why is there no change in blood pressure in prone and supine position?

Hello caru,

The body needs to tightly control blood pressure in order to deliver oxygen to your tissues/cells in a relatively uniform manor, or maintain homeostasis.

A near constant blood pressure, especially if you are talking about minute to minute changes, is maintained by pressure sensors in the body called baroreceptors.  The most important of these are located in the aorta (main artery from the heart) and the carotid arteries (which are the main branches of arteries from the aorta to the brain). 

Due to stretch or relaxation of the walls of the vessel, these sensors send signals to the brain that cause chemicals and nerve signals to be released which affect heart rate, the force of heart contraction, and the amount of constriction of the arteries and veins in all parts of your body. 

By changing these variables, your body can keep a near constant blood pressure.

To your health,

Neil MD