The Respiratory System | ICSE Class 9 Biology Notes

This note covers the ICSE Class 10 Biology topic Respiratory System, which the syllabus for the 2028 examination places in Class 10 (earlier syllabuses placed it in Class 9): the difference between breathing and respiration, the organs of the respiratory system from the nose to the alveoli, the lungs, pleura and diaphragm, the role of the diaphragm and intercostal muscles in inspiration and expiration, the bell-jar model of breathing and how to identify a lung, the exchange and transport of oxygen and carbon dioxide, tissue respiration and heat production, anaerobic respiration in plants and in man, respiratory volumes, the effect of altitude on breathing, and asphyxiation and hypoxia.

What is the difference between breathing and respiration?

Cells use oxygen (O₂) to break down simple molecules such as glucose, amino acids and fatty acids indirectly, and so obtain the energy they need. These catabolic reactions also release carbon dioxide (CO₂), which is harmful.

So O₂ has to be provided to the cells continuously, and the CO₂ they produce has to be released out. The exchange of O₂ from the atmosphere with the CO₂ produced by the cells is called breathing, commonly known as respiration.

Definition: Breathing, or pulmonary ventilation, draws atmospheric air into the lungs and releases CO₂-rich alveolar air. Cellular respiration is the use of O₂ by the cells to break down food, releasing energy and CO₂.

The five steps of respiration

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

Steps 1 and 2 together are often called external respiration, the exchange of gases between the environment and the blood. Step 5, which takes place inside the cells, is called tissue respiration, also known as internal respiration.

How does air pass through the nose, pharynx and larynx?

Nostrils and nasal cavity

We have a pair of external nostrils opening out above the upper lip. They lead through the nasal passage into the nasal chamber, or nasal cavity, which is divided into right and left halves by the nasal septum.

The air is filtered by fine hairs that line the passage. The passage is also lined with mucus, which helps by trapping dust and germs.

The lining of the nasal cavity has a rich blood supply that warms the incoming air, and its moist surface adds water vapour to it. Sense cells for smell lie in its upper part. Air breathed in through the mouth misses this filtering, warming and moistening.

Pharynx

The nasal chamber opens into the pharynx, or throat. A portion of the pharynx is the common passage for food and air. Food passes on into the oesophagus, and air into the larynx.

Larynx

The pharynx opens through the larynx into the trachea. The larynx is a cartilaginous box which helps in sound production, and so it is called the sound box. Its vocal cords vibrate as air is breathed out over them.

The opening of the larynx is the glottis. During swallowing, it can be covered by a thin, elastic, cartilaginous flap called the epiglottis, which prevents the entry of food into the larynx.

If a food particle does slip into the larynx, it is usually forced out by coughing.

What are the trachea, bronchi and bronchioles?

The trachea, or windpipe, is a straight tube extending from the larynx to the middle of the thoracic cavity. In the neck it lies in front of the oesophagus.

Its wall is supported by incomplete cartilaginous rings, which ensure that the air passage does not collapse. The rings are C-shaped and open at the back, where the trachea lies against the oesophagus.

The upper part of the respiratory tract is lined with small, hair-like cilia, together with mucus-secreting cells. The mucus traps germs and dust, and the cilia sweep it upwards, away from the lungs.

Smoking destroys these cilia. Germs, dust, smoke and harmful chemicals then enter the lungs and cause infection, cough and even lung cancer.

The bronchial tree

At the level of the 5th thoracic vertebra, the trachea divides into a right and a left primary bronchus, one to each lung.

Each bronchus divides repeatedly to form the secondary and tertiary bronchi and the bronchioles, which end in very thin terminal bronchioles. The trachea, the bronchi and the initial bronchioles are supported by incomplete cartilaginous rings.

The path of inspired air

1. External nostrils, nasal passage and nasal chamber.
2. Pharynx, the common passage for food and air.
3. Larynx, entered through the glottis.
4. Trachea, which divides into the two primary bronchi.
5. Secondary and tertiary bronchi, bronchioles and terminal bronchioles.
6. Alveoli, where O₂ and CO₂ are exchanged with the blood.

