The human respiration process

The physiology of respiration

In human physiology, respiration is the transport of oxygen from the clean air to the tissue cells and the transport of carbon dioxide in the opposite direction. This is only part of the processes of delivering oxygen to where it is needed in the human body and removing carbon dioxide waste.

Not all of the oxygen breathed in is replaced by carbon dioxide; around 15% to 18% of what we breathe out is still oxygen. The exact amount of exhaled oxygen and carbon dioxide varies according to the fitness, energy expenditure and diet of that particular person.

Air-breathing of humans, respiration of oxygen includes four stages:

  • Ventilation from the ambient air into the alveoli of the lung.
  • Pulmonary gas exchange from the alveoli into the pulmonary capillaries.
  • Gas transport from the pulmonary capillaries through the circulation to the peripheral capillaries in the organs.
  • Peripheral gas exchange from the tissue capillaries into the cells and mitochondria.
Note that ventilation and gas transport require energy to power mechanical pumps (the diaphragm and heart respectively), in contrast to the passive diffusion taking place in the gas exchange steps.

Nasal breathing of respiration process refers to the state of inhaling and exhaling through the nose.

It is considered superior to mouth breathing for several reasons. Breathing through the nose has numerous health benefits due to the fact that the air travels to and from the external environment and the lungs through the sinuses as opposed to the mouth. The sinuses do a better job of filtering the air as it enters the lungs.

In addition, the smaller diameter of the sinuses creates pressure in the lungs during exhalation, allowing the lungs to have more time to extract oxygen from them. When there is proper oxygen-carbon dioxide exchange, the blood will maintain a balanced pH. If carbon dioxide is lost too quickly, as in mouth breathing, oxygen absorption is decreased.

Nasal breathing is especially important in certain situations such as dehydration, cold weather, laryngitis, and when the throat is sore or dry because it does not dry the throat as much.

Nasal breathing in public is considered to be more socially acceptable and attractive than mouth breathing.

The major function of the respiratory process is gas exchange. As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained two opposing conditions could occur: 1) respiratory acidosis, a life threatening condition, and 2) respiratory alkalosis.

The Lungs are the human organs of respiration.

Human body have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately 1500 miles (2,400 km) of airways and 300 to 500 million alveoli, having a total surface area of about 75 m2 in adults — roughly the same area as a tennis court. Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about 620 miles.

The lung capacity depends on the person's age, height, weight, sex, and normally ranges between 4,000 and 6,000 cm3 (4 to 6 L).

For example, females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Smokers have a lower capacity than non-smokers. Lung capacity is also affected by altitude.

People who are born and live at sea level will have a smaller lung capacity than people who spend their lives at a high altitude. This is because the atmosphere is less dense at higher altitude, and therefore, the same volume of air contains fewer molecules of all gases, including oxygen. In response to higher altitude, the body's diffusing respiration capacity increases in order to be able to process more air.

When someone living at or near sea level travels to locations at high altitudes (eg. the Andes, Denver, Colorado, Tibet, the Himalayas, etc.) s/he can develop a condition called altitude sickness because their lungs cannot respirate sufficiently in the thinner air.

Human lungs are to a certain extent 'overbuilt' and have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. This is the reason that individuals can smoke for years without having a noticeable decrease in lung function while still or moving slowly; in situations like these only a small portion of the lungs are actually perfused with blood for gas exchange.

As oxygen requirements increase due to exercise, a greater volume of the lungs is perfused, allowing the body to reach its CO2/O2 exchange respiration requirements.

Mechanism of respiration

Under normal conditions, humans cannot store much oxygen in the body. Apnea of more than approximately one minute's duration therefore leads to severe lack of oxygen in the blood circulation. Permanent brain damage can occur after as little as three minutes and death will inevitably ensue after a few more minutes unless ventilation is restored. However, under special circumstances such as hypothermia, hyperbaric oxygenation, apneic oxygenation (see below), or extracorporeal membrane oxygenation, much longer periods of apnea may be tolerated without severe consequences.

Untrained humans cannot sustain voluntary apnea for more than one or two minutes. The reason for this is that the rate of breathing and the volume of each breath are tightly regulated to maintain constant values of CO2 tension and pH of the blood. In apnea, CO2 is not removed through the lungs and accumulates in the blood. The consequent rise in CO2 tension and drop in pH result in stimulation of the respiratory centre in the brain which eventually cannot be overcome voluntarily.

When a person is immersed in water, physiological changes due to the mammalian diving reflex enable somewhat longer tolerance of apnea even in untrained persons. Tolerance can in addition be trained. The ancient technique of free-diving requires breath-holding and world-class free-divers can indeed hold their breath underwater up to depths of 214 metres and for more than nine minutes. Apneists, in this context, are people who can hold their breath for a long time.


Many people have discovered, on their own, that voluntary hyperventilation before beginning voluntary apnea allows them to hold their breath for a longer period. Some of these people incorrectly attribute this effect to increased oxygen in the blood, not realizing that it is actually due to a decrease in CO2 in the blood and lungs.

Blood leaving the lungs is normally fully saturated with oxygen, so hyperventilation of normal air cannot increase the amount of oxygen available.

Lowering the CO2 concentration increases the time before the respiratory center becomes stimulated, as described above.

This error has led some people to use hyperventilation as a means to increase their diving time, not realizing that there is a danger that their body may exhaust its oxygen while underwater, before they feel any urge to breathe, and that they can suddenly lose consciousness — a shallow water blackout — as a result. If a person loses consciousness underwater, especially in fresh water, there is a considerable danger that they will drown. An alert diving partner would be in the best position to rescue such a person.

Apnea, is a technical term that means suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged. Depending on the patency (openness) of the airways there may or may not be a flow of gas between the lungs and the environment; gas exchange within the lungs and cellular respiration is not affected.

Apnea can be voluntarily achieved (e.g., "holding one's breath"), drug-induced (e.g., opiate toxicity), mechanically induced (e.g., strangulation), or it can occur as a consequence of neurological disease or trauma.

Apneic respiration and oxygen uptake

Because the exchange of gases between the blood and airspace of the lungs is independent of the movement of gas to and from the lungs, enough oxygen can be delivered to the circulation even if a person is apneic. This phenomenon (apneic oxygenation) is explained as follows:

With the onset of apnea, an under pressure develops in the airspace of the lungs, because more oxygen is absorbed than CO2 is released. With the airways closed or obstructed, this will lead to a gradual collapse of the lungs. However, if the airways are patent (open), any gas supplied to the upper airways will follow the pressure gradient and flow into the lungs to replace the oxygen consumed.

If pure oxygen is supplied, this process will serve to replenish the oxygen stores in the lungs. The uptake of oxygen into the blood will then remain at the usual level and the normal functioning of the organs will not be affected.

However, no CO2 is removed during apnea. The partial pressure of CO2 in the airspace of the lungs will quickly equilibrate with that of the blood. As the blood is loaded with CO2 from the metabolism, more and more CO2 will accumulate and eventually displace oxygen and other gases from the airspace. CO2 will also accumulate in the tissues of the body, resulting in respiratory acidosis.

Under ideal conditions (i.e., if pure oxygen is breathed before onset of apnea to remove all nitrogen from the lungs, and pure oxygen is insufflated), apneic oxygenation could theoretically be sufficient to provide enough oxygen for survival of more than one hour's duration in a healthy adult. However, accumulation of carbon dioxide (described above) would remain the limiting factor.

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