Cardiac output and control of heart rate and breathing

Cardiac output

Cardiac output = stroke volume x heart rate

• Stroke volume: volume of blood pumped by one ventricle per beat (cm³)
• Heart rate: beats per minute

At rest a typical adult might have a stroke volume of about 70 cm³ and a heart rate of about 70 beats per minute. During exercise both increase.

Minute ventilation (pulmonary ventilation) = tidal volume x breathing rate.

Controlling heart rate

The cardiovascular control centre in the medulla oblongata receives information from:

• chemoreceptors in the aorta, carotid arteries and medulla, which detect a rise in CO2 (fall in blood pH) during exercise
• baroreceptors (pressure receptors) in the aorta and carotid arteries
• stretch receptors in muscles and joints, which detect movement

To speed up the heart, impulses travel along the sympathetic (accelerator) nerve to the SAN. To slow it, impulses travel along the parasympathetic (vagus) nerve. Adrenaline also increases heart rate.

Controlling breathing

The ventilation centre in the medulla sends regular impulses to the intercostal muscles and diaphragm, causing inspiration.

• During exercise, CO2 rises and blood pH falls. Chemoreceptors detect this and send impulses to the ventilation centre, which increases the rate and depth of breathing. More CO2 is removed and more O2 taken in.
• Stretch receptors in the bronchioles are stimulated as the lungs inflate and send impulses that inhibit inspiration, so expiration follows.
• Stretch receptors in muscles detect movement and increase breathing rate at the start of exercise.

Worked example

A person has a stroke volume of 75 cm³ and a heart rate of 68 beats per minute. Calculate the cardiac output.

1. Cardiac output = 75 x 68
2. = 5100 cm³ per minute

Answer: 5100 cm³ min-1 (5.1 dm³ min-1).

Check yourself

1. Stroke volume is 110 cm³ and heart rate is 150 beats per minute during exercise. Calculate the cardiac output in cm³ min-1.
   Show answer

   16500 cm³ min-1. 110 x 150 = 16 500 cm³ min-1.

2. Cardiac output is 6000 cm³ min-1 and heart rate is 80 beats per minute. Calculate the stroke volume in cm³.
   Show answer

   75 cm³. 6000 / 80 = 75 cm³.

3. Tidal volume is 0.5 dm³ and breathing rate is 14 breaths per minute. Calculate the minute ventilation in dm³ min-1.
   Show answer

   7 dm³ min-1. 0.5 x 14 = 7 dm³ min-1.

4. Where is the cardiovascular control centre?
   • Cerebellum
   • Hypothalamus
   • Medulla oblongata
   • Right atrium
   Show answer

   Medulla oblongata. The medulla controls automatic functions such as heart rate and breathing.

5. What do chemoreceptors in the carotid arteries detect during exercise?
   • A rise in CO2 concentration and a fall in blood pH
   • A fall in body temperature
   • A rise in blood glucose only
   • Light intensity
   Show answer

   A rise in CO2 concentration and a fall in blood pH. Extra CO2 lowers blood pH.

6. Impulses along which nerve slow the heart rate?
   • Sympathetic accelerator nerve
   • Optic nerve
   • Parasympathetic (vagus) nerve
   • Motor neurone to the biceps
   Show answer

   Parasympathetic (vagus) nerve. Parasympathetic stimulation slows the SAN.

7. What is the role of stretch receptors in the bronchioles?
   • They send impulses that inhibit inspiration as the lungs inflate
   • They detect CO2
   • They make the diaphragm contract harder
   • They detect blood pressure
   Show answer

   They send impulses that inhibit inspiration as the lungs inflate. This stops over-inflation and allows expiration.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.