Resp 4 Flashcards
fractional components of air
-Oxygen comprises 21% of the molecules in air.
-Concentration of gases is measured as partial pressure (P) or tension. For O2 the value is “PO2 ”. Alveolar PO2 : PAO2 , Arterial PO2 : PaO2 , Venous PO2 : PvO2 . Same for other gases (eg. CO 2 ), substituting their specific symbol for O2
effect of barometric pressure/altitude on oxygen tension
In dry air, P O2 = PB x FO2
where
-PB = barometric pressure (760 mm Hg at sea level)
-FO2 = the fraction of oxygen in the gas mixture (21% or 0.21) PO2 = 760 x 0.21 = 160 mm Hg
-Oxygen tension decreases at high elevations because barometric
pressure decreases
physiological factors affecting alveolar O2 concentration
1) Humidity: When air is inhaled, it is humidified -> concentration of oxygen falls because it is diluted by water vapour
-PO2 of humidified air = (PB – PH2O) x FiO2
-So PO2 of humidified air = (760 – 50)x 0.21 = 149 mm Hg
~~PO2 of inspired air is lower than the environmental PO2 because
air is humidified
2) CO2 Level
-In the alveoli, inspired air mixes with CO2 which is simultaneously diffusing out of alveolar capillaries as oxygen enters.
-Alveolar partial pressure of carbon dioxide (PACO2 ) is determined by the rate of CO2 production (VCO2 ) in relation to the rate of alveolar ventilation (VA)
~~PO2 of inspired air is lower that that of environmental air because
it contains CO2
~~To keep PACO2 relatively constant, alveolar ventilation increases when CO2 production rises (e.g. during exercise)
-Oxygen Diffusion into Bloodstream. O2 rapidly diffuses from the alveolus into the bloodstream, lowering the concentration in the alveolus
3) Alveolar oxygen tension (PAO2 ) is lower than that of the inspired air because oxygen is continuously diffusing out of the alveoli into blood
-Overall, during a breathing cycle, PAO2 fluctuates around an average value, rising during inhalation, falling during exhalation.
-Taking all three factors into consideration, the average oxygen tension in the alveoli can be calculated from the alveolar gas equation:
PAO2 = [(PB – PH2O) x FiO2 ] - PACO2/R
-So alveolar oxygen tension rises with barometric pressure and inspired oxygen tension, and depends on the rate of exchange of oxygen for carbon dioxide (which determines PCO2 ) and humidity
-PACO2 is usually ~40 mm Hg, so PAO2 averages about 100 mm Hg at sea level
-A second implication is that, since the PACO2 is inversely related to the PAO2 and since PACO2 , PaCO2 and PvCO2 are typically similar values, PCO2 (arterial or venous) are often used as measures of alveolar ventilation.*
*Arterial samples are the best measure, but venous samples are
much easier to obtain and provide a reasonable estimate.
-A major implication of this equation is that, when PACO2 rises, PAO2 must fall, and vice versa.
alveolar hypoventilation
-A decrease in alveolar ventilation elevates PACO2 and decreases PAO2
-Elevated PACO2 leads to elevated PaCO2 and PvCO
causes
1. damage to CNS (drugs, trauma)
2. peripheral nerve injury
3. damage to pump (e.g., muscle paralysis, trauma to chest, bloated abdomen)
4. lung resisting inflation (airway obstruction, decreased lung compliance)
5. resp compensaiton for metabolic acidosis
alveolar hyperventilation
- Causes a decrease in PACO2 because ventilation is increased in relation to the level of CO2 production.
