Fundamentals of physiology: a human perspective
by Lauralee Sherwood, pub. 2005

Chapter 13: The Urinary System, page 436

Urine is temporarily stored in the bladder,
from which it is emptied by micturition.

Once urine has been formed by the kidneys, it is transmitted through the ureters to the urinary bladder. Urine does not flow through the ureters by gravitational pull alone. Peristaltic contractions of the smooth muscle within the ureteral wall propel the urine forward from the kidneys to the bladder. The ureters penetrate the wall of the bladder obliquely, coursing through the wall several centimeters before they open into the bladder cavity. This anatomic arrangement prevents backflow of urine from the bladder to the kidneys when pressure builds up in the bladder. As the bladder fills, the ureteral ends within its wall are compressed closed. Urine can still enter, however, because ureteral contractions generate enough pressure to overcome the resistance and push urine through the occluded ends.

Role of the Bladder

The bladder can accommodate large fluctuations in urine volume. The bladder wall consists of smooth muscle, which can stretch tremendously without building up bladder wall tension (see p.234). In addition, the highly folded bladder wall flattens out during filling to increase bladder storage capacity. Because the kidneys are continuously forming urine, the bladder must have enough storage capacity to preclude the need to continually get rid of the ruine.
The bladder smooth muscle is richly supplied by parasympathetic fibers, stimulation of which causes bladder contraction. If the passageway through the urethra to the outside is open, bladder contraction empties urine from the bladder. The exit from the bladder, however, is guarded by two sphincters, the internal urethral sphincter and the external urethral sphincter. See here for diagrams.

Role of the Urethral Sphincters

A sphincter is a ring of muscle that, when contracted, closes off passage through an opening. The internal urethral sphincter — which is smooth muscle and, accordingly, is under involuntary control — is not really a separate muscle but instead consists of the last part of the bladder. Although it is not a true sphincter, it performs the same function as a sphincter. When the bladder is relaxed, the anatomic arrangement of the internal urethral sphincter region closes the outlet of the bladder.
Farther down the passageway, the urethra is encircled by a layer of skeletal muscle, the external urethral sphincter. This sphincter is reinforced by the entire pelvic diaphragm, a skeletal muscle sheet that forms the floor of the pelvis and helps support the pelvic organs. The motor neurons that supply the external sphincter and pelvic diaphragm fire continuously at a moderate rate unless they are inhibited, keeping these muscles tonically contracted so they prevent urine from escaping through the urethra. Normally, when the bladder is relaxed and filling, closure of both the internal and external urethral sphincters keeps urine from dribbling out. Furthermore, because they are skeletal muscles the external sphincter and pelvic diaphragm are under voluntary control. The person can deliberately tighten them to prevent urination from occurring even when the bladder is contracting and the internal sphincter is open.

Micturition Reflex

Fig.13-21Micturition, or urination, the process of bladder emptying, is governed by two mechansims: the micturition reflex and voluntary control. The micturition reflex is initiated when stretch receptors within the bladder wall are stimulated (Figure 13-21). The bladder in an adult can accommodate up to 250 to 400 ml of urine before the tension within its walls begins to rise sufficiently to activate the stretch receptors. The greater the distension beyond this, the greater the extent of receptor activation. Afferent fibers from the stretch receptors carry impulses into the spinal cord and eventually, by inter-neurons, stimulate the parasympathetic supply to the bladder and inhibit the motor neuron suppply to the external sphincter. Parasympathetic stimulation of the bladder causes it to contract. No special mechanism is required to open the internal sphincter; changes in the shape of the bladder during contraction mechanically pull the internal sphincter open. Simultaneously, the external sphincter relaxes as its motor neuron supply is inhibited. Now both sphincters are open, and urine is expelled through the urethra by the force of bladder contraction. This micturition reflex, which is entirely a spinal reflex, governes bladder emptying in infants. As soon as the bladder fills enough to trigger the reflex, the baby automatically wets.

Voluntary Control of Micturition

In additioinal to triggering the micturition reflex, bladder filling also gives rise to the conscious urge to urinage. The perception of bladder fullness appears before the external sphincter reflexly relaxes, warning that micturition is imminent. As a result, voluntary control of micturition, learned during toilet training in early childhood, can override the micturition reflex so that bladder emptying can take place at the person's convenience rather than when bladder filing first activates the stretch receptors. If the time when the micturition reflex is initiated is inopportune for urination, the person can voluntarily prevent bladder emptying by deliberately tightening the external sphincter and pelvic diaphragn. Voluntary excitatory impulses from the cerebral cortex override the reflex inhibitory input from the stretch receptors to the involved motor neurons (the relative balance of EPSPs and IPSPs ), keeping these muscles contracted so that no urine is expelled (see p.90).
Urination cannot be delayed indefinitely. As the bladder continues to fill, reflex input from the stretch receptors increases with time. Finally, reflex inhibitory input to the external-sphincter motor neuron becomes so powerful that it can no longer be overriden by voluntary excitatory input, so the sphincter relaxes and the bladder uncontrollably empties.
Micturition can also be deliberately initiated, even though the bladder is not distended, by voluntarily relaxing the external sphincter ad pelvic diaphragm. Lowering of the pelvic floor allows the bladder to drop downward, which simultaneously pulls open the internal urethral sphincter and stretches the bladder wall. The subsequent activation of the stretch receptors brings about bladder contraction by the micturition reflex. Voluntary bladder emptying may be further assisted by contracting the abdominal wall and respiratory diaphragm. The resulting increase in intra-abdominal pressure squeezes down on the bladder to facilitate its emptying.