Breathing and Exchange of Gases | CBSE Class 11 Biology Notes

This note covers breathing and the exchange of gases: the respiratory organs of different animals, the human respiratory system, the mechanism of inspiration and expiration, respiratory volumes and capacities, the exchange of gases at the alveoli and the tissues, the transport of oxygen and carbon dioxide in blood, the regulation of respiration, and the disorders of the respiratory system.

What is breathing, and why do cells need it?

Organisms use oxygen (O₂) to break down simple molecules such as glucose, amino acids and fatty acids indirectly. This breakdown releases the energy needed to perform various activities.

These catabolic reactions also release carbon dioxide (CO₂), which is harmful. So O₂ has to be provided to the cells continuously, and the CO₂ produced by the cells has to be released out.

The process of exchange of O₂ from the atmosphere with CO₂ produced by the cells is called breathing. It is commonly known as respiration.

You can feel it happening. Place your hands on your chest and you will feel the chest moving up and down. That movement is due to breathing.

Definition: Breathing is the exchange of O₂ from the atmosphere with CO₂ produced by the cells. It is commonly called respiration.

The five steps of respiration

In humans, respiration is not a single event. It involves the following steps, in this order.

1. Breathing or pulmonary ventilation, by which atmospheric air is drawn in and CO₂-rich alveolar air is released out.
2. Diffusion of gases (O₂ and CO₂) across the alveolar membrane.
3. Transport of gases by the blood.
4. Diffusion of O₂ and CO₂ between blood and tissues.
5. Utilisation of O₂ by the cells for catabolic reactions and the resultant release of CO₂. This last step is cellular respiration.

How do different animals exchange gases?

Mechanisms of breathing vary among different groups of animals. They depend mainly on the animals' habitats and their levels of organisation.

Lower invertebrates such as sponges, coelenterates and flatworms exchange O₂ with CO₂ by simple diffusion over their entire body surface.

Earthworms use their moist cuticle. Insects have a network of tubes, the tracheal tubes, which transport atmospheric air within the body.

Most aquatic arthropods and molluscs use special vascularised structures called gills. Breathing through gills is branchial respiration.

Terrestrial forms use vascularised bags called lungs for the exchange of gases. Breathing through lungs is pulmonary respiration.

Among vertebrates, fishes use gills, whereas amphibians, reptiles, birds and mammals respire through lungs. Amphibians such as frogs can also respire through their moist skin. This is cutaneous respiration.

What are the parts of the human respiratory system?

From the nostrils to the trachea

We have a pair of external nostrils opening out above the upper lips. They lead to a nasal chamber through the nasal passage.

The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air. The pharynx opens through the larynx region into the trachea.

The larynx is a cartilaginous box which helps in sound production, and hence it is called the sound box.

During swallowing, the glottis can be covered by a thin, elastic, cartilaginous flap called the epiglottis. This prevents the entry of food into the larynx.

The bronchial tree

The trachea is a straight tube extending up to the mid-thoracic cavity. At the level of the 5th thoracic vertebra, it divides into a right and a left primary bronchus.

Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and the bronchioles. These end in very thin terminal bronchioles.

The trachea, the primary, secondary and tertiary bronchi, and the initial bronchioles are supported by incomplete cartilaginous rings.

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.

The path followed by inspired air is:

1. External nostrils, then the nasal passage and the nasal chamber.
2. Pharynx, the common passage for food and air.
3. Larynx (the sound box), whose glottis can be covered by the epiglottis during swallowing.
4. Trachea, which divides into the right and left primary bronchi at the level of the 5th thoracic vertebra.
5. Secondary and tertiary bronchi, then bronchioles and the terminal bronchioles.
6. Alveoli, where O₂ and CO₂ are exchanged with the blood.

The lungs and the pleura

We have two lungs. They are covered by a double-layered pleura, with pleural fluid between the layers. The fluid reduces friction on the lung surface.

The outer pleural membrane is in close contact with the thoracic lining. The inner pleural membrane is in contact with the lung surface.