- As PACO2 decreases, PAO2 rises (according to the alveolar gas
equation)
Causes of alveolar hyperventilation
* Hypoxia in environment (altitude)
* Metabolic Acidosis
* Increase in body temperature (attempt at cooling)
oxygen and carbon dioxie diffusion gradient
Oxygen
* Moves from alveoli into blood
* Blood is almost completely saturated with oxygen when it leaves the capillary (less saturated if flow rate is high)
* Oxygen tension (PO2 ) in peripheral blood decreases because of mixing with deoxygenated blood
* Oxygen moves from tissue capillaries into the tissues
Carbon dioxide
* Moves from tissues into tissue capillaries
* Moves from pulmonary capillaries into the alveoli
Exchange of O2 and CO 2 in the alveoli relies on diffusion across the alveolar and endothelial walls
diffusion of gases; what is it dependent on
-Diffusion of gases across the alveolar and endothelial walls is passive and is dependent on:
- Relative diffusion coefficient of gas (D)
-CO2 D = 20
-O2 D = 1 - Surface area available for diffusion (A)
-Diffusion coefficient depends on molecular weight of gas and solubility of gas
-Diffusion through cell membranes is ~instantaneous
-Diffusion through tissues is similar to diffusion through water
-CO2 diffuses far more rapidly than O2 - Distance between air and blood (X)
- Driving pressure (pressure gradient) [PA(gas) - Pcap(gas)]
oxygen gas exchange; levels, pressure, direction
-In mixed venous blood returning to the heart, PvO2 ~ 40 mmHg
-PAO2 - PvO2 = 100 – 40 = 60 mmHg
->Strong pressure difference driving O2 into capillaries
CO2 gas exchange; levels, pressure, direction
-In mixed venous blood, PvCO2 ~46 mmHg
-PACO2 - PvCO2 = 40 – 46 = -6 mmHg
->A small partial pressure difference, yet the high diffusion coefficient of CO2 causes very rapid equilibration of PCO2
exchange of O2 and CO2 in the tissues
-Exchange of O2 and CO 2 in the tissues is also due to diffusion
-Tissue partial pressures (eg. concentrations):
~PtO2 ~ 40 mm Hg at rest (↓ during exercise)
~PtCO2 ~ 45 mm Hg at rest (↑ during exercise)
-Variables affecting PtO2 and PtCO2
* X, distance between blood and tissue, depends on blood supply
* Driving pressure gradient depends on
* Cellular metabolic rate
* O2 and CO 2 tensions in the blood
O2 and CO2 during exercise
-During exercise, active muscle uses
more O2 , produces more CO2 . Blood flow and ventilation then increase
- Cardiac output is high, velocity of blood flow is high
- More O2 should be transferred but has less time to do so.
- Diffusion equilibrium may not occur in alveoli, which decreases efficiency.
- Ultimately this helps dictate the upper limit of how much O2 the system can deliver to tissues.
gas exchange and capillaries
Gas exchange improves as capillaries are recruited during exercise (dilation and recruitment).
* Distance between tissue and capillary is reduced
* Blood flow slows as the number of vessels dilating rises -> more time for gas exchange
back to the lung; ventilation/perfusion ratios
-Considering all the factors discussed, there is a relatively fixed relationship between, Ventilation (V) and Perfusion (Q) for each alveolus and for the animal overall.
-This is called the Ventilation/Perfusion (V/Q) Ratio. It tends to differ regionally (dorsal vs ventral) in an animal’s lungs
V/Q ratio is normal animals
-about 0.8. i.e. About 4L of air enters the lungs for every 5L of blood
-When the value changes significantly away from 0.8 it is called “ mismatch ”
-There is normally a small mismatch because of gravity effects on lung perfusion – some areas have lower V/Q than other areas.
-In disease states, mismatch may become extreme, and hypoxemia (low arterial oxygen tension, Pa O2 ) may develop at the individual alveoli, and, if extensive enough, in the whole animal.
V/Q mismatch
In alveolus (A) in Figure below, ventilation and perfusion are approximately matched
* Mixed venous blood arrives with PO2 = 40 & P CO2 = 46 mm Hg
* Gas tensions equilibrate
* Blood leaving alveolus (“end capillary” blood) has PO2 = 100 &
PCO2 = 40 mm Hg
On the left, alveolus (B) is supplied by an obstructed bronchiole
* is zero
* Mixed venous blood arrives with PO2 = 40 & P CO2 = 46 mm Hg and
leaves unchanged
* This occurs commonly in lung disease when airways are obstructed
or compliance is reduced
* Impaired ventilation is associated with low ratios
-When ventilation is impaired …..LOW V/Q values are seen in alveoli. Eg. Airway obstruction by exudates in severe pneumonia
->Decreased diffusion of O2
decrease PAO2 and decrease Pa O2
Mismatched V/Q («_space;1)
