Renal Blood Flow & Glomerular Filtration

Question 1

Define glomerula filtration?

What does an abrupt fall in glomerula filtration indicate?

Answer 1

Question 2

What is primary urine?

Answer 2

Question 3

What is the amount excreted of a substance equal to?

Answer 3

Question 4

Write an equation showing the net ultrafiltration pressure?

Answer 4

Question 5

Define glomerular filtration rate (GFR)

Answer 5

Question 6

List 4 factors that gfr depends on?

Answer 6

• Glomerular capillary pressure (Pgc)
• Plasma oncotic pressure (πgc)
• Tubular pressure (Pt)
• Glomerular capillary surface area or permeability (Kf).

GFR is not a fixed value but is subject to physiological regulation. This is achieved by neural or hormonal input to the afferent/efferent arteriole resulting in changes in Puf.

Question 7

What is the myogenic mechanism of autoregulation in the kidney?

Answer 7

1. Myogenic mechanism:

Vascular smooth muscle constricts when stretched.

Keeps GFR constant when blood pressure rises.

Arterial pressure rises → afferent arteriole stretches →

arteriole contracts → (vessel resistance increases)→ blood flow reduces and GFR remains constant:

Question 8

What is the normal gfr value and state 4 things that effect gfr?

Answer 8

Severe haemorrhage

Obstruction in nephron tubule

Reduced plasma protein concentration

Small increase in blood pressure

In a normal individual carrying out a daily routine, GFR will be maintained at 120ml/min.

Question 9

Define renal clearence and state the equation

Answer 9

Question 10

Kidneys receive …% of the cardiac output

Answer 10

Kidneys receive 20% of the cardiac output

Question 11

What is the tubulogomerular feedback mechanism of autoregulation?

Answer 11

o Tubuloglomerular feedback – macula densa response to NaCl increase

NaCl conc in tubular fluid sensed by macula densa in juxtaglomerular apparatus – release ATP as signaling molecule which signals afferent arteriole to vasoconstrict

 reduced filtration (with associated negative feedback loop)

Question 12

 Regulation of GFR is achieved by …………… or …………… input to the afferent/efferent artriole resulting in changes to the glomerular capillary pressure

What happens to decrease gfr and what happens to increase GFR?

When is autoregulation required?

Answer 12

 Regulation of GFR is achieved by neural or hormonal input to the afferent/efferent artriole resulting in changes to the glomerular capillary pressure

 Autoregulation ensures fluid + solute excretion remain reasonably constant (without which urine production + ion loss will vary)

o To decrease GFR, constrict the afferent arteriole or dilate the efferent arteriole

o To increase CFR, construct the efferent arteriole or dilate the afferent arteriole

 Autoregulation is required e.g. during exercise, as bp increases but you don’t want to be producing large amounts of urine

Question 13

Pressure is not the only thing that influences the glomerular filtration rate. Other factors include:

Name them?

Answer 13

Glomerular filtration rate (GFR)

 Pressure is not the only thing that influences the glomerular filtration rate. Other factors include:

o Permeability of tubule membrane

 Clinical correlation: kidney disease may result in a change in membrane permeability

o Surface area of membrane available for filtration

 Clinical correlation: kidney disease may damage nephrons, thus reducing the number of functioning glomeruli

o reducing SA available for ultrafiltration

 These factors are considered in the ultrafiltration coefficient – written as Kf

Question 14

Clinical relevance of GFR

 In a normal individual carrying out a daily routine, GFR will be maintained at 120ml/min

 Severe haemorrhage  decreased GFR (result of decreased blood pressure

 Obstruction in nephron tubule  decreased GFR (due to increased hydrostatic pressure in tubule = opposing pressure)

 Reduced plasma protein concentration  increase GFR (due to increased oncotic pressure = opposing pressure

)  Small increase in blood pressure, e.g. during exercise  GFR remains constant (result of autoregulation)

Answer 14

Question 15

Answer 15

Question 16

However, if a molecule is freely filtered (i.e. neither reabsorbed nor secreted following its ultrafiltration), clearance = ……

What might a low creatine clearence indicate

What might a high creatine clearence indicate

Answer 16

However, if a molecule is freely filtered (i.e. neither reabsorbed nor secreted following its ultrafiltration), clearance = GFR (i.e. amount excreted = amount filtered)

o Example of this is inulin; a plant polysaccharide that is measurable in both urine + plasma and gives a clearance value of 120ml/min = GFR

o However in humans, GFR is estimated from creatinine clearance  Creatinine is a waste product from creatine in muscle metabolism

 The amount of creatinine is fairly constant, thus is renal function is stable, the amount of creatinine in urine is stable (120ml/min)

 A low creatinine clearance may indicate renal failure

 A high plasma creatinine may also indicate renal failure

Question 17

What is used to measure renal plasma flow?

Define renal plasma flow?

Answer 17

Renal plasma flow (RPF)

 The volume of plasma (component of blood) reaching the kidney per unit time

 PAH (para aminohippurate) is completely removed from the plasma passing through the kidney, therefore its clearance = renal plasma flow = 625ml/min

 With other substances, the amount of the substance appearing in the urine reflects the combined effects of filtration, reabsorption + secretion, so that:

o Amount excreted = amount filtered – amount reabsorbed + amount secreted o Thus most solutes have aclearance of < 120ml/min (GFR) = controlled excretion

Question 18

Amount excreted =

finish the equation

Answer 18

Question 19

RENAL DIAGNOSTICS

What is the cardinal feature of renal disease?

Answer 19

A fall in GFR is the cardinal feature of renal disease

If GFR falls, excretory products will build up in the plasma.

A raised plasma concentration of creatinine is diagnostic of renal disease.

Excretion of many other substances will also be impaired in renal failure - including some drugs. This needs to be taken into account when calculating drug doses - PHARMACOKINETICS.

Question 20

Answer 20