Human Gas Exchange

• If you remember one sentence from this chapter, make it this one: “Ventilation is air moving; gas exchange is diffusion at the alveolus; respiration is a chemical reaction inside a cell.” Half the marks lost in this topic are lost by mixing those three up.

• Distinguish ventilation, gas exchange, transport and respiration, and say where each one happens.
• State the features of an efficient gas-exchange surface and explain how each one increases the rate of diffusion.
• State the approximate composition of atmospheric air, and say why the values are approximate.
• Compare inspired and expired air for oxygen, carbon dioxide, nitrogen, water vapour and temperature, giving a reason for each difference.
• Describe investigations that show expired air contains more carbon dioxide and more water vapour, and separate the observation from the conclusion.
• Identify the nose, mouth, larynx, trachea, bronchi, bronchioles, alveoli, lungs, ribs, external and internal intercostal muscles, diaphragm and capillary network on a diagram.
• Trace the route of air from the nose or mouth to an alveolus.
• Describe the structure of an alveolus and its capillary, and explain four adaptations for gas exchange.
• Explain the diffusion of oxygen and of carbon dioxide at the alveolus in terms of concentration gradients.
• Give the complete inhalation mechanism as a causal chain from muscle action to air movement.
• Give the complete exhalation mechanism, and say what changes in forced exhalation.
• Explain why an increase in thoracic volume causes a decrease in pressure inside the lungs.
• Explain why physical activity increases the rate and the depth of breathing.
• Define breathing rate, breathing depth and ventilation rate, and calculate each from data.
• Design a safe, standardised investigation into the effect of activity on breathing.
• Interpret a breathing-rate graph, identify an anomaly and calculate a percentage increase.
• Explain the roles of goblet cells, mucus and ciliated cells in protecting the airways.
• Explain how damage to cilia by smoking increases the risk of infection.

Breathing does not return to the resting rate the instant the exercise ends. It falls back over a period of recovery, which is longer after harder or longer exercise. During that time the extra carbon dioxide that built up in the blood and tissues is still being removed, and the body is still restoring itself to its resting state. This is why an investigation into exercise and breathing should measure during recovery as well as immediately after the activity — the shape of the recovery curve carries as much information as the peak.

Gas Exchange

The movement of oxygen and carbon dioxide between an organism and its environment by diffusion across a specialised surface. In humans this occurs at the alveoli of the lungs: oxygen diffuses from the alveolar air, where its concentration is higher, through the alveolar wall and the capillary wall into the blood, while carbon dioxide diffuses in the opposite direction, from the blood where its concentration is higher into the alveolar air. Both gases move down their own concentration gradients, so the process is passive and requires no energy from respiration.

Gas Exchange Surface

A specialised region of an organism where gases are exchanged with the environment by diffusion. An efficient gas-exchange surface has a large surface area, so that many molecules can cross at once; a barrier only one or two cells thick, so that the diffusion distance is short; a moist lining, so that gases dissolve before they diffuse; a dense supply of blood or other transport medium, which removes the diffusing substance and so keeps the concentration gradient steep; and a means of renewing the medium on the other side, which in humans is ventilation. In humans the gas-exchange surface is the total surface of the alveoli in the lungs.

Expired Air

The air breathed out of the lungs. Compared with inspired air it contains less oxygen, approximately 16 per cent rather than approximately 21 per cent, because oxygen has diffused from the alveolar air into the blood and been used in respiration in the body's cells. It contains much more carbon dioxide, approximately 4 per cent rather than approximately 0.04 per cent, because carbon dioxide produced by respiration has diffused from the blood into the alveolar air. Its nitrogen content is approximately unchanged at about 78 per cent, because nitrogen is not exchanged in the lungs. It carries more water vapour, because the air has passed over the moist linings of the airways and alveoli, and it is close to body temperature, because it has been warmed on the way through.

