Summary and Exam Tips

The Chapter in One Page

Oxygen is utilised by organisms to indirectly break down simple molecules like glucose, amino acids and fatty acids to derive energy, and carbon dioxide, which is harmful, is released during these catabolic reactions. Oxygen therefore has to be continuously provided to the cells and the carbon dioxide produced by them has to be released out, and this process of exchange of oxygen from the atmosphere with the carbon dioxide produced by the cells is called breathing, commonly known as respiration.

Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation. Lower invertebrates like sponges, coelenterates and flatworms exchange gases by simple diffusion over their entire body surface. Earthworms use their moist cuticle. Insects have a network of tubes, the tracheal tubes, to transport atmospheric air within the body. Special vascularised structures called gills - branchial respiration - are used by most of the aquatic arthropods and molluscs, whereas vascularised bags called lungs - pulmonary respiration - are used by the terrestrial forms. Among vertebrates, fishes use gills, while amphibians, reptiles, birds and mammals respire through lungs, and amphibians like frogs can also respire through their moist skin, which is cutaneous respiration.

The human passage runs in a single line and it is worth being able to recite it. A pair of external nostrils opens above the upper lips and leads 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 is therefore called the sound box. During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called the epiglottis, to prevent the entry of food into the larynx. The trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and a left primary bronchus. Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up 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, and the branching network of bronchi, bronchioles and alveoli comprises the lungs. The two lungs are covered by a double layered pleura with pleural fluid between them, which reduces friction on the lung surface; the outer pleural membrane is in close contact with the thoracic lining and the inner pleural membrane with the lung surface.

That passage has two functional parts, and this is the division the exam turns on. The part starting with the external nostrils up to the terminal bronchioles constitutes the conducting part, and the alveoli and their ducts form the respiratory or exchange part. The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies it and brings it to body temperature. The exchange part is the site of the actual diffusion of oxygen and carbon dioxide between blood and atmospheric air. The lungs sit in the thoracic chamber, which is anatomically an air-tight 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. Any change in the volume of the thoracic cavity is reflected in the pulmonary cavity, and that arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Breathing involves two stages - inspiration, during which atmospheric air is drawn in, and expiration, by which the alveolar air is released out. The movement of air is carried out by creating a pressure gradient between the lungs and the atmosphere. Inspiration can occur if the intra-pulmonary pressure is less than the atmospheric pressure, that is a negative pressure in the lungs with respect to the atmosphere; expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis, and the contraction of the external inter-costal muscles lifts up the ribs and the sternum, increasing the volume in the dorso-ventral axis. The overall increase in thoracic volume causes a similar increase in pulmonary volume, which lowers the intra-pulmonary pressure below atmospheric and forces air in. Expiration needs no contraction at all - relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions, reduces the thoracic and thereby the pulmonary volume, and raises the intra-pulmonary pressure slightly above atmospheric, expelling the air. We can increase the strength of inspiration and expiration with the help of additional muscles in the abdomen. On an average a healthy human breathes 12-16 times per minute, and the volume of air involved in breathing movements can be estimated using a spirometer, which helps in the clinical assessment of pulmonary functions.

Four respiratory volumes and five capacities follow, and they are the most heavily examined pages of this chapter. Tidal volume, TV, is the volume of air inspired or expired during a normal respiration, approx. 500 mL. Inspiratory reserve volume, IRV, is the additional volume a person can inspire by a forcible inspiration, 2500 mL to 3000 mL. Expiratory reserve volume, ERV, is the additional volume a person can expire by a forcible expiration, 1000 mL to 1100 mL. Residual volume, RV, is the volume of air remaining in the lungs even after a forcible expiration, 1100 mL to 1200 mL. By adding up a few of these volumes one derives the pulmonary capacities, which can be used in clinical diagnosis - IC, EC, FRC, VC and TLC, each of them a sum and each of them set out in the number sheet below.

