CO2 + H2O  H2CO3 — Page 26, Lesson 39

I want to walk you through a key chemical relationship in your blood, then into the circulation system itself. Let’s start with that equation: CO₂ + H₂O → H₂CO₃. Carbon dioxide plus water forms carbonic acid. Your blood is naturally acidic because of the presence of this carbonic acid. That acidic basis is required — it’s not a problem, it’s essential — to allow the easy release of oxygen from oxyhaemoglobin to the tissues. Oxyhaemoglobin is the compound formed when oxygen binds to haemoglobin in your red blood cells. For that oxygen to let go and move into the tissues where it’s needed, the blood needs to be slightly acidic. Now, what happens if the acidity drops? A reduction in the acidity of the blood — which can occur when there is insufficient carbon dioxide in the blood, as when hyperventilating — interferes with the release of oxygen to the tissues. Hyperventilation means breathing faster or deeper than necessary, which blows off too much CO₂. Less CO₂ means less carbonic acid, so the blood becomes less acidic. That shift makes it harder for oxyhaemoglobin to release oxygen. I’ll cover hyperventilation in more detail when we get to Chapter 3 on Oxygen and Respiration, but for now, hold that link: CO₂ level controls blood acidity, and blood acidity controls oxygen release. Now let’s move into the circulation system itself. The function of the circulatory system that particularly concerns you as an aviator is the carriage of oxygen to the tissues and the removal of carbon dioxide. Oxygen is required by the tissues for oxidation of food — that’s the chemical burning of fuel to release energy. The major source of energy for the body is carbohydrates, which are a component of our food. But energy can also be derived from proteins and fats. That’s why those on hunger strike inevitably utilize proteins for energy — when you stop eating carbohydrates, the body breaks down its own protein stores. Carbohydrates themselves are composed only of the elements carbon, hydrogen, and oxygen. In the tissues, these foodstuffs combine with oxygen to give energy, and the equation looks like this: Carbohydrates + Oxygen → Energy + CO₂ + H₂O. So the process consumes oxygen and produces carbon dioxide and water as waste products. This process whereby energy is released from food takes place in the cells and is called Internal Respiration. That’s distinct from external respiration, which is the exchange of gases between the lungs and the atmosphere. Internal respiration is the cellular-level burning of fuel. Oxygen is obtained from the atmosphere, and the blood picks up the oxygen from the lungs for transport around the body. Let’s follow the path of the circulatory system as shown in Figure 2.2. The largest and most muscular part of the heart is the left ventricle. Blood containing oxygen is sent around the body from the left ventricle when it contracts. A system of one-way valves in the heart prevents blood going the wrong way — so when the ventricle squeezes, blood goes forward into the arteries, not backward into the atrium. The oxygenated blood passes through the aorta — that’s the biggest artery leaving the heart — into the major arteries, which divide into the smaller arteries before arriving at the smallest vessels of the system: the capillaries. The capillaries have very thin walls, only one cell thick. That thinness allows the passage of oxygen from the blood into the tissues by diffusion, governed by Fick’s Law. Diffusion means molecules move from an area of higher concentration to lower concentration. The capillaries also allow carbon dioxide and water vapour to diffuse in the reverse direction — from the tissues back into the blood, to be carried away and exhaled. So to tie it all together: the left ventricle pumps oxygen-rich blood out through the aorta, down through progressively smaller arteries, into the capillaries. At the capillary walls, oxygen diffuses out to the tissues, and CO₂ and water vapour diffuse in. That CO₂ then travels back to the lungs, where it combines with water to form carbonic acid in the blood — maintaining that acidic environment needed for the next round of oxygen release.