The physiological definition of respiration differs from the biochemical definition, which refers to a metabolic process by which an organism obtains energy (in the form of ATP and NADPH) by oxidizing nutrients and releasing waste products. 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 organism, while physiologic respiration concerns the diffusion and transport of metabolites between the organism and the external environment.
Gas exchanges in the lung occur by ventilation and perfusion. Ventilation refers to the in and out movement of air of the lungs and perfusion is the circulation of blood in the pulmonary capillaries. In mammals, physiological respiration involves respiratory cycles of inhaled and exhaled breaths. Inhalation (breathing in) is usually an active movement that brings air into the lungs where the process of gas exchange takes place between the air in the alveoli and the blood in the pulmonary capillaries. Contraction of the diaphragm muscle cause a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of the respiratory system. In contrast, exhalation (breathing out) is usually a passive process.
The process of breathing does not fill the alveoli with atmospheric air during each inhalation (about 350 ml per breath), but the inhaled air is carefully diluted and thoroughly mixed with a large volume of gas (about 2.5 liters in adult humans) known as the functional residual capacity which remains in the lungs after each exhalation, and whose gaseous composition differs markedly from that of the ambient air. Physiological respiration involves the mechanisms that ensure that the composition of the functional residual capacity is kept constant, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thus throughout the body. Thus, in precise usage, the words breathing and ventilation are hyponyms, not synonyms, of respiration; but this prescription is not consistently followed, even by most health care providers, because the term respiratory rate (RR) is a well-established term in health care, even though it would need to be consistently replaced with ventilation rate if the precise usage were to be followed.
Classifications of respiration
There are several ways to classify the physiology of respiration:
By intensive care and emergency medicine
- Mechanical ventilation
- Iron lung
- Intensive care medicine
- Liquid breathing
- Oxygen toxicity
- Medical ventilator
- Life support
- General anaesthesia
By other medical topics
- Respiratory therapy
- Breathing gases
- Hyperbaric oxygen therapy
- Gas embolism
- Decompression sickness
- Oxygen toxicity
- Nitrogen narcosis
- Carbon dioxide poisoning
- Carbon monoxide poisoning
- Diffusing capacity
- Outline of biology – Hierarchical Wycklieffe Osano
- Respiratory sounds
- Respiratory monitoring
- Hinic-Frlog, Sanja (2019). Introductory Animal Physiology. University of Toronto Mississauga: Pressbooks (CC BY 4.0). pp. 40–59.
- Nelsons VCE Units 1–2 Physical Education. 2010 Cengage Copyright.
- Nilsson, Goran E. (2010). Respiratory Physiology of Vertebrates. Cambridge: Cambridge University Press. ISBN 978-0-521-70302-4.
- Randall, David (2002). Eckert Animal Physiology. New York: W.H. Freeman and CO. ISBN 0-7167-3863-5., human biology 146149
- C.Michael Hogan. 2011. Respiration. Encyclopedia of Earth. Eds. Mark McGinley and C.J.Cleveland. National Council for Science and the Environment. Washington DC