Figure 48-15 shows the three main parts of the kidney. The renal cortex, the outermost portion of the kidney, makes up about a third of the kidney's tissue mass. The renal medulla is the inner two-thirds of the kidney. The renal pelvis is a funnel-shaped structure in the center of the kidney. Also, notice in Figure 48-15 that blood enters the kidney through a renal artery and leaves through a renal vein. The renal artery transports nutrients and wastes to the kidneys. The nutrients are used by kidney cells to carry out their life processes. One such process is the removal of wastes brought by the renal artery.

The most common mammalian metabolic waste is urea (yoo-REE-uh), a nitrogenous product made by the liver. Nitrogenous wastes are initially brought to the liver as ammonia, a chemical compound of nitrogen so toxic that it could not remain long in the body without harming cells. The liver removes ammonia from the blood and converts it into the less harmful substance urea. The urea enters the bloodstream and is then removed by the kidneys.


• Identify the major parts of the kidney.

• Relate the structure of a nephron to its function.

• Explain how the processes of filtration, reabsorption, and secretion help maintain homeostasis.

• Summarize the path in which urine is eliminated from the body.

• List the functions of each of the major excretory organs.

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