Physiology - Equations

Question 1

Intrapleural pressure (PIP)

Answer 1

Negative pressure within the pleural space, generated by deviation of the lung and chest wall from their intrinsic equilibrium positions

Varies from -5cm H20 (end expiration) to -30cm H2O (end inspiration)

Question 2

Transpulmonary Pressure (PTP)

Answer 2

The pressure exerted on the lung by the pleural space; provides the driving force for changing lung volume during breathing

PTP = PL - PIP

Question 3

Compliance

Answer 3

C = dV/dP

Determined by plotting lung volume as a function of PTP

Dependent on lung and chest wall compliance

Question 4

Hysteresis

Answer 4

Compliance of the lung is lower during inspiration; a greater change in PTP is required to produce a given change in lung volume during inspiration than during expiration

This is a function of the elasticity of the lung

Question 5

Laminar Flow

Answer 5

Flow rate is proportional to the pressure difference

Resistance is proportional to tube length and viscosity, and is inversely proportional to radius ^ 4

Question 6

Turbulent Flow

Answer 6

Seen at higher flow rates through larger diameter tubes

Turbulent flow is proportional to the square root of the pressure difference; it is therefore less efficient than laminar flow

Question 7

Dynamic airway collapse

Answer 7

Tendency of the airway to remain open is defined by the PTP = PAW - PIP

During normal expiration, negative PIP ensures that PTP remains positive and airways are open

Forced expiration causes the chest wall to exert an extra force on the intrapleural space, producing a positive PIP and a negative PTP leading to airway collapse

Question 8

Dalton’s law for inspired air

Answer 8

PIO2 = (PB - 47Torr) x 0.21

PB at sea level = 760 Torr
PB in Denver = 620 Torr

Question 9

Alveolar Gas Equation

Answer 9

PAO2 = PIO2 - (PACO2/R) +K

Question 10

Alveolar ventilation equation

Answer 10

PACO2 = (VCO2/VA) x K

and PACO2 = PaCO2

Describes the effect of ventilation rate on PaCO2

Question 11

Relationship between PaO2 and SO2 - Arterial

Answer 11

Normal arterial PaO2 = 100Torr

Normal arterial SO2 = 97.5%

Question 12

Arterial oxygen content (CaO2)

Answer 12

Hemoglobin-bound O2 + freely dissolved O2

9. 1mM
10. 7 ml O2 / 100 ml blood

Question 13

Relationship between PO2 and SO2 - Venous

Answer 13

PO2 = 40 Torr
SO2 = 75%

Question 14

Fick’s Law of Diffusion

Answer 14

VG = dP x (A/d) x k

Gas transfer rate is proportional to the prssure difference and the surface area for diffusion; gas transfer is inversely proportional tot he thickness of the gas transfer surface