Gas exchange and transport Flashcards
what are the main function of the lungs
Ventilation – movement of air in and out
Gas exchange – the exchange of oxygen and carbon dioxide between the airspace of the alveoli and the blood
how does the partial pressure of oxygen change as it goes from atmosphere to mitochondria
Breathe in and out to continually add and remove gas to alveoli so pp of oxygen decreases at each tissue level -
Cascade effect
how does gas exchange occur in the lungs
Air is inspired into the lungs, uneven distribution throughout the lung (lower zones more compliant)
Pulmonary circulation is a low-pressure system, upright position barely enough pressure to perfuse oxygen the apices with overperfusion of the bases (pressure equal to air distribution)
Aka known as ventilation/perfusion (V/Q) matching
what is a steady state condition
the amount of CO2 produced by the body and the amount of O2 absorbed depends on metabolic activity of the body – respiratory quotient (RQ) (match up CO2 and O2)
how is the respiratory quotient calculated
RQ= CO2 produced/O2 absorbed
Varies, about 0.8 at rest (fat metabolism at 0.7 or carbs at 1.0)
How does the partial pressure of CO2 change in the body
CO2 is produced at a relatively constant state by the body, the pp of CO2 (PCO2) of alveolar air (PACO2) depends on alveolar ventilation
AV up then PAC02 down and vice versa
what is the difference between PCO2, PACO2 and PaCO2
Partial pressure in the atmosphere
Partial pressure in the alveolar air
Partial Pressure in arterial blood
How does the partial pressure of O2 change in the body
Partial pressure of O2 also varies with AV
AV up then PAO2 up and vice versa
Big enough AV increase will allow PAO2 to approach PO2
what determines the rate of gas entering the blood
Fick’s Law (diffusion through tissues)
The rate of transfer of gas through a tissue is proportional to the tissue area and the difference of the gas pp between the two sides and inversely proportional to tissue thickness
what effect do local differences have on V/Q
In the normal lung, most alveoli receive V/Q in correct proportion
Low V/Q, low ventilation relative to blood flow. PCO2 increase and PO2 decrease (dissociation curve reflects this with arterial content of each gas)
High V/Q shows the opposite
what is the alveolar gas equation
PAO2=PIO2 - (PaCO2/0.8)
The relationship between PaCO2 and PaO2 is summarised
What is the A-a (alveolar-arterial) gradient
PaCO2 is equal to PACO2 but Pa02 is not equal to PAO2 (Cascade effect)
Healthy young adults = A-a gradient is small (<2 kPa)
if A-a gradient is greater, the abnormality in blood gas cannot be explained by a change in ventilation (must be intrinsic, lung vasculature, causing V/Q mismatch)
how does respiratory failure relate to the alveolar gas equation
Pure underventilation =increased PaCO2 + proportionate fall in PaO2 – type 2 respiratory failure
Disturbance in V/Q matching = fall in PaO2 but no change in PaCO2 – type 1 respiratory failure
Both can occur concurrently – the alveolar gas equation allows determination of reduced PaO2 due to underventilation or an intrinsic lung issue
what is forced vital capacity
total lung capacity - residual volume
what ratio determines the 3 physiological zones of the lung
the ratio between capillary perfusion and alveolar ventilation
