Physiology 7 - RFTs & Micturation Flashcards

1
Q

What is normal GFR?

A

~125mls/min/1.73m2 surface area

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2
Q

Whats GFR important for?

A

Monitoring progression of renal disease:

  • The function of the nephron will eventually decrease
  • GFR is dependent on the total filtration of the nephrons
  • If GFR decreases this is an indication of decrease in nephron function

Monitoring renal function for drug dosing:

  • Some drugs such as digitalis are removed by excretion/ filtration
  • If GFR decreases the amount of excretion may decrease so there would be higher levels in the plasma
  • In this case the concentration of drug would have to be adjusted
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3
Q

How do we measure GFR?

A

Through Plasma clearance (Cx) of a specific substance where:
• Cx= [Ux] V/[Px]

Ux is urine conc of x
V is urine flow rate
Px is plasma conc of x

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4
Q

What substance do we routinely use to measure GFR?

A

Inulin is gold standard but needs to be injection IV

Creatinine is used to measure eGFR because you should produce a fairly constant amount of it, therfore plasma creatinine is all thats needed to tell how well the kidney is excreting it

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5
Q

How do we measure renal plasma flow and what is normal?

A

~660ml/min

Measured using clearance of PAH (Para-amino-hippuric) acid.
PAH is freely filtered and secreted so plasma clearance is >90%

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6
Q

Dsecirbe the structure of the bladdeR?

A

1) Found ant to repro system & rectum
2) Found post to pubic bones
3) Mostly smooth muscle (Detrusor)
4) lined with transitional epithelium
5) Trigone contains 2 vesicouretic openings and 1 urethral opening

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7
Q

How do we prevent reflux up the vesicouretic opening?

A

They pass obliquely through the bladder wall so pressure of urine closes off the opening preventing backflow

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8
Q

describe the graph of intravesicle pressure vs volume of urine in bladdeR?

A

ITs a sigmoid curve
I.e. it rises fast at first and you get the urge to void at ~100mls

Then pressure remains fairly constant till around 400mls when the pressure rises fast, along with the sense of urgency

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9
Q

What volume of urine in the bladder triggers spinal reflex micturition?

A

300-350mls

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10
Q

Describe the afferent sensory supply of micturition?

A

Stretch receptors in bladder wall discharge triggering the spinal reflex:

  • Excitation of parasymp outflow
  • Inhibition of sympathetic & somatic outflow

Also conveys the sensation of fullness to the brain

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11
Q

Describe the motor supply of micturition?

A

Parasymp pelvic nerves (S2-4) - Contract detrusor

Symp Hypogastric nerves (L1-3) - Relax detrusor and contract Internal urethral sphincter

Somatic pudendal nerves (S2-4) contract external urethral sphincter

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12
Q

Sympathetic supply is much weaker than parasympathetic, what is its main function?

A

To close the internal urethral sphincter so you don’t reflux semen into your bladder

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13
Q

How does voluntary delay/initiation of micturition work?

A

Your higher brain centres can inhibit parasympathetic outflow (S2-4) and stimulate somatic outflow (S2-4) to delay or vice versa to initiate micturition

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14
Q

How do we clear the last urine left in our urethra?

A

In women its expelled by gravity

In men its by the bulbocavernosus muscle

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15
Q

What kind of neural lesions can cause abnormal micturition?

A

Neural lesions can:

1) interrupt afferent nerves
2) Interrupt afferent & efferent
3) interrupt descending pathways

Net result from any of them is too weak bladder contraction so you dont fully empty it

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16
Q

What is a mass refleX?

A

For patients with a spinal section (paraplegia) some afferent stimuli can irradiate from one reflex centre to another.
Some paraplegics can control bladder/rectal voiding by pinching their thigh to initiate a mild mass reflex that irradiates to the autonomic centres.

17
Q

Summary of micturition control

A

Mainly a spinal reflex through L1-3 afferents detecting stretch in the bladder and S2-4 parasymp efferents causing contraction of the bladder and relaxation of the internal urethral sphincter.
Can be overridden by descending pathways

18
Q

What happens in Babies?

A

No higher centres so controlled at a local level

  • Increased parasympathetic activity
  • Decrease somatic motor control

Potty training involves the development of descending brain pathways

19
Q

How much does GFR decrease by after the age of 30?

A

GFR declines by 1ml/min/year after 30.

20
Q

Difference in plasma clearance in substances which are reabsorbed/secreted compared with inulin

A

Substances which are filtered AND REABSORBED:
• lower Ux than Inulin
• higher Px than Inulin

Substances which are filtered AND SECRETED
• higher Ux than inulin
• lower Px than inulin

21
Q

What factors can affect creatinine?

What implications does this have?

A

What factors may affect serum creatinine?
Muscle mass
Dietary intake
Drugs
So does have its faults in measuring plasma clearance