Why You Breathe Faster When Carbon Dioxide Rises, Not When Oxygen Falls
Learn the human respiratory system and how gases are exchanged in the alveoli, how oxygen and carbon dioxide travel in blood, respiratory volumes, capacities and the control of breathing, and disorders of the respiratory system.
What happens to the air you breathe in?
You breathe in and out many times a minute without thinking about it. Each breath carries oxygen deep into the lungs, where it crosses into the blood, while carbon dioxide made by your cells moves the other way.
This lesson covers the respiratory system and gas exchange, the transport of oxygen and carbon dioxide, lung volumes and the control of breathing, and respiratory disorders.
This lesson covers the respiratory system and gas exchange, the transport of oxygen and carbon dioxide, lung volumes and the control of breathing, and respiratory disorders.
What are the parts of the human respiratory system, and how are gases exchanged in the lungs?
Air passes through the nostrils, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, where oxygen and carbon dioxide diffuse across a very thin membrane along their partial pressure gradients.
The respiratory tract:
- Nasal chamber — filters, warms and moistens air
- Larynx — the sound box; the epiglottis covers the glottis during swallowing
- Trachea, bronchi and bronchioles — tubes held open by incomplete rings of cartilage
- Alveoli — tiny, thin-walled air sacs rich in capillaries, where exchange happens
- Lungs — wrapped in a double-layered, fluid-filled pleura, sitting in the thoracic chamber above the diaphragm
Breathing. In inspiration, the diaphragm and external intercostal muscles contract, enlarging the chest so air rushes in; in expiration, they relax and air is pushed out.
Gas exchange:
- Oxygen — partial pressure 104 mm Hg in alveoli and 40 mm Hg in deoxygenated blood, so it diffuses into blood
- Carbon dioxide — 45 mm Hg in deoxygenated blood and 40 mm Hg in alveoli, so it diffuses out
An everyday example. Choking on food while laughing at the dinner table happens when the epiglottis does not close the windpipe in time.
The substance. Breathing and respiration are not the same — breathing moves air in and out of the lungs, while respiration releases energy inside cells.
The respiratory tract:
- Nasal chamber — filters, warms and moistens air
- Larynx — the sound box; the epiglottis covers the glottis during swallowing
- Trachea, bronchi and bronchioles — tubes held open by incomplete rings of cartilage
- Alveoli — tiny, thin-walled air sacs rich in capillaries, where exchange happens
- Lungs — wrapped in a double-layered, fluid-filled pleura, sitting in the thoracic chamber above the diaphragm
Breathing. In inspiration, the diaphragm and external intercostal muscles contract, enlarging the chest so air rushes in; in expiration, they relax and air is pushed out.
Gas exchange:
- Oxygen — partial pressure 104 mm Hg in alveoli and 40 mm Hg in deoxygenated blood, so it diffuses into blood
- Carbon dioxide — 45 mm Hg in deoxygenated blood and 40 mm Hg in alveoli, so it diffuses out
An everyday example. Choking on food while laughing at the dinner table happens when the epiglottis does not close the windpipe in time.
The substance. Breathing and respiration are not the same — breathing moves air in and out of the lungs, while respiration releases energy inside cells.
How are oxygen and carbon dioxide transported in the blood?
About 97 per cent of oxygen travels bound to haemoglobin in red blood cells and about 3 per cent dissolves in plasma, while carbon dioxide travels about 70 per cent as bicarbonate, 20 to 25 per cent bound to haemoglobin and about 7 per cent dissolved in plasma.
Oxygen transport:
- Each haemoglobin molecule can carry up to four oxygen molecules as oxyhaemoglobin
- Binding depends mainly on the partial pressure of oxygen — high in the alveoli, so oxygen binds, and low in tissues, so it is released
- The oxygen dissociation curve, plotting percentage saturation of haemoglobin against partial pressure of oxygen, is sigmoid
- In tissues, higher carbon dioxide, more hydrogen ions and higher temperature help haemoglobin release oxygen
Carbon dioxide transport:
- In tissues, carbonic anhydrase in red blood cells speeds up
- In the lungs, the reactions reverse and carbon dioxide is released into the alveoli
An everyday example. Trekkers arriving in Leh feel breathless at first because the lower partial pressure of oxygen means haemoglobin loads less oxygen in the lungs.
The substance. Carbon monoxide binds haemoglobin far more tightly than oxygen does — so a charcoal stove burning in a closed room can block oxygen transport even when air seems plentiful.