Goblet Cell

A specialised cell found in the epithelium lining the airways, including the trachea, the bronchi and the bronchioles. It is named for its shape, being wide at the surface and narrow at the base, and its function is to secrete mucus onto the inner surface of the airway. Inside the cell the mucus is stored as granules packed into the wide upper part, above a nucleus that is pushed down to the narrow base. The mucus released is a sticky fluid that traps dust particles, pollen and pathogens carried in with the inhaled air, so that they do not reach the delicate gas-exchange surface of the alveoli. Goblet cells produce mucus but do not move it; the mucus is moved by the beating of cilia on the neighbouring ciliated cells.

Exhalation

The phase of ventilation in which air moves out of the lungs. In quiet breathing it is a passive process: the external intercostal muscles relax so the ribs move downwards and inwards under their own weight and the elastic recoil of the tissues, and the diaphragm relaxes and returns to its dome shape. Both changes decrease the volume of the thoracic cavity, so the pressure inside the lungs rises above atmospheric pressure and air moves out down that pressure difference. In forced exhalation, such as during coughing or vigorous exercise, the internal intercostal muscles also contract to pull the ribs downwards and inwards more strongly and more quickly, so that air is expelled faster and in greater volume.

Bronchiole

One of the fine tubes into which each bronchus divides repeatedly inside a lung. Bronchioles have much narrower diameters than the bronchi and their walls contain no cartilage rings. They form a branching network that distributes air throughout the lung tissue and delivers it to the clusters of alveoli at their ends. No gas exchange takes place across the wall of a bronchiole; like the trachea and bronchi it is part of the conducting airway, whose job is to carry air, warm it, moisten it and trap particles in mucus before the air reaches the exchange surface.

Concentration Gradient

A difference in the concentration of a substance between two regions. Particles move randomly, and the net result of that random movement is a flow of particles from the region where they are more concentrated to the region where they are less concentrated: that net movement is diffusion, and it continues until the concentrations are equal. At the alveolus, oxygen is at a higher concentration in the alveolar air than in the blood arriving in the capillary, so oxygen diffuses into the blood; carbon dioxide is at a higher concentration in that blood than in the alveolar air, so carbon dioxide diffuses out. Both gradients are maintained by ventilation on one side and by blood flow on the other, and no energy from respiration is used to move either gas.

Ciliated Cell

A cell of the epithelium lining the airways whose exposed surface carries many cilia, which are fine hair-like projections a few micrometres long. The cilia beat rhythmically and together within a watery layer of fluid, and their tips engage the thicker sticky layer of mucus resting on top, so that their coordinated beating moves the whole sheet of mucus upward along the airway toward the throat, carrying with it the dust particles and pathogens the mucus has trapped. At the throat the mucus is swallowed, and many of the trapped pathogens are then destroyed in the acidic conditions of the stomach. Ciliated cells do not produce mucus, which is secreted by the neighbouring goblet cells, and their cilia can be damaged by smoking, which reduces the clearing of mucus from the airways.

Limewater Test

A laboratory test for carbon dioxide. Limewater is a solution of calcium hydroxide, and it is clear and colourless. When carbon dioxide is bubbled through it, insoluble calcium carbonate is formed as a fine suspension, and the limewater turns milky or cloudy. The more concentrated the carbon dioxide in the gas being bubbled through, the more quickly the cloudiness appears. In gas-exchange work the test is used comparatively: air breathed out is bubbled through one sample of limewater and atmospheric air through an identical sample, and the sample exposed to expired air becomes cloudy first. The result shows that expired air contains a higher concentration of carbon dioxide; it does not measure that concentration.

Inspired Air

The air breathed into the lungs, which in ordinary conditions is atmospheric air. Dry atmospheric air contains approximately 78 per cent nitrogen, approximately 21 per cent oxygen, approximately 0.04 per cent carbon dioxide and approximately 1 per cent argon together with other gases present in trace amounts; the quantity of water vapour it carries is variable and depends on the humidity of the surroundings. These percentages are rounded approximations used for teaching and for reasoning about diffusion gradients, and any data supplied within an examination question takes priority over them.

