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C3.1.15—Ventilation rate feedback control

Ventilation rate feedback control links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.15
Level
HL

Exam analysis

Chance of appearing8%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–6

Common command terms

  • Explain
  • State
  • Outline
  • Identify

Scoring notes

Common mistake
Treating low oxygen as the only trigger and omitting carbon-dioxide-driven pH change.

Recent exam appearances

May 2025Paper2 ["HL"] · TZ34(c)[ 2 ]C3.1.15—Ventilation rate feedback control
May 2025Paper2 ["HL"] · TZ34(b)[ 1 ]C3.1.15—Ventilation rate feedback control
May 2025Paper2 ["HL"] · TZ34(a)[ 1 ]C3.1.15—Ventilation rate feedback control
May 2025Paper2 ["HL"] · TZ210(c)[ 8 ]C3.1.15—Ventilation rate feedback control
May 2024Paper3 ["HL"] · TZ120(c)[ 2 ]C3.1.15—Ventilation rate feedback control
Practice this objective

Coverage 2014–2025 · Updated 16 Jul 2026

Ventilation feedback matches breathing to CO₂ demand

Ventilation rate is controlled by negative feedback from chemoreceptors that detect blood-pH changes linked mainly to carbon dioxide.

CO₂ dissolves and forms carbonic acid, increasing H⁺ and lowering pH. Chemoreceptors in the brainstem detect the change; the respiratory control centre sends nerve impulses to the diaphragm and intercostal muscles to alter breathing rate and depth.

Raised CO₂ → lower pH → brainstem chemoreceptor input → stronger or more frequent signals to diaphragm/intercostals → increased ventilation → more CO₂ removed → pH moves back toward normal.

During exercise, increased respiration produces extra CO₂. Ventilation rises, increasing gas exchange and limiting the fall in blood pH.

The lungs do not directly decide how to correct pH. Chemoreceptor detection and nervous output to breathing muscles coordinate the response.

Ventilation rate feedback control

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / State / Outline.

Command terms

Explain / State / Outline / Identify

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Watch for

Treating low oxygen as the only trigger and omitting carbon-dioxide-driven pH change.

Representative question

Question 1

[Maximum number: 8]

Explain the causes of a decreased blood pH and its effects on the ventilation rate in humans.

Body-System Integration

  • Nervous signals, hormones and blood transport integrate organs into coordinated systems; emergent functions arise from their interactions.
  • Sensory neurons carry receptor input to the CNS; motor neurons carry output to effectors. Mixed nerves contain both fibre types.
  • Reflex arcs provide rapid involuntary responses through sensory, relay and motor neurons. The spinal cord also links the brain and peripheral nerves.
  • Cerebral hemispheres support conscious processing; the cerebellum coordinates movement and balance; the medulla adjusts ventilation and heart activity.
  • The hypothalamus links nervous and endocrine control through the pituitary. Pineal melatonin helps time sleep; adrenal epinephrine supports acute stress responses.
  • Baroreceptors and chemoreceptors provide feedback about pressure, CO2, pH and O2.
  • The CNS controls voluntary swallowing and egestion, while the enteric nervous system coordinates gut peristalsis.

Concept essentials

  • Ventilation rate feedback control depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for ventilation rate feedback control must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give ventilation rate feedback control its biological specificity.
  • Evidence about ventilation rate feedback control is strongest when structure, mechanism and outcome are connected.