Alveoli are the primary sites of exchange of gases, and exchange also occurs between blood and tissues. Oxygen and carbon dioxide are exchanged at these sites by simple diffusion, mainly based on the pressure or concentration gradient, and the solubility of the gases as well as the thickness of the membranes involved in diffusion are also important factors that affect the rate of diffusion. The pressure contributed by an individual gas in a mixture of gases is called the partial pressure, written $p\mathrm{O_2}$ for oxygen and $p\mathrm{CO_2}$ for carbon dioxide. The values show a gradient for oxygen from alveoli to blood and from blood to tissues, and a gradient for carbon dioxide in the opposite direction, from tissues to blood and from blood to alveoli. As the solubility of $\mathrm{CO_2}$ is 20-25 times higher than that of $\mathrm{O_2}$, the amount of carbon dioxide that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher. The diffusion membrane is made up of three major layers - the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance in between them - and its total thickness is much less than a millimetre. All the factors in our body are therefore favourable for the diffusion of oxygen from alveoli to tissues and of carbon dioxide from tissues to alveoli.

Blood is the medium of transport for both gases. Haemoglobin is a red coloured iron containing pigment present in the RBCs, and oxygen binds with it in a reversible manner to form oxyhaemoglobin; each haemoglobin molecule can carry a maximum of four molecules of oxygen. Binding is primarily related to the partial pressure of oxygen, with the partial pressure of carbon dioxide, the hydrogen ion concentration and the temperature as the other factors that can interfere with it. Plotting the percentage saturation of haemoglobin with oxygen against $p\mathrm{O_2}$ gives a sigmoid curve, the oxygen dissociation curve. In the alveoli, where there is a high $p\mathrm{O_2}$, a low $p\mathrm{CO_2}$, a lesser hydrogen ion concentration and a lower temperature, the factors favour the formation of oxyhaemoglobin; in the tissues, where there is a low $p\mathrm{O_2}$, a high $p\mathrm{CO_2}$, a high hydrogen ion concentration and a higher temperature, the conditions favour the dissociation of oxygen from oxyhaemoglobin. Carbon dioxide is carried by haemoglobin as carbamino-haemoglobin, and this binding is related to the partial pressure of carbon dioxide, with $p\mathrm{CO_2}$ a major factor; more binding occurs in the tissues and dissociation occurs at the alveoli. RBCs contain a very high concentration of the enzyme carbonic anhydrase, and minute quantities of it are present in the plasma too, and it facilitates the following reaction in both directions:

$$\mathrm{CO_2 + H_2O} \xrightleftharpoons{\text{carbonic anhydrase}} \mathrm{H_2CO_3} \xrightleftharpoons{\text{carbonic anhydrase}} \mathrm{HCO_3^- + H^+}$$

At the tissue site, where the partial pressure of carbon dioxide is high because of catabolism, carbon dioxide diffuses into the blood and forms bicarbonate and hydrogen ions. At the alveolar site, where $p\mathrm{CO_2}$ is low, the reaction proceeds in the opposite direction, forming carbon dioxide and water, so the carbon dioxide trapped as bicarbonate at the tissue level and transported to the alveoli is released out as $\mathrm{CO_2}$.

Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues, and this is done by the neural system. A specialised centre in the medulla region of the brain, the respiratory rhythm centre, is primarily responsible for this regulation. Another centre in the pons region, the pneumotaxic centre, can moderate the functions of the respiratory rhythm centre, and neural signals from it can reduce the duration of inspiration and thereby alter the respiratory rate. A chemosensitive area is situated adjacent to the rhythm centre, and it is highly sensitive to carbon dioxide and hydrogen ions; an increase in these substances activates the area, which signals the rhythm centre to make the adjustments by which they can be eliminated. Receptors associated with the aortic arch and the carotid artery also recognise changes in carbon dioxide and hydrogen ion concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

Finally, the disorders. Asthma is a difficulty in breathing causing wheezing, due to inflammation of the bronchi and bronchioles. Emphysema is a chronic disorder in which the alveolar walls are damaged, due to which the respiratory surface is decreased, and one of the major causes of this is cigarette smoking. 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, and workers in such industries should wear protective masks. Hypoxia, asked at the end of the chapter, is a condition in which the tissues receive an inadequate supply of oxygen, caused by high altitude, lung disease such as emphysema, anaemia or carbon monoxide poisoning.