Oxygen transport:
- Each haemoglobin molecule can carry up to four oxygen molecules as oxyhaemoglobin
- Binding depends mainly on the partial pressure of oxygen — high in the alveoli, so oxygen binds, and low in tissues, so it is released
- The oxygen dissociation curve, plotting percentage saturation of haemoglobin against partial pressure of oxygen, is sigmoid
- In tissues, higher carbon dioxide, more hydrogen ions and higher temperature help haemoglobin release oxygen
Carbon dioxide transport:
- In tissues, carbonic anhydrase in red blood cells speeds up
- In the lungs, the reactions reverse and carbon dioxide is released into the alveoli
An everyday example. Trekkers arriving in Leh feel breathless at first because the lower partial pressure of oxygen means haemoglobin loads less oxygen in the lungs.
The substance. Carbon monoxide binds haemoglobin far more tightly than oxygen does — so a charcoal stove burning in a closed room can block oxygen transport even when air seems plentiful.
What are respiratory volumes and capacities, and how is breathing regulated?
Respiratory volumes measure single parts of a breath, capacities are sums of volumes, and breathing is controlled by the respiratory rhythm centre in the medulla, adjusted by the pneumotaxic centre in the pons and by chemical signals such as carbon dioxide and hydrogen ions.
Volumes in a healthy adult:
- Tidal volume (TV) — air breathed in or out normally; about 500 mL
- Inspiratory reserve volume (IRV) — extra air that can be forced in; 2500 to 3000 mL
- Expiratory reserve volume (ERV) — extra air that can be forced out; 1000 to 1100 mL
- Residual volume (RV) — air left after the most forceful expiration; 1100 to 1200 mL
Capacities:
- Functional residual capacity = ERV + RV
- Vital capacity = IRV + TV + ERV
- Total lung capacity = vital capacity + RV
Regulation:
- The respiratory rhythm centre in the medulla sets the basic rhythm
- The pneumotaxic centre in the pons can shorten inspiration and change the rate
- A chemosensitive area beside the rhythm centre responds to rising carbon dioxide and hydrogen ions
- Receptors in the aortic arch and carotid artery detect the same changes; oxygen plays only a minor role
An everyday example. Holding your breath under water in a swimming pool becomes unbearable mainly because carbon dioxide builds up, not because oxygen runs out first.
The substance. Residual volume can never be breathed out — so vital capacity is always smaller than total lung capacity.
Volumes in a healthy adult:
- Tidal volume (TV) — air breathed in or out normally; about 500 mL
- Inspiratory reserve volume (IRV) — extra air that can be forced in; 2500 to 3000 mL
- Expiratory reserve volume (ERV) — extra air that can be forced out; 1000 to 1100 mL
- Residual volume (RV) — air left after the most forceful expiration; 1100 to 1200 mL
Capacities:
- Functional residual capacity = ERV + RV
- Vital capacity = IRV + TV + ERV
- Total lung capacity = vital capacity + RV
Regulation:
- The respiratory rhythm centre in the medulla sets the basic rhythm
- The pneumotaxic centre in the pons can shorten inspiration and change the rate
- A chemosensitive area beside the rhythm centre responds to rising carbon dioxide and hydrogen ions
- Receptors in the aortic arch and carotid artery detect the same changes; oxygen plays only a minor role
An everyday example. Holding your breath under water in a swimming pool becomes unbearable mainly because carbon dioxide builds up, not because oxygen runs out first.
The substance. Residual volume can never be breathed out — so vital capacity is always smaller than total lung capacity.
What are the main disorders of the respiratory system?
Respiratory disorders such as asthma, bronchitis, emphysema and occupational lung diseases narrow the airways, damage the alveoli or scar lung tissue, and so reduce gas exchange.
- Asthma — inflammation of the bronchi and bronchioles makes breathing difficult and causes wheezing; attacks can be set off by allergens such as pollen and dust
- Bronchitis — inflammation of the bronchi with swelling and thick mucus, causing a persistent cough
- Emphysema — a chronic disorder in which alveolar walls are damaged, reducing the surface area for gas exchange; cigarette smoking is a major cause
- Occupational respiratory disorders — long exposure to dust in industries such as stone grinding and breaking causes inflammation and fibrosis, or scarring, of lung tissue, as in silicosis and asbestosis
An everyday example. Workers in stone-crushing units are advised to wear masks because fine silica dust can cause silicosis after long exposure.
The substance. Emphysema cannot be reversed by opening the airways — unlike asthma, the problem is lost alveolar surface, not a narrowed tube.
