Respiratory system

What the exam asks

Expect an oxygen dissociation curve, a spirometry trace, a table of volumes, or partial pressures at two points in a circuit. The commonest items by far are curve-reading: identify a shifted curve, name what could have caused the shift, and say what it does to loading at the lungs versus unloading at the tissues. The trap that catches most candidates is the axis. The y-axis is percentage saturation, so it reports how full the haemoglobin is and says nothing about how much haemoglobin there is. In anaemia the curve lies exactly where a normal one does, arterial oxygen tension is normal, saturation is normal, and the blood still carries half the oxygen it should. If a stem asks about oxygen content or delivery, saturation is only one of the two numbers you need. The second trap is reading a right shift as damage. At the tissues it is help: it is the mechanism that hands more oxygen to the muscle that is producing the acid and the heat, and it costs the lungs almost nothing because loading happens on the flat part of the curve.

The respiratory topic is two machines. One moves air, and it runs on Boyle's law: muscles change the volume of the chest, pressure follows volume, and air slides down the gradient that creates. The other moves oxygen from that air into blood and out again at the tissues, and it runs on haemoglobin.

The oxygen dissociation curve is the single highest-yield figure in the section, and it is worth understanding rather than remembering. It is sigmoid because haemoglobin's subunits cooperate: the first oxygen to bind makes the next one easier, and the loss of the first makes the rest easier to lose. That gives a curve with a flat top and a steep middle, and those two regions do different jobs. The flat top is at the lungs, where it means loading is protected against a fall in alveolar oxygen. The steep middle is at the tissues, where it means a modest fall in oxygen tension unloads a great deal of oxygen. The shape matters where the slope is, and the slope is at the tissues.

A right shift is lower affinity: haemoglobin holds oxygen less tightly, so at any given tension it is less saturated and gives more up. The things that cause it are the things a working muscle produces: heat, carbon dioxide, acid. That is not a coincidence to be memorised but a mechanism to be read. Tissue that is working hardest advertises itself chemically, and the blood passing through hands over more oxygen there without any control system being involved.

Ventilation questions are usually arithmetic in disguise. Some of every breath never reaches an alveolus, so what matters is not how much air moves but how much of it arrives somewhere useful, and two people moving identical volumes per minute can be ventilating very differently.

What to hold

• Inspiration is active: the diaphragm and external intercostals enlarge the thorax, pressure in the lungs falls below atmospheric, and air flows in down the gradient. Quiet expiration is passive elastic recoil. The lung never pulls air in: volume changes first and pressure follows, which is Boyle's law.
• The lung follows the chest wall only because the pressure in the pleural space is below atmospheric. Open that space to the air and the coupling is lost, so the lung recoils inwards and collapses.
• Surfactant lowers surface tension, and because pressure in a bubble rises as radius falls, it is what stops small alveoli emptying themselves into large ones.
• The oxygen dissociation curve is sigmoid because of cooperative binding: each oxygen bound raises the affinity of the remaining sites.
• The plateau above about 60 mmHg means saturation is well defended against falls in alveolar oxygen tension, and also that raising tension above normal adds very little.
• The steep segment sits in the range of tissue oxygen tensions, so a small fall in tension releases a large amount of oxygen. This is where the sigmoid shape earns its keep.
• A right shift means reduced affinity and a higher P50: less saturated at any tension, and more readily unloaded.
• Increased carbon dioxide, increased acidity, increased temperature and increased 2,3-BPG all shift the curve right. The first three are what active tissue produces.
• The y-axis is percentage saturation, not oxygen content. Anaemia lowers content with a normal curve and a normal tension, so the curve alone cannot show it.
• Residual volume cannot be exhaled, so spirometry cannot measure it, nor any capacity that contains it, which rules out functional residual capacity and total lung capacity.
• Alveolar ventilation is tidal volume minus dead space, times rate. Rapid shallow breathing wastes a larger fraction of every breath on dead space than slow deep breathing at the same minute ventilation.
• Diffusion across the alveolar barrier rises with surface area and falls with barrier thickness. Carbon dioxide is far more soluble than oxygen, so it crosses more easily despite its smaller gradient.