The Number Sheet

Every figure in the chapter, in one place. Learn them as they are printed - ranges stay ranges, and the approximate values keep the word approximately in front of them.

The four respiratory volumes

The five capacities, formula and meaning together

The partial pressure table, exactly as printed - partial pressures in mm Hg of oxygen and carbon dioxide at the different parts involved in diffusion, compared with those in the atmosphere.

Read it as four gradients, not as ten numbers. Oxygen, alveoli to deoxygenated blood: 104 against 40. Oxygen, oxygenated blood to tissues: 95 against 40. Carbon dioxide, tissues to oxygenated blood: 45 against 40. Carbon dioxide, deoxygenated blood to alveoli: 45 against 40.

The rest of the numbers

• Solubility: the solubility of $\mathrm{CO_2}$ is 20-25 times higher than that of $\mathrm{O_2}$, so the amount of carbon dioxide that can diffuse per unit difference in partial pressure is much higher.
• Diffusion membrane: three major layers, and a total thickness much less than a millimetre.
• Transport of oxygen: about 97 per cent is transported by RBCs; the remaining 3 per cent is carried in a dissolved state through the plasma. Each haemoglobin molecule can carry a maximum of four molecules of oxygen.
• Transport of carbon dioxide: nearly 20-25 per cent is transported by RBCs as carbamino-haemoglobin; 70 per cent is carried as bicarbonate; about 7 per cent is carried in a dissolved state through plasma. These are the printed figures and they are what you reproduce.
• The two delivery figures, and they are not interchangeable: every 100 mL of oxygenated blood delivers around 5 mL of oxygen to the tissues, and every 100 mL of deoxygenated blood delivers approximately 4 mL of carbon dioxide to the alveoli. Oxygen 5, carbon dioxide 4.
• Breathing rate: on an average a healthy human breathes 12-16 times per minute, and a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.
• The one anatomical number: the trachea divides at the level of the 5th thoracic vertebra into a right and a left primary bronchus.

A worked set of totals to keep in your head as a check. Take TV 500 mL, IRV 3000 mL, ERV 1100 mL and RV 1200 mL. Then IC = 500 + 3000 = 3500 mL, EC = 500 + 1100 = 1600 mL, FRC = 1100 + 1200 = 2300 mL, VC = 1100 + 500 + 3000 = 4600 mL and TLC = 4600 + 1200 = 5800 mL. If a calculation in an exam lands far from these, you have used the wrong formula.

The Address Sheet

One line per structure - where it is and what it does. Most of what is asked about this chapter beyond the numbers is an address, and the paper tests it by swapping two of them.

