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In the absence of surfactant...
- the smaller alveolus will have a larger inward pressure than the larger alveolus. The smaller alveolus will collapse and empty air into the larger alveolus.
In the presence of surfactant...
- inward pressure in the smaller alveolus is reduced to a level comparable to that of the larger alveolus. Thus, the smaller alveolus will not collapse.
- helps to keep alveoli expanded. Each alveolar is surrounded by other alveoli and interconnected with them by connective tissue. If an alveolus starts to collapse, the surrounding alveoli are stretched as their walls are pulled in the direction of the caving-in alveolus.
Variations in lung volume
- tidal volume- variation in lung volume with normal quiet breathing (average male= 2,700ml-2,200ml=500ml)
- vital capacity- air exchange between max inflation and max deflation (average male=5,700ml-1,200ml=4,500ml)
- a device for measuring the volume of air breathed in and out.
- air filled drum floating in a water filled chamber
- as person breathes in and out of the drum through a connecting tube, the resultant rise and fall of the drum are recorded as a spirogram.
For an average male this is a normal spirogram...
- TV= Tidal volume (500ml), variation in lung volume with normal breathing.
- IRV= Inspiratory reserve volume (3,000ml), the extra volume of air that can be maximally inspired over and above the typical resting tidal volume.
- IC= Inspiratory capacity (3,500ml), max volume of air that can be inspired at the end of a normal, quiet expiration (IC=IRV+TV).
- ERV= Expiratory reserve volume (1,000ml), the extra volume of air that can be actively expired by maximal contraction of the expiratory muscles.
- RV= Residual Volume (1,200ml), the minimum volume of air remaining in the lungs even after a maximal expiration.
- FRC=Functional residual capacity (2,200ml), the volume of air in the lungs at the end of a normal passive expiration (FRC=ERV+RV)
- VC=Vital capacity (4,500ml), the maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC=IRV+TV+ERV)
- TLC=Total lung capacity(5,600ml), the maximum volume of air that the lungs can hold (TLC=VC+RV)
- the continuous exchange of O2 and CO2 between the external environment and the tissues of the body.
- movement of the gases occurs by passive diffusion down partial pressure gradients.
Partial Pressure (PG)
- the independent pressure exerted by a particular gas contained w/in a mixture of gases.
- The partial pressure of a gas is directly proportional to the percentage of that gas in the total air mixture:
- Partial pressure of N2 in atmospheric air: PN2=760mm Hg x 0.79=600mm Hg
- Partial pressure of O2 in atmospheric air: PO2=760mm Hg x 0.21=160mm Hg
Oxygen and CO2 exchange across pulmonary and systemic capillaries caused by partial pressure gradients.
- Across the pulmonary capillaries:
O2 and CO2 exchange across pulmonary and systemic capillaries caused by partial pressure gradients.
- Across the systemic capillaries:
Partial pressures relevant to respiration
- atmospheric PO2 =160mm Hg
- atmospheric PCO2 =0.03mm Hg
- alveolar PO2 =100mm Hg
- alveolar PCO2 =40mm Hg
- systemic tissue PO2 < 40mm Hg
- systemic tissue PCO2 > 46mm HgAlveolar PO2 < atmospheric PO2 because of teh partial pressure of water vapor in the lungs, and mixing of inspired air with residual alveolar air.
- Systemic PCO2 is higher in the tissues because of the production of CO2 during oxidative metabolism.
3 other factors influencing gas exchange
- 1) alveolar-capillary distance-gas exchange is proportional to the diffusion distance.
- 2) alveolar surface area- gas exchange is proportional to the surface area over which diffusion can take place.
- 3) diffusion coefficient- rate of gas diffusion in a liquid depends on the solubility of the gas and its molecular weight.
- -The solubility of CO2 in the body tissues is approximately 20x higher than O2.
- -The increased solubility of CO2 offsets the greater partial pressure gradient of O2 and enables approximately equal amounts of O2 and CO2 to be exchanged during respiration.
the process by which O2 and CO2 are transported between the systemic tissues and the lungs.
- O2 is transported in two forms:
- 1) dissolved in the blood (1.5%)
- 2) chemically bound to hemoglobin (98.5%).
- -Only the dissolved O2 contributes to the partial pressure.
- hemoglobin serves as a reservoir for O2 without affecting the partial pressure gradient that is necessary for gas exchange.