RENAL: Control of Renal Blood Flow and Glomerular Filtration Rate

Question 1

Describe the structure and function of the glomerulus

Answer 1

• Network of capillaries at the beginning of the renal tubule
• Involved in filtration process of blood contents into tubule lumen

Question 2

What governs glomerulus filtration?

Answer 2

• Net filtration rate (NFR) from capillaries to renal proximal greatly determined by pressure difference b/w afferent and efferent arterioles
• Glomerular filtration rate (GFR) needs to be kept around >60-90 ml/min
• Below this could lead to potential renal failure

Question 3

What controls renal blood flow and glomerular filtration rate?

Answer 3

To increase GFR:

• DIlate afferent arterioles OR constrict efferent arterioles

To decrease GFR:

• Constrict afferent arterioles OR dilate efferent arterioles

Question 4

Describe the autoregulation and myogenic response that controls RBF

Answer 4

Macula densa - Cells in DCT, generate paracrine chemical signals that control RBF, GFR and renin release. They sense changes in NaCl levels, and from this can signal whether these ions need to be increased or decreased as well as water reabsoprtion to help adjust blood volume and BP

Renal afferent arteriole smooth muscle cells contain stretch receptors, an increase in BP is detected by these. This induces vasoconstriction, leads to reduced blood pressure in glomerulus - less RBF/GFR

Reduced BP - vasodilation

Increased RBF/GFR:

• If there’s increased RBF/GFR (e.g hypertension), less time for NaCl reabsorption, increased NaCl load at macula dense, inducing adenosine release.
  
  • Adenosine preferentially constricts afferent arterioles, decreasing RBF/GFR

Decreased RBF/GFR (e.g. hypervolaemia, HF)
- Decreased NaCl load, induces release of prostaglandins (PGs)
- Preferentially dilate afferent arterioles - Increases RBF/GFR

Question 5

Describe the action of sympathetic nerves and RAAS on RBF/GFR

Answer 5

Sympathetic nerves release noradrenaline on B1 receptors on the kidneys, this happens in response to a drop in RBF/GFR. This induces the release of renin.

RAAS:

• Renin release leads to the production of Ang II and the release of aldosterone (from adrenal gland)
• Ang II → Preferentially constricts efferent arterioles, increases RBF/GFR

Vasopressin (ADH) and natriuretic peptides (ANP) also regulate renal circulation

Question 6

How is GFR relate to stages of kidney disease?

Answer 6

GFR defines how well your kidneys are filtering your blood

Calculates fluid that filters through the glomerulus into Bowman’s capsule over time

• If GFR 90ml/min and over - Stage 1 kidney disease with normal or high GFR
• GFR 60-89 ml/min - Stage 2 kidney disease (mild)
• 45-59 ml/min - Stage 3A kidney disease (moderate)
• 31-44 ml/min - Stage 3B kidney disease (moderate)
• 15-29 ml/min - Stage 4 kidney disease (severe)
• <15 ml/min - Stage 5 kidney disease (End-Stage Renal Disease- ESRD)

Question 7

How is GFR measured?

Answer 7

For a substance that is freely filtered, not reabsorbed by tubules or secreted by tubules

[Substance in plasma] x GFR = [Substance in urine] x Urine flow rate (UFR)

GFR (renal clearance) = ([Urine] x UFR)/ [Plasma]

Useful substances:

• Creatinine - freely filtered (a little secreted), increase lvls in plasma ca estimate drop in GFR
• Inulin - freely filtered (not secreted)

Often use radiolabelled-DTPA (diethylenetriaminepenta-acetic acid)

Measure fall in plasma levels over time

Calculate rate of removal

Also, scans of radiolabelled-DTPA show renal blood flow