This book is the life's compilation of Dr. Rai's research and experiments to establish the function of the atrial diastole of the human heart with intriguing historical connections of these discoveries and the motion of blood in the venous system. Sir William Harvey was the first to establish the ventricle's mechanical function and the motion of blood in the arterial system. After a 400-years span, this book will further complement and complete our knowledge on the mechanical function of the atrial chambers of the human heart and open new doors in the world of research for a better understanding and treatment of ailments affecting the human heart. The book explores the origins and history of circulation to current newly discovered findings of the motion of blood in the venous system. The book also contains discoveries and revelations Dr. Rai made interconnected to each during his experimental exploration of the function of the Atrial diastole. Some of which are; venous flue of blood is pulsatile, the motion of venous valves a new discovery, cardiac muscle contracts during systole and expands during diastole, measurement of the velocity of the blood. All this provides a new parameter to understand hemodynamics This book is for anyone who has a passion for learning about the wonderful phenomenon of heart and blood circulation. However, the primary audience that will benefit from this book is vascular surgeons, angiologists, phlebologists, cardiovascular disease specialists, cardiac surgeons, and any specialists devoted to circulation. Dr. Rai's humble effort is to simplify the complex topic relating to the circular system for students and fellows who are doing Master's and post-doctorate in relative fields. This book contains many useful and novel researches in basic physiology, and as long as humankind is there, these researches will benefit everyone. In the future, we see this book being translated into many languages to be utilized to its full potential.