Physiology II Flashcards
How is the “volume of the physiologic dead space” estimated?
(Pco2 of systemic arterial blood - Pco2 of mixed expired air)*(tidal volume/Pco2 of systemic arterial blood)
or tidal volume times the dilution of alveolar Pco2 by dead space air (which contributes no CO2)
What is ventilation rate?
the volume of air moved into and out of the lungs per unit time
What are some ways to express ventilation rate?
- minute ventilation
- alveolar ventilation
What is minute ventilation?
the total rate of air movement into and out of the lungs
What is the eqn. for minute ventilation?
tidal volume*(Breaths/min)
How is alveolar ventilation different from minute ventilation?
alveolar corrects from the physiological dead space
What is the eqn for alveolar ventilation?
(tidal volume - physiologic dead space)* (breaths/min)
or
Vco2(Rate of CO2 production) * K (863mmHg) / PAco2(alveolar PCo2)
So Pco2 can be predicted using what two variables?
1) rate of CO2 production (Vco2)
2) alveolar ventilation (which excretes the produced co2)
T or F. If CO2 production (Vco2) is constant, then PAco2 is determined by alveolar ventilation
T.
Arterial Pco2 always equals alveolar Pco2. Why?
CO2 always equilibrates between pulmonary capillary blood and alveolar gas
In asthma are FEV1 and FVC decreased or increased?
both decrease but FEV1 is decreased more so the ratio decreases as well (typical of obstructive disease where airway resistance is increased)
How do FEV1 and FVC change in restrictive disease?
both decrease but FEV1 is decreased LESS so the ratio increases
For a given pressure the volume of a lung is larger during expiration than inspiration. Why?
Hysteresis. Compliance is higher during expiration
What is responsible for hysteresis?
difference in surface tension at the liquid-air interface of the air-filled lung
So why is compliance less during inspiration?
when the lung volume is small, more tension is required to overcome surface tension
When is surfactant active, inspiration or expiration?
inspiration
What creates the negative pressure of the intrapleural space?
two opposing forces pulling on the space, the lungs tend to collapse and the chest wall tends to pull out
NOTE: this negative pressure helps keep the chest wall from popping out of place or the lungs from collapsing
What is a consequence of pneumothorax?
the lung collapses because its no longer opposed by the IP vacuum
How does emphysema affect lung compliance?
Increases it (fibrosis decreases it)
T or F. In emphysema, the tendency for the lungs to collapse is less than for the chest wall to expand
T. So the body compensates with increased lung volumes (seeks a HIGHER FRC)
fibrosis is the opposite (seeks a lower FRC)
What is the eqn for collapsing pressure on alveoli by surface tension?
P=2T/r (this equals the pressure needed to keep it from collapsing)
When an alveoli collapses it is known as _____
Atelectasis
What is the most important constituent of surfactant?
dipalmitoyl phosphatidylcholine (DPPC)
When does surfactant production begin in gestation?
as early as 24 weeks
What is the eqn for transmural pressure?
Palveolar- Pip
Why is Pip negative?
The opposing forces of the lungs trying to collapse and the chest wall trying to expand create a negative pressure. This negative pressure keeps the lungs inflated
When does Palveolar become less than Patm and thus drive air entering the lung?
halfway through inspiration
During inspiration, Pip becomes even more negative. Why?
1) as lung volume increases, the elastic recoil of the lungs also increases and pull more against the IP space
2) airways and alveolar pressures become negative
T or F. Alveolar pressure is negative during expiration
F.
During forced expiration, the alveolar pressure becomes very positive. Why doesnt the lung collapse?
As long as transmural pressure remains positive the lungs stay open
What is a possible consequence of forced expiration in someone with emphysema?
The positive alveolar pressures generated may cause the lungs to collapse
Why would the lungs collapse during forced expiration in emphysema?
Pip raises to the same value as a normal person (~+20) but because elastic recoil is gone, alveolar pressure and airway pressure are lower than normal. The alveoli can remain open at a relatively normal transmural pressure but the larger airways cannot maintain the positive transmural gradient
How do emphysema patients combat the potential for airway collapse upon expiration?
They expire slow and with pursed lips to raise airway pressure
What is the eqn for converting partial pressure of a gas to conc of gas in liquid phase?
Cx= Px*Solubility constant
What is the driving force for diffusion of gas?
partial pressure difference of the gas across the membrane
Which diffuses more readily, Co2 or O2?
Co2 by up to 20x
What forms of gas contribute to its partial pressure?
Only dissolved forms (i.e. not Hb bound, protein bound, etc.)
What is the only constituent of air NOT bound in any form in blood?
Nitrogen
