gas exchange

Question 1

describe the oxygen pathway into the body

Answer 1

1. atmosphere
2. into the lung by ventilation (tidal volume)
3. into the blood by diffusion
4. heart
5. circulated to tissues in the blood
6. into the tissues by diffusion
7. into mitochondria (oxidative phosphorylation
8. PO2 reduces as travels through system
9. PCO2 increases as travels through system

Question 2

what factors could affect the pathways (or partial pressures) of O2 and CO2

Answer 2

• altitude
• temperature
• asthma
• size of lungs
• second hand smoke
• blood flow rate

Question 3

how is oxygen transported through the body

Answer 3

• 98.5 % is transported through the blood by being bound to haemoglobin in the erythrocytes (RBC)
• 1.5 % is dissolved in plasma

Question 4

how does temperature and pH affect the oxy-hemoglobin dissociation curve

Answer 4

at high temps (low pHs) hemoglobins affinity for O2 reduces in order to allow the O2 to unbind from the hemoglobin and diffuse into the tissue
- pH works the same => pH is higher in metabolically active tissues

reduction in afinity to bind to O2 = right shift of curve (= O2 unloaded)

normal pH arterial blood = 7.4, venous blood = 7.36

Question 5

how is CO2 transported through the body

Answer 5

• physically dissolved in the blood (10%)
• carboxyhemaglobin on proteins (20%)
• bicarbonate ions in the plasma (70%)

CO2 enters the RBC where it reacts with water to form carbonic acid
carbonic acid dissociates to:
- bicarbonate ions (transported to plasma)
- hydrogen ions (buffered by Hb)

Question 6

what are the driving frces of gas transport

Answer 6

• diffusion due to differences in partial pressures
• hemaglobin affinity for O2, CO2 and H+
• CO2 content of deoxygenated blood is higher than oxygenated blood
• O2 diffuses out of blood and into tissues because partial pressure of O2 is higher in the blood (high pressure to low pressure)

Question 7

what is ficks law of diffusion

Answer 7

• links together the nature of the membrane gradient and the membrane thickness
• rate of diffusion is inversely proportional to the distance over which the diffusion occurs

Question 8

how much oxygen does the air contain

Answer 8

21%

at altitude partial pressure decreases

Question 9

how do you determine oxygen tension (partial pressure)

Answer 9

determined by barometric pressure x fraction of oxygen
PO2 = Pbaro x FO2
But: water vapour pressure humidifies inspired air, reducing PO2
PO2 humidified gas: PO2 = (pbaroxPH2) x FO2

Question 10

why is PO2 lower in the alveolus than in inspired air

Answer 10

O2 and CO2 exhange occurs continuously in the alveolus

Question 11

what factors govern diffusion of gas at the tissues

Answer 11

• dissociation curve not fixed: increased heat and pH shift curve right
• the “bohr shift”: increased CO2 = decreased pH = unloading 2, 3 diphosphoglycerate (DPG) shifts curve to the right

Question 12

describe how a pulse oximeter works

Answer 12

compares infra-red and red light:
- oxygenated hemoglobin absorbs more infra-red light than red light
- deoxygenated hemoglobin absorbs more red than infra red

Question 13

what is hyperventilation? hypoventilation?

Answer 13

hyperventilation: increased alveolar ventilation that leads to REDUCED arterial pressure of CO2 (gets rid of too much CO2) =>
- decreased CO2 = decrease H+ = increased pH = respiratory alkalosis
- caused by anxiety, fear, pain, hypoxia, altitude, excessive mechanical breathing (breath too FAST)
hypoventilation: reduced alveolar ventilation leads to increased arterial pressure of CO2 (not blowing off enough CO2) =>
- increase CO2 = increased H+ = decrease pH = respiratory acidosis
- cuased by respiratory failure, obstructive respiratory diseases, depression of respiratory centres (trauma, drugs), inadequate mechanical ventilation