A front view of the chest. The epiglottis, larynx and ringed trachea run down the middle and branch into a bronchus to each lung, with the heart between the lungs and the curved diaphragm below.

The lung on the right of the drawing (the left lung) is cut open to show branching bronchioles ending in clusters of alveoli, surrounded by the pleural membranes with pleural fluid between them. Cut ends of ribs are drawn along both sides.

See Fig. 14.1 in your NCERT textbook

What are the conducting and exchange parts, and how is the thorax built?

The human respiratory system has two functional parts.

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 respiratory part, or exchange part, is formed by the alveoli and their ducts. It is the site of the actual diffusion of O₂ and CO₂ between blood and atmospheric air.

The thoracic chamber

The lungs are situated in the thoracic chamber, which is anatomically an air-tight chamber. It is formed:

• dorsally by the vertebral column;
• ventrally by the sternum;
• laterally by the ribs;
• on the lower side by the dome-shaped diaphragm.

The lungs are set in the thorax in such a way that any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity.

This arrangement is essential for breathing, because we cannot directly alter the pulmonary volume. We change it by changing the volume of the thorax, as the next two sections show.

How does inspiration take place?

Breathing involves two stages. In inspiration, atmospheric air is drawn in. In expiration, the alveolar air is released out.

Air moves into and out of the lungs because a pressure gradient is created between the lungs and the atmosphere. The pressure within the lungs is the intra-pulmonary pressure.

Inspiration can occur if the intra-pulmonary pressure is less than the atmospheric pressure. In other words, there is a negative pressure in the lungs with respect to atmospheric pressure.

These gradients are generated by the diaphragm and a specialised set of muscles between the ribs: the external intercostals and the internal intercostals.

Steps of inspiration

1. Inspiration is initiated by the contraction of the diaphragm. This increases the volume of the thoracic chamber in the antero-posterior axis.
2. The external intercostal muscles contract and lift up the ribs and the sternum. This increases the volume of the thoracic chamber in the dorso-ventral axis.
3. The overall increase in thoracic volume causes a similar increase in pulmonary volume.
4. The increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure.
5. This pressure difference forces air from outside to move into the lungs. This is inspiration.

A side view of the chest, with the vertebral column at the back and the rib cage at the front. Arrows point outwards from the rib cage, labelled "Ribs and sternum raised".

Arrows point downwards from the diaphragm, labelled "Diaphragm contracted". The enlarged chest cavity is labelled "Volume of thorax increased", and an arrow at the top shows "Air entering lungs".

See Fig. 14.2a in your NCERT textbook

Note: The muscles act on the thorax, not directly on the lungs. The lungs follow the thorax because of the way they are set in the air-tight thoracic chamber.

How does expiration take place, and how fast do we breathe?

Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. The muscles that enlarged the thorax now relax.

Steps of expiration

1. The diaphragm and the intercostal muscles relax.
2. The diaphragm and the sternum return to their normal positions.
3. The thoracic volume is reduced, and thereby the pulmonary volume is reduced.
4. The intra-pulmonary pressure rises to slightly above the atmospheric pressure.
5. Air is expelled from the lungs. This is expiration.

The same side view of the chest. The rib cage is labelled "Ribs and sternum returned to original position", with arrows pointing inwards.

The diaphragm is labelled "Diaphragm relaxed and arched upwards", with arrows pointing up. The smaller chest cavity is labelled "Volume of thorax decreased", and an arrow at the top shows "Air expelled from lungs".

See Fig. 14.2b in your NCERT textbook

Forceful breathing and breathing rate

We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.

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

What are the respiratory volumes?

The volumes of air involved in breathing movements are estimated with a spirometer. Four respiratory volumes are described below.

Tidal volume (TV) is the volume of air inspired or expired during a normal respiration. It is approximately 500 mL. So a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.

Inspiratory reserve volume (IRV) is the additional volume of air a person can inspire by a forcible inspiration. It averages 2500 mL to 3000 mL.

Expiratory reserve volume (ERV) is the additional volume of air a person can expire by a forcible expiration. It averages 1000 mL to 1100 mL.

