Indirect respiration means the exchange of gases through some specialized structure and involves two phases, external and internal respiration.
External respiration: External respiration includes the exchange of gases between the external environment and the blood of the respiration organ.
Internal respiration: Internal respiration, or cellular respiration involves the exchange of gases between the bloodstream and the body cells. It is also known as tissue respiration and the oxygen consumption per milligram of tissue per hour is denoted as Qo2. Since the respiration rate of each tissue depends on its demand for ATP, there are considerable variations in Qo2 of various tissues.
In eukaryotic cells, the oxidative part of cellular respiration occurs inside the mitochondria, while the glycolysis part occurs inside the cytoplasm. Although very different in mechanism, cellular respiration is in principle similar to combustion of gasoline in an automobile engine. Food provides the fuel for respiration and the exhaust is carbon dioxide and water. The overall process can be summarized as follows:
Organic compounds (from food) + oxygen → carbon dioxide + water + energy
Although carbohydrates, proteins and fats can all be processed and consumed as fuel, the steps of cellular respiration can be easily understood by tracking the degradation of glucose, C6H12O6, the fuel that cells most often use:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + Heat)
The reactants as well as products of respiration are transported to and from the cell by the bloodstream, through processes like diffusion and osmosis. Oxygen enters the blood from air that animals inhale, through the alveoli and capillaries of the lungs. Carbon dioxide enters the bloodstream from the cells, transported to the lungs and exchanged there in the air only to be finally exhaled out. All these gas exchanges take place by diffusion, owing to difference in partial pressures of respective gases on the two sides of the transport medium.
Water molecules are also transported out of the cell and are either recycled in to feed other physiological processes requiring water, or excreted/exhaled out of the body.
The energy released in respiration is stored in the form of chemical energy in small denominations of ATP (38 moles of ATP are formed in total when one mole of glucose is degraded inside the cell, each one carrying 7.3 Kcal of free energy). Thus, ATP is the workable change the cell uses to power its various works and functions. Some amount of energy is utilized to produce heat that keeps the cells warm.