Breathing and Exchange of Gases

Every cell in the body oxidises food to release energy. In that process the cell uses oxygen to break down glucose, amino acids and fatty acids, and it gives out carbon dioxide, which is harmful if it accumulates. So the body needs a continuous supply of O₂ to the cells and a continuous removal of CO₂ from them. The process of exchange of O₂ from the atmosphere with CO₂ produced by the cells is called breathing, commonly known as respiration. NCERT uses that exact sentence, and a 1-mark question can ask you to reproduce it.

Keep two words apart from the start, because the first question in many school papers is exactly this distinction. Breathing is the physical, mechanical process of taking air into the lungs and pushing it out; no energy is released, no enzymes act, and it happens at the level of the organism. Cellular respiration is the biochemical, enzyme-driven oxidation of food inside the cell that actually releases energy as ATP. Breathing supplies the O₂ that cellular respiration uses and removes the CO₂ that it produces; the two are linked but not the same thing.

The chapter then moves in NCERT order: the organs that different animals use to breathe, the human respiratory system in detail, how the air is moved in and out, how much air moves, how the gases cross into the blood, how the blood carries them, how the brain sets the rhythm, and what goes wrong in disease. Read it as one journey of a molecule of O₂ from the nostril to a mitochondrion, and one molecule of CO₂ making the reverse trip.

The mechanism of breathing varies among animals depending on their habitat and their level of organisation. The examiner expects you to match each animal group with its organ, so learn this list as a table.

• Lower invertebrates such as sponges, coelenterates and flatworms exchange O₂ and CO₂ by simple diffusion over the entire body surface. They are small or thin enough that no cell is far from the outside.
• Earthworms use their moist cuticle. Gases dissolve in the film of moisture and diffuse into the blood vessels just beneath the skin.
• Insects have a network of tubes, the tracheal tubes, opening to the outside through spiracles and carrying atmospheric air directly to the tissues. Blood plays no part in gas transport in insects.
• Aquatic arthropods and molluscs use special vascularised structures called gills; this is branchial respiration.
• Terrestrial forms use vascularised bags called lungs; this is pulmonary respiration.

Among vertebrates, fishes use gills, while amphibians, reptiles, birds and mammals respire through lungs. Amphibians like frogs can also respire through their moist skin, which is cutaneous respiration. Note the pattern: as animals get larger and more complex, the exchange surface moves from the general body surface to a specialised, folded, richly vascularised organ, because diffusion alone cannot serve a large body.

Two sentences to keep for 1-mark questions: the site of gaseous exchange in an insect is the tracheal tube system (the exchange itself happens where the fine tracheoles meet the tissues), and a frog uses both lungs and skin. Mammals, including humans, use lungs alone.

Trace the path of air in order, because the diagram and the sequence are what the 5-mark question rewards. Air enters through a pair of external nostrils into the nasal chamber, then the nasal passage, which opens into the pharynx. The pharynx is a common passage for food and air. From the pharynx air enters the larynx, a cartilaginous box that helps in sound production and is therefore called the sound box. During swallowing the epiglottis, a thin elastic cartilaginous flap, closes the glottis so that food does not enter the larynx. Below the larynx is the trachea, a straight tube that runs down to the mid-thoracic cavity and at the level of the fifth thoracic vertebra divides into a right and a left primary bronchus. Each primary bronchus divides repeatedly into secondary and tertiary bronchi and then into bronchioles, ending in very fine terminal bronchioles. The trachea, primary, secondary and tertiary bronchi and the initial bronchioles are supported by incomplete cartilaginous rings, which keep the airway open without preventing it from flexing. Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles and alveoli makes up the lungs.

We have two lungs covered by a double-layered pleura, with pleural fluid between the two layers. The fluid reduces friction on the lung surface as it expands and relaxes. The outer pleural membrane lies in close contact with the thoracic lining and the inner one with the lung surface.

NCERT divides the system into two functional parts, and this division is a very common 2-mark question. The conducting part runs from the external nostrils up to the terminal bronchioles. It transports atmospheric air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature. The exchange part is the alveoli and their ducts, the site of actual diffusion of O₂ and CO₂ between blood and atmospheric air. No exchange happens in the conducting part because its walls are neither thin enough nor vascularised.