Residual volume (RV) is the volume of air remaining in the lungs even after a forcible expiration. It averages 1100 mL to 1200 mL.

Tidal volume per minute and per hour

The per-minute figure follows from the breathing rate. At 12 to 16 breaths per minute, 500 mL per breath gives 6000 to 8000 mL of air per minute.

Worked example 1. Find the approximate volume of air a healthy human breathes in (tidal volume) in an hour.

Answer: Air breathed per minute is 6000 to 8000 mL. In 60 minutes this is 6000 × 60 = 360,000 mL to 8000 × 60 = 480,000 mL, that is about 360 to 480 litres per hour.

What are the pulmonary capacities, and how are they calculated?

By adding up some of the respiratory volumes, one can derive various pulmonary capacities. These capacities can be used in clinical diagnosis.

Inspiratory capacity (IC) is the total volume of air a person can inspire after a normal expiration. It includes tidal volume and inspiratory reserve volume (TV + IRV).

Expiratory capacity (EC) is the total volume of air a person can expire after a normal inspiration. It includes tidal volume and expiratory reserve volume (TV + ERV).

Functional residual capacity (FRC) is the volume of air that will remain in the lungs after a normal expiration. It includes ERV + RV.

Vital capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration. It includes ERV, TV and IRV. It is also the maximum volume of air a person can breathe out after a forced inspiration.

Total lung capacity (TLC) is the total volume of air accommodated in the lungs at the end of a forced inspiration. It includes RV, ERV, TV and IRV, or vital capacity + residual volume.

The air left in the lungs after a normal breath out is the FRC, that is ERV + RV.

How are gases exchanged at the alveoli and the tissues?

The alveoli are the primary sites of exchange of gases. Exchange of gases also occurs between blood and tissues.

At both sites, O₂ and CO₂ are exchanged by simple diffusion, mainly based on the pressure or concentration gradient. The solubility of the gases and the thickness of the membranes involved are also important factors that can affect the rate of diffusion.

The pressure contributed by an individual gas in a mixture of gases is called its partial pressure. It is written pO₂ for oxygen and pCO₂ for carbon dioxide.

Definition: Partial pressure is the pressure contributed by an individual gas in a mixture of gases.

Reading the gradients

The values show a concentration gradient for oxygen from alveoli to blood, and from blood to tissues. Oxygen moves along it as follows.

1. Alveolar air has a pO₂ of 104 mm Hg, while deoxygenated blood arriving at the alveoli has a pO₂ of 40 mm Hg.
2. O₂ therefore diffuses from the alveoli into the blood.
3. The blood leaves as oxygenated blood with a pO₂ of 95 mm Hg.
4. In the tissues the pO₂ is 40 mm Hg, so O₂ diffuses from the blood into the tissues.

A gradient for CO₂ is present in the opposite direction, from tissues to blood and from blood to alveoli. Deoxygenated blood carries a pCO₂ of 45 mm Hg, above the alveolar value of 40 mm Hg.

At the top, an alveolus receives inspired air and gives out expired air; it is labelled "Alveolar air, pO₂ = 104 mm Hg, pCO₂ = 40 mm Hg", with CO₂ passing into it and O₂ passing out of it.

A pulmonary artery (blue) brings blood to the alveolus and a pulmonary vein (red) takes it to the heart in the centre. On the left, systemic veins carry deoxygenated blood (pO₂ = 40 mm Hg, pCO₂ = 45 mm Hg).

On the right, systemic arteries carry oxygenated blood (pO₂ = 95 mm Hg, pCO₂ = 40 mm Hg). At the bottom, O₂ passes from the blood into the body tissues and CO₂ passes from the tissues into the blood.

See Fig. 14.3 in your NCERT textbook

Why is the alveolar membrane suited to diffusion?

Solubility of the two gases

The solubility of CO₂ is 20 to 25 times higher than that of O₂. So the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than that of O₂.

This is why a small pCO₂ difference, such as 45 mm Hg in deoxygenated blood against 40 mm Hg in the alveoli, is enough to move CO₂ out of the blood.