The part from the external nostrils to the terminal bronchioles is the conducting part. It transports air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature.

An outline of a human head and chest. The nasal passage, mouth cavity, pharynx and larynx are labelled on the left, with the bronchi, bronchioles and an alveolar sac lower down.

On the right are the trachea with its rings of cartilage, a lung, the ribs and the diaphragm below the lungs. An arrow leads from the lung to an enlarged view of clusters of alveoli at the ends of respiratory bronchioles.

See Fig. 5.9 in your NCERT textbook

How are the alveoli suited to the exchange of gases?

Each terminal bronchiole gives rise to a number of very thin, irregular-walled, vascularised, bag-like structures called alveoli (singular alveolus). These balloon-like air sacs are the primary sites of exchange of gases.

The alveoli and their ducts form the exchange part of the respiratory system, the site of the actual diffusion of O₂ and CO₂ between the blood and the air.

Features that make exchange efficient

• Huge surface area: if the alveolar surface were spread out, it would cover about 80 m².
• Very thin barrier: the alveolar wall is one cell thick, and the whole diffusion membrane is much less than a millimetre thick.
• Rich blood supply: the walls contain an extensive network of blood vessels.
• Moist lining: a thin film of moisture lines each alveolus, so the gases dissolve before they diffuse.
• Air always present: the lungs always contain a residual volume of air, giving time for O₂ to be absorbed and CO₂ released.

The diffusion membrane has three layers: the thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary, and the basement substance between them.

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

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

See Fig. 14.4 in your NCERT textbook

How are the lungs, pleura and diaphragm arranged in the chest?

We have two lungs, one on either side of the heart. The branching network of bronchi, bronchioles and alveoli makes up the lungs, so they are soft, spongy and elastic.

The right lung is larger and has three lobes. The left lung is smaller and has two lobes, with a notch on its inner side where the heart lies.

The pleura

Each lung is covered by a double-layered pleura, with pleural fluid between the layers that reduces friction on the lung surface. The outer layer is in close contact with the chest wall and the inner layer with the lung surface.

The thoracic chamber

The lungs lie in the thoracic chamber, an air-tight chamber formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and below by the dome-shaped diaphragm.

The diaphragm is a sheet of muscle separating the thoracic cavity from the abdominal cavity. Between the ribs lie the external and internal intercostal muscles.

Any change in the volume of the thoracic cavity is reflected in the lungs. This is essential for breathing, because we cannot directly alter the volume of the lungs.

Blood vessels of the lungs

The pulmonary artery brings deoxygenated blood from the right ventricle of the heart to the lungs. The pulmonary veins return oxygenated blood from the lungs to the left atrium.

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

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

See Fig. 14.1 in your NCERT textbook

How do the diaphragm and intercostal muscles bring about inspiration?

Breathing has two stages: inspiration, in which atmospheric air is drawn in, and expiration, in which alveolar air is released out.

Air moves because a pressure gradient is created between the lungs and the atmosphere. Inspiration can occur if the pressure within the lungs, the intra-pulmonary pressure, is less than the atmospheric pressure.

The diaphragm and the external and internal intercostal muscles generate these gradients.

Steps of inspiration

1. The diaphragm contracts and flattens, increasing 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, increasing the volume in the dorso-ventral axis.
3. The overall increase in thoracic volume causes a similar increase in pulmonary volume.
4. The intra-pulmonary pressure falls below the atmospheric pressure.
5. Air from outside is forced 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 under the diaphragm, labelled "Diaphragm contracted". The 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 breathing muscles act on the walls of the thorax, not directly on the lungs. The lungs follow the thorax because they sit in an air-tight chamber.

How does expiration take place, and what happens in forceful breathing?

Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. In quiet breathing, the muscles that enlarged the thorax simply relax.

Steps of expiration

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

The same side view. 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 cavity is labelled "Volume of thorax decreased", and an arrow shows "Air expelled from lungs".

See Fig. 14.2b in your NCERT textbook

Forceful breathing and breathing rate

We can increase the strength of inspiration and expiration with additional muscles in the abdomen. In a forceful expiration, the internal intercostal muscles also contract and pull the ribs down and in.

