What are alveoli?
Alveoli are terminal air sacs in the lungs that are communications between the arterial and venous systems. Respiratory gases(oxygen and carbon dioxide)and other substances like glucose and waste products are transferred from one system to the other in the alveolus(plural). When you exhale you blow off carbon dioxide, when you inhale you take in oxygen contained in the air. The lungs contain literally millions of alveoli.
Atelectasis is an airless condition of the alveoli or other structures in the lung parenchyma which is due to disease, infection, or trauma. This condition is commonly called a "collapsed lung" and can be quite serious because it interferes with the exchange of these respiratory gases which in turn diminishes the oxygen content delivered to all the tissues in the body.
Alveoli are found in the lungs, at the tips of the branching bronchioles. They have the form of a hollow cavity and are composed of a single layer of epithelial tissue. The adult lung consists of approximately 300-400 million alveoli. They are surrounded by pulmonary capillaries. The pulmonary capillaries function very closely to the alveoli. Together there is a rapid exchange of CO2 and O2 occurs.
When people have asthma attacks it is because the alveoli get stiff and cannot perform as well as they should. Others medical conditions relating to alveoli are chronic bronchitis, emphysema, cystic fibrosis, and pneumonia.
Alveoli is the scientific name given to air sacks that form the extreme ends of the branched structure of lungs. These form the the respiratory units of the lung. The walls of each alveoli contain networks of extremely small blood vessels called pulmonary capillaries. It is here that gas exchange takes place.
Oxygen from the air passes through the walls of the capillaries that surround each alveoli and binds with haemoglobin. Some oxygen also dissolves in the plasma.
As the blood in the pulmonary capillaries moves along the walls of the air sacs, oxygen from the inhaled air leaves these alveoli and enters the blood. At the same time, carbon dioxide moves from the blood into the alveoli. Carbon dioxide from the alveoli is carried through the airways to the nose and mouth and then out of the body when a person exhales.
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Within the lungs, the air tube is divided into smaller tube known as bronchi and still smaller tubes known as bronchioles. These bronchioles terminate in balloon like structure called alveoli. these alveoli provide
a) a large surface for the exachange of gases .
b) are highly vascular(richly supplied with blood vessels)
c) have permeable thin walls
Each human lung contains about 300 million alveoli. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 to 300 microns, with an increase in diameter during inhalation.
The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between alveoli called pores of Kohn.
There are three major alveolar cell types in the alveolar wall (pneumocytes):
- Type I (Squamous Alveolar) cells that form the structure of an alveolar wall
- Type II (Great Alveolar) cells that secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate, thereby increasing the capability to exchange gases. Surfactant is continuously released by exocytosis. It forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed primarily of dipalmitoyl phosphatidylcholine.
- Macrophages that destroy foreign material, such as bacteria.
Reinflation of the alveoli following exhalation is made easier by pulmonary surfactant, which is a phospholipid and protein mixture that reduces surface tension in the thin fluid coating within all alveoli. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air. The surfactant is produced by great alveolar cells (granular pneumonocytes, a cuboidal epithelia), which are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous alveolar cells (a squamous epithelium).
Great alveolar cells also repair the endotheilium of the alveolus when it becomes damaged. Insufficient pulmonary surfactant in the alveoli can contribute to atelectasis (collapse of part or all of the lung). Without pulmonary surfactant, atelectasis is a certainty; however, there are other causes of lung collapse such as trauma (pneumothorax), COPD, and pleuritis.