The three layers of the diffusion membrane

The diffusion membrane is made up of three major layers:

1. the thin squamous epithelium of the alveoli;
2. the endothelium of the alveolar capillaries;
3. the basement substance in between them.

The basement substance is composed of a thin basement membrane supporting the squamous epithelium, and the basement membrane surrounding the single layer of endothelial cells of the capillaries.

However, the total thickness of the membrane is much less than a millimetre. So all the factors in our body are favourable for the diffusion of O₂ from the alveoli to the tissues, and of CO₂ from the tissues to the alveoli.

A rounded alveolar cavity opens at the top, where arrows show air moving in and out. Its wall is labelled "Squamous epithelium of alveolar wall (one-celled thick)".

A blood capillary wraps around the alveolus, containing red blood cells. Between the alveolar wall and the capillary lies the basement substance. The capillary wall is labelled "Endothelium of blood capillary".

See Fig. 14.4 in your NCERT textbook

Why exchange happens in the alveolar region

The alveoli and their ducts are the exchange part of the respiratory system. The alveoli are very thin, irregular-walled and vascularised, and the membrane between air and blood is much less than a millimetre thick.

The conducting part, from the nostrils to the terminal bronchioles, carries, cleans, humidifies and warms the air, and its larger tubes are supported by cartilaginous rings.

How is oxygen transported in the blood?

Blood is the medium of transport for O₂ and CO₂. About 97 per cent of O₂ is transported by RBCs in the blood. The remaining 3 per cent is carried in a dissolved state through the plasma.

Haemoglobin is a red-coloured, iron-containing pigment present in the RBCs. O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.

Each haemoglobin molecule can carry a maximum of four molecules of O₂.

The binding of oxygen with haemoglobin is primarily related to the partial pressure of O₂. The partial pressure of CO₂, the hydrogen ion concentration and the temperature are other factors which can interfere with this binding.

The oxygen dissociation curve

When the percentage saturation of haemoglobin with O₂ is plotted against pO₂, a sigmoid curve is obtained. This is the oxygen dissociation curve.

The curve is highly useful in studying the effect of factors such as pCO₂ and H⁺ concentration on the binding of O₂ with haemoglobin.

The x-axis is the partial pressure of oxygen (mm Hg), from 0 to 100. The y-axis is the percentage saturation of haemoglobin with oxygen, from 0 to 100.

The red curve is S-shaped. It rises gently at first, then steeply between about 10 and 30 mm Hg, and then more slowly, levelling off near 100 per cent saturation at about 100 mm Hg.

See Fig. 14.5 in your NCERT textbook

Loading in the lungs and unloading in the tissues

This shows that O₂ gets bound to haemoglobin at the lung surface and gets dissociated at the tissues.

Under normal physiological conditions, every 100 mL of oxygenated blood can deliver around 5 mL of O₂ to the tissues.

How is carbon dioxide transported in the blood?

Carbon dioxide is carried in three ways. Nearly 20 to 25 per cent is transported by RBCs, about 70 per cent is carried as bicarbonate, and about 7 per cent is carried in a dissolved state through the plasma.

As carbamino-haemoglobin

CO₂ is carried by haemoglobin as carbamino-haemoglobin. This binding is related to the partial pressure of CO₂, and pO₂ is a major factor which could affect 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 CO₂ bound to haemoglobin in the tissues is delivered at the alveoli.

As bicarbonate, with carbonic anhydrase

RBCs contain a very high concentration of the enzyme carbonic anhydrase, and minute quantities of it are present in the plasma too. The enzyme facilitates the following reaction in both directions.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ (carbonic anhydrase acts on both steps)

1. At the tissue site, the partial pressure of CO₂ is high due to catabolism.
2. CO₂ diffuses into the blood (RBCs and plasma) and forms HCO₃⁻ and H⁺.
3. CO₂ is thus trapped as bicarbonate at the tissue level and transported to the alveoli.
4. At the alveolar site, where pCO₂ is low, the reaction proceeds in the opposite direction, forming CO₂ and H₂O.
5. The CO₂ is released out at the alveoli.

Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

How is breathing regulated?

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.

The centres in the brain

A specialised centre in the medulla region of the brain, called the respiratory rhythm centre, is primarily responsible for this regulation.

Another centre in the pons region of the brain, the pneumotaxic centre, can moderate the functions of the respiratory rhythm centre. A neural signal from it can reduce the duration of inspiration and thereby alter the respiratory rate.

A chemosensitive area is situated adjacent to the rhythm centre. It is highly sensitive to CO₂ and hydrogen ions.

How a rise in CO₂ is corrected

1. The concentration of CO₂ and hydrogen ions increases.
2. The increase activates the chemosensitive area next to the rhythm centre. Receptors associated with the aortic arch and carotid artery can also recognise the change.
3. These send signals to the respiratory rhythm centre.
4. The rhythm centre makes the necessary adjustments in the respiratory process, by which these substances can be eliminated.

The role of oxygen in the regulation of the respiratory rhythm is quite insignificant.

What are the disorders of the respiratory system?

Asthma is a difficulty in breathing that causes wheezing. It is due to inflammation of the bronchi and bronchioles, the air tubes of the conducting part.

Emphysema is a chronic disorder in which the alveolar walls are damaged, so the respiratory surface is decreased. One of the major causes of emphysema is cigarette smoking.

Occupational respiratory disorders

In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defence mechanism of the body cannot fully cope with it.

Long exposure can give rise to inflammation leading to fibrosis, the proliferation of fibrous tissues, and thus cause serious lung damage. Workers in such industries should wear protective masks.

Glossary

• Breathing — The exchange of O₂ from the atmosphere with CO₂ produced by the cells; it is commonly known as respiration.
• Epiglottis — A thin, elastic, cartilaginous flap that can cover the glottis during swallowing and prevents the entry of food into the larynx.
• Alveoli — Very thin, irregular-walled, vascularised bag-like structures arising from each terminal bronchiole; they are the primary sites of exchange of gases.
• Pleura — The double-layered membrane covering each lung, with pleural fluid between its layers that reduces friction on the lung surface.
• Intra-pulmonary pressure — The pressure within the lungs; it falls below atmospheric pressure during inspiration and rises slightly above it during expiration.
• Tidal volume — The volume of air inspired or expired during a normal respiration, approximately 500 mL in a healthy person.
• Residual volume — The volume of air remaining in the lungs even after a forcible expiration, averaging 1100 mL to 1200 mL.
• Vital capacity — The maximum volume of air a person can breathe in after a forced expiration; it equals ERV + TV + IRV.
• Partial pressure — The pressure contributed by an individual gas in a mixture of gases, written pO₂ for oxygen and pCO₂ for carbon dioxide.
• Oxyhaemoglobin — The compound formed when O₂ binds reversibly with haemoglobin in RBCs; each haemoglobin molecule carries at most four O₂ molecules.
• Oxygen dissociation curve — The sigmoid curve obtained when percentage saturation of haemoglobin with O₂ is plotted against pO₂.
• Carbamino-haemoglobin — The form in which about 20 to 25 per cent of CO₂ is carried, bound to haemoglobin in the RBCs.
• Carbonic anhydrase — An enzyme found in very high concentration in RBCs and in minute quantities in plasma that facilitates the formation and breakdown of bicarbonate.
• Pneumotaxic centre — A centre in the pons that moderates the respiratory rhythm centre and can reduce the duration of inspiration.

