The higher you are in the sky, the less oxygen your body will carry, and less oxygen means higher blood pressure. If you typically have a regular blood pressure or even a low blood pressure, this increase will likely have no effect on you.

Long-duration flights can indeed cause temporary fluctuations in blood pressure due to a combination of environmental and physiological factors. The primary culprit is the lower air pressure and reduced oxygen levels in a pressurized cabin, which can cause the heart to work harder to circulate oxygenated blood. Furthermore, the prolonged immobility of sitting in a cramped seat leads to "blood pooling" in the lower extremities, increasing the risk of peripheral edema and potentially affecting systemic pressure. Psychological stressors such as "white-knuckle" flying anxiety, dehydration from the dry cabin air, and the high sodium content typically found in processed airplane meals further contribute to elevated readings. For individuals with pre-existing hypertension, these factors can be significant, making it vital to stay hydrated, perform "seated calf raises" to encourage circulation, and avoid excessive caffeine or alcohol during transoceanic travel.