrease osmolarity.
What is the net movement of sodium and water between the vasa recta and medullary interstitial space?
List the contributing forces to this net movement.
- There is a net inward movement for both sodium and water.
- This is due to the low hydrostatic pressure in the vasa recta, high hydrostatic pressure in the interstitial space and high oncotic pressure in the vasa recta.
What are the flow changes in the vasa recta?
- Flow rate in the descending vasa recta must decrease as water leaves the capillaries.
- Flow rate in the ascending vasa recta must increase as water enters the capillaries.
List the 2 autoregulation mechanisms of renal blood flow.
How do they differ functionally?
1 - The myogenic response, which regulates the total renal blood flow.
2 - The tubuloglomerular reflex, which regulates a single nephron GFR, but affects total renal blood flow if many individual nephrons are affected.
What is the (hypothetical) relationship between flow rate and pressure in a non-elastic tube?
They are directly proportional.
What is the (hypothetical) relationship between flow rate and pressure in an elastic tube?
Flow rate increases exponentially with pressure.
Describe and explain the relationship between renal flow rate and blood pressure.
- On a graph of renal blood flow (y axis) over blood pressure (x axis), the graph is a sigmoid.
- This is because the autoregulatory range of blood pressure begins and ends at the beginning and end of the plateau.
Explain how the myogenic response works to autoregulate renal blood flow.
- Flow Rate = πΔP*R^4 / 8ηl
- R=radius, ΔP=Pressure difference, η=viscosity, l=length of vessel.
- When the afferent arterioles are stretched, they contract (an increase in perfusion pressure causes vasoconstriction) as stretch-activated cation channels depolarise the smooth muscle cells by increasing calcium influx. This results in a decreased radius and increased resistance, therefore reducing flow rate.
- This reduction in radius offsets the increase in flow you might expect from an increase in pressure.
What is the Bayliss effect?
Another term for the myogenic response.
What is the macula densa?
Where is it found?
- An area of closely-packed cells that sense changes in Na+ and Cl- osmolality in the filtrate.
- They line the wall of the distal tubule at the point where the thick ascending limb meets the distal convoluted tubule.
Explain how the tubuloglomerular reflex works to autoregulate renal blood flow.
1 - High sodium in the filtrate of the distal tubule (an indicator of high GFR) is sensed at the macula densa by an NKCC2-dependent mechanism.
2 - Cells of the macula densa release ATP that is broken down into adenosine.
3 - Adenosine causes vasoconstriction of afferent arterioles, as the distal tubule runs close to the glomerulus.
4 - This leads to a fall in glomerular hydrostatic pressure and a fall in GFR.
List 2 factors that oppose renal autoregulation of blood flow.
1 - Renal innervation.
2 - Circulating hormones.
- These factors help to match the ‘needs’ of the body against the ‘wants’ of the kidneys.
Are most of the efferent nerves supplying the kidney sympathetic or parasympathetic?
What is the consequence of stimulating these nerves?
What is the key stimulus for stimulation of these nerves and why?
- Sympathetic.
- Stimulation results in noradrenaline release and vasoconstriction.
- The key stimulus for simulating these nerves is hypotension, as vasoconstriction causes a decrease in renal blood flow in an attempt to retain volume, and shunt blood flow to muscle.
