However, if an artery is partially blocked or narrowed, the heart muscles need to work harder to push through the same volume of blood, which increases the blood pressure. In people suffering from chronic high blood pressure, the readings are generally above 140/90 mm Hg. This is called as hypertension and is typically caused by conditions such as arteriosclerosis, where the arteries are clogged up with plaque. The main culprit for this is high cholesterol. Such an individual is then at the risk of developing serious cardiovascular diseases, and perhaps even a heart attack if an artery is completely blocked.
However, blood vessels can be temporarily blocked or constricted due to physical reasons. One example is that of combat aircraft pilots, who undergo rapid changes in gravitational forces (G-forces) while maneuvering. This can cause blood to flow either to or away from the extremities, including the hands, feet, and head. If the blood flow to the brain is interrupted, it reduces the supply of oxygen, leading to dizziness and fainting. As you can imagine, fainting while flying at the speed of sound is hardly safe! This is the reason why combat pilots wear special g-suits, which automatically squeeze near critical veins in the limbs and torso. This increases the blood pressure, allowing the heart to pump additional oxygenated blood to the brain, saving the pilot from passing out. Crossing the legs causes a minor effect of the same kind. When the leg muscles are tensed, it squeezes the large veins, constricting them. This increases the blood pressure and makes the heart work harder. A recent study with a group of test subjects showed that crossing legs while getting a blood pressure reading increased both systolic and diastolic blood pressures in patients with existing hypertension. However, healthy individuals showed a rise mainly in the systolic blood pressure.
answered by M W