Glomerular Filteration process

1. GFR

2. Glomerular Filtration Rate (GFR)
• The amount of filtrate formed per minute by two kidneys combined;
• For every 1 mm Hg of net filtration pressure, the kidneys produce
about
12.5 mL of filtrate/minute = filtration coefficient (K f ).
• GFR = NFP x K f ≈125 ml/min or 180 L/day, male
• GFR = NFP x K f ≈105 ml/min or 150 L/day, female
– filtration coefficient (K f ) depends on permeability and surface area
of
filtration barrier
• 99% of filtrate reabsorbed, 1 to 2 L urine excreted

3. REGULATION OF GLOMERULAR FILTRATION
• GFR must be precisely controlled:
↑GFR, urine output rises → dehydration, electrolyte depletion;
↓GFR → wastes reabsorbed (azotemia possible)
• GFR controlled by adjusting glomerular blood pressure
• This is achieved by 3 homeostatic mechanisms:
1. Renal autoregulation
2. sympathetic control
3. hormonal control (renin and angiotensin)

4. RENAL AUTOREGULATION
• The ability of the nephrons to adjust their own blood flow and GFR
without external (nervous, hormonal) control.
• It enables them to maintain a stable GFR even when BP rises. (100mm
→ 125mm = 2L/day → 45L/day)
• Helps to ensure stable fluid and electrolyte balance.
2 Mechanisms:
1. Myogenic mechanism
2. Tubuloglomerular feedback

5. MYOGENIC MECHANISM
• Stabilizes GFR on the premise that smooth muscle will contract when
stretched.
• So as BP rises, it will stretch the afferent arteriole;
• It will contract resulting in ↓ blood flow into glomerulus preventing
the change of blood flow;
• Conversely, when BP drops the smooth muscle relaxes so blood flows
easier into glomerulus.
• Either way blood flow and filtration remain fairly stable.

6. Juxtaglomerular Apparatus

7. RENAL AUTOREGULATION OF GFR
• ↑ BP → constrict afferent arteriole, dilate efferent
• ↓ BP → dilate afferent arteriole, constrict efferent
• Stable for BP range of 80 to 170 mmHg (systolic)
• Cannot compensate for extreme BP

8. Renal Autoregulation of GFR
• Myogenic mechanism
↑ BP → stretches afferent arteriole → afferent arteriole constricts →
restores GFR
• Tubuloglomerular feedback
– Macula densa on DCT monitors tubular fluid and signals
juxtaglomerular cells (smooth muscle, surrounds afferent arteriole)
to constrict afferent arteriole to ↓ GFR

9. Negative Feedback Control of GFR

10. RENIN-ANGIOTENSIN MECHANISM
• As BP drops the SNS stimulates juxtaglomerular cells to produce renin
(enzyme); converting…
• Angiotensinogen (plasma protein) to angiotensin I;
• In the lungs angiotensin-converting enzyme (ACE) converts angiotensin I to
angiotensin II.
• Angiotensin II has multiple effects
• they collectively act to raise BP by reducing water loss, encouraging water
intake, and constricting blood vessels.