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By the end of this section, you will be able to:
  • Identify the following structures of the urinary system and describe their function: kidney, ureters, bladder, and urethra
  • Describe the structure of the kidneys and the functions of the parts of the kidney
  • Describe how the nephron is the functional unit of the kidney and explain how it actively filters blood and generates urine
  • Detail the three steps in the formation of urine: glomerular filtration, tubular reabsorption, and tubular secretion

Anatomy of the urinary system

The kidneys , illustrated in [link] , are a pair of bean-shaped structures that are located just below and behind the liver in the abdominal cavity. The adrenal glands sit on top of each kidney and function as a component of the endocrine system. Kidneys filter blood and purify it. All the blood in the human body is filtered many times a day by the kidneys; these organs use up almost 25 percent of the oxygen absorbed through the lungs to perform this function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine    . Urine is carried from the kidneys to the urinary bladder    via the ureters , which are approximately 30 cm long. As urine passes through the ureters, it does not passively drain into the bladder but rather is propelled by waves of peristalsis (smooth muscle contractions). The bladder collects urine from both ureters . During late pregnancy, its capacity (typically several hundred milliliters) is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. The urethra    transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical. In females, the urethra is relatively short length, about 4 cm, and is less of a barrier to fecal bacteria than the longer male urethra (approximately 20 cm). This length difference is the best explanation for the greater incidence of urinary tract infections (UTIs) in women. The urethra in males also has a reproductive function, as it transports semen (sperm and accessory fluids).

Illustration shows the placement of the kidneys and bladder in a human man. The two kidneys face one another and are located on the posterior side, about halfway up the back. A renal artery and a renal vein extend from the inside middle of each kidney, toward a major blood vessel that runs up the middle of the body. A ureter runs down from each kidney to the bladder, a sac that sits just above the pelvis. The urethra runs down from the bottom of the bladder and through the penis. The adrenal glands are lumpy masses that sit on top of the kidneys.
Kidneys filter the blood, producing urine that is stored in the bladder prior to elimination through the urethra. (credit: modification of work by NCI)

Kidney structure

Internally, the kidney has three regions—an outer cortex , a medulla    in the middle, and the renal pelvis    in the region called the hilum    of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters. The renal cortex is granular due to the presence of renal corpuscles; nephron tubules can be found throughout the renal cortex and renal pyramids , the multiple tissue masses that make up the majority of the renal medulla. There are, on average, eight renal pyramids in each kidney. Urine that is produced by the nephrons travels into the renal pelvis and then into the ureters, which carry the urine to the bladder.

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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