oxygen and carbon dioxide exchange in the lung

Question 1

what does fick’s law** state**?

Answer 1

• This states that the rate of transfer of a gas through a sheet of tissue is** proportional to the tissue area** and the different in partial pressures between the 2 sides, and** inversely proportional to the tissue thickness**

Question 2

how does oxygen move in gas exchange in the lung?

Answer 2

• oxygen must move from the alveolar air into the pulmonary capillaries (down its partial pressure gradient)
• it is then transported by the blood to the tissues
• leaves the tissue capillaries and enters the ECF
• crosses plasma membrane and enters the body’s cells

Question 3

How does carbon dioxide move in gas exchange of the lung?

Answer 3

• carbon dioxide (produced by cells in body) must move from the pulmonary capillaries into the alveolar air to be exhaled
• diffuses from high PaCO2 to low PACO2

Question 4

how much oxygen in mls crosses the diffusion barrier?

Answer 4

250ml

Question 5

How much CO2 leaves the blood per minute to the lungs?

Answer 5

200ml

Question 6

what is the typical PAO2?

capital A = alevolar

Answer 6

100mmHg or 14 kilopascals

Question 7

what is the typical PACO2?

Answer 7

40mmHg or 5 kilopascals

Question 8

why is the alveolar (PAO2) lower than atmospheric PO2?

Answer 8

• as some of the oxygen in the air entering the alveoli leaves to enter the pulmonary capillaries

Question 9

what** 3 factors **is the alveolar partial pressure of oxygen (PAO2) affected by?

Answer 9

1. the PO2 of atmospheric air
2. the rate of alveolar ventilation
3. the rate of total body 02 consumption

Question 10

what occurs to the PAO2 at high altitudes?

hint: think about the 3 factors that affect PA02

Answer 10

• at high altitudes, there is less oxygen in the atmosphere
• therefore this will decrease the PAO2

Question 11

what effect does a decrease in alveolar ventilation have on the PAO2?

Answer 11

• it decreaes the PAO2

Question 12

what effect does an increasing in the rate of oxygen consumption (eg during intense exercise) have on PAO2?

Answer 12

• a increase in the rate of oxygen consumption will decrease the O2 content of the** blood** returning to the lungs
• this will increase the concentration gradient of oxygen from the lungs to the pulmonary capillaries resulting in an increase in oxygen diffusion
• if alveolar ventilation does not change, this will lower the PAO2 as a large amount of O2 will be inhaled and used up by tissues

Question 13

what effect does a decrease in alevolar ventilation have on PACO2?

Answer 13

• a decrease in alveolar ventilation willl increase PACO2 as there is a decrease of CO2 exhaled and an increase of CO2 in alveoli

Question 14

what effect does an increase in the production of CO2 by cells have on the PACO2?

Answer 14

• it will** increase PACO2**, as more CO2 will be diffusing from the blood into the alveolar air down uts concentration gradient

Question 15

what is hypoventilation?
ie what happens to the CO2

Answer 15

• an increase in the ratio of CO2 production in alveolar ventilation
• ie a person is hypoventilating** if the alveolar ventilation cannot keep pace with CO2 production**
• the** alveolar PACO2 increases above the normal value**

Question 16

what is hyperventilation?

IE IN RELATION TO THE CO2 / ALVEOLAR VENTILATION

Answer 16

• a decrease in the ratio of CO2 production to alveolar ventilation
• ie when the alveolar ventilation is actually too great for the amount of CO2 produced
• the PACO2 is below the normal value

Question 17

what is mixed venous blood?

Answer 17

a mixture of venous blood drainage from a number of different organs

Question 18

What does blood contain (in terms of partial pressure of O2 vs CO2) after coming from the tissues?

Answer 18

• it has a relatively high PCO2 and a low PO2

Question 19

how many ‘cells thick’ is the diffusion barrier?

Answer 19

2 cells thick
* capillary endothelial cell
* alevolar epithelial cell

Question 20

In what 2 ways can the diffusion of the gases be impaired?

Answer 20

• **a decreased total surface area **(eg pulmonary aedema - alveoli become filled with fluid and decrease diffusion barrier
• or if the **alevolar walls become severely thickened **(eg pulmonary fibrosis)

Question 21

what effect does exercise have on oxygen consumption and CO2 production?

Answer 21

• exercise increases O2 comsumption and CO2 production

Question 22

what** key mechanism **underlies the increase in the rate of O2 delivery to muscle and CO2 removal?

Answer 22

increase in cardiac output

Question 23

during exercise, blood flow through the lungs is greatly increased,… true or false?

Answer 23

True
the cardiac output increases during exercise and therefore the pulmonary blood flow also increases

Question 24

why does alveolar ventilation increase during exercise?

Answer 24

• alveolar ventilation increases to prevent an increase in arterial CO2 and reduction in arterial O2
• however they must be ‘matched’ to increase CO2 delivery and O2 removal

Question 25

what is ventilation-perfusion inequality?

Answer 25

mismatching of the alveolar airflow (ventilation) and capillary blood flow (perfusion)

Question 26

what is the **major effect **of ventilation - perfusion inequality?

Answer 26

decrease the PO2 of systemic arterial blood

Question 27

how does the gas exchange surface area increase during exercise?

compare at rest vs exercise

Answer 27

• at rest, only a restricted proportion of pulmonary capillaries are perfused
• as CO is increased, a greater number of capillaries are recruited and perfused - which increases the gas exhcnage surface area

Question 28

what is maximal oxygen consumption?

Answer 28

as the intensity of any endurance exercise increases, oxygen consumption also increases proportionally** until reaching a point when it fails to increase despite an increase in workload**

Question 29

what is maximal oxygen consumption limited by?

Answer 29

cardiac output - lung perfusion

Question 30

what properties of a gas are needed in order to assess gas diffusion in the lung?

Answer 30

• the gas must be** diffusion limited** (when its** effective solubility in blood** is significantly greater than its solubility in the blood-gas barrier)
• it must be** extremely soluble in blood **
• the partial pressure of test gas in blood must not equilibrate with that in alveolar gas during transit time