AS · Practice questions · Transport of oxygen and carbon dioxide

Read the curve, then trace the carbon dioxide.

Six original Cambridge-style questions on gas transport in blood, from reading saturation off a dissociation curve to explaining the chloride shift.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Curve, shift, then ions.

Sigmoid curve: cooperative binding of four O₂ per haemoglobin.
Lungs: high pO₂, near full saturation. Tissues: steep part, big unloading.
Bohr shift: more CO₂, curve to the right, lower affinity, more O₂ released.
CO₂: carbonic anhydrase, H⁺ to HHb, HCO₃^- out, Cl^- in.

Recall

[3 marks]

State three ways in which carbon dioxide is transported in the blood, and where each is carried.

As hydrogencarbonate ions (most, about 85%), carried in solution in the plasma. ✓
As carbaminohaemoglobin (about 10%), CO₂ bound to haemoglobin in red blood cells. ✓
Dissolved as CO₂ in the plasma (about 5%). ✓

Explain

[5 marks]

Describe the role of red blood cells in the transport of carbon dioxide from respiring tissues, including the chloride shift.

CO₂ diffuses from respiring cells into the plasma and into red blood cells. ✓
Carbonic anhydrase catalyses the reaction of CO₂ with water to form carbonic acid, which dissociates into H⁺ and HCO₃^-. ✓
H⁺ combines with haemoglobin to form haemoglobinic acid, buffering the pH and causing haemoglobin to release oxygen. ✓
HCO₃^- diffuses out of the red blood cell into the plasma. ✓
Cl^- ions diffuse in to maintain electrical neutrality: the chloride shift. Some CO₂ also binds haemoglobin to form carbaminohaemoglobin. ✓

Explain

[3 marks]

Explain the importance of the chloride shift.

As negatively charged HCO₃^- ions leave the red blood cell, Cl^- ions enter to balance the charge. ✓
This keeps the red blood cell (and plasma) electrically neutral. ✓
It allows HCO₃^- to keep leaving, so HCO₃^- does not build up and carbonic anhydrase can keep converting CO₂, letting large amounts of CO₂ be carried in the plasma. ✓

Explain

[4 marks]

Explain the importance of the shape of the oxygen dissociation curve at the partial pressures of oxygen found in the lungs and in respiring tissues.

In the lungs pO₂ is high, where the curve is flat near the top, so haemoglobin becomes almost fully saturated. ✓
Small changes in pO₂ in the lungs hardly reduce loading. ✓
In respiring tissues pO₂ is low, on the steep part of the curve, so a small fall in pO₂ causes a large fall in saturation. ✓
So a large amount of oxygen is unloaded where it is needed, and more in very active tissues with lower pO₂. ✓

Analysis

[3 marks]

Describe the Bohr shift and explain its importance during exercise.

High carbon dioxide concentration (more H⁺, lower pH) lowers the affinity of haemoglobin for oxygen. ✓
The dissociation curve moves to the right, so at any pO₂ the saturation is lower. ✓
During exercise muscles respire faster and make more CO₂, so more oxygen is unloaded to the muscles that need it. ✓

Calculation

[3 marks]

Blood leaves the lungs 97% saturated. In resting muscle (pO₂ 5 kPa) it is 75% saturated; in exercising muscle with high CO₂ (pO₂ 2.5 kPa) it is 20% saturated. Fully saturated blood carries 20 cm³ O₂ per 100 cm³. Calculate the oxygen unloaded per 100 cm³ of blood in each muscle.

Resting: (97 - 75)% of 20 = 0.22 × 20 = 4.4 cm³ per 100 cm³. ✓
Exercising: (97 - 20)% of 20 = 0.77 × 20 = 15.4 cm³ per 100 cm³. ✓
So exercising muscle receives 3.5 times as much oxygen, because of the lower pO₂ and the Bohr shift. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.