The lungs sit in the thoracic chamber, which is anatomically an airtight chamber formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm. This setup is important: any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity. We cannot directly alter the volume of the lungs, so we alter the volume of the chest and the lungs follow.

Finally, NCERT lists the five steps that together make up respiration. Write them as a numbered list whenever the question says 'steps of respiration'. (i) Breathing or pulmonary ventilation, by which atmospheric air is drawn in and CO₂-rich alveolar air is released out. (ii) Diffusion of O₂ and CO₂ across the alveolar membrane. (iii) Transport of gases by the blood. (iv) Diffusion of O₂ and CO₂ between blood and tissues. (v) Utilisation of O₂ by the cells for catabolic reactions with the resultant release of CO₂, that is, cellular respiration.

Breathing has two stages: inspiration, during which atmospheric air is drawn in, and expiration, by which alveolar air is released out. Air moves only because of a pressure gradient between the lungs and the atmosphere. Inspiration occurs when the pressure within the lungs, the intra-pulmonary pressure, is less than atmospheric pressure, that is, there is a negative pressure in the lungs with respect to the atmosphere. Expiration occurs when the intra-pulmonary pressure is higher than atmospheric pressure. The gradient is created by the diaphragm and a specialised set of muscles, the external and internal intercostal muscles that lie between the ribs.

Inspiration is initiated by contraction of the diaphragm. The dome flattens and moves downward, which increases the volume of the thoracic chamber along the antero-posterior axis. At the same time contraction of the external intercostal muscles lifts the ribs and sternum up and outward, increasing the volume of the thoracic chamber along the dorso-ventral axis. The overall increase in thoracic volume produces a similar increase in pulmonary volume, because the lungs follow the chest wall. A larger volume means a lower pressure, so the intra-pulmonary pressure falls below atmospheric pressure and air is forced from outside into the lungs. That is inspiration: an active process, driven by muscle contraction.

Expiration under normal conditions is passive. The diaphragm and the external intercostal muscles relax, the diaphragm returns to its dome shape and the ribs and sternum return to their normal positions. Thoracic volume decreases, pulmonary volume decreases, intra-pulmonary pressure rises slightly above atmospheric pressure, and air is expelled. We can increase the strength of both inspiration and expiration with the help of additional muscles in the abdomen; forced expiration uses the internal intercostals and abdominal muscles.

Two facts finish this section. On average a healthy human breathes 12 to 16 times per minute. The volume of air involved in breathing movements can be measured with a spirometer, which helps in the clinical assessment of pulmonary functions.

For a 3-mark answer on inspiration the marking scheme wants, in this order: contraction of diaphragm, contraction of external intercostals lifting ribs and sternum, increase in thoracic volume in the two axes, increase in pulmonary volume, fall in intra-pulmonary pressure below atmospheric, air rushes in. Miss the pressure line and you lose a mark; the pressure gradient is the whole point.

NCERT defines four volumes and then builds five capacities by adding volumes together. Memorise the four volumes with their values; the capacities you can then derive in the exam rather than memorise.

Tidal Volume (TV) is the volume of air inspired or expired during a normal respiration, approximately 500 mL. A healthy man can inspire or expire roughly 6000 to 8000 mL of air per minute (500 mL × 12 to 16 breaths). Inspiratory Reserve Volume (IRV) is the additional volume of air a person can inspire by a forcible inspiration, on average 2500 to 3000 mL. Expiratory Reserve Volume (ERV) is the additional volume of air a person can expire by a forcible expiration, on average 1000 to 1100 mL. Residual Volume (RV) is the volume of air remaining in the lungs even after a forcible expiration, on average 1100 to 1200 mL. The residual volume can never be breathed out; it keeps the alveoli from collapsing and is the one volume a spirometer cannot measure directly.