On average, a healthy human breathes 12 to 16 times per minute. The volumes of air involved can be estimated with a spirometer.

How does the bell-jar model show the mechanism of breathing?

The bell-jar model shows how a change in the volume of a closed chamber draws air into, and pushes air out of, the lungs.

A glass bell jar has its open bottom closed by a thin rubber sheet, which can be pulled down by a string or knob at its centre. A cork at the top carries a Y-shaped glass tube, and a balloon is tied to each lower branch of the tube inside the jar.

Draw two views of the bell jar, each with the cork, the Y-tube, a balloon on each branch and the rubber sheet across the bottom.

In (a), show the sheet pulled down, the balloons inflated and air entering the tube. In (b), show the sheet pushed up, the balloons deflated and air leaving. Label the jar, rubber sheet, Y-tube and balloons.

Working of the model

1. The rubber sheet is pulled down, as the diaphragm moves down when it contracts.
2. The volume inside the jar increases, so the air pressure around the balloons falls.
3. Air from outside rushes in through the Y-tube and the balloons inflate, representing inspiration.
4. The sheet is pushed up, as the diaphragm arches up when it relaxes.
5. The volume inside the jar decreases and the pressure rises, so air is pushed out and the balloons deflate, representing expiration.

Limitations of the model

• The rigid jar cannot show the raising and lowering of the ribs by the intercostal muscles.
• The rubber sheet is pulled by hand and is flat, whereas the diaphragm is a dome-shaped sheet that moves by the contraction of its own muscle.
• The balloons are hollow bags, not spongy organs full of alveoli, and no exchange of gases takes place in them.
• The balloons can collapse completely, whereas real lungs keep a residual volume of air.

Identifying a lung from a specimen, model or chart

A lung is cone-shaped, with a narrow apex and a broad base resting on the diaphragm. It is soft, spongy and elastic, and its smooth outer surface is covered by the pleura.

A bronchus and the pulmonary blood vessels enter on its inner side, facing the heart. In a cut section, the air tubes are seen branching like a tree, and the tissue around them is spongy because it is made of tiny alveoli, which charts and models draw as clusters.

How are oxygen and carbon dioxide exchanged and carried in the blood?

At the alveoli and at the tissues, O₂ and CO₂ move by simple diffusion, mainly according to the pressure or concentration gradient. The pressure contributed by one gas in a mixture is its partial pressure (pO₂ or pCO₂).

The pO₂ is 104 mm Hg in the alveoli but 40 mm Hg in deoxygenated blood, so O₂ diffuses into the blood. Oxygenated blood (95 mm Hg) then gives O₂ to the tissues (40 mm Hg). CO₂ moves the opposite way, from the tissues (45 mm Hg) to the blood and from the blood to the alveoli (40 mm Hg).

The solubility of CO₂ is 20 to 25 times higher than that of O₂, so a small difference in pCO₂ moves a lot of CO₂.

Transport of oxygen

Haemoglobin is a red, iron-containing pigment in the red blood cells (RBCs). O₂ binds with it reversibly to form oxyhaemoglobin, and each haemoglobin molecule can carry a maximum of four O₂ molecules.

About 97 per cent of O₂ is carried by the RBCs in this way; the remaining 3 per cent is dissolved in the plasma.

In the alveoli, high pO₂ and low pCO₂ favour the formation of oxyhaemoglobin. In the tissues, low pO₂, high pCO₂, high H⁺ concentration and higher temperature favour the release of O₂. Every 100 mL of oxygenated blood delivers around 5 mL of O₂ to the tissues.

Transport of carbon dioxide

• As bicarbonate (about 70 per cent): the enzyme carbonic anhydrase, present in very high concentration in RBCs, speeds up CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.
• As carbamino-haemoglobin (about 20 to 25 per cent), bound to haemoglobin.
• Dissolved in plasma (about 7 per cent).

