The urinary system, also known as the urinary tract, acts as a filtering unit for the body’s blood, excreting waste products and excess water as urine. The system consists of a pair of kidneys; the bladder; the ureters, which connect each kidney to the bladder; and the urethra, through which urine leaves the body. As the urinary system filters the blood, it regulates body water levels and maintains the balance of body fluids. The kidneys also produce hormones, one of which helps to control blood pressure. Every day the body’s entire volume of blood passes through the kidneys over 300 times – a flow of about 1,700 litres (375 gallons).
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Every action of our daily lives, from eating and breathing to walking and running, is made possible by chemical reactions in our body cells. Wastes produced as a result of these reactions collect in the blood. In order for us to remain healthy, these wastes must be filtered out and excreted. This is the main function of the urinary system.
The kidneys are connected to the aorta, the body’s main artery that runs directly from the heart. When the body is at rest, the kidneys receive about a quarter of the blood pumped by the heart. Filtered blood is returned to the heart through the inferior vena cava, the largest vein in the body.
Each kidney contains about 1 million nephrons, mini-filtering units that consist of a knot of capillaries called the glomerulus and a long, thin tube called the renal tubule. Pores in the glomerulus allow only some of the molecules in the blood to pass through, depending on their size and shape. For example, red blood cells and proteins are much too large to pass through these pores and do not enter the filtered fluid. Smaller molecules pass into the renal tubule, where useful substances, such as glucose, are reabsorbed into the bloodstream. The fluid remaining in the tubule, called urine, is a mixture of wastes, such as urea, and other substances that are not required by the body, such as excess water and salts.
This constant filtration of blood not only removes harmful wastes from the body but also helps to regulate water levels. If, for example, you drink more water than is needed for the body’s requirements, the excess is excreted in urine. However, if you need to conserve water, the kidneys make the urine more concentrated and excrete as little water as possible. Furthermore, if your blood becomes too acidic or too alkaline, the kidneys change the urine’s acidity level to restore the correct balance.
The kidneys produce a number of hormones, each with different roles. Their functions include stimulating the formation of blood cells and controlling blood pressure by helping to regulate the blood flow through arteries.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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