The capacities are sums. Inspiratory Capacity (IC) is the total volume of air a person can inspire after a normal expiration: IC = TV + IRV. Expiratory Capacity (EC) is the total volume a person can expire after a normal inspiration: EC = TV + ERV. Functional Residual Capacity (FRC) is the volume of air that remains in the lungs after a normal expiration: FRC = ERV + RV. Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration, or equivalently the maximum volume a person can breathe out after a forced inspiration: VC = ERV + TV + IRV. Total Lung Capacity (TLC) is the total volume of air accommodated in the lungs at the end of a forced inspiration: TLC = RV + ERV + TV + IRV, which is the same as VC + RV.

Put in typical numbers so the sums feel real. Taking TV = 500, IRV = 3000, ERV = 1100 and RV = 1200 mL: IC = 3500 mL, EC = 1600 mL, FRC = 2300 mL, VC = 4600 mL and TLC = 5800 mL. Any capacity that includes RV (FRC and TLC) cannot be measured by a spirometer; the rest can.

Vital capacity is the one most often asked, together with its significance: it tells us the maximum amount of air that can be exchanged in one breath, so it is a measure of the strength of the respiratory muscles and the elasticity of the lungs. It is higher in athletes, mountain dwellers and young adults, and lower in smokers, the elderly and patients with emphysema or fibrosis. Doctors use it to assess lung function.

Alveoli are the primary sites of exchange of gases, and exchange also occurs between blood and tissues. In both places O₂ and CO₂ move by simple diffusion, mainly down a pressure or concentration gradient. Two other factors affect the rate of diffusion: the solubility of the gas and the thickness of the membranes involved.

The pressure contributed by an individual gas in a mixture of gases is its partial pressure, written pO₂ for oxygen and pCO₂ for carbon dioxide. NCERT Table 14.1 gives the values in mm Hg and you must know them. Atmospheric air: pO₂ 159, pCO₂ 0.3. Alveoli: pO₂ 104, pCO₂ 40. Deoxygenated blood: pO₂ 40, pCO₂ 45. Oxygenated blood: pO₂ 95, pCO₂ 40. Tissues: pO₂ 40, pCO₂ 45. Read the gradients off the table. For O₂ the pressure falls from alveoli (104) to deoxygenated blood (40), so O₂ diffuses into the blood; it falls again from oxygenated blood (95) to tissues (40), so O₂ diffuses into the tissues. For CO₂ the gradient runs the other way: tissues (45) to blood (40 in arterial blood arriving), and deoxygenated blood (45) to alveoli (40), so CO₂ diffuses out of the tissues into blood and out of blood into the alveoli.

Notice that the CO₂ gradients are small, only about 5 mm Hg, while the O₂ gradient at the alveolus is about 64 mm Hg. Yet roughly as much CO₂ leaves as O₂ enters. The reason is solubility: the solubility of CO₂ is 20 to 25 times higher than that of O₂, so the amount of CO₂ that can diffuse across the membrane per unit difference in partial pressure is much higher than that of O₂. This is a favourite Assertion-Reason item.

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 (basement membrane) in between them. Its total thickness is much less than a millimetre. Thin membrane, large gradient for O₂, high solubility for CO₂: all the factors are favourable for the diffusion of O₂ from alveoli to tissues and of CO₂ from tissues to alveoli.

Why does alveolar air have a lower pO₂ (104) than atmospheric air (159) and a higher pCO₂ (40 against 0.3)? Because fresh air mixes with the residual air already in the lungs, which has given up O₂ to the blood and received CO₂ from it, and because the air is humidified so water vapour takes part of the total pressure. NCERT exercise question 5 tests exactly this comparison.

Blood is the medium of transport for both gases, and NCERT gives a clear percentage split that examiners ask as MCQs and 2-mark questions.

About 97 per cent of O₂ is transported by the RBCs in the blood, bound to haemoglobin, and the remaining 3 per cent is carried in a dissolved state through the plasma. O₂ is poorly soluble in water, so a carrier pigment is essential.

CO₂ is carried in three ways. Nearly 20 to 25 per cent is carried by RBCs bound to haemoglobin as carbamino-haemoglobin, about 70 per cent is carried as bicarbonate ions (HCO₃⁻), and about 7 per cent is carried in a dissolved state through the plasma. CO₂ is far more soluble than O₂, which is why a larger fraction can travel dissolved and why it can be converted to bicarbonate in the blood.

