Respiration (physiology)

In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the removal of carbon dioxide in the opposite direction.^[1]

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 exchange in the lung occur by ventilation and perfusion.^[1] Ventilation refers to the in and out movement of air of the lungs and perfusion is the circulation of blood in the pulmonary capillaries.^[1] 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 species

Aquatic respiration
Buccal pumping
Animal respiration
Human respiration

By mechanism

Breathing
Gas exchange
Arterial blood gas
Control of respiration
Apnea

By experiments

Huff and puff
Spirometry
Selected ion flow tube mass spectrometry

By intensive care and emergency medicine

By other medical topics

Respiratory therapy
Breathing gases
Hyperbaric oxygen therapy
Hypoxia
Gas embolism
Decompression sickness
Barotrauma
Oxygen toxicity
Nitrogen narcosis
Carbon dioxide poisoning
Carbon monoxide poisoning
HPNS

References

^ ^a ^b ^c Hinic-Frlog, Sanja (2019). Introductory Animal Physiology. University of Toronto Mississauga: Pressbooks (CC BY 4.0). pp. 40–59.

External links

Overview at Johns Hopkins University