Breathing and Exchange of Gases: NEET notes

1. Respiratory organs in animals

NCERT §14.1

• Breathing, or respiration in the everyday sense, is the exchange of O₂ from the atmosphere with the CO₂ produced by cells. The mechanism varies mainly with habitat and level of organisation.
• Lower invertebrates such as sponges, coelenterates and flatworms exchange gases by simple diffusion over the whole body surface.
• Earthworms use their moist cuticle; insects have a network of tubes called tracheae that carry air within the body.
• Special vascularised structures called gills are used by most aquatic arthropods and molluscs (branchial respiration); lungs are used by terrestrial forms (pulmonary respiration).
• Among vertebrates, fishes use gills, while amphibians, reptiles, birds and mammals breathe through lungs.
• Amphibians such as frogs can also exchange gases through their moist skin (cutaneous respiration).

2. Human respiratory system

NCERT §14.1.1

• Air enters through a pair of external nostrils above the upper lip, runs through the nasal passage into the nasal chamber, then opens into the pharynx, a passage shared by food and air.
• From the pharynx, air passes through the larynx into the trachea. The larynx, a box made of cartilage, produces sound, which is why it is known as the sound box.
• During swallowing, the glottis (the opening of the larynx) can be covered by the epiglottis, a thin elastic cartilaginous flap, so food does not enter the larynx.
• The trachea is a straight tube running down to the mid-thoracic cavity, where at the level of the 5th thoracic vertebra it splits into right and left primary bronchi.
• Each primary bronchus branches repeatedly into secondary and tertiary bronchi and then bronchioles, ending in very thin terminal bronchioles.
• The trachea, the primary, secondary and tertiary bronchi and the initial bronchioles are held open by incomplete cartilaginous rings.
• Every terminal bronchiole ends in a cluster of alveoli, thin, irregular-walled sacs richly supplied with blood vessels. Together, the branching bronchi, bronchioles and alveoli form the lungs.
• A double-layered pleura covers each lung, and the pleural fluid between its layers lowers friction on the lung surface. Of the two pleural membranes, the outer is in contact with the thoracic lining and the inner with the lung surface.
• The conducting part (external nostrils up to terminal bronchioles) carries air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature. The respiratory or exchange part is the alveoli and their ducts, where O₂ and CO₂ actually diffuse between blood and air.
• The lungs lie in the thoracic chamber, which is air-tight. Its walls are the vertebral column at the back (dorsally), the sternum in front (ventrally), the ribs on the sides and the dome-shaped diaphragm below. Whatever change occurs in thoracic volume is transmitted to the lung (pulmonary) cavity, so we can breathe even though lung volume cannot be changed directly.

3. Steps of respiration

NCERT §14.1.1

• Breathing or pulmonary ventilation: atmospheric air is drawn in and CO₂-rich alveolar air is released.
• Diffusion of O₂ and CO₂ across the alveolar membrane.
• Transport of the gases by the blood.
• Diffusion of O₂ and CO₂ between the blood and the tissues.
• Use of O₂ by cells for catabolic reactions, with release of CO₂; this last step is cellular respiration, dealt with in Chapter 12 (Respiration in Plants).

4. Mechanism of breathing

NCERT §14.2

• There are two phases of breathing: inspiration draws atmospheric air in and expiration releases alveolar air. Air flows in and out along a pressure gradient set up between the lungs and the atmosphere.
• Inspiration happens when the pressure inside the lungs (intra-pulmonary pressure) falls below atmospheric pressure; expiration happens when intra-pulmonary pressure rises above atmospheric pressure.
• These gradients are produced by the diaphragm together with a specialised set of muscles between the ribs, the external and internal intercostals.
• Inspiration starts with contraction of the diaphragm, which increases the volume of the thoracic chamber along the antero-posterior axis.
• Contraction of the external intercostal muscles lifts the ribs and sternum, increasing thoracic volume along the dorso-ventral axis.
• The rise in thoracic volume increases pulmonary volume, which lowers intra-pulmonary pressure below atmospheric pressure, so air rushes into the lungs.
• Expiration: relaxing the diaphragm and intercostals brings the diaphragm and sternum back to their resting positions, thoracic and then pulmonary volume shrink, intra-pulmonary pressure climbs slightly above atmospheric, and air is driven out.
• Additional muscles in the abdomen can increase the strength of both inspiration and expiration.
• A healthy person breathes about 12–16 times a minute. A spirometer can estimate the volumes of air involved and is useful clinically for assessing lung function.

