The Effect of G-forces and Blood Pressure — Page 31, Lesson 54

I want to walk you through the effect of G-forces on the circulation system, and then we'll cover two important operational topics: blood donation for aircrew and pulmonary embolism. Let's start with G-forces and blood pressure. When a pilot experiences positive g — that's the force pushing you down into your seat, like during a pull-up or a tight turn — the blood pressure in the area of the brain decreases. Your body has special sensors called pressoreceptors — these are pressure-sensitive receptors located in certain blood vessels. Their job is to detect a drop in pressure and try to protect the brain by increasing the blood pressure. But at high g-forces, the pressoreceptors can no longer cope. When that happens, the pilot may suffer from greyout — where vision dims or turns grey — or blackout, which is a complete loss of vision and possibly consciousness. Now, the opposite case: negative g. That's the force that pushes you up out of your seat, like during a pushover or an outside loop. Here, the reverse process takes place. Instead of blood draining away from the brain, it rushes to the head. The pilot experiences what's called redout — a reddening of vision. This happens because blood vessels in the eye and face can burst under large negative g-forces. Additionally, the lower eyelids are pushed upwards, which physically obscures vision. Positive g-forces also increase what's called the hydrostatic variation of the circulatory blood system. Hydrostatic variation refers to the effect of gravity on the column of blood in your body — when you pull positive g, the blood column effectively becomes heavier, making it harder for the heart to pump blood up to the brain. This is explained in more detail in Chapter 6 of your book, which covers Flying and Health. Let's move on to a practical operational topic: donating blood as aircrew. Many aircrew want to donate blood, either to support the National Blood Transfusion Service or for personal reasons. But there's a small risk involved: after giving blood, you might experience post-transfusion faintness, which is medically called syncope — that's a temporary loss of consciousness due to reduced blood flow to the brain. To prevent this, the recommendation is that after giving blood, aircrew should drink plenty of fluids and rest supine — that means lying flat on your back — for a short time, specifically 15 to 20 minutes. Critically, you must refrain from flying duties for a minimum of 24 hours after donating blood. It's also advisable to seek advice from an aviation specialist before you donate blood in the first place. Finally, let's talk about pulmonary embolism. The blood supply to the lungs can be interrupted by a blockage to the pulmonary artery — that's the major artery carrying deoxygenated blood from the heart to the lungs for oxygenation. This blockage not only causes death of lung tissue, but it also prevents the blood from being oxygenated. The most common cause of this condition is a pulmonary embolism, which results from a blood clot from the leg — that clot is called a thrombus — becoming detached and travelling through the bloodstream to the lungs, where it becomes lodged, causing the blockage. The immobility associated with long-haul flights may predispose the formation of blood clots in the lower limbs. To reduce this risk, exercising the legs helps. Both crew and passengers should be encouraged to walk around the cabin from time to time throughout the flight.