L36. Kidney (2)

Question 1

What does the final composition of the urine to be excreted depend on?

Answer 1

1. Filtration of the kidney - what is filtered into the nephrons from the blood
2. Secretion of the kidney - what is secreted into the nephrons via the blood
3. Re-absorption - what is reabsorbed from the nephron into the blood

Question 2

Tell me a bit about filtration in the kidney?

Answer 2

• Many substances are filtered with a constant rate at the renal corpuscle (glomerulus); EXCEPTION: substances bound to protein
• Some substances need to be partly (Na+, K+) or entirely (glucose) re-absorbed
• Other substances need to be entirely secreted (PAH: represents many drugs)

Question 3

Which ions/substances move from blood into nephron?

Answer 3

• Small substances (low molecular mass) are freely filtered into the nephron
• Large substances (high molecular mass) are NOT filtered and stay in the blood

Question 4

What are the filtration driving forces?

Answer 4

• Blood pressure (PGC) is the main driver for filtration
• Forces opposing filtration are osmotic pressure in the glomerular capillary (TTGC) and fluid pressure in Bowman capsule/space (PBS)

Question 5

What would indicate that the filtration barrier is damaged?

Answer 5

Blood cells or protein in the urine

Question 6

What are the properties a substance must have to be suitable for determination of the filtration rate?

Answer 6

• Freely filtered
• Not re-absorbed
• Not secreted
• Not metabolised
• Not toxic (inert)

These are needed to measure GFR of a substance

e.g. insulin (exogenous) or creatinine (endogenous metabolite)

Question 7

How do we measure renal filtration?

Answer 7

We want to know the volume of plasma that was cleared of a substance S per time

Question 8

What is the clinical relevance of GFR?

Answer 8

• Plasma creatinine is an indicator for kidney function; plasma creatinine is low, if both kidneys are working (125mL/min)
• Plasma creatinine is fairly normal, even if only one kidney is still working (60mL/min, redundancy!!)
• If it gets to 25, kidneys are not functioning healthily

Question 9

What is secretion with the example of PAH?

Answer 9

Clearance of PAH = 600 mL/min (~5 x GFR)
- Cleared from plasma in one perfusion round

CPAH = renal plasma flow (RPF)

RPF = RPF/(1-haematocrit) = 1.2 L/min

Question 10

What is PAH?

Answer 10

PAH is a so called ‘model organic anion’ that does not exist in our body, but it represents a wide range of drugs
- Antibiotics
- Diuretics
- Thiazides
- NSAIDs
- Anti-viral drugs
- Folate-derivatives
- Metal chelators