- Asthma — inflammation of the bronchi and bronchioles makes breathing difficult and causes wheezing; attacks can be set off by allergens such as pollen and dust
- Bronchitis — inflammation of the bronchi with swelling and thick mucus, causing a persistent cough
- Emphysema — a chronic disorder in which alveolar walls are damaged, reducing the surface area for gas exchange; cigarette smoking is a major cause
- Occupational respiratory disorders — long exposure to dust in industries such as stone grinding and breaking causes inflammation and fibrosis, or scarring, of lung tissue, as in silicosis and asbestosis
An everyday example. Workers in stone-crushing units are advised to wear masks because fine silica dust can cause silicosis after long exposure.
The substance. Emphysema cannot be reversed by opening the airways — unlike asthma, the problem is lost alveolar surface, not a narrowed tube.
Exam tip
What earns full marks on breathing and exchange of gases?
Write every lung capacity as a sum of named volumes before giving a number, so the method earns credit even if a value slips.
- Partial pressure of oxygen: 104 mm Hg in alveoli, 40 in tissues; carbon dioxide: 40 in alveoli, 45 in tissues
- Vital capacity = IRV + TV + ERV
The trap. Adding residual volume to vital capacity. Vital capacity excludes RV; only total lung capacity includes it.
- Partial pressure of oxygen: 104 mm Hg in alveoli, 40 in tissues; carbon dioxide: 40 in alveoli, 45 in tissues
- Vital capacity = IRV + TV + ERV
The trap. Adding residual volume to vital capacity. Vital capacity excludes RV; only total lung capacity includes it.
Did you know
How do people cope with the thin air high in the Himalayas?
At high altitude each breath contains fewer oxygen molecules, because the air pressure is lower. Visitors often feel breathless, get headaches and tire quickly.
Over days and weeks the body adjusts: the kidneys release more erythropoietin, which raises red blood cell production, and breathing becomes deeper.
This is why mountaineers and soldiers posted at high altitudes spend time acclimatising before hard physical work.
Over days and weeks the body adjusts: the kidneys release more erythropoietin, which raises red blood cell production, and breathing becomes deeper.
This is why mountaineers and soldiers posted at high altitudes spend time acclimatising before hard physical work.
Exam relevance
How does NEET test gas exchange, gas transport and lung volumes?
Breathing and Exchange of Gases is a recurring NEET chapter, and its questions mix physiology with simple calculations.
What gets asked. Partial pressures of oxygen and carbon dioxide at different sites, the oxygen dissociation curve and what shifts it, definitions and sums of lung volumes and capacities, and the roles of the medulla, pons and chemoreceptors.
Question types. Mostly statement-based and match-the-column questions, with short capacity calculations and graph-based questions on the dissociation curve.
Why it matters later. Oxygen transport links to Body Fluids and Circulation, and control of breathing to Neural Control and Coordination.
The trap that costs marks. Thinking that falling oxygen is the main driver of breathing — rising carbon dioxide and hydrogen ions are the chief stimulus.
What gets asked. Partial pressures of oxygen and carbon dioxide at different sites, the oxygen dissociation curve and what shifts it, definitions and sums of lung volumes and capacities, and the roles of the medulla, pons and chemoreceptors.
Question types. Mostly statement-based and match-the-column questions, with short capacity calculations and graph-based questions on the dissociation curve.
Why it matters later. Oxygen transport links to Body Fluids and Circulation, and control of breathing to Neural Control and Coordination.
The trap that costs marks. Thinking that falling oxygen is the main driver of breathing — rising carbon dioxide and hydrogen ions are the chief stimulus.
Key takeaways
What must you be able to do from this lesson?
- Respiratory system and gas exchange: nostrils to alveoli, breathing by the diaphragm and intercostal muscles, and diffusion along partial pressure gradients
- Transport: oxygen mostly as oxyhaemoglobin; carbon dioxide mostly as bicarbonate, helped by carbonic anhydrase
- Volumes and control: TV, IRV, ERV and RV with their capacities; rhythm set by the medulla and adjusted by the pons and chemoreceptors
- Disorders: asthma, bronchitis, emphysema and occupational lung diseases
With TV 500 mL, IRV 3000 mL, ERV 1100 mL and RV 1200 mL, can you work out the total lung capacity?
- Transport: oxygen mostly as oxyhaemoglobin; carbon dioxide mostly as bicarbonate, helped by carbonic anhydrase
- Volumes and control: TV, IRV, ERV and RV with their capacities; rhythm set by the medulla and adjusted by the pons and chemoreceptors
- Disorders: asthma, bronchitis, emphysema and occupational lung diseases
With TV 500 mL, IRV 3000 mL, ERV 1100 mL and RV 1200 mL, can you work out the total lung capacity?