The Circulation System — Page 31, Lesson 50

I want to walk you through a section of the circulation system that deals with two important topics: the effects of carbon monoxide and smoking on your body as a pilot, and then how blood pressure is measured and what those numbers mean. Let’s start with carbon monoxide. Many aircraft are equipped with carbon monoxide detectors. As a pilot, you need to check these detectors regularly during flight, and they must be correctly maintained by engineering. Carbon monoxide is a serious hazard because it binds with the haemoglobin in your blood, and that prevents the haemoglobin from carrying oxygen. Now, smoking tobacco is a major source of carbon monoxide. Apart from the addictive properties of nicotine—which itself is thought to increase the risk of cardiovascular disease—and tar, which is carcinogenic, meaning it increases the risk of cancer, the carbon monoxide from smoking directly affects your oxygen carriage. If a person smokes 20 cigarettes a day, their carboxyhaemoglobin level will be raised by about 7%. Carboxyhaemoglobin is the compound formed when carbon monoxide binds to haemoglobin. That 7% rise equates to a reduction in oxygen carrying capacity equivalent to being at an altitude of 4000 to 5000 feet. Now add that to a typical cockpit altitude of 6000 to 8000 feet, and the smoker would react as if they were at an altitude up to 12 000 feet. That results in anaemic hypoxia—a type of hypoxia where the blood simply cannot carry enough oxygen—leading to reduced performance and slower reactions. Passive smoking also affects individuals who are not smoking themselves. That is why there is a tendency for modern airlines to move more and more towards a non-smoking policy on board aircraft. The excerpt also lists several other effects of smoking. It leads to lung cancer, breathing problems, circulatory problems, reduced tolerance to g-forces, and an increased risk of heart attack. There is also a stimulation of the secretion of adrenaline and an increase of vigilance caused by the nicotine—but as stated, nicotine is the substance that causes addiction. Additionally, smoking degrades night vision, which is critical for pilots. Some airlines may not accept into their training programmes pilots who smoke. So the advice is clear: if you smoke, stop; if you don’t smoke, don’t start. Now let’s move to blood pressure. As part of any medical examination, a doctor will measure your blood pressure. The result is given as two numbers, for example 120 over 80. The higher figure is the systolic pressure. That is the pressure exerted by the heart when it contracts to send blood around the body. The lower figure is the diastolic pressure, which is the permanent pressure within the arterial system—the pressure that remains in the arteries between heartbeats. These pressures are measured in millimetres of mercury, often written as mmHg. So to summarise: carbon monoxide from smoking robs your blood of oxygen, effectively raising your physiological altitude and causing hypoxia. And blood pressure is recorded as two numbers—systolic over diastolic—measured in millimetres of mercury, with systolic being the pressure when the heart contracts and diastolic being the resting pressure in the arteries.