Gas Exchange: As oxygen is a key component in cellular respiration, a cell needs to constantly bring in oxygen so that it can go through cellular respiration. Also, each cell needs to eliminate its carbon dioxide, a waste product of cellular respiration. Therefore, gas exchange— the process by which oxygen enters a cell and carbon dioxide leaves it— is necessary.
Requirements for gas exhange:
- large surface area: so that it can occur at a fast-enough speed
- moist environment
Diffusion
- Diffusion is a natural process in which molecules travel from areas of high concentration to low concentration
- It is only effective if an organism is only a few cells think
- Therefore, only uni-cellular organisms and very thin multi-cellular organisms (such as planarian) rely on diffusion
Specialized Respiratory Systems
- As organisms become larger, diffusion becomes inefficient.
- Also, many cells become specialized for other functions (e.g., reproduction)
- Therefore, a respiratory system is needed — a system that consists of respiratory surfaces, and muscles and tubes that allow air to contact the surface.
- Different types:
- Skin Respiration: The skin is lined with capillaries so that air can diffuse from the skin into the blood. The blood then transfers the oxygen to rest of the organism. Since diffusion must occur in a moist environment, organisms that rely on skin respiration must live in moist/damp environments. Examples: annelids (including earthworms, leeches)
- Gills: gills are organs that have high surface area and a lot of capillaries. They allow diffusion and gas exchange to occur efficiently for aquatic animals.
- Tracheal Respiratory System: This is for insects. Many insects have spiracles (external pores) that allow the air to go into their trachea, which come in close contact to all the living cells.
- Lungs: organs that forcibly bring in air so that gas exchange can occur at the capillaries located on the lung surface. The blood then transports the oxygen to other parts of the body.
