Data analysis: exchange and transport

Question 1

The alveoli are lined with a thin film of water. Surface tension where this water meets the air tends to make the alveoli collapse. Cells in the alveolar walls secrete surfactant, a mixture that consists mainly of phospholipids. Surfactant forms a layer at the surface between the air and the water and reduces surface tension. Babies born very early often produce too little surfactant.

Doctors randomly allocated 260 babies born very early to two groups. Within an hour of birth, babies in one group were given surfactant through a tube into the trachea; babies in the other group were given a small volume of air in the same way. The table shows the results.

Calculate the percentage reduction in the death rate (deaths per baby) in the surfactant group compared with the control group. Give your answer to 2 significant figures.

Use your knowledge of the structure of phospholipids to explain how they form a layer at the surface between air and water.

The doctors concluded that giving surfactant improves the survival of babies born very early. Evaluate this conclusion.

Accepted Answer

Death rate with surfactant = 14 ÷ 140 = 0.10 (10%)
Death rate in control group = 30 ÷ 120 = 0.25 (25%)
Percentage reduction = (0.25 - 0.10) ÷ 0.25 × 100 = 60%

The phosphate head of a phospholipid is hydrophilic, so it stays in the water, while the two fatty acid tails are hydrophobic and point away from the water into the air. The phospholipids therefore form a single layer at the surface.

The death rate was less than half that of the control group (10% compared with 25%) and a smaller proportion needed a ventilator (30% compared with 60%), and the random allocation and the matching procedure in the control group make it likely that the surfactant caused the difference. However, no statistical test is given, so the difference may be due to chance, and only survival to 28 days was recorded.

Question 2

In some countries, food is cooked on open wood fires inside homes, and the smoke contains small particles and carbon monoxide. Scientists carried out a trial in one rural region. They randomly allocated homes with a child under 18 months old to two groups. One group was given a wood-burning stove with a chimney that takes smoke out of the house; the other group continued to use open fires. Over the next 18 months, doctors recorded every case of pneumonia, a lung infection, in the children. The scientists also measured carbon monoxide in each child's exhaled breath as a measure of exposure to smoke.

The table shows their results. A child-year is one child observed for one year. A statistical test showed that the difference in the rate of pneumonia was not significant (P = 0.09).

Calculate the number of cases of pneumonia per 100 child-years for each group.

In pneumonia, some alveoli fill with fluid and white blood cells. Explain why this reduces the uptake of oxygen into the blood.

The scientists concluded that replacing open fires with chimney stoves reduces pneumonia in young children. Evaluate this conclusion.

Accepted Answer

Chimney stove: 159 ÷ 530 × 100 = 30 cases per 100 child-years
Open fire: 189 ÷ 525 × 100 = 36 cases per 100 child-years

When alveoli fill with fluid, fewer alveoli are ventilated, so the surface area for gas exchange is smaller, and the fluid increases the diffusion distance between the air and the blood. Less oxygen diffuses into the blood.

The rate of pneumonia was lower with chimney stoves (30 compared with 36 per 100 child-years), exposure to smoke was almost halved (carbon monoxide 2.4 compared with 4.6 ppm), and the homes were allocated at random. However, the difference was not significant (P = 0.09), so it may be due to chance. The families and doctors knew which group each child was in, and the children with stoves were still exposed to some smoke, so the conclusion is not fully supported.

Question 3

Scientists pumped solutions through short sections of the ileum of anaesthetised rats. They measured the rate at which water and glucose were absorbed from each solution. The solutions contained: A, sodium chloride; B, sodium chloride and glucose; C, glucose but no sodium ions; D, sodium chloride, glucose and an inhibitor of the sodium-potassium pump. All four solutions had the same water potential; where needed, a sugar that is not absorbed was added. The table shows the results.

Calculate the percentage increase in the rate of water absorption when glucose was added to the sodium chloride solution.

Explain why all four solutions had the same water potential.

Use your knowledge of co-transport to explain the results for solutions C and D.

Oral rehydration solutions, containing glucose and sodium chloride, are given to children who are losing water because of severe diarrhoea. Evaluate the use of these data to support this treatment.

Accepted Answer

Percentage increase = (85 - 20) ÷ 20 × 100 = 325%

The solutions had the same water potential so that any difference in water absorption was caused by the solutes being absorbed, not by different water potential gradients between the solutions and the cells.

Glucose enters the epithelial cells together with sodium ions through a co-transport protein. In solution C there are no sodium ions, so very little glucose is absorbed. The sodium-potassium pump normally uses ATP to transport sodium ions out of the epithelial cells into the blood, keeping their concentration in the cells low. In solution D the pump is inhibited, so there is no sodium ion gradient into the cells and co-transport of glucose almost stops. In both C and D little solute is absorbed, so the water potential of the cells and blood is not lowered and little water follows by osmosis.

The data support the treatment, because sodium chloride with glucose (B) gave about four times as much water absorption as sodium chloride alone. However, these were healthy, anaesthetised rats, not children with diarrhoea, and only one glucose concentration was tested; too much glucose could draw water into the gut by osmosis.

Question 4

A snorkel is a tube that lets a swimmer breathe with their face under water. After each breath out, the airways and the snorkel are left full of air that has just been breathed out, and this air is the first to be breathed in again. The volume of these tubes is called the dead space.

Scientists investigated the effect of breathing through snorkels with different internal volumes. Twelve adult volunteers sat at rest and breathed through each snorkel, in a random order. The scientists made their measurements after each volunteer had breathed through a snorkel for 10 minutes. The table shows the mean results.

Alveolar ventilation (the volume of fresh air reaching the alveoli each minute) = (tidal volume - dead space) × breathing rate, where the dead space is 150 cm³ plus the volume of the snorkel.

Calculate the alveolar ventilation with the 450 cm³ snorkel. Give your answer in dm³ min⁻¹.

Explain why the concentration of carbon dioxide in the alveolar air increased as the volume of the snorkel increased.

Suggest why the scientists waited until each volunteer had breathed through a snorkel for 10 minutes before making their measurements.

A safety organisation recommends that snorkels for children should have a volume of no more than 150 cm³. Use the data and your own knowledge to evaluate this recommendation.

Accepted Answer

Dead space with the 450 cm³ snorkel = 150 + 450 = 600 cm³
Alveolar ventilation = (850 - 600) × 15 = 3750 cm³ min⁻¹ = 3.75 dm³ min⁻¹

The first air breathed in comes from the snorkel and has just been breathed out, so it has a high concentration of carbon dioxide. Although the volunteers breathed more deeply and faster, less fresh air reached the alveoli each minute (3.75 dm³ min⁻¹ compared with 4.2 dm³ min⁻¹ without a snorkel), so carbon dioxide was removed from the alveoli more slowly than it diffused in from the blood.

The scientists waited 10 minutes so that each volunteer's breathing, and the concentration of carbon dioxide in the alveolar air, had time to adjust to the new snorkel and become steady.

For adults the data support the limit: the 150 cm³ snorkel reduced alveolar ventilation by only about 1% (4.2 to 4.16 dm³ min⁻¹) and raised carbon dioxide only from 5.3% to 5.4%, while larger snorkels had much bigger effects. However, the volunteers were adults at rest. Children have smaller tidal volumes, so 150 cm³ would be a larger fraction of each breath and could reduce their alveolar ventilation much more, and swimming increases the demand for oxygen. Only 12 volunteers were tested and no statistical test was used, so the limit for children may need to be lower.

Sample questions with model answers

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Question 2

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Question 3

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Question 4

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