At the tissues, where pCO₂ is high, CO₂ is trapped as bicarbonate; at the alveoli, where pCO₂ is low, the reaction reverses and CO₂ is released. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

At the top, an alveolus labelled "Alveolar air, pO₂ = 104 mm Hg, pCO₂ = 40 mm Hg" takes in inspired air and gives out expired air; CO₂ enters it and O₂ leaves it.

A pulmonary artery (blue) brings blood to the alveolus, and a pulmonary vein (red) takes blood to the heart. Systemic veins (pO₂ = 40, pCO₂ = 45 mm Hg) and systemic arteries (pO₂ = 95, pCO₂ = 40 mm Hg) link the heart with the body tissues at the bottom.

See Fig. 14.3 in your NCERT textbook

What is tissue respiration, and how does it produce heat?

Tissue respiration is the breakdown of food, mainly glucose, inside the cells to release energy. When O₂ is present, glucose is broken down completely into carbon dioxide and water. This is aerobic respiration:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Stages of aerobic respiration

1. Glucose, a six-carbon molecule, is broken down into pyruvate, a three-carbon molecule, in the cytoplasm.
2. Pyruvate is broken down using oxygen in the mitochondria.
3. Each pyruvate gives three molecules of carbon dioxide, and water is formed.
4. The energy released is used to make ATP from ADP and inorganic phosphate.
5. ATP is broken down wherever energy is needed in the cell.

ATP is the energy currency for most cellular processes. Breaking its terminal phosphate linkage with water releases energy equivalent to 30.5 kJ/mol, used for muscle contraction, protein synthesis, nerve impulses and other activities.

Heat production

Not all the energy released from glucose is trapped in ATP; a part is given off as heat. In human beings this heat keeps the body temperature steady, at about 37 °C.

During exercise the muscles respire faster and release more heat, so the body warms up. Heat production is a direct sign of the energy released in the cells by tissue respiration.

How does anaerobic respiration in plants differ from that in man?

Some organisms and tissues break down glucose without using oxygen. This is anaerobic respiration. In every case, the first step is the breakdown of glucose into pyruvate in the cytoplasm; what happens next depends on the organism.

In plants and yeast

In yeast, in the absence of oxygen, pyruvate is converted into ethanol (a two-carbon molecule) and carbon dioxide. This is fermentation. Plant tissues short of oxygen, such as roots in waterlogged soil and some germinating seeds, respire in the same way:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy

In human muscles

When there is a lack of oxygen in our muscle cells, pyruvate is converted into lactic acid, also a three-carbon molecule, and no carbon dioxide is produced:

C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + energy

The build-up of lactic acid in the muscles during sudden activity causes cramps. Afterwards, fast breathing continues for a while to supply the extra oxygen needed to remove the lactic acid, called the oxygen debt.

"Glucose (6-carbon molecule)" leads, "In cytoplasm", to "Pyruvate (3-carbon molecule) + Energy", from which three arrows branch.

"Absence of oxygen (in yeast)" leads to "Ethanol + Carbon dioxide + Energy (2-carbon molecule)". "Lack of oxygen (in our muscle cells)" leads to "Lactic acid + Energy (3-carbon molecule)". "Presence of oxygen (in mitochondria)" leads to "Carbon dioxide + Water + Energy".

See Fig. 5.8 in your NCERT textbook

Aerobic respiration releases far more energy: up to about 38 ATP per glucose molecule, against a net 2 ATP in anaerobic respiration.

What are the respiratory volumes?

The volumes of air moved in breathing are measured with a spirometer. They are called respiratory volumes, and sums of them are called capacities.

Tidal volume (TV) is the volume of air inspired or expired in a normal breath, approximately 500 mL. So a healthy person breathes about 6000 to 8000 mL of air per minute.

Inspiratory reserve volume (IRV) is the extra air a person can inspire by a forcible inspiration, averaging 2500 mL to 3000 mL.

Expiratory reserve volume (ERV) is the extra air a person can expire by a forcible expiration, averaging 1000 mL to 1100 mL.

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

Vital capacity and total lung capacity

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

Total lung capacity (TLC) is the total volume of air in the lungs at the end of a forced inspiration: RV + ERV + TV + IRV, or VC + RV.

