Pulmonary alveolus

An alveolus (plural:alveoli), is an anatomical structure that has the form of a hollow cavity. In the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood. Alveoli are peculiar to mammalian lungs; different structures are involved in gas exchange in other vertebrates.

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 an 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 and thus does not require energy for exchange. Substances move down a concentration gradient. Oxygen moves from the alveoli (high oxygen concentration) to the blood (lower oxygen concentration, due to the continuous consumption of oxygen in the body). Conversely, carbon dioxide is produced by metabolism and has a higher concentration in the blood than in the air.

Oxygen in the lungs first diffuses through the alveolar wall and dissolves in the fluid phase of blood. The amount of oxygen dissolved in the fluid phase is governed by Henry's Law. Oxygen dissolved in the blood may diffuse into red blood cells and bind to hemoglobin. Binding of oxygen to hemoglobin allows a greater amount of oxygen to be transported in the blood. Although carbon dioxide and oxygen are the most important molecules exchanged, other gases are also transported between the alveoli and blood. The amount of a gas that is exchanged depends on the water solubility of the gas the affinity of the gas for hemoglobin. Water vapor is also excreted through the lungs, due to humidification of inspired air by the lung tissues.

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. Transport of carbon monoxide is thus termed 'diffusion limited'. Gases that reach equilibrium before the blood leaves the alveolar capillaries are 'perfusion limited', since the amount of the gas exchanged depends solely on the volumetric flow rate of blood past the alveoli.

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 tonsils in the nasopharynx which traps germs, 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 105 mmHg and 40 mmHg, respectively. For dry air at sea level, the partial pressures for O₂ and CO₂ are about 160 mmHg and 0.3 mmHg, respectively. The alveolar oxygen pressure is lower than the atmospheric O₂ partial pressure for two reasons. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg. The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO₂ out of the capillaries into the alveoli.

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

The alveolar pO₂ is not routinely measured but is calculated from blood gas measurements by the Alveolar Gas Equation:

$p_AO_2 = p_IO_2 - p_ACO_2/R + F \,$

where: R is the Respiratory Quotient (normally about 0.8)

p_AO₂ is the Alveolar pO₂

p_IO₂ is the Inspired pO₂, equal to about 150 mm Hg (0.21 x 713 mmHg at sea level). The given pressure at sea level is due to atmospheric pressure (760 mmHg) minus the partial pressure of water vapor (47 mmHg), as alveolar gas is completely saturated with water. The mole fraction of oxygen is about 0.21 in dry atmospheric gas.

p_ACO₂ is the Alveolar pCO₂ (assumed to be equal to the measured arterial pCO₂)

F is a correction factor (usually less than 2 mmHg)

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

Exchange between blood and gas

The blood that enters the pulmonary capillaries is the systemic venous blood that 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 certain 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 is most commonly lowered by pulmonary embolism. A VQ scan is performed to detect imbalances in ventilation ('V') and perfusion ('Q').

Homeostatic responses in the lungs minimize the mismatch 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 elastin in the walls of the alveoli is broken down by an imbalance between the production of neutrophil elastase (elevated by cigarette smoke) and alpha-1-antitrypsin (the activity varies due to genetics or reaction of a critical methionine residue with toxins including cigarette smoke). The resulting loss of elasticity in the lungs leads to prolonged times for exhalation, which occurs through passive recoil of the expanded lung. This leads to a smaller volume of gas exchanged per breath.

Chronic bronchitis occurs when an abundance 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 a genetic condition caused by the dysfunction of 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 tissues.

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.

References

External links

Respiratory system

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