Common errors and misconceptions

• Misconception: Breathing and cellular respiration are the same process. Correct: Breathing (pulmonary ventilation) is the first step of respiration. Cellular respiration is the last step, in which cells use O₂ for catabolic reactions and release CO₂.
• Misconception: The diaphragm relaxes during inspiration. Correct: Inspiration is initiated by contraction of the diaphragm, which increases thoracic volume in the antero-posterior axis. The diaphragm relaxes and arches upwards during expiration.
• Misconception: A forcible expiration empties the lungs completely. Correct: The residual volume, averaging 1100 mL to 1200 mL, remains in the lungs even after a forcible expiration.
• Misconception: Vital capacity is the total volume of air the lungs can hold. Correct: That is total lung capacity. Total lung capacity equals vital capacity plus residual volume.
• Misconception: Most CO₂ is carried by haemoglobin as carbamino-haemoglobin. Correct: About 70 per cent of CO₂ is carried as bicarbonate; 20 to 25 per cent is carried as carbamino-haemoglobin, and about 7 per cent is dissolved in plasma.
• Misconception: A fall in oxygen is the main signal that changes the breathing rate. Correct: The chemosensitive area and the aortic and carotid receptors respond to CO₂ and H⁺. The role of oxygen in regulating the respiratory rhythm is quite insignificant.
• Misconception: CO₂ diffuses poorly because its partial pressure gradient is small. Correct: CO₂ is 20 to 25 times more soluble than O₂, so much more CO₂ diffuses per unit difference in partial pressure.

Exam-style questions with model answers

Q1. What will be the pO₂ and pCO₂ in atmospheric air compared with those in alveolar air? Support your answer with values. [2 marks]

1. Atmospheric air has a higher pO₂ and a lower pCO₂ than alveolar air.
2. pO₂ is 159 mm Hg in atmospheric air against 104 mm Hg in the alveoli; pCO₂ is 0.3 mm Hg in atmospheric air against 40 mm Hg in the alveoli.

Q2. Define vital capacity. What is its significance? [2 marks]

1. Vital capacity is the maximum volume of air a person can breathe in after a forced expiration, or breathe out after a forced inspiration. It includes ERV, TV and IRV, about 4000 to 4600 mL from the average values.
2. It is a pulmonary capacity used in clinical diagnosis, because it measures the largest volume of air the lungs can move in a single breath.

Q3. Explain the process of inspiration under normal conditions. [3 marks]

1. Inspiration occurs when the intra-pulmonary pressure is less than the atmospheric pressure.
2. It is initiated by contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis.
3. The external intercostal muscles contract and lift the ribs and sternum, increasing thoracic volume in the dorso-ventral axis.
4. The overall increase in thoracic volume causes a similar increase in pulmonary volume, which lowers the intra-pulmonary pressure below atmospheric pressure.
5. Air from outside is forced into the lungs.

Q4. Distinguish between (a) IRV and ERV, (b) inspiratory capacity and expiratory capacity, (c) vital capacity and total lung capacity. [3 marks]

1. (a) IRV is the additional air a person can inspire by a forcible inspiration, 2500 to 3000 mL; ERV is the additional air a person can expire by a forcible expiration, 1000 to 1100 mL.
2. (b) Inspiratory capacity is the total air a person can inspire after a normal expiration (TV + IRV); expiratory capacity is the total air a person can expire after a normal inspiration (TV + ERV).
3. (c) Vital capacity is the maximum air a person can breathe in after a forced expiration (ERV + TV + IRV); total lung capacity is the total air in the lungs at the end of a forced inspiration (VC + RV).

Q5. How is respiration regulated in humans? [3 marks]

1. The respiratory rhythm centre in the medulla region of the brain is primarily responsible for regulating the respiratory rhythm.
2. The pneumotaxic centre in the pons moderates the rhythm centre; its signal can reduce the duration of inspiration and so alter the respiratory rate.
3. A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO₂ and H⁺. A rise in these activates it, and it signals the rhythm centre to adjust breathing so they are eliminated.
4. Receptors of the aortic arch and carotid artery also detect changes in CO₂ and H⁺. The role of oxygen is quite insignificant.

