Oxygen and Carbon Dioxide Transport

Question 1

Give the equation for the reaction between one O2 and one heme unit.

Answer 1

O2 + Hb ⇄ HbO2

Question 2

What is PaCO2?

Answer 2

Partial pressure of arterial CO2

Question 3

What is PACO2?

Answer 3

Partial pressure of alveolar CO2

Question 4

How many oxygen molecules can one molecule of haemoglobin hold?

Answer 4

4

Question 5

When does haemoglobin dissociate oxygen?

Answer 5

In areas of low PO2 i.e. metabolically active tissue

Question 6

What effect does an increase in temperature have on the oxygen dissociation curve?

Answer 6

Shifts curve to the right. At any given PO2, haemoglobin will have a lower affinity for oxygen. This is because the haemoglobin binding sites become distorted.

Same effect is achieved by a decrease in PH (increase in PCO2). Clever, as it means more oxygen is dissociated by haemoglobin.

Question 7

What effect does an decrease in temperature have on the oxygen dissociation curve?

Answer 7

Shifts curve to the left. At any given PO2, haemoglobin will have a higher affinity for oxygen.

Lower acidity (higher PH) also has this effect.

Question 8

How does CO affinity for oxygen compare to haemoglobin?

Answer 8

CO has an affinity 200x greater for oxygen than haemoglobin. It competes with haemoglobin for oxygen binding bites.

Question 9

What effect does CO have on the oxygen dissociation curve?

Answer 9

Shifts curve to the left. This decreases dissociation of O2 from haemoglobin into the tissues.

Question 10

Name the three ways in which CO2 is carried in the blood and give equations.

Answer 10

1. Bound to haemoglobin.
   CO2 + Hb ⇄ HbCO2
2. CO2 is dissolved in plasma
3. Travels as HCO3- (most CO2 is carried in blood this way).
   CO2 + H2O ⇄ H2CO3 ⇄ HCO3- + H+
   Carbonic anhydrase is needed for the first part of the reaction.

Question 11

What is hypoxia?

Answer 11

Deficiency of oxygen at the tissue level

Question 12

Name the common causes of hypoxia.

Answer 12

1. Hypoventilation
2. Diffusion impairment
3. Shunting
4. Ventilation-perfusion mismatch

Question 13

Describe how hypoventilation leads to hypoxia.

Answer 13

• increased PaCO2
• failure to ventilate alveoli adequately
• caused by muscular weakness, loss of respiratory drive, obesity

Question 14

Describe how diffusion impairment leads to hypoxia.

Answer 14

• PaO2 and PAO2 are not equilibrated
• thickening of alveolar membranes
• caused by pulmonary oedema and interstitial fibrosis

Question 15

Describe how shunting leads to hypoxia.

Answer 15

• one cause is mixed venous blood bypassing ventilated alveoli, passing from right side of heart to left side e.g. ventricular septal defect
• one cause is where mixed venous blood perfuses unventilated alveoli - an intrapulmonary defect

Question 16

Describe how ventilation-perfusion mismatch leads to hypoxia.

Answer 16

-caused by pulmonary embolism, asthma, pneumonia + pulmonary oedema - essentially blockages and obstructions of airways

Question 17

What is hypercapnia?

Answer 17

CO2 retention, increased PaCO2

Question 18

What causes hypercapnia?

Answer 18

Hypoventilation.

Question 19

Describe the features of Type 1 respiratory failure in terms of partial pressures.

Answer 19

pO2 is low

pCO2 is low or normal

Question 20

What is the most common cause of Type 1 respiratory failure?

Answer 20

Pulmonary embolism

Question 21

Describe the features of Type 2 respiratory failure in terms of partial pressures.

Answer 21

pO2 is low

PCO2 is high

Question 22

What causes Type 2 respiratory failure?

Answer 22

Hypoventilation