Unit 2: Nutrition                                                                  1   2   3 

2.4. Excretion

Excretion is the process of collecting waste products and expelling them outside the body.

Wastes include carbon dioxide (CO₂) and substances rich in nitrogen, such as ammonia and urea. Carbon dioxide is eliminated by the respiratory system and nitrogen-containing substances are eliminated by the excretory system.

Simplest animals (Poriferans and Cnidarians) expel waste substances directly from each cell to the external medium, so that they do not need an excretory system. The rest of animals need a system which filters the blood and expel these wastes outside of their bodies.

Different types of invertebrates have different excretory organs. For example:

-  Insects have Malpighian tubules. They are tiny blind tubules distributed inside

   the abdominal cavity. These tubules filter the hemolymph and pour the waste

   substances to the digestive tract.

- Crustaceans have two excretory organs nearby antennae (green glands)

   that make the same function.

Vertebrates have different ways to expel waste substances. The main one is the urinary system. It is composed by the kidneys, the ureters, the urine bladder and the urethra. Kidneys filter blood and produce urine constantly. Urine is stored in the urine bladder and when it is full, urine is expelled to the exterior.

Mammals also have sweat glands which produce sweat, a substance similar to urine but much more diluted.

Marine Birds and Reptiles and some Fish which drink salt water have
specialised organs (salt glands) which expel the excess of salt from their bodies.

2.13. Indicate why:

a. It is said that the respiratory system have excretory function.

b. Marine birds can drink salt water.

c. Although kidneys produce urine constantly, this is eliminated only

   from time to time.