Pulmonary Ventilation (V)
It is the amount of air moved into or out of the lungs per minute; normal value is about 6 L/min. ．
V = f x TV
where f, frequency of breathing (breaths/min)
TV, tidal volume (L)
Alveolar Ventilation (VA)
It is the amount of air reaching the functioning alveoli (exchange surface) per minute; normal value is about 4.2 L/min.
VA = f x VA
or f x (TV - VD)
where VA, volume of air reaching the functioning alveoli, L. VD, volume of dead space, L.
PACO2 - VA Relationship
At steady state, amount of CO2 exhaled per minute equals to amount of CO2 produced per minute. At a constant level of CO2 production, the PACO2 varies inversely with the level of alveolar ventilation.
PAO2 - VA Relationship
At steady state, the amount of O2 uptake per minute equals to the amount of O2 consumed per minute. At a constant level of O2 consumption and a fixed inspired O2 concentration, there is a direct relationship between PAO2 and the level of alveolar ventilation. Physiological Dead Space Volume (VD)
It is the space in the lungs where gas exchange cannot take place. It oncludes: 1.
Anatomical Dead Space - those parts of the respiratory tract (nose, pharynnx, trachea, bornchi, bronchioles) which act as passageways.
Alveolarl Dead Space – alveolar space that does not receive blood supply and/or space in which ventilation is in excess of the need to arterialize the blood.
You should now be able to:
define pulmonary ventilation and state the formula relating pulmonary ventilation, tidal volume and breathing frequency.
define alveolar ventilation and state the formula for calculating alveolar ventilation. 3.
state the effects of alveolar ventilation on PCO2 and PO2 in the body. 4.
define physiological dead space.
PULMONARY GAS DIFFUSION
Simultaneous processes of pulmonary ventilation, perfusion and alveolo-capillary gas transfer accomplish the continuous exchange of O2 and CO2 between blood and atmosphere. Hence, gas diffusion is an important step in respiratory gas exchange.
Factors affecting gas diffusion through respiratory membrane Fick's Law of Diffusion:
Vg = D.A. (P1 - P2)
where Vg, rate of gas transfer through a sheet
Diffusion constant (D) i.
molecular weight of gas (O2 diffuses slightly faster than CO2). solubility coefficient of gas (CO2 diffuses much faster than O2).
Considering molecular weight and solubility coefficient together, CO2 diffuses much faster than O2. Patient with diffusion impairment normally has problem with O2 diffusion but not with CO2 diffusion, resulting in a fall on PO2 but normal PCO2. Even after ventilatory compensation caused by hypoxemia stimulation, the PO2 is still not back to normal but PCO2 will fall.
2. Surface area (A) The total area of alveolar space in contact with capillary blood is about 50 - 100 sg.m. 3. Distance for diffusion (T) Thickness of the alveolo-capillary membrane varies from 0.2 - 0.5 micron. 4. Transmembrane pressure gradient (P1 -P2)
You should now be able to:
state the factors affecting gas diffusion across the alveolocapillary membrane. 2.
explain why CO2 diffuses faster than O2 across alveolocpaillary membrane. 3.
state the effects of diffusion impairment on PO2 and PCO2 in the body.
VENTILATION AND PERFUSION MISMATCHING
Matching of ventilation and blood flow within various regions of the lung is critical for adequate gas exchange. If ventilation and blood flow are mismatched in various regions of the lung, impairment of gas transfer ． ． They key to understanding how this happens is the alveolar results.
ventilation/perfusion ratio (VA/Q). The normal value of VA/Q for the lung as a whole is 0.8 (VA, 4 ．
L/min; Q, 5 L/min).
Effect of altering VA/Q...
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