Physiology of the urinary system

The urinary system is the major system involved in the excretion of metabolic waste products which accumulate in the blood and excess of water from the body, regulates blood volume and pressure, and regulates blood pH. A type of waste called urea is removed from the blood by the urinary system. Urea is produced when foods containing protein, such as meat, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

The kidneys remove urea from the blood through nephrons. And then urea, together with water and other waste substances, forms the urine and passes through the nephrons and then it is excreted from the kidney via the ureters.

Wastes in the blood come from the normal breakdown of active tissues and from food. The body uses food for energy and self-repairs. After the body has taken what it needs from food, wastes are sent to the blood.

Ureters are two narrow tubes that propel urine towards the urinary bladder, where it is stored and subsequently expelled by urination. Muscles in the ureter walls continually tighten and relax forcing urine away from the kidneys. If urine backs up or stands still, a kidney infection can develop.

The urinary bladder is a temporary storage for the urine, its reservoir. The bladder's walls relax and expand to store urine, and contract to empty it. Nerves in the bladder alert a person when it is time to urinate.

The urethra passes the urine outside of the body. The brain signals the bladder muscles to tighten, which squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.

Facts about urine:

• Average urine production in adults is about 1 – 2 l per day. The amount of urine depends on the amounts of fluid and food a person consumes and how much fluid is lost through sweat and breathing.

• Normal urine is sterile fluid. It contains fluids, salts and waste products, but it is free of bacteria, viruses and fungi.

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