Excretion

Anatomy of the excretory system:

When the bladder fills up, the micturition reflex makes the bladder contract and push the urine out via the urethra. At the beginning of the urethra there is a sphincter (ring of smooth muscle) which stays closed as long as the micturition reflex does not occur. 

 After watching these videos, what do you make? Is urination voluntary or involuntary? Why urinary infections are more common in females than in males?