The urinary tract consists of the organs involved in the production and elimination of liquid waste (urine) from the body: the kidneys, ureters, bladder, and urethra.
There are also two adrenal glands, one on top of each kidney, that produce important hormones the body needs. Men also have a gland called the prostate, located in the pelvis, below the bladder and in front of the rectum. It surrounds the upper part of the urethra and is part of the male reproductive system.
The kidneys are a pair of bean shaped organs in the back of the abdomen. They clean the body of excess water, salt and waste products. As blood circulates through the kidneys, it passes through more than a million tiny filtering units within them. These filtering units produce urine.
Urine flows from the kidneys through a pair of thin tubes, the ureters, to the bladder. (There is one ureter for each kidney.) The bladder, located in the pelvis, is a hollow, muscular, balloon shaped organ that stores urine. Normally, people can hold urine in their bladder until they want to urinate.
During urination, muscles in the wall of the bladder contract, forcing urine out of the bladder and into a tube called the urethra. At the same time, the sphincter muscles that surround the urethra relax. The pelvic floor muscles, below the bladder, assist the sphincter muscles by supporting the bladder and helping it hold or release urine.
One end of the urethra is connected to the bladder; the other end is open. In women, the opening is located just above the vagina. In men, it s at the tip of the penis. When the bladder muscles tighten and the sphincter muscles relax, urine leaves the body by passing through the opening of the urethra.
Communication between the urinary tract and the brain controls the storage and release of urine. Nerves, running from the bladder and surrounding muscles through the spinal cord to the brain, carry messages between them, telling the brain that the bladder is full. The brain sends messages back to the muscles, telling them either to tighten or release. For the urinary system to work right, the muscles and nerves must work together to hold urine in the bladder and then release it at the right time.
Excretion can be defined as the removal of toxic waste products of metabolism from the body. These wastes can be either solid, liquid or in the gaseous state. The liquid wastes are ammonia and urea, which exist in the blood along with the nutrients and other useful substances. So there is a need of complex organ that may separate or filter out the dissolved excretory wastes from blood while retaining the nutrients in the latter. Two kidneys in human beings are such organs that perform this task. There is a distinct advantage of the two kidneys in our body. If one kidney fails, the other can still deal with functions of excretion and regulation.
The kidneys are solid; bean shaped, reddish brown-paired structure, which lie in the abdominal cavity one on either side of the vertebral column. The kidneys collect the excretory products and eliminate them in the form of urine, which then passes down the two tubular ureters into the collapsible urinary bladder, which is a muscular reservoir of urine. The urine is released periodically to the outside via the urethra.
A section through a kidney shows a darker outer region, the cortex and a lighter inner zone, the medulla, where the ureter leaves the kidneys is a space called the pelvis. Each kidney is made of numerous (about 1 million) called tubules known as nephrons, which are the functional unit of the kidneys. About 180 litres of blood, which run through these nephrons, are reduced to urine by the process of filtration, reabsorbtion and secretion by the nephrons. The urine enters the pelvis of the kidney where it collects and continues down the ureters to the bladder. In the urinary bladder urine is temporarily stored and is finally eliminated from the body. Human being on an average excrete about 1 to 1.5 litres of urine per day.