• Larynx - a cartilaginous box between the pharynx and the trachea; helps in sound production, and is therefore called the sound box.
• Epiglottis - a thin elastic cartilaginous flap that covers the glottis during swallowing, to prevent the entry of food into the larynx. It is the lid, not the box.
• Terminal bronchiole - the very thin final bronchiole, and the last structure of the conducting part. No diffusion happens here. Each terminal bronchiole gives rise to a number of alveoli.
• Alveoli - very thin, irregular-walled and vascularised bag-like structures; the alveoli and their ducts form the respiratory or exchange part, and alveoli are the primary sites of exchange of gases.
• Pleural fluid - between the two layers of the double layered pleura covering each lung; it reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining, the inner one with the lung surface.
• Diaphragm - the dome-shaped muscle forming the lower side of the thoracic chamber; its contraction increases the volume of the thoracic chamber in the antero-posterior axis, and this is what initiates inspiration.
• External inter-costal muscles - between the ribs; their contraction lifts up the ribs and the sternum, increasing the volume of the thoracic chamber in the dorso-ventral axis. Diaphragm - antero-posterior. External inter-costals - dorso-ventral.
• Diffusion membrane - at the alveolus, between alveolar air and capillary blood; three major layers - the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance in between (a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of the capillaries). Total thickness much less than a millimetre.
• Haemoglobin - in the RBCs; a red coloured iron containing pigment that binds oxygen reversibly to form oxyhaemoglobin, a maximum of four molecules of oxygen per haemoglobin molecule, and also carries carbon dioxide as carbamino-haemoglobin.
• Carbonic anhydrase - in a very high concentration in the RBCs, and in minute quantities in the plasma; it facilitates the interconversion of carbon dioxide and water with bicarbonate and hydrogen ions, in both directions.
• Respiratory rhythm centre - in the medulla region of the brain; primarily responsible for the regulation of the respiratory rhythm.
• Pneumotaxic centre - in the pons region of the brain; can moderate the functions of the respiratory rhythm centre, and its neural signals can reduce the duration of inspiration and thereby alter the respiratory rate.
• Chemosensitive area - situated adjacent to the rhythm centre; highly sensitive to carbon dioxide and hydrogen ions, and an increase in these activates it, so that it signals the rhythm centre to make the adjustments by which they can be eliminated.
• Receptors of the aortic arch and the carotid artery - outside the brain, on the great vessels; they also recognise changes in carbon dioxide and hydrogen ion concentration and send necessary signals to the rhythm centre for remedial actions.

Three pairs to rehearse aloud, because each is examined as a swap. Larynx is the sound box, epiglottis is the flap. Medulla holds the rhythm centre, pons holds the pneumotaxic centre. Diaphragm works the antero-posterior axis, external inter-costals the dorso-ventral axis.

The Mistakes That Cost Marks in This Chapter

1. Answering RV when the question says "after a normal expiration". This is the single most expensive error in the chapter. The volume of air that will remain in the lungs after a NORMAL expiration is the functional residual capacity, FRC, which is ERV + RV. The residual volume, RV, is the volume of air remaining in the lungs even after a FORCIBLE expiration, and averages 1100 mL to 1200 mL. Both are offered as options every time, and the whole question hangs on one adjective. Circle normal or forcible in the stem before you read the options.
2. Swapping the larynx and the epiglottis. The larynx is the cartilaginous box that helps in sound production - the sound box. The epiglottis is the thin elastic cartilaginous flap that covers the glottis during swallowing to prevent the entry of food into the larynx. One is the box, the other is the lid on the way in. Say the pair as one sentence and the swap cannot happen.
3. Swapping the medulla and the pons. The respiratory rhythm centre is in the medulla region and is primarily responsible for the regulation. The pneumotaxic centre is in the pons region and moderates the rhythm centre, reducing the duration of inspiration. The chemosensitive area is a third thing again - it is adjacent to the rhythm centre, so it too is in the medulla.
4. Thinking that falling oxygen is what drives breathing. It is a natural guess and the chapter contradicts it flatly: the role of oxygen in the regulation of respiratory rhythm is quite insignificant. What the regulation responds to is carbon dioxide and hydrogen ions - through the chemosensitive area adjacent to the rhythm centre and the receptors associated with the aortic arch and the carotid artery. In a written answer, name carbon dioxide and hydrogen ions first and give the oxygen sentence as it stands.
5. Thinking a normal expiration needs a muscle to contract. It does not. Normal expiration follows the relaxation of the diaphragm and the inter-costal muscles, which returns the diaphragm and sternum to their normal positions and reduces the thoracic and thereby the pulmonary volume, raising the intra-pulmonary pressure to slightly above the atmospheric pressure. Only when we deliberately increase the strength of expiration do additional muscles in the abdomen come in.
6. Putting diffusion in the conducting part. The part from the external nostrils up to the terminal bronchioles is the conducting part, and it only transports the air to the alveoli, clears it from foreign particles, humidifies it and brings it to body temperature. Diffusion of gases occurs in the alveolar region only - the alveoli and their ducts are the respiratory or exchange part, because that is where the thin diffusion membrane and the capillary blood are.
7. Thinking the percentage of oxygen in the air falls at altitude. It does not. The air is much the same mixture at the top of a hill as at the bottom; what falls is the atmospheric pressure. Since the partial pressure of a gas is its share of the total pressure, a lower total pressure means a lower $p\mathrm{O_2}$ in the inspired air, and that is why less oxygen diffuses into the blood, the saturation of haemoglobin falls, and the tissues receive an inadequate supply of oxygen - hypoxia. Any option that says the air contains a smaller percentage of oxygen at altitude is the wrong one.
8. Mixing the 5 mL and the 4 mL. Every 100 mL of oxygenated blood delivers around 5 mL of oxygen to the tissues. Every 100 mL of deoxygenated blood delivers approximately 4 mL of carbon dioxide to the alveoli. Oxygen goes with 5 and with oxygenated blood; carbon dioxide goes with 4 and with deoxygenated blood. The two figures sit one line apart in the chapter and they are swapped in the options more often than any other pair of numbers here.

