Oxygen in, carbon dioxide out: two systems doing one job

Air travels a fixed route, and each structure on it does a specific job:

• Nasal cavity and mouth — air is warmed, moistened and filtered. Nasal breathing does this better, which is why cold dry air reaching the airways directly through the mouth can trigger exercise-induced asthma.
• Pharynx (throat) and larynx (voice box) — shared passage, then the point where the airway separates from the oesophagus. The epiglottis closes the airway when swallowing.
• Trachea (windpipe) — held permanently open by C-shaped rings of cartilage, and lined with mucus and cilia that trap and sweep out particles.
• Bronchi — two branches, one to each lung, continuing to divide.
• Bronchioles — progressively smaller tubes with smooth muscle in their walls. That smooth muscle is what constricts in asthma and what dilates under adrenaline during exercise.
• Alveoli — roughly 300 million microscopic air sacs, where all gas exchange happens.

Everything above the alveoli is plumbing; the alveoli are the system's entire purpose. Four features make them extraordinarily good at exchanging gas, and each one is a direct application of what makes diffusion fast:

• Enormous surface area — around 70 square metres in total, roughly the floor of a classroom, folded into your chest.
• Extremely thin walls — a single cell thick, so the distance a gas must travel is minimal.
• A dense capillary network surrounding every alveolus, so blood is always immediately adjacent.
• A continuously maintained concentration gradient — ventilation constantly refreshes the air inside and circulation constantly removes the oxygenated blood, so the difference driving diffusion never disappears.

The respiratory system's functions are to move air in and out, to exchange gases with the blood, and to help regulate blood pH — because carbon dioxide dissolved in blood forms an acid, so changing how fast you breathe changes blood acidity. That third function becomes important in Lesson 9, when lactate accumulates.

Air is not pulled into the lungs. It is allowed in, by making the space bigger. Gas moves from higher to lower pressure, so enlarging the thoracic cavity drops the pressure inside it below atmospheric pressure, and air flows in to equalise.

• Inspiration is always active. The diaphragm contracts and flattens downwards, and the external intercostals contract to lift the ribs up and outwards. Thoracic volume increases, pressure falls, air enters. Note that these are skeletal muscles doing ordinary concentric contractions — Lesson 2 applies here directly.
• Quiet expiration is passive. The diaphragm and external intercostals simply relax, and the elastic recoil of the lungs and the ribs returning under gravity reduce thoracic volume. Pressure rises above atmospheric and air leaves. No muscle contracts to push it out.
• Forced expiration is active. During exercise the internal intercostals and the abdominal muscles contract to drive air out faster and more completely. This is the change you felt after the stairs in Card 01, and it is one of the reasons hard breathing feels like work while ordinary breathing does not.

Ventilation rate is the volume of air moved per minute, and it is simply tidal volume multiplied by breathing rate. At rest that is roughly 500 mL per breath at about 12 breaths per minute, or about 6 litres per minute. During maximal exercise a trained athlete can reach 150 litres per minute or more — a twenty-five-fold increase.

The heart is a four-chambered pump made of cardiac muscle, divided down the middle by the septum so that the two sides never mix. The two upper chambers are the atria, which receive blood; the two lower chambers are the ventricles, which pump it out.

• Right atrium receives deoxygenated blood from the body, through the vena cava.
• Right ventricle pumps it to the lungs.
• Left atrium receives oxygenated blood from the lungs.
• Left ventricle pumps it to the entire body, and therefore has by far the thickest muscular wall — it must generate enough pressure to reach your feet, while the right ventricle only has to reach the lungs.

Four valves keep flow one-way: the tricuspid between right atrium and right ventricle, the bicuspid (or mitral) between left atrium and left ventricle, and the two semilunar valves where blood leaves each ventricle. Cardiac muscle is self-exciting — it generates its own contraction rhythm rather than waiting for a nerve signal, which is why a heart continues to beat without conscious control.

Two consequences of that table are worth stating explicitly. First, arterioles are the control system: during exercise they redistribute blood so that working muscle can receive several times its resting share while flow to the digestive organs is reduced — a fact Lesson 6 returns to when explaining why eating immediately before exercise is uncomfortable. Second, veins depend on movement. Blood returning from the legs is pushed upwards by the skeletal muscles squeezing the veins as they contract, with the valves preventing backflow. That is why an athlete who stops dead after a hard run can feel faint, and why a cool-down is a circulatory measure rather than a courtesy.

Blood is roughly 55 per cent plasma, mostly water, carrying dissolved nutrients, hormones, waste and heat. The rest is cells: red blood cells containing haemoglobin, which carry oxygen; white blood cells, which defend against infection; and platelets, which clot. The circulatory system's functions follow directly — transport of gases, nutrients, hormones and waste; regulation of body temperature by moving heat to the skin; and protection through immunity and clotting.

