Urination is the process by which dangerous toxins are changed or diluted and excreted from the body with excess fluids as urine. In some animals it also serves as a way of marking territory. Urination is also known medically as micturition, voiding, uresis, and more rarely as emiction. For healthy animals, including humans, urination is typically a voluntary process. However, in infants, some elderly people, and those with neurological injuries, urination can occur involuntarily as a reflex. When urination occurs involuntarily, it is called urinary incontinence. Even in healthy individuals, incontinence can occur due to refusal to willingly empty an overfull bladder or as a reflex reaction to fear or surprise.
The liver carries out three key functions for excretion, as well as a number of things unrelated to excretion. It converts dangerous ammonia to urea, which is far less dangerous to the body, and is one of the end products that leaves the body in urine. It also cleans the blood of drugs and other harmful substances and clears any built up bilirubin from the body. Once the liver breaks down and removes dangerous substances, some are excreted as bile into the intestines, ultimately to leave the body during defecation. The rest is excreted into blood and sent to the kidneys.
The kidneys are bean shaped organs that regulate the balance of electrolytes and proper pH of blood, as well as filter out the waste substances loaded into blood by the liver. Each human kidney (each individual has two) consists of millions of functional units called nephrons. These nephrons filter the blood to maintain the proper concentration of water and soluble substances, like salts. What is needed is reabsorbed, while what is not needed is excreted as urine.
Once the urine leaves the kidneys, it is piped through the ureters, which are tubes of smooth muscle fibers that run from each kidney to the bladder. The bladder is an elastic muscular organ that collects and stores urine continually throughout the day. When the bladder is full, the individual feels the need to urinate. Bladder afferent signals run up the spinal cord to the pontine micturination center and the cerebrum. At a certain level of afferent activity, it becomes difficult to resist the urge to urinate. Whether by conscious choice or by reflex, neurons in the pontine micturition center fire maximally and cause the excitation of the sacral preganglionic neurons. The firing of these neurons causes the walls of the bladder to contract and the voiding phase begins. Voiding continues until the bladder is empty, or until the individual exerts enough control over the muscles to stop it.
When urine leaves the bladder during the voiding phase, the urinary sphincter relaxes and allows the urine to flow down through the urethra and out of the body. The urethra is the final leg of the journey for urine. After urination, the bulbospongiosus muscle contracts until the urine is completely drained from the urethra.