Ventilation/Diffusion/Gas Transport Flashcards
What is the conventional way fractional gas concentrations are reported?
As though there were no water present Fg = Pg/(Pb-Ph2o)
What does the pressure of water vapor in the airways depend on?
Temperature
What is convention for expressing volumes of ventilated gas (ex: TV)?
BTPS (body temp, ambient pressure, saturated with water vapor)
What is the convention for expressing metabolic rates (O2 consumption, CO2 production)?
STPD (because volumes at STPD direct relate to moles)
What is Henry’s Law?
C(gas) = K*P(gas)
What does K indicate in Henry’s Law?
Solubility (depends on specific gas, solvent, and temperature)
How do the solubilities of O2 and CO2 compare?
CO2 is MUCH more soluble in blood than O2
How does diffusion occur within a medium vs between different media?
In a medium - down concentration gradient Between media - down partial pressure gradient
Why does convection occur?
Breathing makes the air (medium) move in a circulatory way, contributing to gas exchange and diffusion.
What is minute ventilation (V dot)?
Amount of air inspired/minute (V dot I) or expired/minute (V dot E)
How do you calculate minute ventilation?
VT x breathing frequency Normal: 500 mL x 15/min = 7.5 L/min
Where does the last air inspired in each tidal breath go?
It stays in the anatomic dead space (conducting airways)
How do you calculate alveolar ventilation?
f x (VT-VD) VT>VD, usually VT = 3 X VD
What is the effect of decreasing respiratory frequency on alveolar ventilation?
It will increase V dot A (this advantage is limited by mechanical factors, etc though)
What is alveolar dead space?
Alveolar gas volume that is ventilated but not effectively perfused, so it does not participate in gas exchange. Reasons - hydrostatic failure, PE, lung injury (ventilation of non-vascular air space), external obstruction of pulmonary circulation (Ex: tumor).
What is the physiologic dead space?
The total volume of inhaled gas that doesn’t participate in gas exchange - sum of anatomic + alveolar dead space. In health, anatomic = physiologic dead space.
What is the respiratory quotient?
R = V dot CO2/V dot O2
What does RQ depend on?
Diet and metabolism
Rank these diets from highest to lowest RQ: lipid, carbohydrate, protein
Carbohydrate (RQ = 1) > Protein (RQ = 0.83) > Lipid (RQ = 0.7)
What is a normal RQ?
0.8
What are two reasons that alveolar air is different from ambient air?
1) Water vapor 2) CO2 from circulation
What is the partial pressure of water vapor at 37 C (body temperature)?
47 mm Hg
Why can we say that inspired O2 = alveolar O2 and alveolar CO2?
1) No net exchange of N2 by body (partial P equation becomes PIO2 + PICO2 = PAO2 + PACO2) 2) Basically no CO2 in room air (PICO2 = 0)
What is the normal range for PAO2?
90-105 mm Hg
What is the consequence of having an RQ<1?
Expired volume is less than the inspired volume. (This is why we must divide PACO2 by RQ in the alveolar gas equation)
What is the relationship between alveolar ventilation (V dot A) and PACO2?
Inversely hyperbolically related (with constant metabolic rate). However, ventilation increases proportionally with metabolic rate (ex: with exercise) which keeps PACO2 nearly constant.
How do we define hyperventilation and hypoventilation?
Hyperventilation - more alveolar ventilation than needed to maintain normal PCO2
HypOventilation - Less alveolar ventilation than needed to maintain normal PCO2
What is the hallmark of hyperventilation? Hypoventilation?
Hyperventilation: hypocapnia
Hypoventilation: hypercapnia
*need to know metabolic rate to define these though
What is hyperpnea?
Appropriate increase in alveolar ventilation with increased metabolic rate to maintain constant PaCO2.
What is hypopnea?
Appropriate decrease in alveolar ventilation with decreased metabolic rate to maintain constant PaCO2.
What is the relationship between alveolar ventilation and PAO2?
Hyperbolically proportional
What does venous admixture mean?
Mixing of shunted, non-oxygenated blood with normal oxygenated blood
What does the rate of gas diffusion depend on?
Proportional to area and pressure gradient, inversely proportional to thickness.
Also depends on the diffusion coefficient D, which is proportional to gas solubility and inversely proportional to molecular weight.
What influences the diffusion path for O2 between the lung and blood?
Normally none of the first 4 pose significant barriers (path worsens):
Crossing gas space within alveolus (emphysema, loss of lung tissue
Crossing alveolar-capillary membrane (thickening from fibrosis)’
Through plasma from capillary wall into RBCs
Across RBC membrane into cell
O2 combining with Hb ** reaction rate is signicant
What limits the rate of O2 transfer from alveolar gas into blood?
Reaction rate of O2 combining with Hb.
What is the normal transit time for blood in the pulmonary capillaries (at rest)?
0.75 sec
What type of gas transfer exchange does CO follow?
Diffusion limited
Why does a rise in CO concentration in the blood only increase its partial pressure slowly?
The effective solubility (capacitance) is very high because of its tight binding with Hb, and there is normally not any baseline CO present.
Partial P = concentration/capacitance
What does the gas transfer rate of diffusion limited gas depend on?
The diffusional characteristics of the membrane and the gas
How does increasing blood flow rate affect diffusion limited gas transfer?
It does not.
In general, how does partial pressure correlate with gas solubility?
Dissolving more soluble gases raise partial pressure slowly (they need more gas dissolved to raise partial P)
Dissolving insoluble gases can raise partial P very quickly.
(Partial pressure = concentration/capacitance, capacitance = effective solubility)
What type of gas transfer exchange does N2O follow?
Perfusion limited
What determines gas exchange rate for perfusion limited gases?
The rate of blood flow. The partial pressure of such gases increases very fast, so it mainly depends on perfusion rate.
What type of gas transfer exchange does O2 follow?
Perfusion limited (less rapid than N2O though)
What is the diffusing capacity (DL) of the lung?
Rate of gas transfer/pressure gradient across alveolar-capillary membrane.
Summarizes both diffusion through blood-gas barrier and the chemical reactions of O2 and CO2 in the RBCs.
What is the equational analogue of resistance to diffusion in the lung?
1/DL
What limits the rate of CO2 exchange (preventing it from being super fast)?
Chemical reactions of CO2
1) formation of bicarbonate (HCO3-)
2) formation of carbamino compounds with blood proteins (esp Hb)
What is the solubility of O2 in blood?
0.003 mL/100 mL blood/mm Hg
What is the oxygen capacity?
Maximum amount of oxygen that can bind Hb
What is the p50 of Hb?
PO2 when 50% O2 capacity is bound to Hb (indicates affinity of Hb for O2). Normal is about 27 mm Hg.
What does a low P50 for the Hb-O2 curve mean about the affinity of Hb for O2?
High affinity.
How does a right shift in the Hb-O2 curve affect P50?
P50 increases
What is the percent saturation (SO2) equation?
SO2 = (O2 content/O2 capacity) * 100
What are three things that cause a right shift of the Hb-O2 curve?
Increased temperature, decreased pH/increased PCO2, increased 2,3-DPG
What is the expression for O2 delivery to tissues?
O2 delivery = arterial O2 concentration x blood flow
What are the forms of CO2 in the blood?
1) dissolved CO2
2) bicarbonate
3) carbamino compounds
What enzyme catalyzes the conversion of CO2 to H2CO3?
Carbonic anhydrase
