A-LevelBiologyGas exchange9700/21 · May/June 2020 · Q416 Marks

4 Fig. 4.1 shows the site of gas exchange in the mammalian lung. [Figure 4.1]

(a)State two ways in which carbon dioxide is transported in the blood. 1 2[2]
(b)Table 4.1 shows the partial pressures of oxygen (pO₂) and carbon dioxide (pCO₂) at locations W, X and Y in Fig. 4.1. Table 4.1 partial pressure | locations within tissue | W | X | Y pO₂/kPa | 13.87 | 5.33 | 13.87 pCO₂/kPa | 5.33 | 6.00 | 5.55 With reference to Fig. 4.1 and Table 4.1, describe the exchanges that occur as blood flows from X to Y.[4]
(c)Elastic fibres are present in tissues at the site of gas exchange in the lungs. Describe the roles of elastic fibres in the gas exchange system and in the cardiovascular system.[4]
(d)(i)The percentage of mean blood volume that is represented by red blood cells for the samples taken at sea level is 48%. Calculate the percentage of mean blood volume that is represented by red blood cells at 5 to 6 months after living at high altitude. Show your working and give your answer to the nearest whole number. answer = %[2]
(d)(ii)Describe and explain the results in Fig. 4.2.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[2] any four from: carbaminohaemoglobin ; A combined with haemoglobin
hydrogencarbonate ions / HCO3¯ ; R HCO3
CO2 dissolved in the plasma ;
[4] any four from: 1. correct direction of movement for oxygen and carbon dioxide ; e.g. oxygen, moves from, alveolus / W, to, capillary / blood / red blood cells, and carbon dioxide moves from, capillary/ blood /plasma to alveolus / blood to W ;
2. diffusion of, oxygen / carbon dioxide / respiratory gases ; A movement down, partial pressure / concentration, gradient A from high(er), partial pressure / concentration, to low(er), partial pressure / concentration I along a concentration gradient
3. use of data from Table 4.1 to support movement of oxygen or carbon dioxide ; needs ref. to kPa once allow ecf from mp2
4. (diffusion) through, alveolar wall / squamous epithelium ;
5. (diffusion) through, endothelium / capillary wall ;
6. AVP; e.g. haemoglobin binding oxygen
[4] any four from: alveoli / alveolar walls, stretch / expand, during, inspiration / AW ;
arteries / arterioles / blood vessels, stretch / expand, as blood, volume / pressure, increases ;
stretch to prevent (alveoli / arteries), bursting / rupture / AW ; A prevent overstretching
recoil during expiration / AW, to help, expel / force out, air; AW in context of alveoli
recoil to apply pressure to blood so maintaining blood pressure ; in context of arteries
AVP; e.g. allows trachea / bronchus / bronchioles, to expand during inspiration
[2] 50/86 × 100;
58(%); if extracted values are different allow ecf for calculated answer accept ± 1 mm when reading from graph (49 to 51 and 85 to 87)
[4] allow 'volume of blood' for 'mean volume of blood per kg body mass'
any four from: description to max 3
1. trend is / overall / AW, increase in volume of blood, over time / AW or increase in volume of blood over time except between 0 and 1-2 months ;
2. trend is / overall / AW, increase in red blood cell volume, over time / AW ; or increase in red cell volume over time except between 7–8 and 12 months ;
3. ref. to percentage of mean blood volume represented by red blood cells / haematocrit; e.g. fluctuates / mainly increasing / decreases between 1–2 and 3–4months / decreases between 7-8 and 12 months ;
4. any comparative use of, figures / percentages ; to support mp1, mp2 or mp3
explanation to max 3
5. lower partial pressure of oxygen at high altitude / less oxygen in inhaled air;
6. lower oxygen saturation of haemoglobin / haemoglobin has lower oxygen affinity;
7. compensation or idea that changes occur to make up for less, oxygen taken in / transported to tissues R idea of body getting more oxygen than at sea level
8. (over time) more red blood cells produced so more haemoglobin / AW ;
9. (over time) proportion of red cells / concentration of haemoglobin, increases so, take up / AW, greater volume of oxygen; A more red blood cells per unit time passes through, lungs to take up oxygen / tissues to deliver oxygen
10. AVP; e.g. ref. to erythropoietin / EPO