Biology › Exchange surfaces and gas exchange › Lungs: seventy square metres, folded into a chest

Lungs: seventy square metres, folded into a chest

Your gas exchange surface is roughly the area of a badminton court and it fits under your ribs because it is divided into about three hundred million sacs. Getting air to it takes a branching tree of tubes whose walls change composition all the way down, and a set of muscles that never touch the lungs at all.

Before this Surface area to volume ratio and Fick's law · Squamous and ciliated epithelium as tissues

What you should be able to do

• Trace air from the trachea to an alveolus and name the tissues in the wall at each level.
• Recognise trachea, bronchus, bronchiole and alveolus in a section, and justify the identification from what is in the wall.
• Describe inspiration and expiration in the correct causal order: muscles, then volume, then pressure, then air.
• Explain why quiet expiration costs almost nothing while inspiration always costs energy.
• Calculate pulmonary ventilation rate, and read tidal volume, vital capacity and breathing rate off a spirometer trace.

From trachea to alveolus, and what changes on the way

Air entering the nose is warmed, moistened and filtered before it goes anywhere, then passes down the trachea, a tube about 10 cm long held permanently open by rings of cartilage. The rings are C-shaped rather than complete, with the gap at the back where the oesophagus lies against the trachea, so a swallowed mouthful can bulge into that space instead of being crushed against a rigid tube.

The trachea divides into two bronchi, one to each lung, and each bronchus divides again and again into bronchioles of decreasing diameter. Around twenty-three rounds of branching separate the trachea from the smallest air spaces. The finest bronchioles open into clusters of alveoli, and it is only there that any gas exchange happens at all.

Cartilage

A firm, flexible supporting tissue. In the trachea it forms incomplete rings, and in the bronchi irregular blocks, holding the airway open against the pressure drop of inspiration.

Ciliated epithelium

An epithelium whose cells carry cilia that beat in a co-ordinated wave, sweeping mucus and trapped particles up towards the throat.

Goblet cell

A cell in the airway epithelium that secretes mucus, which traps dust, pollen and bacteria.

Smooth muscle

Involuntary muscle in the airway wall; its contraction narrows the lumen, and it is the tissue that contracts inappropriately during an asthma attack.

Elastic fibres

Fibres that stretch as the lungs inflate and recoil as the muscles relax, providing most of the force for quiet expiration.

Squamous epithelium

An epithelium of very flattened cells, giving a wall a fraction of a micrometre thick; the lining of an alveolus.

Cambridge candidates should treat the distribution of these tissues as a learned list, because CAIE 9700 asks for them explicitly and expects the structures to be recognised in slides, photomicrographs and electron micrographs. The identification is easier than it looks if you go in with one question: what holds this tube open? Rings of cartilage means trachea. Blocks of cartilage means bronchus. No cartilage but a conspicuous ring of smooth muscle means bronchiole. No wall to speak of, just a thin sheet against a capillary, means alveolus.

Smooth muscle and elastic fibres run the whole length of the tree, and they work against each other. Smooth muscle contracts to narrow an airway; elastic fibres stretch when it does and recoil to widen it again when the muscle relaxes. Elastic fibres are not antagonistic muscles, and a mark scheme will not accept them described as such.

The surface itself

An adult has somewhere around 300 million alveoli with a combined surface area of roughly 70 m². The folding is the whole point: that area sits inside a chest cavity of a few litres, which is the first term of Fick's law bought at almost no cost in volume.

Each alveolus is lined by a single layer of squamous epithelium and wrapped in a dense network of capillaries so narrow that red blood cells pass through in single file, squeezed against the capillary wall. Between the air and the haemoglobin lie only the alveolar epithelium, a basement membrane shared with the capillary, the capillary endothelium and a film of plasma.

The gradient is held up from both sides at once, which is the second term of Fick's law being managed. Ventilation replaces alveolar air several times a minute, so the oxygen partial pressure in the alveolus stays around 13.3 kPa. Blood arriving from the pulmonary artery has an oxygen partial pressure of about 5.3 kPa and is swept away as soon as it is loaded, so the blood side never has time to equilibrate and stop the process. Carbon dioxide crosses the other way down a much smaller gradient, roughly 6.1 kPa in the arriving blood against 5.3 kPa in the alveolus, and that suffices because carbon dioxide diffuses far more readily than oxygen.

Two details are worth carrying. The alveolar lining is kept moist, because a gas has to dissolve before it can diffuse across a membrane. And the fluid contains a surfactant secreted by specialised cells in the alveolar wall, which lowers surface tension and stops the smallest alveoli collapsing shut between breaths. Premature babies are often short of it, which is why respiratory distress syndrome is a risk of early birth.