Vital capacity is a useful measure of lung fitness. It tends to be higher in athletes and people who exercise regularly, and lower in people with lung disease.

How does altitude affect breathing?

As we go higher above sea level, the atmospheric pressure falls. The air contains about the same proportion of oxygen, but the partial pressure of oxygen is lower, so each breath brings in less oxygen.

The pO₂ in the alveoli falls too, so the gradient driving O₂ into the blood is smaller and haemoglobin is less fully loaded.

Effects of a quick climb

1. The person breathes faster and more deeply.
2. The heart beats faster, to pump more blood to the tissues.
3. Mountain sickness may develop, with headache, breathlessness, nausea, dizziness, tiredness and poor sleep.
4. At very great heights, the shortage of oxygen can cause confusion and loss of consciousness.

Acclimatisation

If a person stays at a high altitude for some days or weeks, the body adjusts. This is acclimatisation: the body makes more red blood cells and haemoglobin, so the blood carries more oxygen. People who live at high altitudes usually have a higher red blood cell count.

Climbers go up in stages with rest days, mountaineers carry oxygen cylinders at very great heights, and high-flying aircraft have pressurised cabins.

What are asphyxiation and hypoxia?

Hypoxia is a condition in which the body, or a part of it, does not receive enough oxygen for its needs. Its signs include breathlessness, a fast heartbeat, headache, confusion and a bluish tinge to the lips and skin.

Causes of hypoxia include breathing air with a low pO₂ at high altitudes; lung diseases such as emphysema, in which alveolar walls are damaged, and asthma, in which the bronchi and bronchioles are inflamed; too little haemoglobin, as in anaemia; and carbon monoxide poisoning.

Asphyxiation (asphyxia, or suffocation) is a condition in which the body is deprived of oxygen because breathing is blocked or the air lacks oxygen. Carbon dioxide also builds up in the blood, and if it lasts more than a few minutes it leads to unconsciousness and death.

Causes of asphyxiation include choking on food, strangulation, drowning, breathing smoke in a fire, and carbon monoxide from coal or charcoal burnt in a closed room.

Carbon monoxide combines with haemoglobin far more readily than oxygen does, forming a stable compound, carboxyhaemoglobin. That haemoglobin can no longer carry oxygen, so the tissues are starved of it even though the person is breathing.

Glossary

• Breathing — The physical process by which atmospheric air is drawn into the lungs and CO₂-rich alveolar air is released out; also called pulmonary ventilation.
• Tissue respiration — The breakdown of food, mainly glucose, inside the cells to release energy, using O₂ and producing CO₂ and water.
• Epiglottis — A thin, elastic, cartilaginous flap that covers the glottis during swallowing and prevents the entry of food into the larynx.
• Alveoli — Very thin, irregular-walled, vascularised air sacs at the ends of the terminal bronchioles; the primary sites of exchange of gases.
• Pleura — The double-layered membrane covering each lung, with pleural fluid between the layers that reduces friction on the lung surface.
• Diaphragm — The dome-shaped sheet of muscle forming the floor of the thoracic chamber; its contraction starts inspiration.
• Oxyhaemoglobin — The compound formed when O₂ binds reversibly with haemoglobin in red blood cells; about 97 per cent of O₂ travels this way.
• Tidal volume — The volume of air inspired or expired during a normal breath, 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 out after a forced inspiration; it equals ERV + TV + IRV.
• Oxygen debt — The extra oxygen taken in after hard exercise to remove the lactic acid that built up in the muscles.
• Hypoxia — A condition in which the body or a tissue does not receive enough oxygen for its needs, as at high altitudes.
• Asphyxiation — Suffocation; deprivation of oxygen because breathing is blocked or the air lacks oxygen, with a build-up of carbon dioxide.

