In an integrated question, breathing, blood flow and respiration are treated as one chain: cells need oxygen and make carbon dioxide, the lungs exchange gases with the blood, and the heart moves the blood between them. If you can state each link in order, most scenario questions become manageable.
This skill belongs to integrated biological reasoning. It appears in Co-ordinated Sciences papers whenever data about exercise, altitude or illness is supplied. Check your own syllabus year on the Cambridge subject page for the exact wording.
How do the three ideas fit together?
Start from the cell, not the lungs. Respiring muscle cells use oxygen and release carbon dioxide. Both gases move down concentration gradients, so oxygen moves from blood into cells and carbon dioxide moves from cells into blood.
The blood carries both gases. Carbon dioxide arriving at the lungs diffuses out into the alveoli, and oxygen diffuses in. Breathing keeps those gradients steep by replacing the air.
So when the cells work harder, three things follow: the breathing rate and depth rise, the heart rate rises, and the blood moves faster. Each one answers the same demand.
Calculating the change from supplied data
Two formulas are useful:
- ventilation rate = breaths per minute × volume per breath
- cardiac output = heart rate × stroke volume (volume pumped per beat)
Worked example (invented data)
A student’s figures at rest and after running on the spot are below. The numbers are invented for this lesson.
| Rest | After exercise | |
|---|---|---|
| Breaths per minute | 14 | 24 |
| Volume per breath (dm³) | 0.50 | 1.00 |
| Heart rate (beats per minute) | 70 | 150 |
| Stroke volume (cm³) | 70 | 100 |
Step 1, ventilation at rest: 14 × 0.50 = 7.0 dm³ per minute.
Step 2, ventilation after exercise: 24 × 1.00 = 24 dm³ per minute.
Step 3, compare: 24 ÷ 7.0 = 3.43, so ventilation rose by a factor of about 3.4.
Step 4, cardiac output at rest: 70 × 70 = 4900 cm³ per minute, which is 4.9 dm³ per minute.
Step 5, cardiac output after exercise: 150 × 100 = 15 000 cm³ per minute, which is 15.0 dm³ per minute.
Step 6, compare: 15.0 ÷ 4.9 = 3.06, so cardiac output rose by a factor of about 3.1.
Step 7, explain: the muscles respire faster, so they need more oxygen. Breathing rises by about 3.4 times and blood flow by about 3.1 times, so both oxygen supply and carbon dioxide removal increase together.
The mistake to watch for
Mistaken answer: “The heart beats faster so that the lungs can make more oxygen.”
The lungs do not make oxygen. They exchange gases between air and blood.
The heart does not feed the lungs either. The correct link is that blood carries oxygen from the lungs to the respiring muscles, so a faster heart delivers it sooner.
A second slip is mixing units. Stroke volume is in cm³ and ventilation is in dm³. Convert (1 dm³ = 1000 cm³) before comparing the two.
Check yourself
1. A person takes 16 breaths per minute, each of 0.45 dm³. Calculate the ventilation rate.
Show answer
16 × 0.45 = 7.2 dm³ per minute.
2. A heart beats 80 times per minute with a stroke volume of 60 cm³. Calculate the cardiac output in dm³ per minute.
Show answer
80 × 60 = 4800 cm³ per minute. Divide by 1000: 4.8 dm³ per minute.
3. Explain why the carbon dioxide concentration in the blood leaving working muscle is higher than in the blood arriving.
Show answer
Muscle cells release carbon dioxide from respiration. It diffuses from the cells, where the concentration is higher, into the blood, where it is lower, so the blood leaving the muscle carries more.
Where this leads next
Next, see how an enzyme pattern affects a larger process. When you want mixed questions, use the integrated biological reasoning practice set, and the scientific investigation critic helps you check your reasoning.
A student who knows the facts but loses marks joining them is the situation our teachers work on in online one-to-one Co-ordinated Sciences tuition.