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.

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:

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• 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.

Related Topics:
Surfactant - Pore

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Details

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

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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.

Related Topics:
Diffusion - Concentration gradient - Hemoglobin - Water vapor

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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'.

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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.

Related Topics:
Pathogen - Cilia - Trachea - Bronchi - Mucus - Reflex - Coughing - Sneezing

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