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Structure and Function

The urinary system consists of the kidneys, ureters, bladder, and urethra. Each structure is uniquely designed and suited to its purpose. The urinary system’s main functions are to filter and excrete waste products from the body, help regulate blood pressure, and maintain an optimal level of fluid and electrolytes within the body (Fig. 10-1).

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The key organs of the urinary system are the kidneys, which are located in the back of the abdominal cavity in the retroperitoneal space, to either side of the vertebral column. The right kidney is slightly lower than the left. Each one is surrounded by a renal capsule, made up of connective tissue and a thick layer of fat. This provides protection by acting as a cushion and a shock absorber. The renal artery, vein, nerves, and ureter exit the kidneys on the medial (inner) side.

Both kidneys are highly vascular organs made up of an outer cortex and an inner medulla. In fact, over 20% of the blood pumped by the heart each minute passes through the kidneys. The vascular nature of the kidneys lends itself to their function, which is to filter blood for the elimination of wastes and excess fluid and to regulate electrolytes (the ions in bodily fluids). Major electrolytes include sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca).

Located primarily within the outer cortex of the kidneys are microscopic structures called nephrons (Fig. 10-2). There are more than one million nephrons in each kidney. Each nephron is composed of an arteriole, venule, Bowman’s capsule, glomerulus (capillary cluster within the Bowman’s capsule), proximal tubule, Henle’s loop, distal tubule, and capillary bed. The nephron has long been called the functional unit of the kidney, because it is where most of the action takes place. To begin the filtration process, blood passes from a tiny arteriole into the glomerulus. The walls of the glomerulus and Bowman’s capsule are designed to permit filtration of water, electrolytes, urea, and other small molecules. A large amount of this fluid (approximately 180 liters), called filtrate, is created each day. However, as it moves on through the proximal tubule, Henle’s loop, and distal tubule, nearly all of the water and useful solutes (99%) are reabsorbed, and additional wastes are excreted. After the kidneys make final adjustments in the composition of the leftover fluid, it is called urine. The kidneys produce and excrete an ...

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