Renal: Physiology - The bladder

Question 1

How is the smooth muscle of the bladder arranged?

Answer 1

In spiral, longitudinal and circular bundles

Question 2

What prevents reflux of urine from the bladder into the ureters?

Answer 2

No sphincters, but the oblique orientation of the passage of the ureters into the bladder wall tends to keep them closed - except during the peristaltic waves which move the urine from the renal pelvis to the bladder (contractions occur 1-5x per min)

Question 3

What is the circular muscle of the bladder called? What is its role?

Answer 3

Detrusor
Contraction is responsible for bladder emptying

Question 4

Describe the two urethral sphincters

Answer 4

Internal: muscle fibres passing on either side (but not encircling) urethra
External: sphincter of skeletal muscle under voluntary control

Question 5

Describe the composition of the bladder epithelium

Answer 5

Superficial layer of flat cells
Deep layer of cuboidal cells

Question 6

At what bladder volume does the first urge to micturate occur?

Answer 6

150ml

Question 7

What happens to intravesical pressure as volume increases? Why?

Answer 7

As volume increases with bladder filling, pressure initially remains constant
Due to the law of Laplace: pressure in a spherical viscus is equal to twice wall tension divided by radius
Tension increases as organ fills, but so does radius - thus pressure increase is slight until the organ is relatively full
Additionally, bladder muscle has property of plasticity: when stretched, the tension initially produced is not maintained

Question 8

What is the role of the internal urethral sphincter?

Answer 8

No role in micturition
Main function in males is the prevention of reflux of semen into bladder during ejaculation

Question 9

How does micturition occur?

Answer 9

Perineal muscles and external urethral sphincter relax
Detrusor contracts
Voluntary contraction of abdominal muscles aids expulsion (but voiding can be initiated without straining these muscles, even when bladder is nearly empty)
Urine passes out through urethra

Question 10

How is urine emptied from the urethra after voiding in men vs women?

Answer 10

Men: expelled by contractions of bulbocavernosus muscle
Women: empties by gravity

Question 11

Describe the reflex control of micturition

Answer 11

Afferent limb: pelvic nerve fibres
Efferent limb: parasympathetic fibres within pelvic nerves
Reflex integrated in sacral portion of spinal cord
Higher centres provide voluntary control (can facilitate or inhibit micturition)

Question 12

What volume of urine in adults initiates reflex contraction of the bladder?

Answer 12

300-400ml

Question 13

What is the role of sympathetic nerves to the bladder?

Answer 13

No role in micturition
In males mediates contraction of the bladder muscle preventing semen entering the bladder during ejaculation

Question 14

What happens to the bladder with interruption of the afferent nerve pathways from the bladder?

Answer 14

Reflex contractions are abolished
Bladder becomes distended, thin-walled and hypotonic
Some contractions still occur due to intrinsic response of smooth muscle to stretch

Question 15

What happens to the bladder with interruption of both afferent and efferent nerve pathways to and from the bladder?

Answer 15

Bladder initially flacid and distended
Bladder muscle later becomes active and can expel dribbles of urine out of urethra
Bladder becomes shrunken and hypertrophic

Question 16

What changes to bladder function are seen in spinal cord transection?

Answer 16

During spinal shock: flaccid, unresponsive bladder becomes overfilled leading to overflow incontinence
After spinal shock passes: voiding reflex returns but without voluntary control, can become hyperactive with reduction in bladder capacity and hypertrophy of bladder wall (“spastic neurogenic bladder”; exacerbated by cystitis)