Writing the Chapter-End Exercises Well

Class 11 has no board paper, but the chapter-end exercises and your school tests are still written answers, marked by a person reading for particular words. Everything below is about getting those words onto the page in the order a marker looks for them.

Give the formula before you give the number. Every capacity question in this chapter is marked on the sum first. Write the formula on its own line, then substitute, then state the answer with its unit - VC = ERV + TV + IRV, then 1100 + 500 + 3000, then 4600 mL. A paragraph that describes vital capacity beautifully and never lands on ERV + TV + IRV loses the mark that the one line of algebra would have earned. The same order works for the definition-plus-significance exercises: define vital capacity as the maximum volume of air a person can breathe in after a forced expiration, or breathe out after a forced inspiration, then give the formula, then the significance - it is measured with a spirometer and used in the clinical assessment of pulmonary functions, and a person with a larger vital capacity can move more air per breath. For the exercise that asks you to distinguish between two of them, answer in a two-column table with the formula in both rows - IC = TV + IRV against EC = TV + ERV, VC = ERV + TV + IRV against TLC = VC + RV - because the difference the marker is looking for is the difference between the two sums.

State the direction and both its ends for a gradient question. Never write that a gas "moves down its gradient" and stop. Name the gas, name where it starts and where it ends, and give both numbers with the unit. Oxygen diffuses from the alveoli, where $p\mathrm{O_2}$ is 104 mm Hg, into the deoxygenated blood, where it is 40 mm Hg; then from oxygenated blood at 95 mm Hg into the tissues at 40 mm Hg. Carbon dioxide diffuses from the tissues at 45 mm Hg into the oxygenated blood at 40 mm Hg, and from the deoxygenated blood at 45 mm Hg into the alveoli at 40 mm Hg. Two numbers and an arrow are worth more than three sentences of prose. When the exercise asks you to compare atmospheric air with alveolar air, answer with both gases in one line - in atmospheric air $p\mathrm{O_2}$ is higher, at 159 mm Hg against 104 mm Hg, and $p\mathrm{CO_2}$ is lesser, at 0.3 mm Hg against 40 mm Hg. For the exercise on the effect of $p\mathrm{CO_2}$ on oxygen transport, give the two sets of conditions as a matched pair - high $p\mathrm{O_2}$, low $p\mathrm{CO_2}$, lesser hydrogen ion concentration and lower temperature in the alveoli favour the formation of oxyhaemoglobin, while low $p\mathrm{O_2}$, high $p\mathrm{CO_2}$, high hydrogen ion concentration and higher temperature in the tissues favour dissociation.

Name the exact structure and its region for an address question. A vague location scores nothing. Say "the respiratory rhythm centre, in the medulla region of the brain", not "a centre in the brain". Say "the pneumotaxic centre, in the pons region of the brain". Say "a chemosensitive area situated adjacent to the rhythm centre", and then that it is "highly sensitive to carbon dioxide and hydrogen ions". Say "receptors associated with the aortic arch and the carotid artery". The exercise on how respiration is regulated is marked as a list of these four addresses plus the closing sentence that the role of oxygen in the regulation of respiratory rhythm is quite insignificant - write them as four bullets and the fifth line, and every mark is on the page. The same discipline answers the exercise on why diffusion happens only in the alveolar region: name the exchange part as the alveoli and their ducts, name the conducting part as everything from the external nostrils to the terminal bronchioles, and give the reason as the three-layered diffusion membrane whose total thickness is much less than a millimetre.

