Resp Physiology - West Flashcards
How do oxygen/CO2 move between air and blood?
Simple Diffusion
Fick’s law of diffusion
Amount of gas that moves across a sheet of tissue = proportional to the area of a sheet, inversely proportional to its thickness
Conducting Airways
Trachea –> left, right main bronchi –> lobar bronchi –> segmental bronchi –> segmental bronchioles (smallest airways without alveoli)
As the airways progress distally, proportion of cartilage decreases - smooth muscle increases
Anatomic Dead Space
Conducting airways = trachea, main bronchi, lobar/segmental bronchi, segmental bronchioles
Lungs that receive ventilation but no blood flow
Vol = 150mL
Respiratory Zone
Respiratory bronchioles - contain occasional alveoli
Alveolar ducts - completely lined with alveoli
Acinus
Portion of the lung distal to a terminal bronchiole that forms an anatomical unit
During inspiration…
- Vol of the thoracic cavity increases
- Air drawn into the lung
- Increase in vol brought about partly by contraction of the diaphragm –> causes it to descend, partially by action of the intercostal muscles –> raise the ribs and increase cross sectional area of the thx
Pressure required to move gas through airways
Normal inspiration: air flow rate of 1L/sec requires a pressure drop of <2cm H20
Diameter of capillary segment
7-10um - just large enough for an RBC
Lengths of the segments so short that dense network forms an almost continuous sheet of blood in the alveolar wall
Consequences of extreme thinness of blood gas barrier (BGB)
Capillaries = easily damaged –> ex increasing pressure in the capillaries to high levels, inflating the lung to high volumes
Can leak plasma/RBCs into the alveolar spaces
Blood-Gas Interface
Extremely thin (0.2-0.3um)
Enormous surface area of 50-100m^2
Large area obtained by having approx 500 million alveoli
So thin that large increases in capillary pressure damage barrier
Blood Vessels
Whole output of right heart goes to the lung
Diameter of capillaries = 7-10um
Thickness of much of the capillary walls <0.3um
Blood spends approx 0.75s in the capillaries –> traverses 2-3 alveoli, sufficient for virtually complete equilibration of oxygen and CO2 btw alveolar gas and cap blood
Resistance of pulmon circuit
Extremely small
Mean pulmonary arterial pressure of only 20cm H20 (15mm Hg) required for flow of 6L/min
Stability of the alveoli
Structure of lung = inherently unstable
Surface tension of liquid lining the alveoli, relatively large forces develop that tend to collapse alveoli
Surfactant –> dramatically lowers
Functional residual capacity (FRC)
Vol of gas in lung after normal expiration
Boyle’s Law
At constant temperature, pressure x volume = constant
How to calculate PO2 given a barometric pressure
PO2 = 0.21 x barometric pressure
If in the airways: PO2 = 0.21 (BP-PH20 vapor)
–> PO2 = 0.21 (BP-47)
Minute Ventilation
AKA total ventilation
TV x RR
Total vol of air leaving lung per minute
Vol of air entering the lung very slightly greater bc more oxygen is taken in than CO2 is given out
Alveolar Ventilation
Amount of fresh inspired air available for gas exchange
Vol of fresh gas entering respiratory zone per minute: (TV-anatomic dead space vol) x RR
Even though smaller amount of vol enters alveoli with each breath, alveolar vol still expands by full size of the TV as the dead space vol left over at the end of the previous exhalation is drawn into the alveoli with each breath before the fresh gas enters
Why is increasing Tv often more effective at increasing alveolar ventilation?
Reduces the fraction of each breath occupied by the dead space ie the dead space fraction
