Renal assessment and micturition Flashcards
Why is it important to assess renal function?
Because of the kidney’s central role in homeostasis
Which 2 clinical situations is the measurement of GFR particularly useful?
- Disease progression of patients with renal disease. 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 indicated by the reduction in GFR.
- Drug toxicity. Many drugs are removed from the body by excretion by filtration. When GFR falls, excretion falls - drug conc in plasma may rise causing toxicity
Describe 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?
CX= [UX] V/[PX]
Ux = urine concentration of X
V = Urine flow rate
Px = Plasma concentration of X
Which molecule is often used for plasma clearance tests?
Inulin
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
What is the ‘normal’ GFR in man?
125 mls/min
Depends on surface area, but values in women are about 10% lower, even after correction for surface area
GFR declines by
about 1ml/min/year after 30.
How does secretion and reabsorption alter clearance rate?
Reabsorbed - lower clearance rate
Secreted - higher clearance rate
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’s the big caution with using Creatinine
Plasma creatinine decreases as GFR increases
BUT
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: creatinine 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 value of Glucose?
0 because normally all of it is reabsorbed
What is the clearance of urea?
Less than that or inulin because 50% urea is reabsorbed
What is used to measure real plasma flow (RPF)?
The organic anion para-amino-hippuric acid (PAH)
What happens to para-amino-hippuric acid (PAH) in the kidney?
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.
What is PAH clearance a measure of?
It is a measure of all the plasma flowing through the kidneys in a given time = renal plasma flow = 660mls/min
How does urine flow from the kidney 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
Why do the ureters enter bladder at an oblique angle?
Prevents reflux of urine
Does the composition of urine does change once it leaves kidneys?
No
What’s the name given to the smooth muscle found in the wall of the bladder?
Detrusor muscle
When the smooth muscle of the bladder is relaxed what does the internal urethral ‘sphincter’ do?
The smooth muscle at the start of the urethra acts as an internal sphincter and passively contracts when the bladder is relaxed/fills
What are the true and fake urethral sphincters?
Fake - internal sphincter
True - external sphincter
What is the external sphincter made up of?
Skeletal muscle
What control is the external sphincter under?
Voluntary somatic control
The bladder lies anterior to which structures?
The reproductive system and rectum
What makes the trigone?
The 2 vesicoureteric openings and the urethral opening makes a smooth triangle
Unilateral renal problems occur when which structure is obstructed?
Ureter
Bilateral renal problems occur when which structure is obstructed?
Urethra
Describe the pressure-volume curve of the bladder?
Has a long flat segment as the initial increments of urine enter the bladder
Then a sudden sharp rise as the micturition reflex is triggered (urgency)
How does the external urethral sphincter stay contracted?
Due to higher CNS input causing tonic discharge of impulses to the bladder
Motor control of micturition
Rich parasympathetic supply (pelvic nerves) increase activity => increased contraction of detrusor muscle =>
increased pressure within the bladder, S2-S4
Sparse sympathetic supply (hypogastric nerves)
Somatic motoneurones (pudendal nerves), innervate the skeletal muscle that forms the external urethral sphincter, keeps the sphincter closed, even against strong bladder contractions S2-S4.
Main function of sympathetic supply (hypogastric nerves) in the bladder?
Main function is to prevent reflux of semen into the bladder during ejaculation L1-L3.
Describe ensory innervation to the bladder by stretch receptors
Stretch receptor afferents from the bladder wall. As the bladder fills there is increased discharge in afferent nerves to spinal cord via interneurones
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?
Inhibits 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- L1-L3
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 yet 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?
They 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 initiation of micturition involve
Descending pathways that 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 detrusor muscle to initiate its contraction
Which muscles cannot be contracted voluntarily? Why?
Perineal muscles and external sphincter muscles
To prevent urine flow flowing down the urethra or interrupting the flow once urination begins
After urination, how does the female urethra empty?
By gravity
How is urine remaining in the male urethra removed after urination?
It is expelled by contractions of the bulbocavernosus muscle
3 major abnormalities of micturition due to neural lesions
Interruption of afferent nerves
Interruption of both afferent and efferent nerves
Interruption 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.
What is a mild mass reflex?
Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs creating a mild mass reflex
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.
