The urinary system is responsible for the production and elimination of urine when a type of waste called urea is removed from the blood. Urea is produced when protein-containing foods are broken down in the body. Urea is carried in the bloodstream to the two kidneys, where urine is formed. The two ureters, the urinary bladder, and the urethra, are responsible for the elimination of urine. Figure illustrates the urinary system in relationship to the body, and Figure illustrates the interior and exterior features of the urinary system organs.
FIGURE. The urinary system
FIGURE. (A) Frontal section of the right kidney showing internal structures and blood vessels. (B) Magnified section of the kidney shows several nephrons. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 422, with permission.)
The kidneys, each about the size of a fist, help to regulate the water, electrolyte (ionized salt), and acid-base content of the blood, and they selectively filter the waste products of metabolism.
They also play an important role in regulating blood pressure. Each kidney contains more than 1 million nephrons, which are the principal filtering units of the kidney. Each nephron houses a ball of tiny blood capillaries called the glomerulus and a renal tubule. It is here that the three-part process of selective filtration of wastes, reabsorption of vital minerals and fluid, and secretion of waste products and other substances takes place. As urine is formed, it passes through the nephrons into the renal tubules of the kidneys.
FIGURE. Nephron with associated blood vessels
From the kidneys, urine travels into the ureters, thin tubes about 8 to 10 inches in length, on its way to the urinary bladder. Small amounts of urine drip into the urinary bladder about every 12 seconds. The urinary bladder is a balloon-shaped muscular organ that stores urine until it is emptied. It can hold about 16 ounces of urine for 2 to 5 hours. Nerves in the urinary bladder indicate when it should be emptied. Sphincter muscles keep urine from leaking into the urethra too soon and relax when it is time to urinate.
It is worth emphasizing the reabsorption process of the kidneys’ nephrons. Were it not for this process, the body would rapidly be depleted of its fluid. Typically, only 1% of the fluid passing through a nephron is excreted as urine.
A routine diagnostic test for suspected urinary disease is a urinalysis, which includes testing the specific gravity; pH; and presence of protein, blood, sugar, and ketones. It includes a microscopic examination for the presence of white blood cells (WBCs) and red blood cells (RBCs), casts, bacteria, and crystals. Normal urine is amber in color with a slightly acid reaction, has a peculiar odor, and frequently deposits a precipitate of phosphates when fresh. The specific gravity varies from 1.005 to 1.030. The greater the rate of urine excretion, the lower is the specific gravity. Refer to Table, Significance of Changes in Urine, throughout the chapter, noting possible abnormalities and their significance to the disease in question.
Table. Significance of Changes in Urine