Nervous control of breathing – Gas Exchange in Animals and Plants (Senior 5 Biology) | Ganzaa

Gas Exchange in Animals and Plants

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Senior 5 · Biology

Gas Exchange in Animals and Plants

5 lessons

Lesson 3 of 5

Nervous control of breathing

20 min

Why can't you hold your breath for long?

Take a breath and hold it. Within a minute or so the urge to breathe becomes overwhelming, although your blood still holds plenty of oxygen. What forces you to breathe is mainly the rising carbon dioxide. Breathing is controlled automatically by the brain, using information from sensors in the brain and in the large arteries. You can override it for a short time, when you speak, sing or swim under water, but not for long. This lesson explains how the control works, and how it copes with exercise and with high places such as the slopes of Karisimbi.

Definition

Respiratory centre: groups of neurones in the medulla oblongata of the brainstem (with the pons) that set the rhythm and depth of breathing.

Chemoreceptor: a receptor that detects chemical changes. Central chemoreceptors in the medulla respond to the hydrogen ion concentration (pH) of the cerebrospinal fluid; peripheral chemoreceptors in the carotid bodies and aortic bodies respond to carbon dioxide, hydrogen ions and a large fall in oxygen in the blood.

Stretch receptors: receptors in the walls of the bronchioles that are stimulated as the lungs inflate.

Negative feedback: a change from a set level triggers a response that reverses the change.

The basic rhythm

The inspiratory centre in the medulla sends bursts of impulses along the phrenic nerves to the diaphragm and the intercostal nerves to the external intercostal muscles. They contract, and you breathe in.
As the lungs inflate, stretch receptors in the bronchiole walls send impulses along the vagus nerve to the medulla. These inhibit the inspiratory centre.
The muscles relax, and expiration follows passively by elastic recoil. When the stretch receptors stop firing, inspiration begins again.

This stretch reflex (the Hering–Breuer reflex) also protects the lungs from over-inflation. During exercise, an expiratory centre also sends impulses to the internal intercostal and abdominal muscles, for forced expiration.

Adjusting breathing to the body's needs

Carbon dioxide dissolves in blood plasma and forms carbonic acid, which releases hydrogen ions: $\mathrm{CO_2} + \mathrm{H_2O} \rightleftharpoons \mathrm{H_2CO_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO_3^-}$ So more carbon dioxide means a lower pH. Carbon dioxide also crosses easily into the cerebrospinal fluid around the brain, where it lowers the pH sensed by the central chemoreceptors.

The chemoreceptors send impulses to the respiratory centre, which increases the rate and depth of breathing. More carbon dioxide is breathed out, the blood pH rises back to normal, and the stimulus is removed: a negative feedback loop (Figure 5). Arterial carbon dioxide is held remarkably steady this way.

Oxygen has a smaller role. The peripheral chemoreceptors respond to oxygen only when it falls a long way, as at high altitude or in lung disease. Figure 6 shows how strongly the body responds to carbon dioxide.

Exercise

Ventilation rises at the very start of exercise, before blood carbon dioxide has had time to change. Impulses from the motor cortex of the brain and from stretch receptors in working muscles and joints stimulate the respiratory centre directly. As exercise continues, extra carbon dioxide and hydrogen ions from respiring muscles keep ventilation high, and adrenaline adds to the effect. After exercise, breathing stays deep for a while as the body repays the oxygen debt and removes lactate.

Worked example 1: oxygen at altitude

Air is about 21% oxygen at every altitude, but the total pressure falls as you climb. The partial pressure of oxygen is $p_{\mathrm{O_2}} = 0.21 \times \text{total pressure}$ . Using approximate standard-atmosphere pressures, calculate $p_{\mathrm{O_2}}$ at sea level (101 kPa), at about 1 500 m, roughly the height of Kigali (85 kPa), and at the summit of Karisimbi, 4 507 m (58 kPa).

Sea level: $0.21 \times 101 = 21.2\,\mathrm{kPa}$ .
About 1 500 m: $0.21 \times 85 = 17.9\,\mathrm{kPa}$ , about 84% of the sea-level value.
Karisimbi summit: $0.21 \times 58 = 12.2\,\mathrm{kPa}$ , about 58% of the sea-level value.

Effect: at the summit the peripheral chemoreceptors detect the low oxygen, and ventilation rises. Over days, the kidneys and bone marrow help the body adapt, for example by making more red blood cells.

Worked example 2: which stimulus matters more?

Illustrative results for volunteers at rest:

Air breathed	Pulmonary ventilation / dm³ min⁻¹
Normal air (21% O₂, 0.04% CO₂)	6
Low oxygen: 15% O₂, no added CO₂	6.5
Normal oxygen with 5% CO₂	28

Lowering oxygen from 21% to 15% raises ventilation by $\frac{6.5 - 6}{6} \times 100 = 8\%$ .
Adding 5% carbon dioxide raises it by $\frac{28 - 6}{6} \times 100 = 367\%$ , more than four and a half times the resting value.

