FUNCTION OF THE URINARY SYSTEM

 

The main urinary system function is to filter the blood of excess water, salts, and waste products, temporarily store these within a reservoir, and intermittently expel these products from the body.

The main urinary system function is to filter the blood of excess water, salts, and waste products, temporarily store these within a reservoir, and intermittently expel these products from the body.

Filtration, Reabsorption, and Secretion

Each kidney contains approximately one million nephrons. Nephrons play a crucial role in removing waste products and adjusting concentrations of water, ions and smaller molecules in the blood. A single kidney contains enough nephrons to filter the blood– once they are damaged, they cannot be replaced. and produce urine – for this reason, kidney transplants can use organs from living donors. When both kidneys are damaged, ions, salts, water, and small molecules accumulate in the body, causing complete organ failure and death if left untreated. A kidney cannot generate new nephrons.

Urine Transportation and Storage

Nephrons empty the final product – urine or ultrafiltrate – into a system of collecting ducts which bring the urine into the ureter. Ureters are two narrows but relatively long tubes (approximately 25 – 30 cm) which leave the kidneys and enter the urinary bladder. Ureter muscle layers help by way of peristalsis – the involuntary contraction and relaxation of smooth muscle – to propel urine towards the urinary bladder. Gravity also plays a part. The function of the urinary bladder is to store and expel urine. 

Micturition

A human bladder usually stores around 300 to 600 ml of urine before micturition and is controlled by the pontine micturition center of the brain.

The triangular shaped bladder is a large muscle into the top of which the two ureters empty the urine. Under the bladder, close to where it connects to a single urethra, lie two circular sphincters – internal and external. These only relax during micturition and otherwise prevent urine from constantly leaking out. The internal sphincter is an involuntary muscle; the external sphincter is voluntary. The urinary bladder is lined by a thick mucosa which forms folds when the bladder is empty but opens out as the bladder fills and expands.

Located within the bladder wall are stretch receptor cells. When the bladder fills to around 250 ml, the mucosal folds open and stimulate these stretch receptors. They send signals to the pontine micturition center of the brain. The brain responds by relaxing the internal urethral sphincter. The external urethral sphincter remains closed until the person in question chooses to urinate, although in babies, the elderly, and those with motor or sensory nerve disorders such as sufferers of multiple sclerosis, this is not always a voluntary action. Upon relaxation of the voluntary external sphincter, urine from the bladder flows through the urethra and is expelled from the body. The image below shows the stages of micturition and the muscles involved, namely the detrusor muscle of the urinary bladder, the internal urethral sphincter, and the pelvic floor muscle which surrounds the external sphincter.