Physiology of Micturition and Assessment of Renal Function Flashcards
Why is it important to be able to asses renal function?
It has a central role in homeostasis
In what situations is the ability to measure GFR useful?
1) In patients with renal disease to measure the disease progression
2) Many drugs are removed by excretion at the kidneys. If GFR falls then the excretion falls and the plasma concentration of the drug may rise causing toxicity
How is the renal function measured?
Plasma clearance tests: these measure the ability of the kidney to clear the plasma of various substances
What is the equation for plasma clearance?
Cx = [Ux]V/[Px]
Ux - urine concentration of x
V = urine flow rate
Px = plasma concentration of x
Which test is the gold standard for measuring renal clearance?
Inulin clearance. IV dose, given time to equilibrate and then plasma and urine are measured simultaneously
What happens to inulin in the kidneys?
It is freely filtered but not reabsorbed or secreted
What will happen to the clearance of substances that are filtered and reabsorbed?
The clearance will be decreased
What is the normal GFR in males and is it different in females?
125ml/min
In females it is 10% lower
Why is inulin no longer used in clinical practice?
It is too cumerbersome and time consuming
What is used instead of inulin to estimate GFR?
Creatinine (clearance is similar to inulin)
What is eGFR?
Estimated GFR
What factors affect serum creatinine?
Muscle mass (more muscle = more creatinine), dietary intake (creatinine) supplements vs vegetarians) and drugs (some cause increases as does ketoacidosis)
What is the clearance of glucose in a healthy kidney?
Zero - normally all glucose is reabsorbed
How does the clearance of urea compare to the clearance of inulin?
Less - some urea is reabsorbed (50%)
Which substance is used to measure real plasma flow and why?
PAH - 90% of the plasma is cleared of its PAH in one transit of the kidneys
How does the clearance of penicillin clearance compare to inulin clearance?
It is greater because the penicillin is filtered and secreted
What causes the urine to flow from the kidneys to the ureters?
Peristaltic contraction of the smooth muscle of the ureters
Does the urine composition change after it has left the kidneys?
No
Which muscle is responsible for emptying the bladder during micturition?
Detrusor muscles (smooth muscle arranged in spirals, longitudinal and circular bundles)
Which sphincter is a true sphincter, internal or external urethral?
External
Where does the bladder lie?
In the midline posterior to the pubic bones
What renal problems arise from urethral obstructions?
Bilateral problems
What renal problems can arise from ureter obstruction?
Unilateral renal problems
What is the normal daily urine production in temperate climates?
750-2500ml
What is the nerve control of micturition?
Parasympatetic control (pelvic nerves) - contraction of detrusor muscle Sympathetic (hypogastric nerves) - inhibit bladder control and close the internal urethral sphincter Somatic Motoneurones: innervate the skeletal muscle keeping the external urethral sphincter closed
What is the sensory innervation of micturition?
Stretch receptors in the bladder wall
Spinal reflex controls micturition - may or may not be influenced by higher centres
In which groups of people would there be no input fro the higher centres in micturition?
‘Leaky’ babies and adults with spinal cord transection
How does the pathway from the brain delay micturition?
Inhibiting the parasympathetic and stimulates the somatic nerves
How does voluntary initiation work?
Stimulates the parasympathetic and inhibits the somatic motor neurones
Which muscles are involved in the initial events of micturition?
Muscles of the pelvic floor
What are the 3 types of neural lesions that can cause abnormal micturition?
Interruption of afferent nerves, interruption of both afferent and efferent nerves and interruption of facilitatory and inhibitory descending pathways from the brain
