Question
Fig. 4.1 shows the site of gas exchange in the mammalian lung.
Y
X
W
red blood cells
elastic fibres
Fig. 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 locations within tissue pressure 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) Scientists in Peru investigated the effect of moving from sea level to high altitude on the composition of blood. The scientists studied 10 students. Each student had lived at sea level since birth and then moved to live in the Andes at a height of 4500 metres above sea level.
The scientists took samples of blood from each student before they moved to live at high altitude. The scientists took further blood samples at intervals after the students had moved to high altitude.
The relative proportions of red blood cells and plasma in each sample were determined.
The total volume of blood for each student was also determined at each sampling time. As the students were all of different body mass, the total volumes were converted to a volume per kg of body mass (cm3 kg–1).
Fig. 4.2 shows for each sampling time:
• the mean volume of blood per kg of body mass
• the proportions of red blood cells and plasma.
120
Key
110 plasma
100 red blood
90 cells
80
mean volume 70 of blood per kg of body mass 60
/ cm3 kg–1
50
40
30
20
10
0
0 (at 1 to 2 3 to 4 5 to 6 7 to 8 12 sea level) time / months after moving to high altitude
Fig. 4.2
(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]
(ii) Describe and explain the results in Fig. 4.2.
…… [4]
[Total: 16]
Mark scheme
4(a) any four from: 2
carbaminohaemoglobin ; A combined with haemoglobin
hydrogencarbonate ions / HCO3- ;
R HCO3
CO2 dissolved in the plasma ;
4(b) any four from: 4
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(c) any four from: 4
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
4(d)(i) 50
× 100 ; 2
86
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(d)(ii) allow ‘volume of blood’ for ‘mean volume of blood per kg body mass’ 4
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