Blood vessels carry oxygen from alveoli to tissues. Cells of tissues take up the oxygen. In the mitochondria of the cells, oxygen acts upon glucose to yield energy and release carbon dioxide. This is called cellular respiration.

In animals, Respiration involves:

  1. breathing or ventilation which happens in lungs
  2. cellular respiration which occurs in cells and produces energy from oxidation of food

Organs of respiration in fish are gills. In humans - nostrils, trachea, bronchi, lungs.

Breathing happens by change in position of ribs and diaphragm. O2 enters blood and reaches cells. CO2 produced by cells goes out into blood and from lungs out of body. Exchange of O2 and CO2 takes place between alveoli and blood. In the cells, energy production by oxidation of glucose occurs in mitochondria.

Gas Exchange between alveoli and blood capillaries

  • Air with more O2 breathed in
  • Reaches alveoli
  • High concentration of O2 in alveoli
  • High concentration of CO2 in tissues
  • The two gases exchanged between blood brought from tissues and alveoli

In plants, oxygen enters through stomata of leaves by diffusion. CO2 diffuses out of cells, also through stomata.