Flashcards in PHYS: Gas Exchange Deck (38):
What is RQ?
respiratory quotient: ratio of CO2 produced to O2 consumed (normally around 0.8)
The respiratory quotient depends on what?
What we eat and burn (RQ is 1 for Carbs, and .7 for Fat)
How do you calculate partial pressure of a gas?
Atmospheric pressure X fraction of gas molecules in the dry air
What is the partial pressure of oxygen in air?
What happens to the partial pressure of oxygen if it is inspired?
it decreases to around 150 mmHg due to humidification (must subtract out water vapor)
How is concentration of a gas related to it partial pressure?
concentration (amount dissolved) is equal to partial pressure X solubility
Diffusion of gas across a membrane depends on what factors?
Which is better at diffusing in blood and tissue, CO2 or O2?
CO2 is 20 times faster at diffusing than O2
What are the Pco2 and Po2 at the alveoli?
Pco2= 40 mmHg
Po2= 105 mmHg
What are the Pco2 and Po2 at the lung capillary after diffusion?
Pco2= 40 mmHg
Po2= 100 mmHg (won't equilibrate due to shunts and slower diffusion)
What are the Pco2 and Po2 in the systemic veins after diffusion into tissue?
Pco2= 46 mmHg
Po2= 40 mmHg
Alveolar partial pressure of oxygen is determined by the ratio of what two values?
CO2 production/ alveolar ventilation
Increased ventilation will do what to PAco2?
decrease it (fresh inspired air dilutes the gas)
Increased metabolism will do what to PAco2?
increase CO2 production so increase PAco2 (more CO2 enters alveoli from blood per unit time)
What is the alveolar gas equation?
PAo2= PIo2 - PAco2/ (CO2 prduction/O2 consumption)
What is hypoxemia?
lower than normal (100 mmHg) arterial Po2
What is the A-a gradient?
It is the difference between Po2 in alveolar gas and arterial blood (tells if O2 has equilibrated between the two and is useful for examining causes of hypoxemia)
What is Pao2/Fio2?
The ratio of arterial Po2 to the fraction of inspired oxygen (useful for measuring acute lung injury--normal is 476 mmHg)
What is the Pao2/Fio2 in ARDS?
less than 100 if severe
What is hypoxia?
decrease in O2 delivery to or utilizaiton by dissues
What is hypercapnia?
higher than normal arterial Pco2 (normal is 40)
What is the most common cause of hypercapnia?
low alveolar ventilation
True or false: virtually all gas exchange is completed in the initial region of the pulmonary capillary.
Diffusion of oxygen and CO2 is described as what type?
Perfusion-limited (blood flow determines amount that diffuses)
Diffusion of carbon monoxide is described as what type?
Diffusion-limited (extremely soluble in blood, so it doesn't matter how much blood is there)
What are the two mechanisms for oxygen transport in the blood?
1) Oxygen is physically dissolved in the blood
2) Oxygen (4 molecules) is bound to Hb
Oxygen binds to what form of heme?
Ferrous (2+) iron portion
What is methemoglobin?
oxidized heme with iron in the ferric (3+) state, so it cannot bind to oxygen
Why does fetal Hb have a higher affinity for oxygen?
it has 2 alpha and 2 gamma subunits (adults have 2 alpha and 2 beta)
What is cooperativity in relation to the Hb dissociation curve?
binding of 1 oxygen to heme increases the affinity for other molecules to the remaining spots (gives sigmoidal shape to curve)
Low Hb saturation in surface capillaries causes what phenomenon?
cyanosis (bluing of lips, ears, nails, tongue)
True or false: all oxygen in the blood contributes to the partial pressure of oxygen.
FALSE: only dissolved, free oxygen (not oxygen bound to hemoglobin)
What causes a right shift in the Hb dissociation curve?
What causes a left shift in the Hb dissociation curve?
A right shift is a _____ in affinity.
decrease (good for unloading)
A left shift is a _____ in affinity.
How does 2,3-DPG decrease Hb affinity for O2?
it binds strongly to deoxygenated Hb and lowers its affinity for oxygen