Q6. What are the major transport mechanisms for CO₂? Explain. [4 marks]

1. About 70 per cent of CO₂ is carried as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin in RBCs, and about 7 per cent dissolved in plasma.
2. Carbamino-haemoglobin: CO₂ binds to haemoglobin where pCO₂ is high and pO₂ is low, as in the tissues, and dissociates where pCO₂ is low and pO₂ is high, as in the alveoli.
3. Bicarbonate: RBCs have a very high concentration of carbonic anhydrase, which facilitates CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ in both directions.
4. At the tissues, high pCO₂ drives the reaction to the right and CO₂ is trapped as bicarbonate. At the alveoli, low pCO₂ reverses it, releasing CO₂ and H₂O.
5. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

Q7. Define the oxygen dissociation curve. Explain how the conditions in the alveoli and the tissues affect the binding of oxygen to haemoglobin, and suggest a reason for the sigmoid shape of the curve. [5 marks]

1. The oxygen dissociation curve is the sigmoid curve obtained when the percentage saturation of haemoglobin with O₂ is plotted against pO₂. It is used to study the effect of factors such as pCO₂ and H⁺ concentration on the binding of O₂ with haemoglobin.
2. Binding of O₂ is primarily related to pO₂; pCO₂, H⁺ concentration and temperature can also interfere with it.
3. In the alveoli, pO₂ is high, pCO₂ is low, H⁺ concentration is lesser and temperature is lower. These favour the formation of oxyhaemoglobin.
4. In the tissues, pO₂ is low, pCO₂ is high, H⁺ concentration is high and temperature is higher. These favour the dissociation of O₂ from oxyhaemoglobin.
5. So O₂ is bound at the lung surface and released at the tissues; every 100 mL of oxygenated blood delivers around 5 mL of O₂ to the tissues.
6. Reason for the sigmoid shape: each haemoglobin molecule carries up to four O₂ molecules, and once the first binds, the next ones bind more easily. So saturation rises slowly at first, then steeply, and levels off near full saturation.

Key takeaways

• Breathing exchanges atmospheric O₂ for the CO₂ produced by cells; respiration has five steps, from pulmonary ventilation to cellular respiration.
• Air passes from the nostrils through the pharynx, larynx, trachea, bronchi and bronchioles to the alveoli; the trachea divides at the 5th thoracic vertebra.
• Inspiration starts with contraction of the diaphragm and external intercostals, which enlarges the thorax and lowers intra-pulmonary pressure below atmospheric pressure.
• Vital capacity is ERV + TV + IRV, and total lung capacity is vital capacity plus residual volume.
• Gases diffuse along partial pressure gradients across a three-layered membrane much less than a millimetre thick; CO₂ is 20 to 25 times more soluble than O₂.
• About 97 per cent of O₂ travels as oxyhaemoglobin in RBCs, while about 70 per cent of CO₂ travels as bicarbonate formed with carbonic anhydrase.
• The rhythm centre in the medulla regulates breathing, the pneumotaxic centre in the pons moderates it, and rises in CO₂ and H⁺ trigger adjustments; the role of oxygen is quite insignificant.
• Asthma inflames the bronchi and bronchioles, emphysema damages alveolar walls, and long dust exposure in grinding or stone-breaking can cause fibrosis.

Test yourself

How is atmospheric air carried within the body of an insect?

Insects have a network of tubes called tracheal tubes, which transport atmospheric air within the body.

What is cutaneous respiration, and which animals show it?

Cutaneous respiration is breathing through moist skin. Amphibians such as frogs can respire through their moist skin as well as through lungs.

What does the pleural fluid do?

The pleural fluid lies between the two layers of the pleura covering each lung, and it reduces friction on the lung surface.

Why can we not breathe by changing lung volume directly?

We cannot directly alter pulmonary volume. The lungs sit in an air-tight thoracic chamber, so a change in thoracic volume is reflected in the pulmonary volume.

How many times does a healthy human breathe in a minute, and what instrument measures the volumes involved?

A healthy human breathes 12 to 16 times per minute on average. The volumes are estimated with a spirometer.

What is the effect of pCO₂ on oxygen transport?

A high pCO₂, as in the tissues, favours the dissociation of O₂ from oxyhaemoglobin. A low pCO₂, as in the alveoli, favours the formation of oxyhaemoglobin.

Name the three major layers of the diffusion membrane.

The thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them.

What is emphysema, and what is one of its major causes?

Emphysema is a chronic disorder in which alveolar walls are damaged, decreasing the respiratory surface. Cigarette smoking is one of its major causes.