Everything the next three lessons say about altitude rests on the machinery in this one. Two transport systems keep every cell supplied: the respiratory system moves air in and out and swaps gases with the blood, and the circulatory system moves that blood to the tissues and back. The exam wants both systems by name and function, the handful of normal values a medical examiner works with, and the ways the pump and its pipes fail.
The air you breathe
Start with the raw material. Dry air is 78 per cent nitrogen, 21 per cent oxygen and about 1 per cent other gases, argon and carbon dioxide among them, and those volume percentages stay constant at all altitudes at which conventional aircraft operate. What changes with altitude is not the recipe but the pressure: the same 21 per cent is taken from an ever thinner total, so the partial pressure of oxygen falls with every foot of climb. That single fact powers the whole hypoxia story of the next lesson.
The respiratory system
Air reaches the blood through a chain of named components, each with its function:
Upper airways — nose, mouth and pharynx warm, moisten and filter the incoming air.
Trachea and bronchi — the branching pipework that conducts air into each lung.
Alveoli — millions of thin-walled air sacs where gas exchange with the capillary blood actually happens.
Diaphragm and intercostal muscles — the pump: contracting enlarges the chest and draws air in, relaxing lets the elastic lungs push it out.
The volumes have names too. A resting breath moves the tidal volume, about half a litre; the greatest volume that can be exchanged with maximum effort is the vital capacity, several litres more; and even the fullest exhalation leaves a residual volume behind, so the total lung capacity of an adult is around six litres. At rest an adult breathes at a normal respiratory rate of about 12 to 16 breaths per minute.
Gas exchange and its control
Respiration happens twice. External respiration is the exchange in the lungs: oxygen diffuses from the alveolar air into the blood while carbon dioxide diffuses out to be exhaled. Internal respiration is the same exchange at the other end, between the blood and the tissues: oxygen is delivered to the cells and the carbon dioxide their metabolism produces is collected. Both are pure diffusion, each gas sliding down its own partial-pressure gradient.
The controller is a surprise the exam loves: the drive to breathe is regulated not by a shortage of oxygen but by carbon dioxide. Chemoreceptors monitor the CO2 level, and through it the acidity, of the arterial blood, and the brain's respiratory centre adjusts the rate and depth of breathing to hold that level; rising CO2 deepens and quickens breathing, falling CO2 suppresses it. Oxygen sensing exists but only takes over at a dangerously low level, which is why the insidious oxygen shortage of altitude fails to ring the alarm, and why over-breathing in the next lesson causes trouble by washing out carbon dioxide rather than by adding oxygen.
The circulatory system
The delivery network has four major components:
The heart — a four-chambered muscular double pump: the right side sends blood through the lungs to load oxygen, the left side sends the oxygenated blood around the body.
Arteries — thick, elastic-walled vessels carrying blood away from the heart under pressure.
Capillaries — vessels one cell thick where internal respiration happens: gases, nutrients and waste cross here.
Veins — thin-walled vessels with one-way valves returning blood to the heart at low pressure.
The numbers a medical examiner expects: a normal resting pulse of about 60 to 80 beats per minute, around 70 on average. The pulse rate is not fixed; it is determined by factors such as physical exertion, emotional stress, temperature, general fitness and health, and altitude, the last through the compensation reflexes the next lesson describes. Multiply the heart rate by the stroke volume, the roughly 70 millilitres each beat ejects, and you get the cardiac output: about 70 beats of 70 ml, close to five litres per minute for an adult at rest, which is roughly the body's entire blood volume circulated every minute.
Each heartbeat writes a pressure wave into the arteries, and the two edges of that wave are the measured values: systolic pressure is the maximum, produced as the heart's contraction ejects blood, and diastolic pressure is the minimum, holding between beats while the heart refills. Both are measured in millimetres of mercury (mm Hg) and written as the pair systolic over diastolic: a healthy resting adult sits near 120/80 mm Hg, with normal ranges of roughly 100 to 140 systolic over 60 to 90 diastolic.
Blood
What the vessels carry is itself a transport system, with a division of labour among its main constituents:
Plasma — the watery medium, a bit over half the volume, carrying nutrients, hormones, waste and dissolved carbon dioxide.
Red blood cells — the oxygen carriers, packed with haemoglobin.
White blood cells — the immune defence against infection.
Platelets — the clotting agents that seal damaged vessels.
The molecule that makes the whole altitude story possible is haemoglobin: the iron-bearing protein in the red cells that binds oxygen in the lungs and releases it in the tissues, and carries almost all of the blood's oxygen; the amount dissolved in plasma alone would not keep you conscious. Too little of it, from a reduced red-cell count, low haemoglobin concentration or abnormal haemoglobin, is anaemia, commonly caused by iron deficiency, blood loss or chronic disease, and an anaemic body tolerates altitude poorly because its oxygen-carrying capacity is already spent.
Haemoglobin's binding curve also explains the shape of trouble at altitude. Because the molecule holds oxygen greedily, saturation stays high while the aircraft climbs through the first thousands of feet: about 93 per cent at 8,000 ft and still near 89 per cent at 10,000 ft, the point where it slips below the 90 per cent mark. Beyond there the fall steepens with every thousand feet: about 87 per cent at 12,000 ft, 83 per cent at 14,000 ft, and down to some 65 per cent at 20,000 ft. The gentle start is why a cabin altitude of 8,000 ft is acceptable; the steepening fall is why the next lesson's thresholds crowd so closely above 10,000 ft.