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Physiology 3120 Lecture Notes - Partial Pressure, Respiratory Tract, Bronchus

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Tom Stavraky

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Human Physiology
Friday, January 29, 2010
“Respiratory I”
Structure & Function of the Lung
Main function of the lung is gas exchange
Occurs by diffusion, which occurs according to Fick’s law of diffusion
Vgas = (constant * (P1 – P2)) * A/T
A = area larger area = faster diffusion
T = thickness bigger thickness = slower diffusion
P1 – P2 = difference in partial pressure larger gradient = faster diffusion
Constant = diffusion constant larger = faster diffusion
oProportional to gas solubility but inversely proportional to the square root
of its molecular weight
CO2 out, O2 in; oxygen from atmosphere into blood, carbon dioxide from blood into atmosphere
The area of the lung where diffusion occurs is the blood-gas barrier has thin interface and large
area (permits fast diffusion)
Partial pressure = pressure exerted by one gas in a mixture of gases; equal to the total pressure of the gas
mixture times the fractional concentration of the gas in the mixture
Partial pressure of a gas in solution
Equal to the partial pressure of the gas in equilibrium with the mixture
Regardless of the solubility of the gas in solution
Partial pressure of oxygen in arterial blood is PaO2
Blood-gas barrier
Very thin (0.5μm thick)
Several cell layers
Very large surface area (50-100m2)
Attained through repeated branching of airways
About 300 million air sacs called alveoli
Maximum 23 branches (not all airways divide 23 times)
Trachea divide into right & left bronchi, which then divide into bronchioles, and
terminal bronchioles constitute the conducting zone
Terminal bronchioles, get some airways called respiratory bronchioles (RBL),
which have some alveoli (small amount of gas exchange)
RBL branch into alveolar ducts (entire wall is covered in alveoli), which end on
alveolar sacs
Parts after the RBL are the transitional and respiratory zone (participates in gas
Physical environment of the lung
Located in thoracic cavity
Space between chest wall and lung (intrapleural space) has subatmospheric pressure;
pressure within the lung (intrapulmonary space) is atmospheric
IPP is negative because. . .
oElastic properties of the lung and tendency of chest wall to move outward
(lung wants to collapse, chest wall wants to expand)
Creating a pneumothorax (hole in the diaphragm), lung collapses, chest wall
expands, and the IPP becomes 0
Blood vessels and blood flow
Extensive network of capillaries that surround alveoli; permits efficient gas exchange
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