Blood pressure measures the amount of force in the flow of blood through the body. The amount of pressure present is the result of the force exerted by the heart as it pumps blood, and the amount that the arteries resist the circulation of blood due to narrowing or other factors. Many things can affect blood pressure, including changes in atmospheric pressure due to deep sea diving, leaving the atmosphere entirely, and changes in the weather.
Deep water diving can cause extreme changes in blood pressure levels. The amount of atmospheric pressure is increased dramatically, due to the pressure exerted by the water over the swimmer. This increased pressure forces an increase of blood pressure, which can be extremely dangerous to anyone with high blood pressure. Individuals with blood pressure problems should consult their physician prior to any deep water diving excursion, to avoid serious risks to their health.
The effect of atmospheric pressure can be seen in reverse, by studying astronauts. These individuals spend long periods of time in space, without gravity, and the pressure exerted by the atmosphere. The greater the length of time spent outside of the Earth's atmosphere, the more likely that the astronaut will experience fainting episodes upon their return to Earth. It is theorized that the increased atmosphere puts a higher demand on the heart and it cannot keep up, which makes the blood pressure lower, which results in fainting. Further studies and reviews are still necessary to fully understand this phenomenon.
Changes in weather, which cause changes in atmospheric pressure, can be reflected in blood pressure fluctuations. According to a study published in the Journal of Biomedicine & Pharmacotherapy in 2001, changes in air pressure do result in changes in blood pressure. This study, titled: "Close Association Between Day-to-day Fluctuation of Atmospheric Pressure and Blood Pressure", studied over 400 individuals, concluding that there was a correlation between the fluctuations of the atmospheric pressure, and blood pressure readings on a day-to-day basis.