Respiration is the process by which living organisms obtain energy from the food they consume. It involves the exchange of gases, specifically oxygen and carbon dioxide.

In aerobic respiration, which occurs in most living organisms, the process can be divided into three main stages:

1. Breathing: This is the process of inhaling oxygen-rich air and exhaling carbon dioxide. In humans and animals, breathing involves the intake of air through the nose or mouth, which then travels down the trachea and into the lungs. In plants, respiration occurs through tiny openings called stomata on the leaves.

2. External respiration: This stage involves the exchange of gases between the organism and its environment. In the lungs of humans and animals, oxygen from the inhaled air passes through the thin walls of the alveoli (tiny air sacs) into the bloodstream. At the same time, carbon dioxide, a waste product of cellular respiration, is released from the bloodstream into the alveoli to be exhaled.

In plants, external respiration occurs through the stomata on the leaves. Oxygen from the air diffuses into the plant cells, while carbon dioxide produced during photosynthesis is released.

3. Internal respiration: This stage involves the exchange of gases between the bloodstream and the body cells. In humans and animals, oxygen from the bloodstream is taken up by the cells, where it participates in cellular respiration, a process that releases energy from glucose molecules. As a result, carbon dioxide is produced as a waste product, which is then transported back to the lungs or gills to be exhaled.

In plants, internal respiration involves the diffusion of oxygen from the plant cells into the mitochondria, where it participates in cellular respiration. Carbon dioxide, produced during cellular respiration, diffuses out of the mitochondria and into the plant cells before being released through the stomata.

Overall, respiration is a vital process that enables living organisms to generate energy for various physiological functions.