In the capillaries over the tissues, haemoglobin meets a very different environment. The tissue cells are continuously using oxygen, hence, the concentration of oxygen is quite low in them. It might be only one third of that in the lungs. As the concentration of oxygen is so low, oxyhaemoglobin releases the oxygen molecule that enters the cells. In the reactions that occur within cells in our bodies, carbon dioxide and water are produced and energy is released to be used up for different purposes.

The term cellular respiration refers to the pathway by which cells release energy by breaking the chemical bonds of glucose molecules. Thus cellular respiration is the essential processes of life. So living cells must carry out cellular respiration. It can be in the presence of oxygen that is 'aerobic respiration', or in the absence of oxygen as in bacterial respiration by anaerobic method which is called fermentation. The substances become sour because of ethyl alchohol or lactic acid is produced in this process. In Animals the anaerobic respiration leads to formation of lactic acid from glucose. In anaerobic repiration few ATP molecules are produced. Cellular respiration in prokaryotic cells like that of bacteria occurs within the cytoplasm. In eukaryotic cells cytoplasm and mitochondria are the sites of the reactions. The produced energy is stored in mitochondria in the form of ATP. That is why mitochondria are called "power houses of the cell".

The exact chemical details of the breakdown of sugar or other foods within a living cell does not take place as a single reaction, but occurs in a series of small steps. How does this affect the energy release? As the change in the chemical nature of the molecule from one stage to the next is slight, in any step small amount of energy is released. The complete breakdown of a sugar molecule with the release of all its available energy