Write these numbers as a small table in your notes and re-read it the night before the exam; a mismatch such as writing '70 per cent as carbamino-haemoglobin' is one of the most common ways students lose a mark in this chapter. The two sub-sections that follow explain the chemistry behind each figure.

Haemoglobin is a red-coloured, iron-containing pigment present in the RBCs. O₂ binds with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O₂, one on each of its four haem groups. Binding of O₂ with haemoglobin is primarily related to the partial pressure of O₂. Three other factors also interfere with the binding: the partial pressure of CO₂, the hydrogen ion concentration and the temperature.

When the percentage saturation of haemoglobin with O₂ is plotted against pO₂, a sigmoid (S-shaped) curve is obtained. This is the oxygen dissociation curve, NCERT Fig. 14.5. The curve is sigmoid because of cooperative binding: the binding of the first O₂ molecule changes the shape of haemoglobin so that the second, third and fourth bind more easily. At low pO₂ saturation rises slowly, then steeply over the middle range, and finally flattens as the molecule approaches full saturation. The shape is very useful physiologically: the flat top means haemoglobin stays almost fully loaded even if alveolar pO₂ drops a little, and the steep middle means a small fall in pO₂ in active tissues unloads a lot of O₂.

The curve is highly useful in studying the effect of pO₂, pCO₂ and related factors on binding. In the alveoli, where there is high pO₂, low pCO₂, lesser H⁺ concentration and lower temperature, the factors all favour the formation of oxyhaemoglobin. In the tissues, where there is low pO₂, high pCO₂, high H⁺ concentration and higher temperature, the conditions favour the dissociation of O₂ from oxyhaemoglobin. On the graph, high pCO₂, high H⁺ (low pH) and high temperature shift the curve to the right, meaning lower affinity and easier unloading, which is exactly what working muscle needs. This shift with pCO₂ and pH is the Bohr effect, a name that is not in the rationalised NCERT but that examiners still use.

Every 100 mL of oxygenated blood can deliver around 5 mL of O₂ to the tissues under normal physiological conditions. Learn this number alongside the 4 mL of CO₂ delivered per 100 mL of deoxygenated blood to the alveoli.

A person going up a hill meets lower atmospheric pressure and therefore lower pO₂. Less O₂ loads onto haemoglobin, the tissues receive less oxygen (hypoxia), and the body responds with a faster, deeper breathing rate and a faster heartbeat; over days it makes more RBCs. This is NCERT exercise question 9.

CO₂ is carried by haemoglobin as carbamino-haemoglobin (about 20 to 25 per cent). This binding is related to the partial pressure of CO₂, and pO₂ is a major factor that affects it. When pCO₂ is high and pO₂ is low, as in the tissues, more binding of CO₂ occurs; when pCO₂ is low and pO₂ is high, as in the alveoli, CO₂ dissociates from carbamino-haemoglobin. So haemoglobin picks up CO₂ where it drops O₂, and drops CO₂ where it picks up O₂.

The bulk of CO₂, about 70 per cent, travels as bicarbonate, and the chemistry depends on one enzyme. RBCs contain a very high concentration of carbonic anhydrase, and minute quantities are present in the plasma too. This enzyme catalyses the following reaction in both directions:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

At the tissue site, where pCO₂ is high because of catabolism, CO₂ diffuses into the blood (into the RBCs and the plasma) and forms HCO₃⁻ and H⁺. At the alveolar site, where pCO₂ is low, the reaction proceeds in the opposite direction, forming CO₂ and H₂O. Thus CO₂ trapped as bicarbonate at the tissue level is transported to the alveoli and released out as CO₂. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

One detail that older NCERT editions and most school papers still ask: the bicarbonate formed inside the RBC diffuses out into the plasma, and to keep the RBC electrically neutral chloride ions move from the plasma into the RBC. This exchange is the chloride shift (Hamburger phenomenon). At the alveoli the movement reverses. The H⁺ released stays inside the RBC and is buffered by haemoglobin, which is also what lowers haemoglobin's affinity for O₂ in the tissues.

