Renal Blood Flow (Friday 03/11/17)

Question 1

Arterioles and BP

Answer 1

Arteries cause blood pressure to drop, have muscle to cause resistance by narrowing.

In normal capillary the blood pressure would drop along the capillary, but not in glomerular capillaries, this is because the efferent arteriole is narrower than afferent arteriole. This is akin to clamping one end of a water hose. This means it is harder for the blood to leave the glomerulus, so renal blood flow decreases and filtration pressure increases. Maintenance of a constant filtration pressure drives the movement of fluid and small molecules from the blood across the glomerular barrier into Bowman’s Capsule where it is termed primary filtrate.

Despite systemic blood pressure changing, renal blood flow remains relatively constant. This means the kidney makes glomerular filtrate at a constant rate despite what happens to systemic blood pressure. If there was no control, when systemic BP increased, like when exercising, renal blood flow would increase, causing more glomerular filtrate to be made. Vica versa for sleeping. Glomerular filtrate needs to be constant, so changing the afferent arteriole (making it stretch) so the efferent is narrower, is key in maintaining a constant GFR.

Question 2

Auto-regulation of RBF

Answer 2

Two mechanisms:

1. Myogenic Feedback: This is how arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow within the blood vessel constant. In blood vessels, if there is a stretch due to increased blood flow, Ca ions are activated and the smooth muscle of the vessel contracts, to increase the resistance to the flow. This smoothes out renal blood flow and protects the glomerular capillary from large changes in filtration pressure – stabilises GFR.
2. Glomerular Tubular Feedback: as systemic blood pressure increases there will be a transient increase in renal blood flow. Increased renal blood flow results in an increase in filtration pressure and a consequent increase in the amount of fluid and small molecules filtered per unit time. An increase in the amount of NaCl filtered is sensed in the distal nephron by specialised Macula Densa cells which make up part of the juxtaglomerular apparatus. It is postulated that Macula Densa cells transport tubular NaCl and that if NaCl delivery increase they respond to transport more. The transport of NaCl is energy expensive and results in generation of adenosine in proportion to the amount transported. Adenosine acts as a local hormone diffusing from the Macula Densa to the adjacent afferent arteriole where it acts on A1 receptors resulting in Ca entry and smooth muscle contraction resulting in an increase in afferent arteriole resistance and a stabilisation of RBF and GFR.

Is autocrine blood flow control mechanism, hence GFR control.

Question 3

Local Hormonal and Neuronal Control of Renal Blood Flow: when autoregulation isn’t enough

Answer 3

Vasoconstrictors and Vasodilators can super-impose on top of the auto-regulators when required, eg. when haemorrage, need to divert the blood flow to brain, so kidney is sacrificed.

To prevent the kidney from ischemic damage, but still abandoning GFR production, Prostaglandin is released.

As systemic blood volume and blood pressure drops, noradrenaline is released from the renal sympathetic nerve, adrenaline is released from the adrenal gland and angiotensin II is generated stimulated by a rise in plasma renin concentration

Question 4

Role of Nitric Oxide

Answer 4

Causes Renal & Peripheral Vasodilation

Question 5

The kidney can cause hypertension, if it thinks there is decreased systemic blood volume.

Answer 5

If have renal artery stenosis (block) the kidney has less blood. The kidney reacts by…see slide

Question 6

Renal Vasculature

Answer 6

· Renal blood supply enters via left and right renal artery

· Branches into segmental, interlobar, arcuate, interlobular arteries

· Each nephron receives one afferent arteriole

· Divides into a capillary network with the glomerulus – capillary ball

· Reunite to form efferent arteriole (unique)

* In juxta-medullary nephrons only: efferent arteriole penetrates deep into medulla as vasa recta vessels

· Divide to form peritubular capillaries surrounding nephrons (both juxta medullary and cortical nephrons have pertibular capillaries- see image).

· Peritubular venule, interlobar vein and renal vein exits kidney

Question 7

Distribution of renal blood flow and the functional significance of the variation in perfusion between the cortex and the medulla.

Answer 7

• Kidneys receive 25% of cardiac output (340ml per minute).
• The high blood flow relates to the kidneys function to filter blood.
• The cortex receives ~93% of the renal blood flow. Where all the glomeruli filter blood. The cortex is also site of proximal tubules where the majority of reabsorption of solutes and water occurs also site where waste products and drug molecules added to filtrate. To function effectively requires high blood supply.
• In contrast the medulla receives ~7% of the renal blood flow. Site of mechanism responsible for making concentrated urine. Key feature is the generation of a hyperosmotic interstitium. (*See counter current multiplier video) Low blood flow ensures that this is not washed out. Medullary blood supply arranged in loop to generate counter current exchange of solutes.
• At the renal papilla the blood flow is around 1% of total (the renal papilla is the location where the renal pyramids in the medulla empty urine into the minor calyx in the kidney).

Question 8

LO’s

Answer 8

1. Describe the renal vasculature with respect to supply to both cortical and juxtamedullary nephrons
2. Describe how the structure of the juxtaglomerular apparatus is suited to feed-back control of renal function
3. Describe the main sites of vascular resistance within the renal circulation and know how renal blood flow may be measured
4. Describe the magnitude and distribution of renal blood flow within the kidney and the functional significance of the variation in perfusion between the cortex and the medulla