Micturition

Question 1

Excretion

Answer 1

• Urine composition is very different to filtrate
– Important molecules are reabsorbed (eg. Glucose, amino acids)
– Waste products are concentrated (eg. Urea, drugs)
– Ions & water vary depending on blood concentration

Question 2

Transport & storage of urine

Answer 2

Ureters
– Transports urine to bladder
• Bladder
– Storageofurine
– Stretches to accommodate urine, contracts during voiding
• Urethra
– Transports urine to exterior
– Transit of urine is controlled by sphincters
Kidneys Ureters Bladder
Urethra

Question 3

Micturition involves the relaxation of two sphincters

Answer 3

Internal urethral sphincter • Smooth muscle
• Involuntary

External urethral sphincter
• Skeletal muscle
• Voluntary

Question 4

Micturition reflex

Answer 4

• Micturition reflex is involuntary & controlled at the spinal cord
• Filling of the bladder stimulates stretch receptors
• Triggers parasympathetic stimulation of the bladder muscle (contraction)
• Internal sphincter opens
• Micturition reflex inhibits motor neurons innervating the external urethral sphincter
• Voluntary signals from cerebral cortex over- rides inhibition of motor neurons

Question 5

reflex control

Answer 5

bladder fills > stimulates stretch receptors > stim. parasympathetic nerve > bladder > bladder contracts > internal urethral sphincter mechanicallyopens when bladder contracts > urination

Question 6

voluntary control

Answer 6

voluntary control by controlling external urethra

cerebal cortex > MN to external scphinter >
1. external urethral scphinter opens when MN is inhibited > urination
or
2. > external urethral scphincter remains closed when MN stimulated > no urination