Alveoli

Examples of Alveoli in the following topics:

• Alveoli

  • Alveoli are hollow cavities in the lung that perform gas exchange with the blood.
  • Its plural is alveoli, from the Latin alveolus, meaning little cavity.
  • The alveoli are highly elastic, so the alveoli can stretch as they are filled with air during inhalation.
  • In some alveolar walls there are pores between alveoli called the pores of Kohn, that connect alveoli in order to equalize air pressure between the different sacs of an alveolus.
  • Besides these epithelium cells, there are many macrophages found in the alveoli that provide immune system defense of the alveoli from pathogens and foreign material.

• Gas Exchange across the Alveoli

  • Differences in partial pressures of O2 create a gradient that causes oxygen to move from the alveoli to the capillaries and into tissues.
  • In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli.
  • More specifically, alveolar PO2 is higher in the alveoli (PALVO2=100mmHg) than blood PO2 in the capillaries (40mmHg).
  • Due to this gradient, CO2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli.
  • The blood enters the lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again .

• Factors Affecting Pulmonary Ventilation: Surface Tension of Alveolar Fluid

  • The alveoli are highly elastic structures in the parenchyma of the lungs that are the functional site of gas exchange.
  • As the alveoli fill with air during inhalation they expand, and as air leaves the lung with exhalation, the alvoli return to their non-inflated size.
  • The reason for the elasticity of the alveoli is a protein found in the extracellular matrix of the alveoli, called elastin, as well as the surface tension of water molecules on the alveoli themselves.
  • Because the alveoli of the lungs are highly elastic, they do not resist surface tension on their own, which allows the force of that surface tension to deflate the alveoli as air is forced out during exhalation by the contraction of the pleural cavity.
  • Fortunately, the type II epithelial cells of the alveoli continually secrete a molecule called surfactant that solves this problem.

• The Work of Breathing

  • The respiratory rate contributes to the alveolar ventilation, or how much air moves into and out of the alveoli, which prevents carbon dioxide buildup in the alveoli.
  • Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli.
  • By lowering the surface tension of the alveolar fluid, it reduces the tendency of alveoli to collapse.
  • A small amount of surfactant on the airway tissues reduces the effort or work needed to inflate those airways and is also important for preventing collapse of small alveoli relative to large alveoli.
  • In emphysema, which mostly arises from smoking tobacco, the walls of the alveoli are destroyed, decreasing the surface area for gas exchange.

• Blood Flow in the Lungs

  • The pulmonary arteries divide into thin-walled capillaries closely associated with the alveoli, small air sacs in the lungs where gas exchange occurs.
  • Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction.
  • The alveoli are the site of oxygen and carbon dioxide exchange in the lungs.

• Development of the Respiratory System

  • By the end of this period, all of the major lung elements, except those required for gas exchange (e.g. alveoli), have appeared.
  • The lungs of pre-term infants therefore may not function well because the lack of surfactant leads to increased surface tension within the alveoli leading to alveoli collapse and no gas exchange, a condition known as respiratory distress syndrome.
  • Lastly, the alveolar period spans from birth to eight years of age and during this stage the terminal saccules, alveolar ducts, and alveoli increase in number.
  • True alveoli appear as indentations in the saccular wall and septae form to produce divisions in the wall.

• Mammalian Systems and Protective Mechanisms

  • Numerous alveoli (sing. alveolus) and alveolar sacs surround the alveolar ducts .
  • Each sac contains 20-30 alveoli that are 200-300 microns in diameter.
  • Alveoli are made of thin-walled, parenchymal cells that are in direct contact with capillaries of the circulatory system.
  • This ensures that oxygen will diffuse from alveoli into the blood and that carbon dioxide produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled.
  • Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes.

• Lungs

  • The exchange of gases is performed by the alveoli, the functional units of the lungs.
  • The interior of the lung contains the alveoli, where gas exchange occurs.
  • The alveoli branch off from the bronchioles and bronchi that connect to the trachea and allow air to pass into the lungs.
  • The major function of the lungs is gas exchange, which occurs in the alveoli of the lung.
  • Only a relatively small proportion of alveoli in the lungs are perfused with blood and actually take part in gas exchange.

• Viral Pneumonia

  • Pneumonia is an inflammatory condition of the lung that particularly affects microscopic air sacs (alveoli).
  • Once there, the virus will invade the cells that line the airways and the alveoli.
  • White blood cells, in particular lymphocytes, are responsible for activating a variety of chemicals (cytokines) which cause fluid to leak into the alveoli.
  • The combination of cellular destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream.

• Systemic and Pulmonary Circulation

  • At the lungs, the blood travels through capillary beds on the alveoli where respiration occurs , removing carbon dioxide and adding oxygen to the blood.
  • The alveoli are air sacs in the lungs that provide the surface for gas exchange during respiration.
  • A diagram of the alveoli, illustrating the capillary beds where gas exchange with the blood occurs.