Question

For each item A–E related to oxygen delivery and uptake by tissues, select the most appropriate option from the list below. 

1. Total available oxygen. 

2. Cardiac output. 

3. Blood hemoglobin concentration. 

4. Arterial blood oxygen saturation. 

5. Venous blood oxygen saturation. 

6. Pulmonary arterial blood oxygen saturation. 

A. Abnormality here is the fundamental cause of hypoxic hypoxia at high altitudes. 

B. The normal value for this in the resting average adult is around one litre per minute. 

C. Abnormality here is the fundamental cause of anaemic hypoxia. 

D. Abnormality here is the fundamental cause of hypoxic hypoxia in respiratory disease. 

E. This is reduced in stagnant hypoxia, in anaemic hypoxia and also in hypoxic hypoxia.

Answer 1

A. Option 4 Arterial blood oxygen saturation. At high altitudes the alveolar oxygen pressure falls to levels at which blood passing through the lungs is inadequately saturated with oxygen. Therefore arterial blood oxygen saturation is seriously reduced and this is the fundamental cause of impaired delivery of oxygen at high altitudes. 

B. Option 1 Total available oxygen. This equals the oxygen in five litres of normally saturated blood with a normal haemoglobin content. 

C. Option 3 Blood haemoglobin concentration. Here the reduction in total available oxygen is reduced in proportion to the reduction in the blood haemoglobin concentration. 

D. Option 4 Arterial blood oxygen saturation. As with (A) the hypoxic hypoxia is due to inadequate saturation of blood in the lungs. However this time the cause is impaired lung function rather than reduced inspired oxygen. 

E. Option 1 Total available oxygen. This depends on cardiac output, blood oxygen capacity and arterial blood saturation percentage. Impaired cardiac output causes stagnant hypoxia, impaired blood oxygen capacity causes anaemic hypoxia and reduced arterial blood saturation causes hypoxic hypoxia.