For each item A–E related to oxygen transport, select the most appropriate option from the list below: 1. Sigmoid. 2. Plateau.

Question

For each item A–E related to oxygen transport, select the most appropriate option from the list below:

1. Sigmoid.

2. Plateau.

3. Oxygen in solution.

4. Percentage saturation.

5. Oxygen content, ml oxygen per

6. Shift of the curve to the right. 5. litre of blood.

7. Shift of the curve to the left.

A. When the y axis of the oxygen dissociation curve of blood is labelled thus, the curve for severe anaemia is very similar to that for normal blood.

B. The form in which oxygen passes from blood to tissues.

C. This leads to proportionately greater release of oxygen to the tissues.

D. A property of the oxygen dissociation curve which is related to the varying affinities of hemoglobin for oxygen particularly at very low oxygen pressures and at the pressures around the normal range for alveolar oxygen pressures.

E. The reason the ‘plateau’ phase of the oxygen dissociation curve is not completely horizontal.

Answer 1

A. Option 4 Percentage saturation. Regardless of the hemoglobin content, the oxygen content of blood at a given oxygen pressure will always be very similar, expressed as a percentage of the capacity. The very slight differences are due to the fact that in severe anaemia the amount of dissolved oxygen relative to oxygen combined with hemoglobin is slightly greater since there is a higher proportion of plasma.

B. Option 3 Oxygen in solution. Only dissolved gases can pass through the tissue fluids.

C. Option 6 Shift of the curve to the right. When the oxygen dissociation curve of blood shifts to the right on the x axis, the oxygen content of blood at a given oxygen pressure shifts downwards on the y axis. Thus the blood retains less oxygen and more is released to the tissues.

D. Option 1 Sigmoid. Sigmoid means -shaped. The first bend of the is produced by the increasing affinity of hemoglobin for oxygen; at low pressures the curve becomes increasingly steep and then straightens to a steep fairly linear rise. The second bend is produced as full saturation is approached and the steep rise curves to join the plateau.

E. Option 3 Dissolved oxygen. As the partial pressure of oxygen increases above the normal alveolar value, the content rises very gradually as the dissolved oxygen increases in proportion to the pressure. In hyperbaric oxygen conditions this dissolved amount can be significant. For example if 3 ml are dissolved per litre when PO2 is 100 mmHg (13.5 kPa), then at two atmospheres of oxygen, with a PO2 around 1500 mmHg (200 kPa), the dissolved oxygen amounts to 45 ml/litre. This can be helpful when hemoglobin has been disabled by combination with carbon monoxide.