Blood Pressure

The main function of the heart is to pump blood through the body in order to transport oxygen and nutrients to organs and working muscle. Blood pressure refers to the pressure exerted by the blood in the body onto the walls of the blood vessels, and is generated by the contraction of the lower part of the heart (ventricles). The pressure in the blood vessels changes according to whether the heart is contracting (systole, higher value) or relaxing (diastole, lower value). Both values are equally important as they give us an idea of the resistance the heart has to work against, i.e. if the blood pressure is low, the heart will have to work against less resistance and will therefore be under less strain. Alternatively, high blood pressure (hypertension) forces the heart to work against a greater pressure and often results in other health complications.

Factors influencing blood pressure include blood viscosity ('thickness'), total blood vessel length and blood vessel radius. The length and radius of the blood vessels are directly related to the amount of resistance that will later be applied to the heart. An increase in the total blood vessel length, as may occur in obesity, will result in an increase in blood pressure, as the blood will now have to travel further against gravity to reach the heart. Similarly, a smaller diameter will also result in an increase in blood pressure by increasing the resistance to flow, provided the volume of blood remains the same. Mark's blood pressure readings show that gravity influences blood pressure as the blood is required to return to the heart against a constant force of gravity. This force is obviously not present in space.

Blood pressure is closely regulated by the nervous system. During any stress, or for example during exercise, more muscles and organs are needed in order to produce the required movement or effect. This then implies that more nutrients and oxygen will be needed for the increasing amount of active tissue. The nervous system instructs the muscle surrounding the blood vessels to constrict, thereby increasing blood pressure, but also increasing the amount of blood returned to the heart. After exercise, blood pressure decreases below the level that it was at before the exercise started, and remains lower for a certain amount of time. This explains why exercise is an important aspect in the maintenance of a healthy blood pressure.