Conclusion: a moderate rise in carbon dioxide is a far stronger stimulus to breathing than a moderate fall in oxygen. Oxygen becomes important only when it falls a long way.

Warning

Common mistakes and a safety warning

"Low oxygen is the main stimulus for breathing." In healthy people at low altitude, the main stimulus is carbon dioxide, acting through the pH of the blood and cerebrospinal fluid.
"The respiratory centre is in the lungs." It is in the medulla oblongata of the brain.
"Expiration is controlled by the diaphragm contracting." At rest, expiration is passive; the diaphragm relaxes.
Safety: never breathe very fast and deeply (hyperventilate) before swimming under water. It lowers your carbon dioxide, so the urge to breathe comes late, and a swimmer can run out of oxygen and lose consciousness under water without warning.

Alila Medical Media: "Control of Ventilation (Respiration), Animation". Watch for the respiratory centres in the brainstem, the central and peripheral chemoreceptors, and why carbon dioxide is the main signal; compare it with Figure 5.

Tip

Exam technique: describe any control system as a chain: stimulus (CO₂ rises, pH falls) → receptor (chemoreceptors) → coordinator (respiratory centre in the medulla) → effectors (diaphragm and intercostals) → response (deeper, faster breathing) → feedback (CO₂ falls). The same chain will serve you for blood glucose and temperature in Unit 9.

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Practice: nervous control of breathing

Where is the respiratory centre, and which nerves carry its impulses to the breathing muscles?
Describe the role of stretch receptors in the rhythm of breathing.
Explain how a rise in carbon dioxide in the blood leads to a fall in the pH of the cerebrospinal fluid.
Using Figure 6, by how many times does ventilation increase between 0% and 6% carbon dioxide?
Calculate the partial pressure of oxygen at a place where the air pressure is 70 kPa.
Explain why breathing rate rises at the start of a race, before blood carbon dioxide has changed.
A patient's breathing is driven mainly by low oxygen after years of lung disease. Suggest why giving them very high concentrations of oxygen must be done with care.
Explain why hyperventilating before an underwater swim is dangerous.

Show answersHide answers

In the medulla oblongata (with the pons). Phrenic nerves to the diaphragm; intercostal nerves to the intercostal muscles.
As the lungs inflate, stretch receptors in the bronchioles send impulses along the vagus nerve that inhibit the inspiratory centre, so inspiration stops and expiration follows; this also prevents over-inflation.
CO₂ diffuses from the blood into the cerebrospinal fluid, where it reacts with water to form carbonic acid, which dissociates into H⁺ and hydrogencarbonate ions; more H⁺ means a lower pH.
$\frac{40}{6} \approx 6.7$ times.
$0.21 \times 70 = 14.7\,\mathrm{kPa}$ .
Impulses from the motor cortex and from stretch receptors in the working muscles and joints stimulate the respiratory centre directly.
If low oxygen is their main stimulus to breathe, removing it may reduce their breathing, so carbon dioxide can build up to dangerous levels; oxygen should be given at a controlled concentration by trained health workers.
It lowers blood carbon dioxide, so the urge to breathe is delayed; oxygen can fall low enough to cause loss of consciousness under water before the swimmer feels the need to surface.

Key points

The respiratory centre in the medulla sets the rhythm through the phrenic and intercostal nerves; stretch receptors and the vagus nerve end each breath in.
Carbon dioxide, acting through pH, is the main chemical stimulus; central and peripheral chemoreceptors detect it.
Breathing is controlled by negative feedback; low oxygen matters only when it falls a long way, as at high altitude.
In exercise, the motor cortex and muscle receptors raise ventilation at once; CO₂ and adrenaline sustain it.

Sources and credits

Written by Ganzaa. Facts were checked against 39.3 Breathing, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax, licensed under CC BY 4.0. No text was copied.
Photo: Karisimbi, SIBOMANA Constantin, CC BY-SA 4.0, via Wikimedia Commons.
Video: Control of Ventilation (Respiration), Animation, Alila Medical Media, embedded from YouTube.

Figures 5 and 6, the table, the worked examples and the practice questions are by Ganzaa. The values in Figure 6 and Worked example 2 are illustrative; the air pressures in Worked example 1 are approximate standard-atmosphere values, and real pressures vary with the weather.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why can't you hold your breath for long?

2The basic rhythm

3Adjusting breathing to the body's needs

Exercise

Why can't you hold your breath for long?

The basic rhythm

Adjusting breathing to the body's needs