Three habits that pay across the whole exercise set. When an exercise asks for a value, do the arithmetic and write the number - tidal volume is approx. 500 mL, so at 12-16 breaths per minute a healthy human moves about 6000 to 8000 mL per minute, and in an hour that is roughly 360000 mL to 480000 mL, that is about 360 to 480 litres. When an exercise names a mechanism, describe it in the order the events happen - for inspiration under normal conditions, write contraction of the diaphragm, increase in thoracic volume in the antero-posterior axis; contraction of the external inter-costal muscles, ribs and sternum lifted, increase in the dorso-ventral axis; increase in pulmonary volume; intra-pulmonary pressure falls below atmospheric; air moves in. And when an exercise gives you a multiple-choice list, state the chosen option in full words and then say in one line why each of the others is wrong - a bare roman numeral is worth very little, while " $p\mathrm{O_2}$ higher and $p\mathrm{CO_2}$ lesser in atmospheric air, because atmospheric air is 159 mm Hg and 0.3 mm Hg against alveolar 104 mm Hg and 40 mm Hg" is worth the whole mark.

The Night Before - What to Revise, in Order

Read in this order and stop when the list runs out. Nothing new goes in tonight.

1. The number sheet, top to bottom. Twenty minutes, and the most valuable twenty in the chapter. The four volumes with their ranges, then the five capacity formulas, then the partial pressure table. Write the volumes out from memory and check that each range is a range - 2500 mL to 3000 mL, 1000 mL to 1100 mL, 1100 mL to 1200 mL - and that tidal volume still carries the word approx. in front of its 500 mL.

2. The five capacities, written out as sums and then worked once with real numbers. Ten minutes. IC = TV + IRV. EC = TV + ERV. FRC = ERV + RV. VC = ERV + TV + IRV. TLC = VC + RV. Then run the check set - TV 500, IRV 3000, ERV 1100, RV 1200 giving IC 3500, EC 1600, FRC 2300, VC 4600 and TLC 5800 mL - and say aloud which one answers "after a normal expiration" and which one answers "after a forcible expiration".

3. The partial pressure table as four gradients. Ten minutes. 104 to 40 for oxygen at the alveoli. 95 to 40 for oxygen at the tissues. 45 to 40 for carbon dioxide at the tissues. 45 to 40 for carbon dioxide at the alveoli. Then the two atmospheric values, 159 and 0.3, which are the ones the comparison questions turn on.

4. The eight mistakes above. Ten minutes. These are the marks you are most likely to lose while knowing the material perfectly well, which makes them the cheapest ones to save.

5. The address sheet, as a recitation. Ten minutes. Go down the list and say where and what it does for each: larynx, epiglottis, terminal bronchiole, alveoli, pleural fluid, diaphragm, external inter-costals, diffusion membrane, haemoglobin, carbonic anhydrase, rhythm centre, pneumotaxic centre, chemosensitive area, aortic and carotid receptors. Pay special attention to the three pairs that get swapped.

6. The transport percentages and the two delivery figures. Five minutes. Oxygen - 97 per cent by RBCs, 3 per cent dissolved in plasma, four molecules per haemoglobin. Carbon dioxide - 70 per cent as bicarbonate, 20-25 per cent as carbamino-haemoglobin, about 7 per cent dissolved in plasma. Then: 5 mL of oxygen per 100 mL of oxygenated blood, 4 mL of carbon dioxide per 100 mL of deoxygenated blood.

If you have ten minutes and no more, read the number sheet twice. This chapter rewards the figures over everything else, and a student who can reproduce the four volumes, the five sums and the partial pressure table will out-score one who has read the whole chapter once more.