Gas Exchange Flashcards
Normal values for flow (uptake)
Resting:
Vdot CO2 = 0.2 L/min
Vdot O2 = 0.25 L/min
R = 0.8
Maximal Exercise:
Vdot CO2 = 5.6 L/min (28X)
V dot O2 = 4 L/min (16X)
R = 1.2
Gas Laws
Boyle’s Law: P1V1 = P2V2
Charles’ Law: V1/T1 = V2/T2
Universal gas law: PV = nRT
Dalton’s law: total pressure of a gas is sum of partial pressures of each gas
Normal partial pressures of gas in atmosphere
N2 = 79% = mmHg
O2 = 21% = 100 mmHg
Normal partial pressure of gas in human lung
N2 = 79% = 573 mmHg
CO2 = 5% = 40 mmHg
O2 = 16% = 100 mmHg
H2O = 47 mmHg
ATPS, BTPS and STPD
ATPS: ambient temperature and pressure, saturated; saturation with water vapor
BTPS: body temperature and pressure, saturated; saturation with water vapor (used for lungs!)
STPD: standard temperature and pressure, dry; no water vapor
Equation to get flow (Vdot) from volume
Vdot = Volume x fR
Vdot = Volume x rate (breaths per minute)
(Normal total minute ventilation Vdot E = 8-12 L/min)
What is anatomical dead space?
Volume of conducting airways (gas exchange does not occur here)
(trachea, bronchi, etc)
What is alveolar dead space?
When you have an alveolus that is not perfused, so doesn’t contribute to gas exchange (has a capillary next to it but the capillary has no blood)
What is physiological dead space?
Anatomical dead space + alveolar dead space
Total volume of lungs that does not participate in gas exhcange
What happens to alveolar ventilation if you have rapid, shallow breathing?
At given level of total minute ventilation, you’ll have less alveolar ventilation if you have rapid shallow breathing
This is because your dead space ventilation increases
Air stays in your alveoli and you’re “ventilating” the trachea, bronchi, terminal bronchioles, places where gas exchange can’t occur –> more wasted ventilation
Respiratory exchange ratio (R)
R = VCO2/VO2 = 200/250 = 0.8
Note: more O2 uptake than CO2 blown off
Alveolar ventilation (VA dot)
VA dot = VE dot - VD dot
Alveolar minute ventilation: volume of air per minute moving in and out of the alveolar compartment
Equation for physiological dead space
VD(physiological) = VT x (PaCO2 - PEbarCO2)/PaCO2
CO2 concentration breathed out into bag is less than partial pressure of CO2 in arterial circulation due to the dilution of exhaled air by the physiological dead space air
Dead space volume is 25-33% of tidal volume at rest, 10-20% during exercise and 45-50% in obstructive diesease or pulmonary vascular occlusive disease
Equation for partial pressure of O2 in alveoli using alveolar ventilation
PAO2 = PIO2 - (863 x VdotO2)/VdotA
Normal alveolar ventilation
VdotA = 4 L/min
Hyperpnea and hypopnea
Increase or decrease in ventilation
Apnea, tachypnea and bradypnea
No breathing, rapid (often shallow), slow breathing
Hyperventilation and hypoventilation
Hyperventilation: Increase in ventilation out of proportion to any increase in metabolic CO2 production/flow, and therefore causes low arterial PCO2
Hypoventilation: Decrease in ventilation out of proportion to any decrease in metabolic CO2 production/flow, and therefore causes high arterial PCO2
Alveolar hypoventilation
Leads to hypoxemia and hypercapnia (too much CO2 in blood)
Seen in diseases that affect medullary respiratory center, respiratory neuromuscular function, severe obstructive or restrictive pulmonary disease
3 things that govern diffusion according to Fick’s law
1) Partial pressure gradient
2) Diffusion coefficient
3) Geometry of the interface (amount of surface area)
Diffusion of O2 and CO2 within the alveolus and across the alveolar capillary membrane
Within alveolus: O2 diffuses 18% faster than CO2 because 2 has smaller MW (diffusion is inversely proportional fo square root of MW = Graham’s Law)
Across alveolar-capillary membrane: for getting into alveoli from blood, CO2 is 19 times more diffusible than O2 in liquid phase because CO2 is way more soluble (Henry’s Law: VO2/Vol Plasma = PO2 x alpha)
Normal gas tensions
Mixed venous blood (coming into lung): PO2 = 40; PCO2 = 46
Alveolar = arterial: PO2 = 100; PCO2 = 40
Time for complete diffusion of O2 and CO2 across alveolar-capillary membrane
CO2 goes from pulm capillary to alveoli and takes <0.1 sec for Alveolar CO2 to equal arterial CO2 (as a result can set PaCO2 = PACO2)
O2 goes from alveoli to pulmonary capillaries and takes 0.33 sec for arterial O2 to approximately equal Alveolar O2 (but Alveolar O2 always 1 mmHg higher than arterial O2 due to diffusion, and 9 mmHg higher because of venous admixture)
Factors causing diffusion impairment
1) Decreased pulmonary capillary transit time (exercise)
2) Increased diffusion path length (alveolar proteinosis, pulmonary edema, interstitial inflammation or fibrosis, pneumocystis carinii pneumonia)
3) Reduction of functioning pulmonary capillary bed (pulmonary embolism, emphysema, pulmonary vascular disease)
4) Reduced alveolar PO2 (high altitude)