Mechanisms
In unicellular organisms, simple diffusion is sufficient for gas exchange: every cell is constantly bathed in the external environment, with only a short distance for gases to flow across.
In plants oxygen is produced in photosynthesis but most oxygen used in plant respiration enters passively by diffusion or through structural openings such as lenticels.
Complex multicellular animals such as humans have a much greater distance between the environment and their innermost cells, thus, a respiratory system is needed for effective gas exchange. The respiratory system works in concert with a circulatory system to carry gases to and from the tissues.
| Organ | Oxygen consumption (ml O2/min per 100g) |
|---|---|
| Heart (rest) | 8 |
| Heart (heavy exercise) | 70 |
| Brain | 3 |
| Kidney | 5 |
| Skin | 0.2 |
| Resting skeletal muscle | 1 |
| Contracting skeletal muscle | 50 |
In air-breathing vertebrates such as humans, respiration of oxygen includes four stages:
- Ventilation, moving of the ambient air into and out of the alveoli of the lungs.
- Pulmonary gas exchange, exchange of gases between the alveoli and the pulmonary capillaries.
- Gas transport, movement of gases within the pulmonary capillaries through the circulation to the peripheral capillaries in the organs, and then a movement of gases back to the lungs along the same circulatory route.
- Peripheral gas exchange, exchange of gases between the tissue capillaries and the tissues or organs, impacting the cells composing these and mitochondria within the cells.
Note that ventilation and gas transport require energy to power a mechanical pump (the heart) and the muscles of respiration, mainly the diaphragm. In heavy breathing, energy is also required to power additional respiratory muscles such as the intercostal muscles. The energy requirement for ventilation and gas transport is in contrast to the passive diffusion taking place in the gas exchange steps.
Respiratory behavior is correlated to the cardiovascular behavior to control the gaseous exchange between cells and blood. Both behaviors are intensified by exercise of the body. However, respiratory is voluntary compared to cardiovascular activity which is involuntary.
Respiratory physiology is the branch of human physiology concerned with respiration.
Read more about this topic: Respiration (physiology)