In animal physiology
is the transport of Oxygen
from the outside air to the cells within tissues
and the transport of carbon dioxide
in the opposite direction. This is in contrast to the biochemical
definition of respiration, which refers to cellular respiration
: the metabolic
process by which an organism
obtains energy by reacting oxygen
to give water
, carbon dioxide
(energy). Although physiologic respiration is necessary to sustain cellular respiration
and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the animal, while physiologic respiration concerns the bulk flow
and transport of metabolites between the organism and external environment.
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 contrast, 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.
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 respitation, mainly the diaphragm. In heavy breathing, energy is also required to power additional respiatory muscles such as the intercostal muscles. The energy requirement for ventiliation and gas transport is in contrast to the passive diffusion taking place in the gas exchange steps.
Respiratory physiology is the branch of human physiology concerned with respiration.
Classifications of respiration
There are several ways to classify the physiology of respiration:
By intensive care and emergency medicine
By other medical topics