VMBO Year 2 · HAVO/VWO Years 2–3 · Cellular respiration and breathing · Section 1.6

Gas exchange in animals

Unicellular organisms

In unicellular organisms, such as amoebae, oxygen and carbon dioxide pass through the cell membrane.

These organisms are small and have a large surface area relative to their volume. Diffusion distances are short, so they do not need specialised respiratory organs.

Insects

Insects exchange gases through tracheae, a highly branched system of air-filled tubes carrying air throughout the body. Air enters through spiracles, openings along the body, including the abdomen.

Many insects ventilate the system by pumping movements of the abdomen. The fine branches bring air close to tissues, allowing efficient gas exchange directly with cells.

Fish

Fish use gills to exchange gases with water. In bony fish, the gills lie in chambers covered by gill covers, or opercula.

Gill arches support many fine filaments and lamellae, which contain tiny blood vessels. Thin walls and a large surface area allow oxygen to enter the blood from water and carbon dioxide to leave it.

Coordinated movements of the mouth and gill covers help keep fresh water flowing over the gills.

Birds

Bird lungs work differently from mammal lungs. They do not have mammalian alveoli. Instead, the lungs are connected to several air sacs that help maintain airflow.

In a simplified model, breathing in draws fresh air towards the rear air sacs and moves air from the lungs towards the front sacs. Breathing out moves air from the rear sacs through the lungs and expels air from the front sacs.

This arrangement allows airflow through the gas-exchange parts of the lungs during both inhalation and exhalation. It supports the high oxygen needs of birds, especially during flight.