Mechanisms Of Respiration Flashcards
What is (expired) minute ventilation (V̇ E)?
Expired Minute Ventilation (V̇ E) = Tidal Volume (V T) * frequency of breaths (f)
V̇ E (ml/min) = V T (ml/breath) * f (breaths/min)
Given that VT = VD + VA,
V̇ E = (f * VD) + V̇A
What is LaPlace’s Law, and how does it relate to surfactant in the lungs?
LaPlace’s Law states that the pressure required to maintain a bubble, or an alveolus by analogy, is proportional to the wall tension and inversely proportional to the radius.
Surfactant reduces the surface tension of an aqueous air-water interface, and its surface tension is proportional to the area that it covers. Thus, the surface tension of an alveolus decreases as the alveolus gets smaller, and its pressure does not increase as it shrinks.
What is the tidal volume (V T)?
The tidal volume (VT) is the volume of air expired during a single respiratory cycle.
Tidal Volume (V T) = Anatomic Dead Space (V D) + Volume of Air Expired Fromm the Alveoli (V A)
VT = VD + VA
What does Poiseuille’s equation state, and how does this relate to air flow in the lungs?
Poiseuille’s equation states that for laminar flow of gases in the respiratory system:
Flow = (Palv - Patm) / R = ∆P / R
Where the resistance to air flow (R) is proportional to the viscosity of air and the length of the airway and is inversely proportional to r4, where r is the radius of the vessel.
What does Henry’s law state?
Henry’s law states that at a constant temperature, the amount of a given gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. The concentration of a gas dissolved in a liquid can be calculated as the product of the partial pressure of the gas times the solubility coefficient of the gas in the liquid of at that temperature.
Cx = Px * Sx
Where S is the solubility coefficient of the gas.
What does the Fick’s law of diffusion state?
Fick’s law of diffusion expresses the volume of gas diffusing through the alveolar-capillary barrier per unit time (V• gas) as:
V• gas = [DSA)*(P1-P2)] / ∆x
Where D is the diffusion coefficient of the particular gas (inversely proportional to the square root of its molecular weight), S is the solubility of the gas in the liquid medium, A is the surface area of the barrier, ∆x is the thickness of the barrier and (P1-P2) is the difference in partial pressure of the gas across the barrier.
What is the respiratory quotient/ratio?
The respiratory quotient/ratio refers to the quantity of CO2 produced by tissues to that of the O2 consumed by that as the result of metabolic processes. The respiratory exchange ratio (R) is the ratio of the flow of CO2 to that of O2 across alveolar membranes. In the steady state, RQ and R are equal. R values between 1 (carbohydrate metabolism) and 0.7 (fat metabolism). The average diet has an R value ~0.8.
What is the alveolar gas equation?
A simplified version of the alveolar gas equation states the following:
PAO2 = PIO2 - (PACO2 / R)
Where PAO2 is the partial pressure of oxygen in the alveoli, PIO2 is the partial pressure of the inspired oxygen, PACO2 is the partial pressure of CO2 in the alveoli, and R is the respiratory ration.
Note that R is equal to 0.8 , and PIO2 is equal to 150 mm Hg at ambient temperature and pressure
What is the alveolar ventilation equation?
The alveolar ventilation equation states the following:
V•A = V•CO2[(PB - 47 mm Hg)/PACO2
Where V•A is minute alveolar ventilation, V•CO2 is the amount of CO2 produced each minute expressed as volume, and PB is barometric pressure (760 mm Hg at sea level).
What is the alveolar-arterial oxygen difference and how does it utilize other respiratory equations?
The alveolar-arterial oxygen is equal to the difference between PAO2 and PaO2 (PAO2 - PaO2). This value can be calculated by utilizing the alveolar gas equation to obtain an estimate of PAO2 and the value of PaO2 obtained from arterial blood gas tests.
