Lecture 18- urinary tract 2- glomerular filtration

Question 1

Where does glomerular filtration occur?

Answer 1

bowmans capsule

Question 2

Is the GFR constant?

Answer 2

Generally, Glomerular Filtration Rate (GFR)
is relatively constant at 125ml/min (even
after drinking a lot, exercise, dehydration).

Question 3

How is GFR controlled?

Answer 3

GFR is controlled by 2 factors:
* Net filtration
* Changes in renal blood flow and blood
pressure (specifically in the glomerulus)
* Filtration Coefficient
* Changes in diameter (podocyle regulated)
of arterioles to alter the GFR

Question 4

What is autoregulation? Why is macula densa important for this?

Answer 4

Autoregulation
* Myogenic response - response to
pressure changes (ascending limb of
loop of Henle)

Question 5

What is tubuloglomerular feedback?

Answer 5

Tubuloglomerular feedback –
macula densa release local paracrine
hormones; cause physical changes in
afferent and efferent arterioles

Question 6

How can the nervous and hormonal system affect the control of glomerular filtration?

Answer 6

Nervous:
* Sympathetic nerves release
noradrenaline –> arteriole constriction

Hormonal: (endocrine, long acting) relates to relaxation and vasodilation
* Angiotensin II
* Prostaglandins

Question 7

What determines the GFR and happens to the GFR when the arterial blood pressure changes?

Answer 7

• Renal blood flow determines GFR
• autoregulation of the GFR

when the arterial blood pressure increases the GFR increases when the BP falls the GFR falls

Question 8

How is renal blood flow kept between 90-180 mhg?

Answer 8

Renal blood flow is autoregulated
between about 90-180 mm Hg
(normal) systolic blood pressure.
* Autoregulation is achieved by varying
renal vascular resistance (RVR).
* RVR is determined by the resistances
(tension) of the afferent and efferent
arterioles.

Question 9

What does renal blood flow determine?

Answer 9

the GFR

Question 10

How is autoregulation achieved and what determines RVR?

Answer 10

Renal blood flow is autoregulated
between about 90-180 mm Hg
(normal) systolic blood pressure.

• Autoregulation is achieved by varying
  renal vascular resistance (RVR).
• RVR is determined by the resistances
  (tension) of the afferent and efferent
  arterioles.

Question 11

How is RVR determined by the resistances of afferent and efferent arterioles?

Answer 11

RVR is determined by the resistances
(tension) of the afferent and efferent
arterioles.

Question 12

Answer the questions relating to what happens when the afferent arteriole dilates.

Answer 12

What happens to the following when afferent arteriole dilates?
* Capillary BP
* GFR
* Renal blood flow

Question 13

Go through the steps of tubuloglomerular feedback.

Answer 13

1. GFR increases
2. flow through the tubule increases
3. flow past macula densa increases
4. afferent arteriole constricts
5. afferent arteriole constricts
6. resistance to afferent arteriole increases
7. hydrostatic pressure in glomerulus decreases
   8.GFR decreases

Question 14

When are renal sympathetic nerves activated?

Answer 14

• Renal sympathetic nerves are
  activated under conditions of stress
  (cold; fear; haemorrhage; pain;
  vigorous exercise)

Question 15

What is the emergency mechanism to increase vascular resistance and why?

Answer 15

• Emergency mechanism to
  increase systemic vascular
  resistance and hence maintain
  blood pressure (compensate
  e.g sweat loss) despite loosing
  fluid

Question 16

What happens if the renal sympathetic nerves are persistently activated?

Answer 16

• If persistent, renal blood flow is
  reduced and renal failure may ensue
  (GFR decreases)

Question 17

What is the kidney’s function?

Answer 17

• Evaluation of kidney function = assessment of filtering capabilities

Question 18

What is the clinical function of the GFR?

Answer 18

Glomerular filtration rate (GFR)
* Estimates the efficiency with which substances are cleared from the blood
by glomerular filtration
* Indication of nephron function
* Glomerular filtration rate does not allow us to diagnose kidney disorders

Question 19

What does GFR estimate?

Answer 19

Estimates the efficiency with which substances are cleared from the blood
by glomerular filtration

Question 20

What are the limitations of GFR?

Answer 20

• Glomerular filtration rate does not allow us to diagnose kidney disorders

Question 21

How can we measure GFR? What are the conditions necessary for us to measure the GFR?

Answer 21

• GFR estimation Can be performed using an exogenous (administered) or endogenous substance (S)
• Clearance of the substance S by the kidney can be measured if substance S fits the
  following criteria:
• It is present at stable concentration in plasma (not synthesized or metabolized)
• It must be physiologically inert (not dissapearing)
• It is freely filtered at the glomerulus (uncharged) remember albumine!
• It is not secreted, reabsorbed, synthesised, or metabolised in the kidney
  Therefore Filtered S = Excreted S in the urine

GFR x [S]plasma = [S]urine x urinary flow rate (V excreted/unit time, catheter)

Question 22

What methods can allow us to estimate the GFR? What criteria does the substance need to follow to be useful for measuring eGFR?

Answer 22

GFR estimation (eGFR): Methods

• GFR estimation Can be performed using an exogenous (administered) or endogenous substance (S)
• Clearance of the substance S by the kidney can be measured if substance S fits the
  following criteria:
• It is present at stable concentration in plasma (not synthesized or metabolized)
• It must be physiologically inert (not dissapearing)
• It is freely filtered at the glomerulus (uncharged) remember albumine!
• It is not secreted, reabsorbed, synthesised, or metabolised in the kidney

Therefore Filtered S = Excreted S in the urine

Question 23

What is the formula for measuring GFR

Answer 23

GFR x [S]plasma = [S]urine x urinary flow rate (V excreted/unit time, catheter)

Question 24

how is eGFR calculated from in regard to individuals’ backgrounds?

Answer 24

using GFR x [S]plasma = [S]urine x urinary flow rate (V excreted/unit time, catheter)

The (estimated)eGFR is then calculated from the subject age, sex and
blood creatinine level. Adjustments are made to reflect ethnic origins.

Question 25

Why is creatinine a good GFR marker?

Answer 25

Creatinine is the breakdown product of creatine & phosphate
(muscle)
* Its production isrelatively constant depending on the amount of
muscle mass (broken down in specific rate)
* Passesthrough the glomerulusinto the filtrate during glomerular
filtration (small molecule)
* A tiny amount is also excreted in the proximal convoluted tubule
* Virtually no creatinine is reabsorbed

Question 26

During the stages of renal failure, how can BP increase have an effect?

Answer 26

during stage one BP increases to balance
the GFR drop

during stage 2 there is an Increase in BP, but causes other systemic effects !

Question 27

Answer the glomerular filtration rate questions

Answer 27

Glomerularfiltration rate……

a. is about 20 ml/min in healthy
adult human kidneys
b. can be measured by the clearance
rate of creatinine
c. is dependent on the value of
arterial blood pressure
d. is a measure of active secretion
from blood to the renal nephrons
e. is increased during renal failure