Our cells need oxygen to release usable energy from food. They also produce carbon dioxide, which must be removed. The respiratory system brings air into the lungs, allows gases to move between air and blood, and helps eliminate carbon dioxide.
Breathing means moving air into and out of the lungs. Cellular respiration refers to the chemical reactions in cells that release energy from food.
Table of Contents
Respiratory Organs in Animals
Animals exchange gases in different ways. Earthworms use their moist body surface, insects have a system of air tubes, fish use gills, and humans use lungs. Frogs can also exchange gases through their moist skin.
Human Respiratory System
Air follows this route through the human respiratory system:
Nostrils → Nasal chamber → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
The trachea divides into two main bronchi, one entering each lung. The bronchi branch into smaller bronchioles, which end in tiny air sacs called alveoli.
The alveoli have thin walls and are surrounded by blood capillaries. They are the main sites of gas exchange. The lungs are covered by pleural membranes, with fluid between them that helps reduce friction during breathing.
Mechanism of Breathing
Breathing has two phases: inspiration and expiration.
| Phase | Diaphragm and ribs | Chest volume | Pressure in lungs | Air movement |
|---|---|---|---|---|
| Inspiration | Diaphragm contracts and flattens; ribs move up and outward | Increases | Falls below atmospheric pressure | Air enters |
| Normal expiration | Diaphragm relaxes; ribs return towards resting position | Decreases | Rises above atmospheric pressure | Air leaves |
Air moves because of a pressure difference between the atmosphere and the lungs. During normal quiet breathing, expiration is largely due to relaxation of the breathing muscles and elastic recoil.
Respiratory Volumes and Capacities
A spirometer can be used to measure several breathing volumes. The values below are approximate textbook values; actual measurements vary among individuals.
| Term | Meaning | Approximate value |
|---|---|---|
| Tidal volume (TV) | Air inhaled or exhaled in a normal breath | 500 mL |
| Inspiratory reserve volume (IRV) | Extra air inhaled after a normal inspiration | 2,500–3,000 mL |
| Expiratory reserve volume (ERV) | Extra air exhaled after a normal expiration | 1,000–1,100 mL |
| Residual volume (RV) | Air left in the lungs after a forceful expiration | 1,100–1,200 mL |
Important capacities include:
- Inspiratory capacity = TV + IRV
- Expiratory capacity = TV + ERV
- Functional residual capacity = ERV + RV
- Vital capacity = IRV + TV + ERV
- Total lung capacity = Vital capacity + RV
Exchange of Oxygen and Carbon Dioxide
Gas exchange occurs mainly by diffusion, driven by differences in the gases’ partial pressures.
- In the lungs, oxygen moves from the alveoli into the blood.
- In the body tissues, oxygen moves from blood into cells.
- Carbon dioxide moves in the opposite direction: from tissues into blood and then from blood into the alveoli.
The thin barrier between alveolar air and blood helps these gases diffuse efficiently.
Transport of Oxygen
Most oxygen is carried by haemoglobin in red blood cells. Oxygen binds reversibly to haemoglobin to form oxyhaemoglobin.
In the lungs, a higher oxygen partial pressure favours oxygen binding. In active tissues, lower oxygen partial pressure and higher carbon dioxide levels favour oxygen release.
The relationship between oxygen partial pressure and haemoglobin saturation is shown by the oxygen dissociation curve.
Transport of Carbon Dioxide
Carbon dioxide travels in blood in three main forms:
| Form | Approximate share |
|---|---|
| Bicarbonate ions | About 70% |
| Bound to haemoglobin and other blood components | About 20–25% |
| Dissolved in plasma | About 7% |
The enzyme carbonic anhydrase helps convert carbon dioxide to bicarbonate in blood and supports the reverse reaction in the lungs.
Regulation of Breathing
A respiratory rhythm centre in the medulla of the brain helps maintain breathing. A centre in the pons can modify that rhythm. Chemosensitive regions respond strongly to changes in carbon dioxide and hydrogen ion levels and help adjust breathing to the body’s needs.
Respiratory Disorders
- Asthma: Inflammation and narrowing of air passages can make breathing difficult and cause wheezing.
- Emphysema: Damage to alveolar walls reduces the surface available for gas exchange.
- Occupational respiratory disorders: Long-term exposure to certain workplace dusts can damage lung tissue.
Quick Revision
- Alveoli: Main site of gas exchange in the lungs.
- Diaphragm contracts: Chest volume increases and air enters.
- Oxygen transport: Mainly by haemoglobin.
- Carbon dioxide transport: Mainly as bicarbonate.
- Tidal volume: Air moved during one normal breath.
- Vital capacity: IRV + TV + ERV.
Check Your Understanding
- What is the main site of gas exchange in human lungs?
- Why does air enter the lungs during inspiration?
- What is tidal volume?
- In which form is most carbon dioxide carried in blood?
- Which part of the brain contains the main respiratory rhythm centre?
Answers: (1) Alveoli. (2) Chest expansion lowers lung pressure below atmospheric pressure. (3) Air inhaled or exhaled in a normal breath. (4) Bicarbonate ions. (5) Medulla.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is breathing?
Breathing is the physical movement of air into and out of the lungs.
How is breathing different from cellular respiration?
Breathing moves air; cellular respiration releases usable energy from food within cells.
What is the main site of gas exchange in human lungs?
Gas exchange occurs across the thin walls of alveoli and their surrounding capillaries.
What is inspiration?
It is the movement of air into the lungs when thoracic volume increases and pressure in the lungs falls.
What is expiration?
It is the movement of air out of the lungs when thoracic volume decreases and lung pressure rises.
What does the diaphragm do during inspiration?
It contracts and flattens, increasing the volume of the thoracic cavity.
What is tidal volume?
It is the amount of air inhaled or exhaled during a normal, quiet breath.
What is vital capacity?
It is the maximum air a person can exhale after a maximum inhalation.
What is residual volume?
It is the air remaining in the lungs after a forceful maximum exhalation.
How does oxygen move from alveoli into blood?
It diffuses down its partial-pressure gradient across the respiratory membrane.
How is most oxygen transported in blood?
Most oxygen is transported bound to haemoglobin in red blood cells.
How is most carbon dioxide transported in blood?
Most is transported as bicarbonate ions after conversion in red blood cells.
What mainly regulates the normal breathing rhythm?
Neural centres in the brainstem regulate the rhythm and respond strongly to changes associated with CO₂ levels.
What is asthma?
It is an inflammatory airway condition in which narrowed airways can make breathing difficult.
What is emphysema?
It is damage to alveolar walls that reduces the lung surface area available for gas exchange.