Oxygen and Respiration — Page 37, Lesson 62

I want to walk you through the start of the chapter on Oxygen and Respiration. We've just seen from the previous chapter that every cell and tissue in the body needs oxygen. Some cells are far more sensitive to a lack of oxygen than others. Brain cells, for example, will die if they are deprived of oxygen for as little as two minutes. That's a critical fact for any pilot to understand — the brain cannot tolerate even a short interruption in oxygen supply. Now, the oxygen the body needs comes from the air we breathe. Here's a striking statistic: the brain only makes up about 2% of your total body weight, yet it consumes 20% of all the oxygen required for the body to function normally. So the brain is a very demanding organ in terms of oxygen. Nitrogen also dissolves into the blood to a small extent, but it plays no part in the body's normal processes. However, the nitrogen content becomes very important when we talk about decompression sickness, or DCS, which we'll discuss later in this chapter. The level of carbon dioxide in the bloodstream was mentioned in the previous chapter. It's the carbon dioxide level that actually triggers the brain to increase or decrease your breathing rate. Here's how it works: the higher the carbon dioxide level, the more the brain is stimulated to increase breathing. That increased breathing brings in more oxygen, which in turn reduces the carbon dioxide content. Once the brain senses the carbon dioxide level is back to normal, the breathing rate is reduced. There are also certain cells in the brain that detect a shortage of oxygen in the blood directly, and they will also trigger an increase in respiration. Let's look at the mechanics of how air gets into the lungs. During inspiration — that's the act of breathing in — the intercostal muscles between the ribs work together with the diaphragm. They increase the volume of the chest cavity, which reduces the internal pressure inside the chest. Since air moves from high pressure to low pressure, air is drawn into the lungs. Expiration is the reverse process. In normal, relaxed breathing, expiration happens simply by relaxing those same muscles — the intercostals and the diaphragm — allowing the chest cavity to return to its smaller size. This whole mechanism is sometimes referred to as external respiration. Under normal conditions, external respiration is a subconscious process — you don't have to think about it. It occurs at a rate of 12 to 20 breaths per minute, with an average of about 16 breaths per minute. Normal breathing is a purely automatic process. But in some diseases, such as poliomyelitis, that automatic system fails. In those cases, an artificial respirator is required to maintain respiration for the patient. That figure shows the anatomy involved in this process. So to summarise: oxygen comes from the air, the brain is a heavy consumer of oxygen and is very sensitive to deprivation, carbon dioxide drives the breathing control loop, and the physical act of breathing relies on the intercostal muscles and diaphragm working together to change chest volume and pressure.