Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR Flashcards Preview

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Flashcards in Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR Deck (39):

When is it important to assess renal function?

Renal disease - rate of decline of GFR gives inication of prognosis - renal disease causes damage to nephrons and therefore reduces nephron function

Many drugs are removed by filtration - excretion decreases when GFR decreases meaning drug concentration in the plasma can reach toxic levels


Describe plasma clearance tests

Measure the ability of the plasma to clear various substances

Clearance concerns the volume of plasma cleared and not a quantity of substance removed from the plasma


What is the equation for the plasma clearance of X?

Ux = urine concentration of X

V = Urine flow rate

Px = Plasma concentration of X


What molecule is often used for plasma clearance tests?


In clinical practice, inulin is no longer used, because too cumbersome, 51Cr-EDTA has been used instead, a suitable radioactive substance that is handled by the kidney in the same way as inulin.


What makes inulin an effective measurer of GFR?

Freely filtered at the glomerulus

Not reabsorbed or secreted

Not metabolised by the kidney

Doesn't interfere with normal renal function


How does secretion and reabsorption alter clearance rate?

Reabsorbed - lower clearance rate

Secreted - higher clearance rate


What is GFR in a normal man

GFR in “normal” man = 125mls/min. The magnitude correlates with surface area, but values in women are » 10% lower, even after correction for surface area. GFR declines by  »1ml/min/year after 30


GFR is too complex and expensive to measure, it takes several hours and an injection of isotope 51 Cr EDTA - what is the clinical replacement?

Plasma creatinine (a breakdown product of muscle creatine)


What is the big caution associated with using creatinine as a marker of GFR?

Plasma creatinine decreases as GFR increases 


It is not a linear equation 


The GFR can halve before there is an increase in creatinine

Creatinine plasma concentration is therefore taken into account alongside other variables to make up the estimated GFR



What factors affect serum creatinine?

Muscle mass: athletes vs malnutrition

Dietary intake: creatine supplements vs vegetarians

Drugs: Some lead to spurious increases as does ketoacidosis.


What is normal GFR?

Normal GFR is approximately 100mls/min/1.73m2


What is the clearance of glucose?

0 since it is normally reabsorbed


How much urea is reabsorbed?



What substance is used to measure real plasma flow (RPF)?

PAH - para-amino-hippuric acid - it is freely filtered at the glomerulus and remaining plasma is secreted. Over 90% of the PAH is removed from the plasma in one transmit of the kidney 

Renal plasma flow is about 660 mls/min


In this table - when filtration increases reabsorption also increases 

When excretion increases - secretion also increases




How does urine pass from the kidneys to the bladder?

Urine flows from the kidneys to the ureters via peristaltic contraction of the smooth muscle of the ureters, they enter the bladder at an oblique angle (prevents reflux of urine).


What is the muscle of the bladder?

Detrusor muscle

Contraction of this muscle is mainly responsible for emptying the bladder during micturition.


Describe the internal urethral sphincter

Not a true sphincter

Where the smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed


Describe the muscle found in the external sphincter

The external urethral sphincter is a true sphincter, made up of skeletal muscle under voluntary somatic control.


What is the bladder overlain with?




What type of epithelium is found in the bladder?

Transitional epithelium


What makes up the trigone of the bladder?

2 vesicoureteric openings

Urethral opening


What is normal urine production?

Normal daily urine production varies between 750ml-2500mls in temperate climates.


This graph shows the increase in sense of urgency to void as the bladder fills up

There is a long flat segment as the initial increments of urine enter the bladder and then a sudden sharp rise as the micturition reflex is triggered.


Which nerves are responsible for contraction of the detrusor muscle?

S2 - S4 

Increases the pressure within the bladder


How does the hypogastric nerve influence micturition?

Inhibit bladder contraction and closes the internal urethral sphincter


Cutting the hypogastric nerve causes increase in frequency of micturition


Main function is to prevent reflux of semen into the bladder during ejaculation 



What nerves are associated with the skeletal muscle that forms the external urethral sphincter?

Pudendal nerves - S2 - S4


What happens when stretch receptor afferents start increasing discharge to spinal cord via interneurones?

Excitation of parasympathetic outflow

Inhibition of sympathetic outflow

Inhibition of somatic motoneurones to external sphincter

Pathways to sensory cortex - sensation of fullness


Micturition is basically a spinal reflex which may or may not be influenced by higher centres.


Why do babies wet themselves?

Micturition reflex serves to stimulate parasympathetic contraction and relaxes the external sphincter by inhibiting somatic motoneurones

Higher brain connections have not been established

This is also the case in adult patients with spinal cord transection after the initial period of spinal shock.


What quantitiy of urine in the bladder is enough to initiate the spinal reflex?

300 - 350 mls


Which parts of the brain are involved in delay of voiding?

Descending pathways from many brain centres including the cortex and the brainstem


 What is the effect of descending pathways which cause delay?

inhibit the parasympathetic and:

 stimulate the somatic nerves to the external sphincter, thus over-ridding the input from the bladder stretch receptors.


What does voluntary inititation involve?

Voluntary initiation involves descending pathways which:

stimulate the parasympathetic and:

 inhibit the somatic motor neurones thus summating with the stretch receptor effects.


How are pelvic floor muscles involved in micturition?

Relaxation of pelvic floor muscles can cause sufficient downward tug on the setrusor muscle to initiate its contraction


What parts of the urinary system can be used to prevent urine flow down the urethra/ interrption of urine flow once urination begins

Perineal muscles and external sphincter can be contracted voluntarily, preventing urine flow flowing down the urethra or interrupting the flow once urination begins.


After urination - how does the female and male urethra empty?

Females - by gravity

Males - contractions of the bulbocavernous muscle



What are the abnormalities of micturition?

1.Interruption of afferent nerves

2. interruption of both afferent and efferent nerves

3.interruption of facilitatory and inhibitory descending pathways from the brain.


What is the result of all the abnormalities of micturition?

In all 3 types the bladder contracts but the contractions are generally insufficient to empty the bladder completely and urine is left in the bladder.


Paraplegic patients can pinch/stroke their inner thigh to stimulate voiding - the process is called mild mass reflex - what is the mechanism behind mild mass reflex?

Afferent stimuli irradiate from one reflex centre to another 

Noxious stimuli may irradiate to autonimc centres and evoke bladder or rectal voiding