Flashcards in Diffusion and perfusion Deck (29):
Rate of gas transfer from alveoli into arterial blood depends on what?
Diffusion from alveolar membrane into pulm capillaries and perfusion (extent of blood flow into ventilated regions of lung)
Arterial oxygen content (CaO2)
CaO2 = hemoglobin-bound O2 concentration + freely-dissolved O2 concentration. Total concentration of O2 in blood. Most O2 is hemoglobin bound, but it is free O2 that is measured in partial pressure values
Define solubility coefficient and describe how they differ for oxygen and CO2.
The tendency of any molecule to dissolve in a liquid is given by its solubility coefficient. aO2= 0.0013mM/Torr. aCO2= 0.03mM/Torr. This means CO2 is about 20 times more soluble in blood than O2
Describe the basic properties of the oxy-hemoglobin dissociation curve (ODC).
Curve that relates oxygen saturation of hemoglobin (SaO2) at equilibrium to PO2 in the blood.
What is hemoglobin saturation at normal arterial oxygen pressure?
For PO2 = 100 Torr, which is normal arterial oxygen, SaO2 = 97.5%. This means that in arterial blood, hemoglobin is very close to full saturation
What effect does increasing arterial PO2 have on SO2
IT will not substantially increase amount of O2 carried by hemoglobin because hemoglobin is already near saturation at normal blood oxygen pressures.
What is hemoglobin saturation in mixed venous blood
For PO2=40Torr, SaO2= 75%
What factors affect rate of gas transfer across tissue plane
difference in partial pressure of the gas on two sides of membrane, tissue plane area, tissue thickness, tissue solubility and molecular weight of gas
Equation for gas transfer rate (Ficks law)
Vg (gas transfer rate) = (P1-P2) x (A/d) x k. Where P is pressure, A is area, d is tissue thickness, and k is constant
equation for diffusing capacity
Dm (diffusing capacity) = (A/d) x k, where A is area, d is thickness, k is constant
Describe factors that promote rapid oxygen diffusion between alveoli and pulmonary pulmonary capillaries in healthy individual
Large surface area of alveolar membrane, thin membrane and the large oxygen pressure gradient btw alveoli and capillaries (due to low solubility of O2 in blood and tendency for O2 to bind hemoglobin which both keep O2 pressure in the capillary low)
Describe oxygen diffusion btw alveoli and pulm capillaries in interstitial disease
During interstitial disease, when there is thickening of the alveolar walls (increased d), the rate of diffusion is slowed, sometimes to the point where arterial oxygen levels do not reach alveolar levels
Describe oxygen diffusion btw alveoli and pulm capillaries in emphysema
breakdown in lung tissue decreases the surface area for diffusion (decreased A), which decreases overall diffusion
Describe oxygen diffusion btw alveoli and pulm capillaries in polycythemia and anemia
Diffusion increases in polycythemia and decreases in anemia due to abnormally high or low hemoglobin levels. But note that changes in hemoglobin conc usually alters oxygen delivery more than oxygen diffusion
Compare diffusion of CO2 vs O2
CO2 is 20 times more soluble in blood than O2, so O2 diffusion is somewhat slowed by its poor solubility and thus is susceptible to disease. CO2 diffusion is NOT limited by solubility, thus it is less affected by disease
Perfusion limited O2 transfer rate
The only factor limiting the rate of oxygen uptake and CO2 excretion is pulmonary blood flow. There is rapid equilibration of blood with alveolar air (PaO2= PAO2, and PaCO2=PACO2). This is normal, even during exercise
Diffusion limited O2 transfer rate
Diffusion rate limits the O2 transfer rate - there is not complete equilibration of alveolar air and blood. PO2 from pulmonary blood < alveolar PO2
Perfusion (Q) is the blood flow of the pulmonary circulation available for gas exchange (in one minute).
Perfusion = ?
Cardiac output (5L/min)
Describe pulmonary circulation pressures and why
Very low blood pressure- due to vast number of vessels and normally dilated state. Walls of pulmonary artery are thin
Factors that regulate perfusion
O2 tension, chemical agents, capillary recruitment, gravity
How does O2 tension affect perfusion
Alveolar O2 tension (PAO2) is most important in regulating pulm circulation. Low PAO2 results in constriction of arterioles (hypoxic pulmonary vasoconstriction) which decreases local blood flow and shifts it to other regions of lung.
How do chemical agents affect perfusion
Thromboxane A2 is the most important
vasoconstrictor, and, as with O2, its effect is mainly localized to the region of release. Prostacyclin (prostaglandin I2) is a potent vasodilator.
How does capillary recruitment affect perfusion
During moderate exercise, CO increases and is accomodated by recruitment of new capillaries and distension of previously open microvessels.
How does gravity affect perfusion
Because of gravity, the pulmonary blood pressure is low at the apex of the lung. At the base of the lung, the blood pressure is higher, allowing more capillaries to open and higher blood flow. Thus, perfusion increases as we go from the apex to the bottom of the lung.
Describe the mechanisms by which dead-space impacst gas exchange.
Alveolar dead space occurs when ventilation occurs, but no perfusion (ie. If patient has a blocked pulmonary capillary). Thus they do not engage in gas exchange
Describe the mechanisms by which shunts impact gas exchange.
Blood perfusion without any ventilation results in a shunt. This can reduce arterial oxygenation significantly when well ventilated blood mixes with shunted blood. PCO2 in arteries is NOT raised b/c increased ventilation will prevent that.
Describe the mechanisms by which V/Q mismatch impact gas exchange.
V/Q mismatch is an unequal ratio of ventilation to perfusion, as can happen in obstructive lung disease and from gravity. When V/Q is low (due to alveolar blockage), V/Q in other alveoli may increase, but since hemoglobin is already at near saturation, increased V/Q doesn't fully compensate for the low V/Q. PCO2 does NOT increase PCO2 overall b/c any increases in PCO2 in parts of the lung with low V/Q will be countered by increased ventilation (high V/Q) in other parts of the lung