Elastic fibres in the alveolar walls stretch during inspiration and store energy while they do. Releasing it is what drives quiet expiration. In emphysema those fibres are destroyed and the alveolar walls break down into fewer, larger spaces, so the surface area falls and the recoil weakens at the same time. Both changes reduce gas exchange, and questions on lung disease almost always want both named.

Ventilation: muscles, then volume, then pressure, then air

The lungs are passive. They sit in a sealed cavity, and the muscles that ventilate them are in the wall of that cavity: the diaphragm underneath and the intercostal muscles between the ribs.

The external and internal intercostals are an antagonistic pair. Externals contract for inspiration, internals for forced expiration, and at any moment one set is relaxed. In quiet breathing the internals do essentially nothing, because recoil of the stretched elastic tissue and the weight of the ribcage are enough. They come into play when you blow out hard, along with the abdominal muscles, which push the abdominal contents up against the diaphragm.

That asymmetry is worth a mark on its own. Inspiration always costs energy, because it always involves muscle contraction. Quiet expiration is largely passive, so an answer describing it as the diaphragm 'pushing up' or 'contracting upwards' is describing something that does not happen: a muscle can only pull, and the diaphragm returns to its dome because it has stopped pulling.

The pressure changes involved are small. Between breaths the pressure in the alveoli equals atmospheric pressure exactly, so no air moves. During quiet inspiration it falls by only a fraction of a kilopascal, which is ample: gases move quickly down even a shallow pressure gradient. Questions sometimes give a trace of alveolar pressure against time and ask you to mark where air is entering. It is entering wherever the trace lies below the atmospheric line, and leaving wherever it lies above it.

Putting numbers on it

AQA names one calculation of its own in this topic, and it is the volume of air moved per minute. OCR A reaches the same quantity through a spirometer trace.

pulmonary ventilation rate = tidal volume × ventilation rateTidal volume in dm³, ventilation rate in breaths per minute, so the answer is in dm³ min⁻¹. AQA names this quantity in its own specification and expects the calculation.

Tidal volume

The volume of air moved in or out in one normal breath at rest, about 0.5 dm³ in an adult.

Ventilation rate

The number of breaths taken per minute, about 12 to 15 at rest. Also called breathing rate.

Vital capacity

The largest volume that can be breathed out after the deepest possible breath in; roughly 4.6 dm³ in a young adult man and 3.1 dm³ in a young adult woman.

Residual volume

The volume left in the lungs after the hardest possible breath out, about 1.2 dm³. It cannot be measured with a spirometer, because it never leaves the lungs.

A spirometer records the volume of air in a closed circuit against time, and OCR A expects lung volumes and oxygen uptake to be read from one. The trick with oxygen uptake is the soda lime in the circuit: it absorbs the carbon dioxide the subject breathes out, so the total volume of gas in the apparatus falls steadily by exactly the volume of oxygen consumed. Draw a line through the midpoints of the oscillations, take its gradient, and that gradient is the rate of oxygen uptake. Without the soda lime the exhaled carbon dioxide would roughly replace the oxygen removed and the trace would stay level.

One refinement that separates a good answer from a correct one. About 150 cm³ of each breath never reaches an alveolus; it stays in the trachea and bronchi, which are conducting tubes with no exchange surface. That volume is the dead space. It means a shallow, rapid breathing pattern moves less useful air than a slow deep one at the same pulmonary ventilation rate, because the dead space is paid for on every breath.

In the exam

• Write the sequence in causal order: muscles contract, thorax volume increases, pressure in the lungs falls below atmospheric, air moves in. Marks are awarded for the links, so a list of facts in the wrong order scores badly.
• Say 'below atmospheric pressure', not 'negative pressure' and never 'a vacuum'. Air moves down a pressure gradient; nothing sucks it.
• Quiet expiration is passive. Name elastic recoil, and do not describe the diaphragm as contracting upwards.
• For pulmonary ventilation rate, check the units before multiplying. Tidal volume is often given in cm³ and the answer wanted in dm³ min⁻¹.
• When identifying an airway from a section, justify it from the wall: rings of cartilage, blocks of cartilage, smooth muscle without cartilage, or a single squamous layer. CAIE 9700 names this skill directly.
• For any question about the rate of gas exchange, come back to Fick's law. Alveoli give area, the squamous epithelium gives thinness, and ventilation plus blood flow give the difference in concentration.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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