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In Depth: Kidneys

Kidneys are the most crucial organs of the urinary system. Their main function is to control water balance in the body by filtering blood and creating urine as a waste product to be excreted from the body.

Adrenal (suprarenal) Glands

Like little hats, the adrenal (or suprarenal) glands sit atop the kidneys. Part of the endocrine system, the glands are divided into two portions, the adrenal cortex and the adrenal medulla, and each synthesizes and secretes a different set of hormones. The various hormones help the kidneys to conserve sodium, thus conserving water. They also play a role in supporting the body’s reproductive functions, among other things.


The kidneys are between 4 and 5 inches long in the average adult and contain an estimated 1 million filtering units called nephrons. The kidneys play a vital role in processing the blood, which the heart pumps before it goes into general circulation. Every minute, between 1.2 and 1.3 Liters (source: Cecil’s Medicine) of blood flows through the kidneys, which is about one-fifth of all the blood pumped from the heart. Blood is pumped from the heart into the kidneys through the renal artery, which branches directly from the abdominal aorta, a section of the body’s main artery.


The ureters are two tubes that carry urine from the kidneys to the urinary bladder.  The ends of each tube act as valves by closing when the bladder is full and preventing backflow of urine. In men, the ureters are close to the seminal vesicles and near the prostate gland. Each ureter contains three layers: a mucous lining, a muscular middle layer, and a fibrous outer layer. The muscular layer helps move the urine via peristalsis (muscular contractions). The rate and strength of these muscular contractions increase the more urine there is. 

Urinary bladder 

The urinary bladder is a collapsible bag of muscles that is located near the vagina and in front of the uterus in women. The bladder’s wall is made mostly of smooth muscle tissue. The urinary bladder contains three openings: one from each ureter and one into the urethra, a small tube that carries urine out of the bladder.

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