The cardiovascular system is responsible for transporting blood around the body and supplying oxygen and nutrients to tissues. Because of this, it is constantly reacting and adapting to changes in the body's position. For example, standing up quickly can result in orthostatic hypotension, a form of low blood pressure that occurs when standing up from sitting or lying down. This is due to the effects of gravity on blood flow and the release of hormones like adrenaline.
Two main types of changes occur in the cardiovascular system during postural changes: in the venous return and in sympathetic activity. Venous return is blood movement from the periphery back toward the heart. It is aided by skeletal muscle contractions (Mumps' Law) and gravity. When you stand up, gravity pulls blood down from your upper body, increasing the amount of blood returning to the heart. This increase in venous return leads to an increased heart rate and stroke volume (the amount of blood pumped by the heart with each beat).
Sympathetic activity is the part of the nervous system that controls the body's "fight or flight" response. When you stand up, sympathetic activity increases to prepare your body for any potential danger. This increase in sympathetic activity leads to an increase in heart rate and blood pressure.