Cardiac output, ventilation and exerciseEdexcel International A Level Biology: Revision notes

Section 1

Cardiac output

Cardiac output is the volume of blood pumped out of each ventricle per minute.

$\text{cardiac output} = \text{heart rate} \times \text{stroke volume}$

Stroke volume is the volume of blood pumped by each ventricle in one beat.

Worked example: heart rate 70 beats per minute and stroke volume 75 cm³ give $70 \times 75 = 5250$ cm³ min⁻¹, or 5.25 dm³ min⁻¹.

During exercise, heart rate and stroke volume both rise, so cardiac output increases. Muscles then receive blood faster, so oxygen and glucose are delivered, and carbon dioxide removed, more quickly.

Key termscardiac outputstroke volumeheart rate

Common mistake

Check the units. 16 500 cm³ min⁻¹ is 16.5 dm³ min⁻¹, not 1.65.

Section 2

Control of heart rate

Heart rate is changed by the cardiovascular control centre in the medulla oblongata, which receives information from chemoreceptors in the carotid arteries and aorta (detecting a rise in CO₂ and fall in pH) and from baroreceptors (detecting blood pressure).

To increase heart rate it sends more impulses along sympathetic nerves to the SAN.
To decrease heart rate it sends more impulses along the vagus (parasympathetic) nerve to the SAN.

The heart is myogenic, so these nerves change the rate of the pacemaker rather than start the beat.

Key termscardiovascular control centremedulla oblongatachemoreceptorsympathetic nervevagus nerve

Section 3

Control of ventilation

The ventilation centre, also in the medulla oblongata, controls breathing. Chemoreceptors detect the rise in blood CO₂ (fall in pH) during exercise.

The ventilation centre sends impulses along nerves to the intercostal muscles and diaphragm.
Breathing becomes faster and deeper, so more O₂ is taken in and more CO₂ removed.

As blood CO₂ returns to normal, impulses reduce: this is negative feedback.

Key termsventilation centretidal volumebreathing rate

Section 4

Adrenaline and the fight or flight response

In stress or danger the adrenal glands release adrenaline. It:

binds to receptors on the SAN and cardiac muscle: heart rate and stroke volume increase
dilates arterioles to the muscles and constricts those to the gut and skin
dilates bronchioles, increasing ventilation
stimulates the liver to convert glycogen to glucose

This gives muscles more oxygen and glucose for rapid action.

Key termsadrenalinefight or flightadrenal gland

Section 5

Core Practical 17: spirometer traces

A spirometer chamber contains soda lime, which absorbs the CO₂ breathed out, so the volume of gas falls only because O₂ is used.

Tidal volume: the volume of air in a normal breath, from the height of a trace peak (calibrated).
Breathing rate: number of peaks per minute.
Minute ventilation = tidal volume × breathing rate.
Oxygen consumption: the volume of O₂ used per minute, from the gradient of the trace.

Worked example: tidal volume 0.50 dm³ and 12 breaths per minute give $0.50 \times 12 = 6.0$ dm³ min⁻¹. Exercise increases all four measurements.

Key termsspirometertidal volumeminute ventilationoxygen consumptionsoda lime

Exam tip

Safety and control: a clean mouthpiece, soda lime that is not used up, and a rest period between rest and exercise readings.

Must know

Cardiac output = heart rate x stroke volume
Medulla: cardiovascular control centre (sympathetic increases, vagus decreases) and ventilation centre
Chemoreceptors detect a rise in CO₂ (fall in pH)
Adrenaline: faster heart rate, vasodilation to muscles, bronchodilation, glycogen to glucose
Spirometer: minute ventilation = tidal volume x breathing rate; oxygen consumption from trace gradient

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Cardiac output, ventilation and exercise

During a fitness test, a student's heart rate and stroke volume were measured at rest and during vigorous exercise. At rest: heart rate 70 beats per minute and stroke volume 75 cm³. During vigorous exercise: heart rate 150 beats per minute and stroke volume 110 cm³.
Explain how the change in cardiac output allows the muscles to respire aerobically at a high rate during exercise.2 marks
A researcher investigates how the body responds when blood carbon dioxide concentration rises during exercise. As the muscles respire faster they release more carbon dioxide into the blood, which lowers the blood pH. Chemoreceptors in the walls of the carotid arteries and the aorta are sensitive to this change.
Explain how a rise in blood carbon dioxide concentration leads to an increase in heart rate.2 marks
A student breathes in and out of a spirometer containing soda lime, first at rest and then during vigorous exercise on a cycle ergometer. The results were: at rest, tidal volume 0.50 dm³, breathing rate 12 breaths per minute and oxygen consumption 0.30 dm³ per minute; during exercise, tidal volume 2.0 dm³, breathing rate 30 breaths per minute and oxygen consumption 2.4 dm³ per minute.
Calculate the respiratory minute ventilation at rest and during exercise, and the percentage increase on exercising.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).