Aerobic Respiration

The chemical breakdown of nutrient molecules using oxygen, releasing energy inside living cells. Glucose reacts with oxygen to produce carbon dioxide and water, and energy is released for processes such as muscle contraction, active transport, growth and the maintenance of body temperature. Aerobic respiration takes place in the cells of the body and not in the lungs. When a person exercises, the muscle cells respire faster, so they take up oxygen from the blood more quickly and release carbon dioxide into it more quickly; the resulting changes in the blood lead to faster and deeper breathing, which supplies the extra oxygen and removes the extra carbon dioxide.

Diaphragm

A sheet of muscle and fibrous tissue that separates the thorax from the abdomen and forms the floor of the chest cavity. At rest it is dome-shaped, curving upward into the thorax; because the liver lies beneath it the right dome sits higher than the left. Its muscle fibres run from the lower ribs, the sternum and the lumbar vertebrae inward to a central tendon, so that when they contract the domes are pulled flatter and downward. That increases the volume of the thoracic cavity and lowers the pressure inside the lungs, drawing air in. When the fibres relax the diaphragm returns to its dome shape, the volume of the thorax decreases, the pressure inside the lungs rises, and air is forced out. It is therefore an active muscle during inhalation and a passive one during normal exhalation.

Ventilation Rate

The total volume of air moved into and out of the lungs in a given time, usually one minute. It is calculated by multiplying the breathing rate, the number of breaths taken per minute, by the volume of air moved in a single breath. Ventilation rate is therefore a more complete measure of breathing than breathing rate alone, because two people can take the same number of breaths per minute while moving very different volumes of air. During physical activity both the rate and the depth of breathing increase, so the ventilation rate increases by more than the increase in rate alone would suggest.

Alveolus

A microscopic air sac at the end of a bronchiole, and the site of gas exchange in the human lung. An alveolus is not a single cell but a small sac, roughly a quarter of a millimetre across, whose wall is made of extremely flattened cells and is only one cell thick. Alveoli occur in clusters that open off a shared alveolar duct, so neighbouring sacs share their walls, and each shared wall carries a dense network of blood capillaries. The inner surface of every alveolus carries a thin film of moisture in which gases dissolve. Because the capillary wall is also one cell thick, a gas crossing between the alveolar air and the blood passes through only two cells, a total distance of well under a thousandth of a millimetre. There are very many alveoli in each lung, and together they provide a very large total surface area for diffusion.

Ventilation

The movement of air into and out of the lungs, also called breathing. Ventilation is produced by muscular changes in the volume of the thoracic cavity: contraction of the external intercostal muscles and of the diaphragm increases that volume, and relaxation decreases it. Because the air already in the lungs then occupies a larger or a smaller space, the pressure inside the lungs falls below or rises above atmospheric pressure, and air moves down the resulting pressure difference. Ventilation does not itself exchange any gas; its function is to renew the air in the alveoli so that the concentration gradients on which gas exchange depends are maintained.

Inhalation

The phase of ventilation in which air moves into the lungs. The external intercostal muscles contract while the internal intercostal muscles relax, so the ribs are pulled upwards and outwards. At the same time the muscle of the diaphragm contracts, so the diaphragm flattens and moves downward. Both movements increase the volume of the thoracic cavity, and because the same quantity of air now occupies a larger space, the pressure inside the lungs falls below atmospheric pressure. Air then moves into the lungs down that pressure difference. Inhalation is therefore an active process, requiring muscular contraction, and the air is drawn in by the pressure difference rather than pulled in by the lungs themselves.

