Explanation
The heart consists of four chambers, 2 auricles and 2 ventricles. Auricles receive oxygenated blood from the lungs and deoxygenated blood from other parts of the body.
Ventricles, on the other hand pump out oxygenated blood to all parts of the body and deoxygenated blood to lungs. So blood pressure actually measures the pressure exerted by the ventricles during contraction and relaxation.