Gas exchange or respiration takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane, but for large organisms it usually is carried out in respiratory systems.

This name can cause problems - in biology the word "respiration" can mean cellular respiration (ATP generation inside cells), however sometimes (such as here) it can also refer to breathing (which is how the word is most often used by non-biologists).

Gases cross the respiratory surface by diffusion, so from Fick's law we can predict that respiratory surfaces must have:

• a large surface area
• a thin permeable surface
• a moist exchange surface

Many also have a mechanism to maximise the diffusion gradient by replenishing the source and/or sink.

Control of respiration is due to rhythmical breathing generated by the nerves. Ventilation is controlled by partial pressures of oxygen and carbon dioxide and the concentration of hydrogen ions. The control of respiration can very in certain curcumstances such as during during exercise.

Gas exchange in humans and mammals

In humans and mammals, respiratory gas exchange or ventilation is carried out by mechanisms of the lungs. The actual exchange of gases occurs in the alveoli.

Convection occurs over the majority of the transport pathway. Diffusion occurs only over very short distances.

Gas exchange occurs only at pulmonary and systemic capillary beds.

Blood always contains both oxygen and carbon dioxide.

Transport of oxygen, carbon dioxide, and hydrogen ions

Blood carries oxygen, carbon dioxide and hydrogen ions between tissues and the lungs.

The alveoli (singular:alveolus), tiny hollow sacs which are continuous with the airways, are the sites of gas exchange with the blood.

Location

The alveoli are found in the respiratory zone of the lungs.

Structure

The alveoli are very small porous air sacs.

The alveoli have an innate tendency to collapse. The pores help to equalize pressures and prevent collapse.

The alveolar walls contain capillaries and a very small interstitial space. In some alveolar walls there are pores between alveoli. There are two major alveolar cell types in the alveolar wall:

• Flat Type I cells forming the structure of a alveolar wall
• Type II cells which secrete surfactant which lowers the surface tension and divide to produce Type I cells

Details

The alveoli are small with very thin walls. They have a radius of 0.1mm and wall thickness of about 0.2µm.

Pulmonary gas exchange is driven by passive osmotic diffusion and does not require ATP-fueled enzyme-based transport. Substances move through the concentration gradient from a higher concentration to a lower concentration. In the alveoli, this means oxygen in the red blood cells will have a lower concentration than in the air. Conversely, carbon dioxide will have a higher concentration in the red blood cells than in the air. This causes the diffusion of oxygen into the blood, binding to haemoglobin protein molecules, and the diffusion of carbon dioxide through to the alveoli to be expelled into the air. Although carbon dioxide and oxygen are the main molecules exchanged, water vapour is also found to be excreted through the lungs.

One of the dangers of this process is that molecules with a high affinity for haemoglobin, such as carbon monoxide, may also bind to red blood cells. Carbon monoxide will readily diffuse past the alveoli in the lungs and into the blood cells. This means that if the concentration of carbon monoxide is high enough, oxygen deprivation will occur.

The lungs contain about 300 million alveoli, each wrapped in a fine mesh of capillaries. The lungs are constantly exposed to airborne pathogens and dust particles. The body employs many defenses to protect the lungs, including small hairs (cilia) lining the trachea and bronchi supporting a constant stream of mucus out of the lungs, and reflex coughing and sneezing to dislodge mucus contaminated with dust particles or micro-organisms.

Alveolar gas pressures

Normal alveolar partial pressures for O₂ and CO₂ are 105mmHg and 40mmHg respectively. For normal air partial pressures for O₂ and CO₂ are 160mmHg and 0.3mmHg respectively. The alvolar oxygen pressure is lower because some oxygen leaves to the pulmonary capillaries. The alveolar carbon dioxide pressure is higher because carbon dioxide enters the alveoli from the pulmonary capillaries.

The factors that determine the values for alveolar PO₂ and PCO₂ are:

• The pressure of outside air
• The rate of alveolar ventilation
• The rate of total body oxygen consumption and CO2 release

Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases. Hyperventilation exists when the same ratio decreases.

Exchange between blood and gas

The blood that enters the pulmonary capillaries is the systemic venous blood which enter the lungs via the pulmonary arteries.

Due to differences in partial pressures across the alveolar-capillary membrane, O2 diffuses into the blood and CO2 diffuses out. Thus, the blood that returns to the heart has the same PO2 and PCO2 as the alveolar air. The more pulmonary capillaries participating in this process, the more total O2 and CO2 can be exchanged.

Matching air supply and blood supply in alveoli

To be most efficient the right proportion of alveolar ventilation and capillary perfusion should be available to each alveolus.

Homeostatic responses in the lungs minimize the mismatching of ventilation and blood flow.

Diseases

Acute respiratory distress syndrome (ARDS) is a severe inflammatory disease of the lung. Usually triggered by other pulmonary pathology, the uncontrolled inflammation leads to impaired gas exchange, alveolar flooding and/or collapse, and systemic inflammatory response syndrome. It usually requires mechanical ventilation in an intensive care unit setting.

In asthma, the bronchioles, or the "bottle-necks" into the sac are restricted causing the amount of air flow into the lungs to be greatly reduced. It can be triggered by irritants in the air, photochemical smog for example, as well as substances that a person is allergic to.

Emphysema is another disease of the lungs, whereby the delicate lining of the alveoli is broken down, greatly reducing the effective surface area for diffusion. The gradual loss of the lungs' ability to draw oxygen into the blood deprives organs of oxygen. The heart attempts to pump more blood through the body in order to satisfy the body's need for oxygen, which in some cases may cause strain on the heart.

Chronic bronchitis occurs when too much mucus is produced by the lungs. The production of this substance occurs naturally when the lung tissue is exposed to irritants. In chronic bronchitis, the air passages into the alveoli, the broncholiotes, become clogged with mucus. This causes increased coughing in order to remove the mucus, and is often a result of extended periods of exposure to cigarette smoke.

Cystic fibrosis is more a genetic condition caused by the dysfunction of the transmembrane conductance regulator, a transmembrane protein responsible for the transport of chloride ions. This causes huge amounts of mucus to clog the bronchiolites, simular to chronic bronchitis. The result is a persistent cough and reduced lung capacity.

Diffuse interstitial fibrosis

Lung cancer is a common form of cancer causing the uncontrolled growth of cells in the lung tissue. It is often difficult to prevent once started, due to the sensitivity of lung tissue to radiological exposure.