• 1. Putting respiration in the lungs. Respiration is a chemical reaction inside living cells — muscle cells, liver cells, root hair cells. The lungs are where gases are exchanged, not where energy is released. Writing “respiration takes place in the lungs” can cost you the mark even in a question that is otherwise correct.
• 2. Saying expired air has no oxygen. It has about 16 %. Only about a fifth of the oxygen you breathe in is absorbed. This is why rescue breathing works at all.
• 3. Skipping the pressure step. “The diaphragm contracts so air rushes in” is a two-mark answer written as a one-mark answer. Volume changes, therefore pressure changes, therefore air moves.
• 4. Getting the diaphragm backwards. When the diaphragm contracts it becomes flatter and moves down, which increases the volume of the thorax. A relaxed diaphragm is the dome-shaped one. Many students write the reverse because a relaxed muscle sounds like it should be flat.
• 5. Treating rate and depth as the same word. Rate is breaths per minute. Depth is the volume of each breath. During exercise both increase, and a question that says “rate and depth” wants two separate statements.
• 6. Reporting a conclusion as an observation. “Limewater went cloudy faster” is what you saw. “Expired air contains a higher concentration of carbon dioxide” is what it means. Practical questions frequently award those two separately, and answers that only give one of them get half the marks.
• Do not force the column to total 100. Rounded values do not have to add up exactly, and the carbon dioxide figure is already included inside the “argon and other gases” one per cent. An answer that writes 78 + 21 + 1 + 0.04 and worries about the extra 0.04 has misread the table, not found an error.
• Nitrogen is not “used up”, and it is not “produced”. Its percentage looks unchanged because nothing exchanges it. Strictly, because the total volume of expired air is slightly different from the volume inspired, the nitrogen percentage can shift by a fraction of a point — but at this level it is treated as unchanged, and you should write “approximately the same” rather than “exactly the same”.
• Antagonistic, not identical. The external and internal intercostals work against each other, like the biceps and triceps at the elbow. Externals raise the ribs; internals lower them. In quiet breathing only the externals do any work — quiet exhalation is caused by muscles relaxing, not by the internals contracting.
• An alveolus is not a cell. It is a small sac, and its wall is made of cells. Writing “the alveolus is one cell thick” is acceptable shorthand only if you mean its wall; writing “an alveolus is a single cell” is wrong, and it makes the rest of your answer hard to mark. Say “the alveolar wall is one cell thick”.
• “The lungs pull air in” is not an explanation. Lungs contain no muscle. They are elastic bags that change shape because the space around them changes shape. If you write that the lungs expand, follow it immediately with because the thorax has expanded.
• Do not invent pressure values. Write “below atmospheric pressure” and “above atmospheric pressure”, or “lower than” and “higher than” the pressure outside. A number such as “the pressure falls to 99 kPa” is not required at this level, and if you invent one you can be marked wrong for a detail that was never asked for. If a question supplies pressure data, use it.
• Breathing does not release the energy. A muscle is not powered by air arriving in the chest; it is powered by respiration inside its own cells. Breathing supplies the oxygen that respiration needs and removes the carbon dioxide it produces. Writing “we breathe faster to release more energy” puts the energy release in the wrong organ and usually loses the mark.
• “I counted my own breathing.” This is a real limitation and it is worth a mark when you name it. Being aware of your breathing alters it, usually making it slower and deeper. Having a partner count, or using equipment, removes the problem.
• The most common error here is dividing by 35 instead of 14, which gives 60 % and describes something else entirely. Ask yourself what the change is being compared with: the value it started from.
• Cilia are not filters. A common wrong answer describes cilia as tiny hairs that filter oxygen or strain the air. They do neither. The mucus traps particles; the cilia transport the mucus. Gas molecules pass straight through both.
• Do not write that “all the oxygen is used up”. About three quarters of the inhaled oxygen is breathed straight back out, and saying otherwise contradicts the data in front of you.
• Do not write that the student breathes faster “to make more energy”. Breathing supplies oxygen; respiration inside the cells releases the energy.

• Topic 9 is unusual in that almost everything in it can be asked as either a recall question or an application question, and the difference is worth several marks.
• muscle action → movement of ribs or diaphragm → change in thoracic volume → change in pressure inside the lungs → movement of air down the pressure difference
• change one variable → observe or measure → repeat → record → evaluate → control everything else → state the timing and the safety precaution