Micturition is the medical term for the process of urination. It is a function of the body’s excretory system, responsible for expelling waste products and surplus water that have been filtered from the bloodstream. This process helps maintain the body’s internal chemical balance, a state known as homeostasis. This ensures that soluble waste, primarily urea, is removed from the body in the form of urine.
The Micturition Reflex
The process of urination begins with the involuntary micturition reflex, which is centered in the urinary bladder. The bladder wall is composed of smooth muscle fibers called the detrusor muscle, which remains relaxed to allow the bladder to store urine. At the base of the bladder, the internal urethral sphincter guards the opening to the urethra and stays contracted during filling.
As the kidneys produce urine, it travels to the bladder for storage. An adult bladder can hold between 300 to 500 milliliters of fluid. When the volume of urine reaches 200 to 400 milliliters, the bladder wall stretches. This stretching activates stretch receptors within the detrusor muscle, which transmit signals to the sacral region of the spinal cord.
The arrival of these signals at the spinal cord initiates a reflex arc. The spinal cord sends signals back to the bladder via parasympathetic nerve fibers. These signals cause the detrusor muscle to contract and the involuntary internal urethral sphincter to relax. In infants, this reflex leads directly to urination without any conscious input.
Voluntary Control of Urination
While the micturition reflex is involuntary, it is subject to conscious oversight in adults. The sensory signals from the bladder’s stretch receptors that travel to the spinal cord also ascend to higher brain centers. These signals reach the brainstem, including the pontine micturition center (PMC), and the cerebral cortex. This communication produces the sensation of needing to urinate.
Upon perceiving the urge to void, the brain can delay urination. This is done through the somatic nervous system, which governs voluntary muscle movements. The brain signals the external urethral sphincter, a consciously controlled muscle, to remain contracted. This muscle’s sustained contraction prevents urine from flowing out, even as the detrusor muscle may begin to contract.
When an appropriate time for urination is determined, the brain initiates the process. The cerebral cortex signals the pontine micturition center, which coordinates the relaxation of the external urethral sphincter. The brain also ceases its inhibitory signals, allowing the micturition reflex to proceed, causing the detrusor muscle to contract and expel urine.
Common Micturition Problems
Disruptions to the process of micturition can lead to common urinary issues. These problems often arise from malfunctions in the muscular or nervous system components that regulate urination. Two of the most frequent issues are urinary incontinence and urinary retention.
Urinary incontinence is the unintentional leakage of urine and can manifest in different forms. Stress incontinence occurs when physical pressure, from coughing or laughing, overwhelms a weakened external urethral sphincter or pelvic floor muscles. Urge incontinence, also called overactive bladder, is caused by sudden, involuntary contractions of the detrusor muscle, creating an intense need to urinate. It is also possible for individuals to experience a combination of both types.
Urinary retention is the inability to completely empty the bladder. This condition can be acute or chronic. A primary cause is an obstruction of the urinary outflow that blocks the path of urine. In men, an enlarged prostate, or benign prostatic hyperplasia (BPH), is a frequent cause, as the gland can compress the urethra. Other causes include nerve problems that interfere with the detrusor muscle’s ability to contract.