Read the two sub-sections together and the logic becomes one story. At the tissue: O₂ unloads, CO₂ loads as carbamino-Hb and bicarbonate, H⁺ rises. At the alveolus: CO₂ unloads from both forms and leaves, H⁺ falls, O₂ loads. The same haemoglobin molecule, the same enzyme, and the direction is set purely by the partial pressures on each side.

Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. This is done by the neural system; there is no hormonal regulation to write about in this chapter.

A specialised centre in the medulla region of the brain, the respiratory rhythm centre, is primarily responsible for the regulation. It sets the basic rhythm of inspiration and expiration. Another centre in the pons region, the pneumotaxic centre, can moderate the functions of the rhythm centre: a neural signal from the pneumotaxic centre can reduce the duration of inspiration and thereby alter the respiratory rate. So the medulla produces the rhythm and the pons fine-tunes it.

The chemical inputs come next. A chemosensitive area situated adjacent to the rhythm centre is highly sensitive to CO₂ and hydrogen ions. An increase in these substances activates this centre, which in turn signals the rhythm centre to make the necessary adjustments in the respiratory process so that the excess CO₂ and H⁺ are eliminated. In addition, receptors associated with the aortic arch and the carotid artery also recognise changes in CO₂ and H⁺ concentration and send the necessary signals to the rhythm centre for remedial action. The role of oxygen in the regulation of respiratory rhythm is quite insignificant. That last sentence is asked almost every year in some form, either directly or as an Assertion-Reason.

Why CO₂ and not O₂? Because CO₂ in blood becomes H₂CO₃ and then H⁺, so a small rise in CO₂ changes blood pH quickly and dangerously; the body guards pH first. O₂ stores on haemoglobin are large and the flat top of the dissociation curve means saturation barely changes with moderate falls in pO₂, so O₂ is a poor signal. Only a severe fall in pO₂ stimulates breathing through the peripheral receptors.

For a 3-mark answer write four points: rhythm centre in medulla; pneumotaxic centre in pons reduces inspiration duration; chemosensitive area near the rhythm centre senses CO₂ and H⁺; aortic and carotid receptors also sense CO₂ and H⁺; oxygen insignificant. A simple flow diagram earns the same marks faster.

NCERT lists three disorders and you should be able to define each in one sentence with its cause, because a 3-mark question of the form 'explain any three respiratory disorders' is standard.

Asthma is a difficulty in breathing that causes wheezing, due to inflammation of the bronchi and bronchioles. The inflamed, narrowed airways with excess mucus resist airflow, especially during expiration, which produces the whistling sound. It is usually an allergic response to dust, pollen or smoke and is treated with bronchodilators.

Emphysema is a chronic disorder in which the alveolar walls are damaged, due to which the respiratory surface is decreased. Neighbouring alveoli merge into larger sacs with much less surface area, the lungs lose elasticity, and the patient cannot exchange enough gas even though the airways are open. One of the major causes is cigarette smoking. Because the loss of alveolar wall is permanent, emphysema does not reverse.

Occupational respiratory disorders arise in certain industries, especially those involving grinding or stone-breaking, where so much dust is produced that the defence mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis, the proliferation of fibrous tissues, and thus cause serious lung damage. The stiff fibrous lung cannot expand, and vital capacity falls. Workers in such industries should wear protective masks. Silicosis and asbestosis are the named examples used in school papers.

Beyond NCERT, two terms appear in questions on this chapter. Hypoxia is a shortage of oxygen at the tissue level; it can be caused by low atmospheric pO₂ at high altitude, by anaemia, by carbon monoxide poisoning (CO binds haemoglobin more strongly than O₂) or by lung disease, and its symptoms are breathlessness, headache, fatigue and, in severe cases, confusion. Pneumonia is an infection (usually bacterial, Streptococcus pneumoniae) in which the alveoli fill with fluid and gas exchange falls; you will meet it again in Class 12 Human Health and Disease.