Renal Blood Supply

Question 1

Describe the structure and function of the vasa recta

Answer 1

• the efferent arterioles from the glomeruli run into portal vessels, which the plunge from the cortex, deep into the medulla, forming a hairpin loop
• take water and solvents from the interstitial space after absorption by the tubules
• supply substances into the interstitial spaces, where they can then be actively secreted by the tubules
• as the capillaries are permeable the osmotic pressure in the vasa recta changes with the local interstitial osmolality, diluting the interstitium on the descending limb, but concentrating it on the ascending limb

Question 2

Describe countercurrent in the vasa recta

Answer 2

Movement is passive
Net movement is inwards, because of the low hydrostatic pressure in the capillaries, high oncotic pressure in the capillaries, and relatively high hydrostatic pressure in the turgid interstitial space
Flow rate in the descending limb must decline as water leaves
Flow rate in the ascending limb must increase as water arrives

Question 3

What two mechanisms are key in autoregulation of renal blood flow

Answer 3

• myogenic response (regulating total renal blood flow)
• tubuloglomerular reflect (regulating single nephron GFR, but affecting renal blood lflow if many single nephrons are affected
• Such auto-regulation is present when the kidney is ex-vivo, so it cannot depend on central neuronal input (intrinsic property of the kidney)

Question 4

Describe the difference in the following graphs

• hypothetical renal blood flow with no auto regulation
• hypothetical renal blood flow assuming elastic arteries
• actual renal autoregulation

Answer 4

• linear relationship between BP and Flow Rate

- Exponential relationship between BP and Flow Rate

Question 5

Describe the actual renal autoregulation response

Answer 5

From 0-80 BP, can increase in BP increases renal blood flow
However at normal blood pressures 80-200, the graph plateaus indicating that an increase in BP does not change flow rate
This means resistance must be increasing

Question 6

Describe the basis of the myogenic response

Answer 6

• ## when the afferent arterioles are stretched, they contract. Hence, an increase in BO causes vasoconstriction, which narrows the vessels, increases resistance and hence reduces the flow rate.

Question 7

Explain mathematically why a only small change in afferent arterial diameter is needed in order to correct for the change in pressure/

Answer 7

flow rate is proportional to r^4

Question 8

What is the cellular mechanism behind the myogenic response?

Answer 8

stretch-activated cation channels depolarising in smooth muscle cells, hence increasing Ca2+ influx and subsequently causing contraction
This type of myogenic effect, which can occur in many arteries, is traditionally known as the Bayliss effect

Question 9

Why is the myogenic response important?

Answer 9

Traditionally, the idea is that the main useful feature is to maintain GFR regardless of the mean arterial pressure, allowing independent regulation of volume and pressure. However some have argued that the main effect is to reduce the impact of high systolic pressures, as the reflex is more sensitive to the peak than to the mean pressure

Question 10

Describe the purpose of and cellular mechanism behind tubuloglomerular feedback

Answer 10

This is the key way in which each nephron can regulate its GFR
- High Na+ in distal tubule is sensed by the macula dense (NKCC2)
- cells of the macula dense release ATP that is brown down to adenosine
- adenosine causes vasoconstriction of afferent arterioles, leading to a fall in glomerular hydrostatic pressure and a fall in GFR
This is negative feedback mechanism

Question 11

What is the macula dense?

Answer 11

The distal tubule runs close to the glomerulus. Here the macula densa is located and regulated tubuloglomerular feedback

Question 12

Describe the 6 stages of tubuloglomerular feedback

Answer 12

1. high single nephron GFR
2. high delivery of Na+ to distal tubule
3. High activity of NKCC2
4. Activation of ATP release from macula densa
5. Adenosine constricts afferent arteriole
6. Lower glomerular hydrostatic pressure –> negative feedback

Question 13

Factors opposing renal auto regulation of blood flow:
While the usefulness of auto regulation is clear ex vivo, in vivo the renal blood flow in also influenced by:
1.
2.
These factors help to match the needs of the body against the wants of the kidneys

Answer 13

1. renal innervation

2. circulating hormones

Question 14

Describe renal innervation

Answer 14

• dense plexus of nerves innervating and regualating renal blood flow
• most efferent nerves are sympathetic, releasing NA and causing vasoconstriction.
  A key stimulus is hypertension, causing a decrease RBF in an attempt to retain volume, and shunt blood flow to muscle for short-term needs
• There are also many sensory afferent neurones