Summary

In this lesson, you explored the following questions:

• What are the principal structures of the excretory system, and how do they function?
• How do nephrons contribute to the function of the excretory system?

The principal structures of the excretory system are the kidneys, ureter, urethra, bladder, renal vein, and renal artery. The kidneys are the filtering/absorbing workstation of this system, while renal blood vessels transport filtered and unfiltered blood to the kidneys. The ureter and urethra are also transport tubes that transfer urine to the bladder and the external environment.

Millions of functional units called nephrons within the kidneys are the site of filtration and absorption. Nitrogenous wastes such as urea, ammonia, and uric acid are filtered from the blood to form urine. The excretory system regulates homeostasis by maintaining blood composition and ridding the body of wastes.

In Lesson 4, you will further explore how the kidneys maintain the composition of the blood plasma by excreting metabolic wastes.

Lesson Glossary

bladder: the organ where urine is stored before being discharged by the urethra

kidneys: two organs that filter wastes from the blood and adjust the concentration of salts in the blood

ureter: in mammals, a pair of muscular tubes that carry urine from the kidneys to the bladder

urethra: the tube through which urine exits the bladder and the body

nephron: a microscopic tube-like filtration unit found in the kidneys that filters and reabsorbs various substances with the blood and produces urine