Respiration takes place in the following two stages:
- External respiration
- Internal respiration
The exchange of gases between the environment and the body is called external respiration or gaseous exchange. This includes the entry of oxygen and the exit of carbon dioxide from the cells.
Internal respiration is also called tissue respiration or cellular respiration. The bio-chemical processes involved in respiration which break down the substrate to release energy take place in the tissues within the cells of an organism. Thus, this is called cellular respiration.
Cellular respiration is common to all organisms - plants or animals. However, the external respiration, that is, the mechanism of gaseous exchange is different for different organisms.
In plants, it is through pores over the body surface. In animals, the various ways of gaseous exchange are also called the breathing mechanisms. There are various structural modifications for gaseous exchange according to the environment in which they live. The area where the gaseous exchange takes place is called the respiratory surface.
The exchange takes place by diffusion and thus, the respiratory surface should be:
- permeable - to allow the gases to pass through
- thin (1mm or less) - to allow effective diffusion
- richly supplied with blood vessels or bodily fluids - to allow maximum uptake of oxygen
- having a large surface area - to allow maximum uptake of oxygen in minimum time.