Common errors and misconceptions

• Misconception: Breathing and respiration mean the same thing. Correct: Breathing is the physical movement of air into and out of the lungs; tissue respiration is the chemical breakdown of food in the cells that releases energy.
• Misconception: The diaphragm relaxes during inspiration. Correct: The diaphragm contracts and flattens during inspiration, and relaxes and arches upwards during expiration.
• Misconception: The lungs expand by themselves and push the ribs out. Correct: The diaphragm and external intercostals enlarge the thorax, and the lungs, in their air-tight chamber, expand as a result.
• Misconception: A forcible expiration empties the lungs completely. Correct: A residual volume of 1100 mL to 1200 mL remains even after a forcible expiration.
• Misconception: Anaerobic respiration in human muscles produces alcohol and carbon dioxide. Correct: Human muscles produce lactic acid and no carbon dioxide; ethanol and carbon dioxide come from yeast and plant tissues.
• Misconception: Air at high altitudes has a lower percentage of oxygen. Correct: The proportion is about the same, but the lower atmospheric pressure lowers the partial pressure of oxygen.
• Misconception: Most carbon dioxide is carried by haemoglobin. Correct: About 70 per cent travels as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin and about 7 per cent dissolved in plasma.

Exam-style questions with model answers

Q1. State two differences between breathing and tissue respiration. [2 marks]

1. Breathing takes place in the air passages and lungs, whereas tissue respiration takes place inside living cells, in the cytoplasm and mitochondria.
2. Breathing is a physical process that releases no energy from food, whereas tissue respiration is a chemical process in which enzymes break down food to release energy.

Q2. What is residual volume? Why is it useful that the lungs always contain some air? [2 marks]

1. Residual volume is the volume of air remaining in the lungs even after a forcible expiration, averaging 1100 mL to 1200 mL.
2. Because air is always present, there is sufficient time for O₂ to be absorbed and CO₂ to be released, and the alveoli do not collapse.

Q3. Distinguish between hypoxia and asphyxiation, giving one cause of each. [2 marks]

1. Hypoxia is a condition in which the body or a tissue receives too little oxygen; one cause is the low pO₂ of air at high altitudes.
2. Asphyxiation is deprivation of oxygen because breathing is blocked or the air lacks oxygen, with CO₂ building up; one cause is drowning.

Q4. In the bell-jar model: (a) what do the bell jar, the rubber sheet, the Y-shaped tube and the balloons represent? (b) What happens to the balloons when the rubber sheet is pulled down, and why? [3 marks]

1. (a) The bell jar represents the wall of the thorax, the rubber sheet the diaphragm, the Y-tube the trachea and the two bronchi, and the balloons the lungs.
2. (b) The balloons inflate. Pulling the sheet down increases the volume inside the jar, so the pressure around the balloons falls below atmospheric pressure, and air from outside rushes in through the tube, as in inspiration.

Q5. Give three differences between anaerobic respiration in plants and in man. [3 marks]

1. Product: in plants and yeast, pyruvate becomes ethanol; in human muscles it becomes lactic acid.
2. Carbon dioxide: released in plants; not released in human muscles.
3. Occurrence and effect: in plants it occurs in tissues short of oxygen, such as waterlogged roots, and ethanol can harm the cells; in man it occurs in muscles during sudden activity, and lactic acid causes cramps.

Q6. Describe briefly how oxygen and carbon dioxide are transported in the blood. [4 marks]

1. About 97 per cent of O₂ is carried by RBCs as oxyhaemoglobin, formed when O₂ binds reversibly with haemoglobin; each haemoglobin molecule carries up to four O₂ molecules. The other 3 per cent is dissolved in plasma.
2. Oxyhaemoglobin forms in the alveoli, where pO₂ is high, and releases O₂ in the tissues, where pO₂ is low and pCO₂ is high.
3. About 70 per cent of CO₂ is carried as bicarbonate, formed with the help of carbonic anhydrase in RBCs.
4. About 20 to 25 per cent travels as carbamino-haemoglobin and about 7 per cent dissolved in plasma; CO₂ is released at the alveoli, where pCO₂ is low.