5. Respiratory volumes and capacities

NCERT §14.2.1

• Tidal Volume (TV): the air breathed in or out in one normal breath, about 500 mL. A healthy person therefore moves roughly 6000–8000 mL of air per minute.
• Inspiratory Reserve Volume (IRV): the extra air that can be forcibly inhaled after a normal inspiration, on average 2500–3000 mL.
• Expiratory Reserve Volume (ERV): the extra air that can be forcibly exhaled after a normal expiration, on average 1000–1100 mL.
• Residual Volume (RV): the air the lungs still hold even after a forcible expiration, on average 1100–1200 mL.
• Inspiratory Capacity (IC) = TV + IRV: total air that can be inhaled after a normal expiration.
• Expiratory Capacity (EC) = TV + ERV: total air that can be exhaled after a normal inspiration.
• Functional Residual Capacity (FRC) = ERV + RV: air left in the lungs after a normal expiration.
• Vital Capacity (VC) = ERV + TV + IRV: the greatest volume of air that can be breathed in following a forced expiration, or breathed out following a forced inspiration.
• Total Lung Capacity (TLC) = VC + RV (equivalently RV + ERV + TV + IRV): all the air the lungs can hold at the end of a forced inspiration.

6. Exchange of gases

NCERT §14.3

• Alveoli are the primary sites of gas exchange; exchange also takes place between blood and tissues. In both places O₂ and CO₂ move by simple diffusion driven by pressure or concentration gradients.
• Diffusion rate also depends on the solubility of the gases and the thickness of the membrane involved.
• Partial pressure, written pO₂ or pCO₂, is the pressure that an individual gas in a mixture contributes.
• Partial pressures of O₂ (mm Hg): atmosphere 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.
• Partial pressures of CO₂ (mm Hg): atmosphere 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.
• So O₂ moves from alveoli into blood and from blood into tissues, while CO₂ moves the opposite way: tissues to blood, and blood to alveoli.
• CO₂ dissolves 20–25 times more readily than O₂, so for the same partial-pressure difference far more CO₂ than O₂ crosses the diffusion membrane.
• The diffusion membrane is made of three major layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance lying between them. Together they are far thinner than a millimetre, which helps diffusion.

7. Transport of oxygen

NCERT §14.4, §14.4.1

• Blood carries O₂ and CO₂. About 97 per cent of O₂ travels bound to haemoglobin in RBCs; the remaining 3 per cent is dissolved in plasma.
• Haemoglobin is a red, iron-containing pigment in RBCs. O₂ binds it reversibly to form oxyhaemoglobin, and each haemoglobin molecule can carry at most four molecules of O₂.
• Binding of O₂ depends mainly on pO₂; pCO₂, hydrogen ion concentration and temperature also affect it.
• Plotting percentage saturation of haemoglobin against pO₂ gives a sigmoid curve, the oxygen dissociation curve, which is very useful for studying how pCO₂, H⁺ concentration and other factors affect O₂ binding.
• In the alveoli, high pO₂, low pCO₂, lower H⁺ concentration and lower temperature all favour formation of oxyhaemoglobin.
• In the tissues, low pO₂, high pCO₂, higher H⁺ concentration and higher temperature favour dissociation of O₂ from oxyhaemoglobin.
• Under normal physiological conditions, every 100 mL of oxygenated blood delivers around 5 mL of O₂ to the tissues.

8. Transport of carbon dioxide

NCERT §14.4, §14.4.2

• Roughly 70 per cent of CO₂ travels as bicarbonate, about 20–25 per cent is bound to haemoglobin as carbamino-haemoglobin, and about 7 per cent is dissolved in plasma.
• CO₂ binding to haemoglobin depends on pCO₂, with pO₂ also a major influence. In the tissues, high pCO₂ and low pO₂ favour more CO₂ binding; in the alveoli, low pCO₂ and high pO₂ make CO₂ dissociate from carbamino-haemoglobin.
• The enzyme carbonic anhydrase is present at very high concentration in RBCs, and in minute amounts in plasma as well.
• Carbonic anhydrase speeds up both directions of the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.
• In the tissues, catabolism raises pCO₂, so CO₂ diffuses into the blood (both RBCs and plasma), where HCO₃⁻ and H⁺ are formed.
• At the alveoli, where pCO₂ is low, the reaction runs the other way, giving back CO₂ and H₂O. CO₂ trapped as bicarbonate at the tissues is thus released at the alveoli.
• Every 100 mL of deoxygenated blood delivers roughly 4 mL of CO₂ to the alveoli.

9. Regulation of respiration

NCERT §14.5

• Humans can adjust their breathing rhythm to match the body's demands; the neural system does this.
• This regulation is mainly the job of a specialised centre in the medulla of the brain, called the respiratory rhythm centre.
• The rhythm centre can be moderated by the pneumotaxic centre in the pons. Its signals can reduce how long inspiration lasts, which changes the breathing rate.
• A chemosensitive area next to the rhythm centre is highly sensitive to CO₂ and hydrogen ions. A rise in these activates it, and it signals the rhythm centre to adjust breathing so these substances are removed.
• Receptors associated with the aortic arch and the carotid artery also detect changes in CO₂ and H⁺ concentration and send signals to the rhythm centre.
• Oxygen plays only a very minor part in setting the rhythm of breathing.

