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Alveolar pressure (P alv) is the pressure of air inside the lung alveoli. When the glottis is opened and no air is flowing into or out of the lungs, alveolar pressure is equal to the atmospheric pressure, that is, zero cmH 2 O .
Alveolar pressure (PA) at end expiration is equal to atmospheric pressure (0 cm H 2 O differential pressure, at zero flow), plus or minus 2 cm H 2 O (1.5 mmHg) throughout the lung. On the other hand, gravity causes a gradient in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position (roughly half of that in the ...
Bronchial anatomy showing terminal bronchioles (BT) leading to respiratory bronchioles (BR) and alveolar ducts (DA) that open into alveolar sacs containing out pockets of alveoli (A) separated by alveolar septa (AS) The alveoli are first located in the respiratory bronchioles as scattered outpockets, extending from their lumens.
The alveolar oxygen partial pressure is lower than the atmospheric O 2 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 alveolar air pressure is therefore always close to atmospheric air pressure (about 100 kPa at sea level) at rest, with the pressure gradients because of lungs contraction and expansion cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa. [18] [19] During exhalation, the diaphragm and intercostal muscles relax.
The surfactant reduces the surface tension at the air-alveolar surface which allows expansion of the alveolar sacs. The alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts, [ 60 ] and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature ...
The red blood cells also carry carbon dioxide (CO 2) away from the cells in the form of carbaminohemoglobin and release it into the alveoli through the alveolar capillaries. When the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide.
Arterial carbon dioxide tension, or partial pressure: P A CO 2: Alveolar carbon dioxide tension, or partial pressure: P v O 2: Oxygen tension of mixed venous blood: P (A-a) O 2: Alveolar-arterial oxygen tension difference. The term formerly used (A-a D O 2) is discouraged. P (a/A) O 2: Alveolar-arterial tension ratio; P a O 2:P A O 2 The term ...