Q7. A person travels from sea level to a high mountain region. Explain the effect on breathing and how the body adjusts over some weeks. [4 marks]

1. At high altitude the atmospheric pressure is lower, so the partial pressure of oxygen in the air and the alveoli is lower.
2. Less oxygen diffuses into the blood, so the person breathes faster and more deeply, and the heart beats faster.
3. A fast climb may cause mountain sickness, with headache, breathlessness, nausea and tiredness.
4. Over days to weeks the body acclimatises by making more red blood cells and haemoglobin, so the blood carries more oxygen.

Q8. Describe the role of the diaphragm and the intercostal muscles in inspiration and expiration. [5 marks]

1. The lungs lie in an air-tight thoracic chamber, so a change in thoracic volume changes the lung volume and the intra-pulmonary pressure.
2. Inspiration: the diaphragm contracts and flattens, increasing thoracic volume in the antero-posterior axis.
3. The external intercostal muscles contract and lift the ribs and sternum, increasing volume in the dorso-ventral axis.
4. The lungs expand, the intra-pulmonary pressure falls below atmospheric pressure and air rushes in.
5. Expiration: the diaphragm and intercostal muscles relax; the diaphragm arches upwards and the ribs and sternum return to their normal positions.
6. The thoracic and lung volumes decrease, the pressure rises slightly above atmospheric pressure and air is expelled. In forceful expiration the internal intercostals and abdominal muscles also contract.

Q9. Define tidal volume, vital capacity and total lung capacity. A person has an IRV of 3000 mL, a TV of 500 mL, an ERV of 1100 mL and an RV of 1200 mL. Calculate the vital capacity and the total lung capacity. [5 marks]

1. Tidal volume is the volume of air inspired or expired during a normal breath, approximately 500 mL in a healthy person.
2. Vital capacity is the maximum volume of air a person can breathe out after a forced inspiration; it is ERV + TV + IRV.
3. Total lung capacity is the total volume of air in the lungs at the end of a forced inspiration; it is vital capacity + residual volume.
4. Vital capacity = 1100 + 500 + 3000 = 4600 mL.
5. Total lung capacity = 4600 + 1200 = 5800 mL.

Key takeaways

• Breathing is the physical movement of air into and out of the lungs; tissue respiration is the breakdown of food inside cells that releases energy.
• Air passes from the nostrils through the pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, being filtered, warmed and moistened on the way.
• Alveoli have a surface of about 80 m², walls one cell thick and a rich network of blood vessels, which suits them to diffusion of gases.
• Inspiration starts with contraction of the diaphragm and external intercostals, which enlarge the thorax and lower the intra-pulmonary pressure below atmospheric pressure.
• In the bell-jar model the rubber sheet stands for the diaphragm and the balloons for the lungs; pulling the sheet down inflates the balloons.
• About 97 per cent of O₂ travels as oxyhaemoglobin, and about 70 per cent of CO₂ travels as bicarbonate formed with carbonic anhydrase.
• Anaerobic respiration gives ethanol and CO₂ in yeast and plants but lactic acid without CO₂ in human muscles, releasing much less energy.
• High altitude lowers the partial pressure of oxygen, causing faster breathing and possible mountain sickness until the body acclimatises.

Test yourself

Which part of the respiratory tract is shared by food and air?

A portion of the pharynx is the common passage for food and air. Food passes on into the oesophagus and air into the larynx.

At the level of which vertebra does the trachea divide, and into what?

At the level of the 5th thoracic vertebra, the trachea divides into a right and a left primary bronchus.

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 during breathing.

How many times does a healthy human breathe in a minute, and which instrument measures breathing volumes?

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

Why does lactic acid build up in muscles during sudden activity?

When muscle cells lack oxygen, pyruvate is converted into lactic acid instead of being broken down in the mitochondria, and the acid causes cramps.

Name the three layers of the diffusion membrane in the lungs.

The thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary, and the basement substance between them.

What is the source of the heat that keeps the human body warm?

Part of the energy released from glucose in tissue respiration is not trapped in ATP but given off as heat, which keeps body temperature steady.

Why is carbon monoxide dangerous to breathe?

Carbon monoxide combines with haemoglobin far more readily than oxygen, forming carboxyhaemoglobin, so the blood cannot carry enough oxygen to the tissues.