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Ventilation graphs and measurements

Joe Wolfensohn

Teacher

Joe Wolfensohn

Contents

What is Ventilation?

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Ventilation Graphs

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Recall Questions

This topic requires prior knowledge of lung volumes and the mechanics of ventilation. You can test your knowledge on these below.

What happens to the diaphragm to cause inhalation?

  • The diaphragm contracts and flattens/ moves down

  • Helping to increase the volume in the thoracic cavity

How do alveoli in the lungs provide an efficient surface for gas exchange?

  • Large surface area (millions of alveoli).

  • Thin walls (one single layer of epithelial cells) for short diffusion distance.

  • Rich blood supply and continuous circulation to maintain the concentration gradient.

What happens to the intercostal muscles during forced expiration?

  • External intercostal muscles relax

  • Internal intercostal muscles contract.

  • This causes the ribs to move further down and in

  • Decreasing the volume in the thorax even further.

Topic Explainer Video

Check out this @JoeDoesBiology video that explains ventilation graphs and measurements or read the full notes below. Once you've gone through the whole note, try out the practice questions!

What is Ventilation?

Ventilation is the mechanical process of breathing, which includes inspiration (inhalation) and expiration (exhalation). It maintains oxygen supply and removes carbon dioxide via diffusion in the alveoli.

 

Key Measurements in Ventilation

Term

Definition

Typical Values

Tidal Volume 

Volume of air inhaled or exhaled in a normal breath

0.5 dm³ (500 cm³)

Breathing Rate 

Number of breaths taken per minute

12–20 breaths per minute

Ventilation Rate

Total volume of air moved in & out per minute

6–10 dm³ min⁻¹

Residual Volume 

Volume of air that remains in the lungs after forced expiration

Around 1.2 dm³

Total Lung Capacity 

Maximum volume of air the lungs can hold

Around 6.0 dm³

Vital Capacity 

Maximum volume of air a person can forcibly exhale after maximum inhalation

Around 4.8 dm³

 

Ventilation Graphs

What do ventilation graphs show?

  • A ventilation graph plots lung volume (y-axis) over time (x-axis).

  • Peaks represent inhalation, and troughs represent exhalation.

How to Calculate Key Measurements from a Ventilation Graph

  1. Breathing Rate

    • Count the number of peaks in one minute (or count over a few seconds and use the below formula).

    • Formula: Breathing Rate= (number of breaths / time in seconds) ​× 60 

  2. Tidal Volume 

    • Measure the height of one normal breath cycle (difference between peak and trough). 

    • In the graph above we could calculate tidal volume by reading off the peak volume (1.32) and subtracting the trough volume (0.08)

    • 1.32 - 0.08 = 1.24 dm³

  3. Ventilation Rate 

    • Formula: Ventilation Rate = Tidal Volume × Breathing Rate

    • Example: If Tidal Volume = 0.5 dm³ and breathing rate = 12 breaths per min: 0.5 x 12 = 6.0 dm³ min-1

Key Terms

  • Tidal Volume: Volume of air inhaled/exhaled in a normal breath.

  • Breathing Rate: Number of breaths per minute.

  • Ventilation Rate: Volume of air moved in/out of the lungs per minute.

  • Vital Capacity: Maximum air exhaled after deep inhalation.

  • Residual Volume: Air remaining in lungs after forced exhalation.

No answer provided.

Exam Tip

Always include units when calculating tidal volume, ventilation rate, or breathing rate.
In graph-based questions, show your working as it may be worth marks even if your final answer is wrong!

No answer provided.

The graph below shows ventilation over 120 seconds. Calculate the breathing rate and ventilation rate when breathing normally. (2 marks)

  1. Breathing Rate Calculation:

    • Number of breaths in 15 seconds = 3.

    • Breathing rate = (3 / 15)×60=12 breaths per minute (1 mark)

  2. Ventilation Rate Calculation:

    • Ventilation Rate = Tidal Volume × Breathing Rate

    • 0.4 dm³ × 12 = 4.8 dm³ min⁻¹
      (1 mark)

Calculate the breathing rate and ventilation rate when exercising intensely. (2 marks)

  1. Breathing Rate Calculation:

    • Number of breaths in 15 seconds = 4.

    • Breathing rate = (4 / 15)×60=16 breaths per minute (1 mark)

  2. Ventilation Rate Calculation:

    • Ventilation Rate = Tidal Volume × Breathing Rate

    • Tidal volume = 4.1- 2.3 = 1.8 dm³

    • 1.8 dm³ × 16 = 28.8 dm³ min⁻¹
      (1 mark)

Practice Question

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!

Ventilation