The heart drives two separate circuits that run in series. Every drop of blood passes through both, alternately, for your whole life.

Pulmonary circulation — heart to lungs and back, the short circuit that collects oxygen:

• Right ventricle contracts, blood leaves through the pulmonary artery to the lungs.
• At the alveolar capillaries it releases carbon dioxide and collects oxygen.
• It returns through the pulmonary vein to the left atrium.

Systemic circulation — heart to body and back, the long circuit that delivers oxygen:

• Left ventricle contracts, blood leaves through the aorta, the body's largest artery.
• Arteries branch into arterioles, then into the capillary beds of every tissue, where oxygen and nutrients are delivered and carbon dioxide and waste collected.
• It returns through venules and veins, into the vena cava, and to the right atrium.

The full pathway, which you should be able to recite in order: vena cava, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary artery, lungs, pulmonary vein, left atrium, bicuspid valve, left ventricle, aortic semilunar valve, aorta, body, and back to the vena cava.

Now the answer to Card 01. Breathing and heart rate rise together because they are two halves of one task — delivering oxygen to muscle and removing carbon dioxide from it. Both are triggered by the same signals: chemoreceptors detect rising carbon dioxide and falling pH in the blood, and receptors in the muscles and joints signal that movement has begun. Carbon dioxide, not oxygen, is the main stimulus. The body monitors what is accumulating rather than what is being used up, which is a more reliable early warning.

Gas exchange is diffusion, and nothing else. Gases move passively from where their partial pressure is higher to where it is lower, and the body's entire respiratory apparatus exists to keep those gradients steep. No energy is spent moving the gases themselves.

External respiration happens at the alveoli:

• Alveolar air has a high partial pressure of oxygen (about 104 mmHg); blood arriving from the body has a low one (about 40 mmHg). Oxygen therefore diffuses into the blood.
• That same blood has a higher partial pressure of carbon dioxide (about 45 mmHg) than the alveolar air (about 40 mmHg). Carbon dioxide diffuses out into the alveoli and is exhaled.

Internal respiration happens at the tissues, and is the same process with the gradients reversed:

• Blood arriving in the muscle capillaries has a high partial pressure of oxygen (about 100 mmHg); working muscle, which is consuming oxygen, has a low one (40 mmHg at rest and far lower during exercise). Oxygen diffuses into the muscle.
• The muscle is producing carbon dioxide, so its partial pressure there is higher than in the blood, and carbon dioxide diffuses into the blood to be carried away.

Getting the gases from one exchange site to the other is a transport problem, and oxygen in particular is very poorly soluble in plasma:

• Oxygen — about 98 per cent is carried bound to haemoglobin in red blood cells, as oxyhaemoglobin. Only about 2 per cent dissolves in the plasma. This is why haemoglobin level is such a direct determinant of how much oxygen you can deliver, which is the whole of Lesson 5.
• Carbon dioxide — about 70 per cent is converted to bicarbonate ions in the plasma, about 23 per cent is bound to haemoglobin, and about 7 per cent is dissolved. The bicarbonate route is also how blood acidity is buffered.

During exercise, exchange gets faster, and for three compounding reasons that are worth having together. The gradients steepen, because working muscle consumes oxygen and produces carbon dioxide faster, so its partial pressures move further from those in the blood. Blood flow to the muscle increases through vasodilation, so more capillaries are open and the exchange surface is larger. And haemoglobin releases oxygen more readily in conditions of higher temperature, higher carbon dioxide and lower pH — exactly the conditions inside working muscle. The tissue that needs oxygen most is the tissue that unloads it most easily, and it is its own activity that causes this.

Write the journey of a single oxygen molecule from the air outside your mouth to the inside of a working quadriceps muscle cell, and then follow a carbon dioxide molecule produced by that cell all the way back out.

This is a small structured measurement, and it is deliberately the shape of the investigation you will design properly in Lesson 15. Take a resting pulse for one minute and count your breaths for one minute while sitting quietly — get someone else to count your breaths if you can, because knowing you are being counted changes your breathing.

Then complete three minutes of continuous moderate activity, and measure both again immediately, then at one, two and three minutes of recovery.

• Plot both on the same time axis. Which rose by a larger proportion of its resting value?
• Which returned to resting first, and what does that suggest about which system is the limiting one?
• Estimate your ventilation rate at rest and after exercise, assuming a tidal volume of 0.5 litres at rest and around 2 litres during exercise. Compare the fold-increase in ventilation rate with the fold-increase in breathing rate alone, and state what the difference tells you.
• Name three sources of error in what you just did — and be specific, because "human error" is not one. Consider counting accuracy, the effect of observation, timing, and whether three minutes of recovery was long enough to see the whole return.