Pulmonary alveolus

An alveolus (plural:alveoli), is an anatomical structure that has the form of a hollow cavity. In the lung, the alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.

Location

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

Structure

The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between alveoli. There are two major alveolar cell types in the alveolar wall:

• Type I cells that form the structure of a alveolar wall
• Type II cells that secrete surfactant to lower the surface tension

The alveoli have an innate tendency to collapse because of their spherical shape, small size, and surface tension. Phospholipids, which are called surfactants, and pores help to equalize pressures and prevent collapse.

Details

The alveoli have radii of about 0.1 mm and wall thicknesses of about 0.2 µm.

Pulmonary gas exchange is driven by passive diffusion, which does not require energy for transport. Substances move down a concentration gradient. Oxygen moves from the alveoli (high oxygen concentration) to the blood (lower oxygen concentration). Conversely, carbon dioxide has a higher concentration in the blood than in the air. The oxygen first dissolves in the fluid in the interstitial tissues and diffuses into the blood. Oxygen binds to hemoglobin in the red blood cells, which allows a greater amount of oxygen to be transported by the blood. Although carbon dioxide and oxygen are the most important molecules exchanged, other gases will also be transported between the alveoli and blood in relation to the water solubility of the gas the ability of the gas to bind to hemoglobin. Water vapor is also excreted through the lungs, due to humidification of inspired air by the lung tissues.

Molecules with a high affinity for hemoglobin, such as carbon monoxide, can be added to the blood in high concentrations. Red blood cells transit the alveolar capillaries in about 3/4 of a second. Most gases reach equilibrium with the blood before the red blood cells leave the alveolar capillaries. However, carbon monoxide is stored in such high concentrations in the blood, due to its strong binding to hemoglobin, that equilibrium is not reached before the blood leaves the alveolar capillary. Thus, the concentration of carbon monoxide in the arterial system can be used to assess the resistance of the alveolar walls to gas diffusion. Thus, transport of carbon monoxide is 'diffusion limited'. Gases that reach equilibrium before the blood leaves the alveolar capillaries are 'perfusion limited'.

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 dry air at sea level, the partial pressures for O₂ and CO₂ are 160mmHg and 0.3mmHg respectively. The alveolar oxygen pressure is lower than the atomospheric pressure because some amount of oxygen enters 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 partial pressures of inspired oxygen and carbon dioxide
• The rates of total body oxygen consumption and carbon dioxide production
• The rates of alveolar ventilation and perfusion

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 nearly the same PO₂ and PCO₂ as the alveolar air. The more pulmonary capillaries participating in this process, the more total O₂ and CO₂ that can be exchanged. The magnitude of the difference between the alveolar PO₂ and arterial PO₂ can be used to detect the presence of some lung diseases.

Matching air supply and blood supply in alveoli

For efficient gas exchange, the ratio of alveolar ventilation and capillary perfusion should be matched for each lung subunit. Ventilation of a subunit can be lowered by obstruction with fluid, particulates, mucous or tumors. Perfusion can be lowered by pulmonary embolism.

Homeostatic responses in the lungs minimize the mismatching of ventilation and blood flow. For example, alveolar epithelia secrete vasodilating substances in response to normal levels of oxygen.

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.

• Infant respiratory distress syndrome (IRDS) is a syndrome caused by lack of surfactant in the lungs of premature infants.

• 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.

• Pneumonia is an infection of the alveoli, which can be caused by both viruses and bacteria. Toxins and fluids are released from the virus causing the effective surface area of the lungs to be greatly reduced. If this happens to such a degree that the patient cannot draw enough oxygen from his environment, then he may need supplemental oxygen.

See also: Pulmonary alveolus, Acute respiratory distress syndrome, Asthma, Bacterium, Blood, Bronchiole, Bronchus, Capillaries, Carbon monoxide, Chronic bronchitis