The Circulation System — Page 31, Lesson 52

Let’s pick up with the circulation system. I want to walk you through blood pressure — what it means, what happens when it goes too high or too low, and how your body regulates it automatically. First, a quick refresher on the two numbers you always see in a blood pressure reading. The top number is the systolic pressure — that’s the peak pressure in your arteries when the heart contracts and pushes blood out. The bottom number is the diastolic pressure — the lower pressure when the heart relaxes between beats. You’ll see them written as, for example, 120 over 80, or 120/80. That’s the typical healthy reading for a young adult. The unit is millimetres of mercury, abbreviated mm Hg. Now, too high a blood pressure — hypertension — is a serious problem. A reading of 160/95 or over is assessed by JAR-FCL3 as unfit for flying. JAR-FCL3 is the Joint Aviation Requirements for Flight Crew Licensing, specifically the medical part. So if your blood pressure hits or exceeds that threshold, you would not pass the medical. Hypertension is a major factor in strokes and can contribute to cardiovascular failure. What causes it? The excerpt lists several factors: stress, smoking, dietary factors — especially excessive fat and/or salt intake — age, obesity, lack of exercise, and narrowing and/or hardening of the arteries. Here’s the tricky part: high blood pressure may give no symptoms at all. That’s why it needs to be detected by routine flight crew screening — you can’t rely on how you feel. When symptoms do appear, the primary ones are: heart palpitations, shortness of breath, angina — which is chest pain — headaches, and nose bleeds. Hypertension can be controlled by drugs, surgery, or a change in lifestyle. And a practical note for pilots: the average pilot’s blood pressure will rise slightly with age as the arteries lose their elasticity. Your aviation medical examiner will take this natural change into consideration during progressive medical tests. Now let’s flip to the other side — hypotension, or low blood pressure. Normally, low blood pressure does not constitute a danger. But if the pressure decreases too much, leading to a shortage of oxygen to the tissues, it can cause problems. Those include: lethargy or tiredness, reduced resistance to the effects of shock — meaning you’re more prone to faints or collapse — congestion of the respiratory system, stagnation in the blood supply, and reduced capability to withstand positive g-forces. That last one is especially relevant for pilots — if you have low blood pressure, you’ll be less able to tolerate pulling positive Gs without passing out. The normal range varies with age, but again, a healthy young adult will typically have a systolic pressure of about 120 mm Hg and a diastolic of about 80 mm Hg — that’s 120/80. Here’s the bottom line for your medical: both hypertension and hypotension may disqualify a pilot from obtaining a medical clearance to fly. So keeping your blood pressure in the normal range is critical. Now, how does your body keep blood pressure stable automatically? That’s where pressoreceptors come in. These are pressure sensors located in the wall of the carotid sinus in the neck, upstream of the brain. They are part of the pressure-regulating system for the blood supply to the brain. Here’s how they work. If the pressoreceptors detect an increase in blood pressure — hypertension — they send impulses to the brain. The brain then responds by causing a reduction of the heart rate and a relaxation of the blood vessels. Both of those effects lower the blood pressure back down. If the pressoreceptors detect a decrease in blood pressure — hypotension — they send impulses to the brain, which causes an increase of the heart rate and a tightening of the blood vessels. That raises the blood pressure back up. So the primary function of the pressoreceptors is maintaining homeostasis — that is, keeping the internal environment stable, in this case keeping your blood pressure within a safe range. That figure shows you the diastolic and systolic pressures visually — the peaks and valleys of the pressure wave in the artery.