Alveoli

Alveoli

Alveoli is “sac.”  An alveolus is one of these sacks here whereas this cluster contains many alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood..  There’s the difference between singular and plural for you. Alveolus is singular and alveoli is plural. Alveoli never function alone they always function in a cluster. They are lined with simple squamous epithelium .

The thinness of these cellsThe basic structural and functional units of life. allows for a very short distance of diffusionPassive movement of molecules from areas of high to low concentration. for oxygen and carbon dioxide. You can see in this picture here that there are holes between each alveolus. These are called alveolar poresSmall openings between adjacent alveoli that allow air to circulate between them, helping to equaliz and they equate the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. in each alveolus so that they all inflate together.

Alveolar Cells

Lining each alveolus are cells called type 1 pneumocytes or alveolar cells. These are simple squamous epithelial cells through which gas exchange is happening. The type 1 pneumocytes or alveolar cells stack up on top of each other. There is the slightest of overlap among adjacent cells.

This is a vessel here containing red blood cells wanting to pick up oxygen from the alveolus.  Red blood cells are moving through these capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. slow enough to allow gas exchange to happen.

Alveolar macrophages were once monocytes in the blood that came into the lung and took up permanent residence. Their function is to eliminate any particulate matter. This matter escaped the filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre methods of the nasal cavityThe internal space behind the nose that filters, warms, and humidifies incoming air., trachea, bronchiThe large airways that branch from the trachea into the lungs, dividing into smaller bronchioles., and conducting tubes.

There is one more population of cells in alveoli. These are cells called type 2 pneumocytes or alveolar great cells.  These cells secrete surfactantA substance secreted by Type II pneumocytes that reduces alveolar surface tension..

Surfactant it is a substance that like many laundry detergents can break up the surface tension of waterThe universal solvent essential for life.. If this alveolus collapsed, the surfactant secreted on all these substances would allow you to inflate this alveolus again. You could do this without much resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter..

The Respiratory Membrane

We’ve mentioned this word before the respiratory membrane. This refers to the mucusy carpet starting at the nasalTwo small rectangular bones forming the bridge of the nose. vestibuleThe area between the labia minora.. It extends all the way down here to one alveolus. At the alveoli, the type 1 pneumocytes or alveolar cells are fused to the endothelial cells of the blood capillary. These two cell types are closely connected.

This is where oxygen goes from a gas like environment to a liquid environment and carbon dioxide does the opposite. This is called the air blood barrier. We want our air blood barrier for the respiratory membrane in our alveoli to be thin with no mucus. To achieve this thinness, the type 1 pneumocytes or alveolar cells merge with the endothelial cells of the blood capillary. This thinness represents a small distance over which gases have to diffuse to be exchanged. I want the smallest distance as possible because this is a passive process of diffusion.

Pneumonia is characterized by excess mucus production. This occurs in the lower parts of the respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide). that are involved in gas exchange. Remember that there are fewer mucus cells scattered among the epithelium as we move into the alveoli. 

The mucus from pneumoniaA lung infection causing inflammation and fluid buildup in the alveoli. fills each alveolus reducing the surface area over which gases can exchange. Pneumonia also increases the distance over which the gases have to diffuse for exchange. This is why pneumonia is a double punch. But as we saw before, there are bronchopulmonary segmentsFunctional units of the lungs, each served by its own tertiary bronchus and blood supply.. Hopefully, they quarantine the bacterial infection causing pneumonia into one pulmonary segment. This allows the others to function as normal.

High Surface Area:Volume Ratio

Your lungs are not as big as you think. Your thoracic cavityThe body cavity housing the heart and lungs. is not as big as you think. So how do the lungs of limited volume oxygenate all those tissues in your peripheral circuit. Lungs have an enormous surface area to volume ratio. I love the textbooks. They say that if you were to take the surface area of lungs, it would cover a tennis court. Who spread it out? I mean seriously who did that who spread it out all over a tennis court to figure that out. Never mind.

Diseases like tuberculosis and emphysemaA lung disease in which alveoli are damaged, reducing surface area for gas exchange and causing shor both reduce the surface area to volume ratio of the lung. They do this by specifically eliminating the surface area of the lung but keeping the same volume of the lung. By doing so this reduces the ratio altogether. Tuberculosis causes nodules to build up in your lungs. These nodules occupy space that gas-exchanging type 1 pneumocytes or alveolar cells should occupy. Emphysema is at the other end of the spectrum. It destroys your alveoli, fusing them together and making large alveoli. This reduces the overall surface area to volume ratio. So with both diseases this ratio is reduced but in two different ways.