Excretion

Introduction
Elimination
- Biological significance of elimination
- Types of waste: metabolic and nonmetabolic
  - Nonmetabolic wastes
  - Metabolic wastes
    - Gaseous wastes
    - Liquid wastes
    - Solid wastes
- Methods of waste disposal
  - Specific elimination mechanisms
    - Alimentary canal
    - Respiratory system
    - The kidneys
  - Nonspecific mechanisms of waste disposal
- Comparative overview of eliminatory mechanisms from protists to vertebrates
  - Protista
  - Plants
  - Animals
    - Sponges
    - Cnidarians
    - Flatworms
    - Nemertine worms
    - Nematodes
    - Other invertebrates
    - Vertebrates
General features of excretory structures and functions
- Products of excretion
- Excretory mechanisms
  - Osmotic pressure
  - Regulation of water and salt balance
  - Principal excretory structures
- Invertebrate excretory systems
  - The contractile vacuoles of protozoans
  - The nephridia of annelids, nemertines, flatworms, and rotifers
  - The renal glands of mollusks
  - The coxal glands of aquatic arthropods
  - The malpighian tubules of insects
- Vertebrate excretory systems
  - Mammals
  - Birds and reptiles
  - Amphibians
  - Fishes
  - Evolution of the vertebrate excretory system

References & Edit History Quick Facts & Related Topics

Images

Line graph showing plasma concentration of a drug over 20 hours after oral administration, with a rapid rise during absorption, a peak, and gradual decline during elimination.

Absorption, distribution, and excretion of toxicants

At a Glance

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excretion summary

Related Questions

Excretion

biology

Also known as: elimination, waste disposal

Written by

Fenton Crosland KelleyAll

Fact-checked by

Britannica Editors

Last updated

Nov. 6, 2026 •History

Excretion, the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment. Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called...

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