The Urinary System
The first successful human organ transplant was a kidney transplant performed in 1953. Because the donor and recipient were identical twins, rejection was not a problem. Thousands of kidney transplants have been performed since then, and the development of immunosuppressive medications has permitted many people to live normal lives with donated kidneys. Although a person usually has two kidneys, one kidney is sufficient to carry out the complex work required to maintain homeostasis of the body fluids.
The urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra (Fig. 18–1). The formation of urine is the function of the kidneys, and the rest of the system is responsible for eliminating the urine.
Body cells produce waste products such as urea,
Figure 18–1. The urinary system shown in anterior view.
QUESTION: Why is blood pressure relatively high in the kidneys? What do you see that would suggest this?
creatinine, and ammonia, which must be removed from the blood before they accumulate to toxic levels. As the kidneys form urine to excrete these waste prod-ucts, they also accomplish several other important functions:
1. Regulation of the volume of blood by excretion or conservation of water
2. Regulation of the electrolyte content of the blood by the excretion or conservation of minerals
3. Regulation of the acid–base balance of the blood by excretion or conservation of ions such as H+ ions or HCO32 ions
4. Regulation of all of the above in tissue fluid
The process of urine formation, therefore, helps maintain the normal composition, volume, and pH of both blood and tissue fluid by removing those sub-stances that would upset the normal constancy and balance of these extracellular fluids.
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