10. Disorders of the respiratory system

NCERT §14.6

• Asthma: difficulty in breathing with wheezing, caused by inflammation of the bronchi and bronchioles.
• Emphysema: a chronic disorder that damages the alveolar walls, so less respiratory surface is left for gas exchange. One of its major causes is cigarette smoking.
• Occupational respiratory disorders: work such as grinding or stone-breaking throws up more dust than the body's defence mechanisms can fully deal with.
• Long exposure to such dust causes inflammation that leads to fibrosis, a proliferation of fibrous tissue, which causes serious lung damage.
• Workers in these industries should wear protective masks.

Must-know facts

1. Trachea divides into primary bronchi at the level of the 5th thoracic vertebra.
2. Incomplete cartilaginous rings give support to the trachea, the bronchi (primary to tertiary) and the initial bronchioles.
3. Epiglottis covers the glottis during swallowing; the larynx is the sound box.
4. Conducting zone ends at terminal bronchioles; the exchange zone is the alveoli and their ducts.
5. Inspiration: diaphragm contracts (antero-posterior axis) and external intercostals contract (dorso-ventral axis); intra-pulmonary pressure falls below atmospheric.
6. Normal breathing rate: 12–16 breaths per minute.
7. TV ≈ 500 mL; IRV 2500–3000 mL; ERV 1000–1100 mL; RV 1100–1200 mL.
8. VC = IRV + TV + ERV; TLC = VC + RV; FRC = ERV + RV; IC = TV + IRV; EC = TV + ERV.
9. A healthy person breathes in or out roughly 6000–8000 mL of air per minute (about 500 mL per breath × 12–16 breaths).
10. pO₂ (mm Hg): atmosphere 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.
11. pCO₂ (mm Hg): atmosphere 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.
12. CO₂ is 20–25 times more soluble than O₂.
13. Diffusion membrane = alveolar squamous epithelium + capillary endothelium + basement substance; much thinner than 1 mm.
14. O₂ transport: 97% as oxyhaemoglobin, 3% dissolved in plasma. One haemoglobin carries at most 4 O₂.
15. CO₂ transport: about 70% as bicarbonate, 20–25% as carbamino-haemoglobin, about 7% dissolved.
16. Per 100 mL blood: about 5 mL O₂ delivered to tissues; about 4 mL CO₂ delivered to alveoli.
17. Oxygen dissociation curve is sigmoid.
18. Rhythm centre = medulla; pneumotaxic centre = pons; chemosensitive area responds to CO₂ and H⁺; O₂ has an insignificant role.
19. Emphysema = alveolar walls damaged, less exchange surface; occupational dust exposure leads to fibrosis.

Common traps

Key terms

Glottis

The opening of the larynx, which the epiglottis closes during swallowing.

Epiglottis

A thin, elastic cartilage flap that stops food from entering the larynx.

Alveoli

Very thin, vascularised air sacs at the ends of terminal bronchioles where gas exchange occurs.

Pleura

The double membrane around each lung, with lubricating pleural fluid between its layers.

Conducting part

The airway from external nostrils to terminal bronchioles that carries air to the alveoli, clears it of particles, humidifies it and brings it to body temperature, without exchanging gases.

Intra-pulmonary pressure

The air pressure inside the lungs, whose difference from atmospheric pressure drives air movement.

Spirometer

An instrument for measuring volumes of air breathed in and out.

Tidal Volume

The volume of air moved in or out in one ordinary breath, about 500 mL.

Residual Volume

The air that stays in the lungs even after the strongest possible exhalation.

Vital Capacity

The largest volume a person can exhale after a maximal inhalation, or inhale after a maximal exhalation.

Partial pressure

The share of total pressure contributed by one gas in a mixture.

Oxyhaemoglobin

Haemoglobin with O₂ reversibly bound to it.

Carbamino-haemoglobin

Haemoglobin carrying bound CO₂.

Carbonic anhydrase

An enzyme, abundant in RBCs, that speeds up the interconversion of CO₂ + H₂O and HCO₃⁻ + H⁺.

Oxygen dissociation curve

The sigmoid graph of haemoglobin's percentage O₂ saturation against pO₂.

Pneumotaxic centre

A centre in the pons that moderates the medullary rhythm centre by shortening inspiration.

Chemosensitive area

A region beside the medullary rhythm centre that is highly sensitive to CO₂ and H⁺.

Fibrosis

Overgrowth of fibrous tissue in the lungs after long-term inflammation, as in dust exposure.