gas exchange and gas transport

Question 1

what are the functions of the lung? 2

Answer 1

• ventilation= movement of air in and out of the gut

- gas exchange= the exchange of oxygen and carbon dioxide between the airspace the alveoli and the blood

Question 2

how is air distributed inside the lungs?

Answer 2

unevenly, lower zones are more compliant

Question 3

what type of pressure is in the pulmonary circulation? 3

Answer 3

• low pressure system
• upright position is barely enough pressure to perfuse the apices of the lungs with overperfusion of the bases
• otherwise known as ventilation/perfusion matching (V/Q)

Question 4

what is the relationship between partial pressures of oxygen and carbon dioxide? 4

Answer 4

• steady state condition- the amount of oxygen produced by the body and amount of oxygen absorbed depends upon the metabolic activity of the body
• the respiratory quotient (RQ)= CO2 produced/ O2 absorbed
• fat metabolism= 0.7
• carbohydrate=1.0

Question 5

explain the pressures of carbon dioxide? 3

Answer 5

• if CO2 is produced at a constant rate in the body, then the partial pressure of CO2 (PCO2) of alveolar air (PACO2) is dependent on alveolar ventilation
• with increase alveolar ventilation we decrease (PACO2)
• with decrease in alveolar ventilation we increase (PACO2)

Question 6

explain the pressures of oxygen? 4

Answer 6

• partial pressure of oxygen (PAO2) also varies with alveolar ventilation
• increase in alveolar ventilation increases PAO2
• a big enough increase will allow the PAO2 to approach PO2
• a decrease in alveolar ventilation will decrease PAO2

Question 7

how does gas get into the blood? 2

Answer 7

• diffusion through tissues is described as fick’s law
• the rate of transfer of gas through a tissue is proportional to the tissue area and the difference in gas partial pressure between the two sides and inversely proportional to the thickness of the tissue

Question 8

what is the effect local differences on V/Q? 3

Answer 8

• in the normal lung, most alveoli receive V/Q in correct proportion
• low V/Q= decreased ventilation relative to blood flow, increased arterial PCO2 and decreased PO2
• high V/Q= increased ventilation relative to blood flow, decreased arterial PCO2 and increased PO2

Question 9

what is the alveolar gas equation? 5

Answer 9

• PAO2= P1O2-(PACO2/0.8)
• summarises the relationship between PACO2 and PO2
• pure underventilation leads to an increase in PACO2 and a proportionate fall in PAO2- type 2 respiratory failure
• disturbance in V/Q matching leads to a fall in PAO2 with no change in PACO2- type 1 respiratory failure
• both can occur concurrently- alveolar gas equation allows determination if there is an observed reduction in PAO2 is related to underventilation alone or whether there is an intrinsic lung tissue

Question 10

what can we do now we know the PAO2? 4

Answer 10

• unlike CO2 there is normally a difference between alveolar and arterial PO2
• this is known as the alveolar-arterial (A-a) gradient
• in healthy adults breathing in the A-a gradient is small
• if the A-a gradient is bigger than this, the abnormality in the blood cannot be explained by a change in ventilation, therefore something abnormal must be going on