Renal Assessmemt and Micturition Flashcards
Why do we need to assess renal function?
Central role in homeostasis
The ability to measure GFR is particularly useful in 2 clinical situations.
What are they?
- In patients with renal disease, progression of the underlying disease process results in nephron destruction and decreased nephron function.
Total GFR = sum of all filtration by functioning nephrons so progression of disease would be indicate by the reduction in GFR.
- Many drugs e.g. digitalis and many antibiotics are removed from the body by excretion by filtration. When GFR falls, excretion falls so that [drug] in plasma may rise causing toxicity
- Therefore may need to adjust dose appropriate to decreased renal function
What is plasma clearance?
Plasma clearance tests used to measure renal function.
They measure the ability of the kidney to clear the plasma of various substances.
N.B. It is the plasma that is important NOT the urine
Clearance relates to a volume of plasma cleared NOT a quantity of substance removed from the plasma.
What is the gold standard of plasma clearance tests?
Inulin clearance, polyfructose,
Loading IV dose of inulin
Allow time to equilibrate
Sample simultaneously plasma and urine (during a timed urine sample)
Why is inulin so good for measuring plasma clearance?
Freely filtered at the glomerulus and neither reabsorbed nor secreted.
It is not metabolised by the kidney, nor does it interfere with normal renal function.
Inulin clearance is therefore a great measure of GFR
How will the clearance of a substance that is filtered and reabsorbed compare to the clearance of inulin?
Lower clearance
How will the clearance of a substance filtered and secreted compare to inulin?
Highre clearance
What is the normal GFR?
“Normal” man = 125mls/min
The magnitude correlates with surface area, but values in women are around 10% lower, even after correction for surface area.
How does GFR decline as you age?
Declines by around 1ml/min/year after 30
What is the equation for measuring clearance?
Cx = [Ux] V/[Px]
Ux = Urine concentration of x
V = urine flow rate
Px = plasma concentration of x
Why is inulin clearance no longer used in clinical practice?
Explain the use of 51Cr-EDTA
Too cumbersome, 51Cr-EDTA has been used instead, a suitable radioactive substance that is handled by the kidney in the same way as inulin.
Now it is considered that GFR is usually too complex and expensive to measure
Takes several hours and requires injection of isotope 51Cr-EDTA.
What plasma clearance test is used more nowadays?
Creatinine clearance routinely used to estimate GFR.
Creatinine is endogenous (breakdown produce of muscle creatine).
Good agreement with inulin clearance…
Why is creatinine clearance very much an estimate of GFR?
How do we get around this?
We would expect a linear relationship between plasma creatinine and GFR but this is not true.
We can half the GFR before there is any elevation in plasma creatinine levels.
There are formulae using serum creatinine value which can take into account confounding variables.
Using these formulae can estimate GFR
Writen as eGFR (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 the clearance of glucose in health?
0 because normally its all reabsorbed
How does the learance of urea compare to inulin?
Less than inulin because some urea is reabsorbed
What is PAH clearance?
What is it used to measure and how can it do this?
The organic anion Para-Amino- Hippuric acid (PAH) is used to measure renal plasma flow (RPF)
PAH is freely filtered at the glomerulus and then the PAH remaining in the plasma is actively secreted into the tubule so that >90% of plasma is cleared of its PAH content in one transit of the kidney.
Therefore PAH clearance is a measure of all the plasma flowwing through the kidneys in a given time = renal plasma flow around 660mls/min
How does the clearance of penicillin compare to the clearance of inulin?
Greater clearance than inulin because its filtered and secreted
How does urine flow from the kidneys to the bladder?
Via ureters
Peristaltic movement of the ureters, they enter the blade at an oblique angle
How does the composition of the urine change once it leaves the kidney?
It doesn’t
How does the internal urethral sphincter compare to the external urethral sphicter?
NOT a true sphincter, but is where the smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed
The external urethral sphincter is a true sphincter, made up of skeletal muscle under voluntary somatic control
What is the characteristic shape of the Pressure-Volume curve of the bladder?
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
Describe the motor innervation of the bladder what helps in control of micturition
- Rish parasympathetic supply (pelvic nerves) Increase activity -> increase contraction of detrusor muscle -> increase pressure within the bladder
- Sparse sympathetic supply (hypogalstric nerves), inhibit bladder contraction and closes the internal urethral “sphincter”.
- Somatic motoneurones (pudendal nerves), innervate the skeletal muscle that forms the external urethral sphincter, keeps the pshincter closed, even against strong bladder contractions
What spinal nerves control parasympathetic nerve supply to detrusor of bladder?
S2-S4
What effect does cutting the hypogastric nerve have on micturition?
Sympathetic supply is not very important to bladder function but cutting the hypogastric nerve -> Increased frequency of micturition
What is the main function of the sympathetic system in bladder control?
Prevent reflux of semen into the bladder during ejactulation
L1-L3
What spinal nerves carry somatic motoneurones that keeps the external sphincter closed when we dont want to pee?
S2-S4
Describe the sensory innervation of the bladder
- Stretch receptor afferents from the bladder wall.
As the bladder fills -> increased discharge in afferent nerves 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
How is micturition controlled at its very simplest?
Basically a spinal reflex which may or may not be influences by higher centres
How does the operation of the local spinal reflex work?
As the bladder fills, it becomes distended and the stretch receptors are increasingly stimulated, until their output becomes great enough to cause bladder contraction via:
- Stimulation of the parasympathetic
- Relaxation of external sphincter by inhibiting somatic motoneurones.
Why do babies “leak”?
The micturition reflex operates at the spinal level because the higher brain connections have yet to be established.
This is also the case in adult patients with spinal cord transection after the initial period of spinal shock.
How is delay in micturition achieved?
Descending pathways from many brain centres, including cortex and brainstem, which:
-Inhibit the parasympathetic and stimulate the somatic nerves to the external sphincter, thus overridding the input from the bladder stretch receptors.
What is one of the initial events in voluntary urination which may cause a sufficient downwards tug on the detrusor muscle to initiate its contraction?
Relaxation of the muscles of the pelvic floor
What muscles can be contracted voluntarily, preventing urine flow flowing down the urethra or interupting the flow once urination begins?
Perineal muscles and external sphincter
After urination how does the male and female urethra empty?
After urination, female urethra empties by gravity
Urine remaining in the male urethra is expelled by contractions of the bulbocavernosus muscle
What 3 major types of abnormalities of micturition are due to neural lesions?
- Interuption of afferent nerves
- Interuption of both afferent amd efferent nerves
- Interuption of facilitatory and inhibitory descending pathways from the brain
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
how can some paraplegic patients train themselves to initiate voiding?
Pinching or stroking their thighs -> mild mass reflex
After spinal section, afferent stimuli irradiate from one reflex centre to another.
When a relatively minor noxious stimulus is applied to the skin it may irradiate to autonomic centres and evoke bladder or rectal voiding.
Returns some measure of “voluntary” control by allowing them to induce an intentional mass reflex
