A multibranched and flexible network of tubular blood vessels runs through the human body. Blood runs through the interconnected vessels and is circulated in the body in a steady flow by the rhythmical pumping of the heart. This mechanism is very important, as the blood is a means of transport for vital nutrients and oxygen, supplying various organs such as the liver, kidneys and lungs for example. At the same time the waste products and used carbon dioxide are evacuated from the tissue with the blood.
The blood vessels through which blood flows to organs are called arteries. Vessels, which transport the blood from the organs back to the heart, are called veins.
Because of the pumping action of the heart and the different diameters of the blood vessels, a higher pressure (high-pressure system) is generated in the arteries than in the veins (low-pressure system). That pressure exerted by the blood flow on the walls of the blood vessels is called blood pressure. In addition, the blood vessels themselves can also produce pressure by narrowing their diameter. Arteries can narrow and expand through the muscles in their walls. Narrowing increases pressure, while expansion causes pressure to drop. Blood pressure therefore depends on cardiac output and the diameter of the blood vessels. While the actual cardiac output only is a functional requirement for the maintenance of blood pressure, the vascular resistance regulates how high the blood pressure is. In other words, blood pressure is a physiological quantity that can be influenced by both physical and psychological tension, without necessarily being a health hazard.
The heart can constrict and relax, like any other muscle. Through this mechanism, it pumps the blood into every area of the body. The rhythmic pressure wave generated by this process can be felt in the arteries and as a pulse. A resting pulse repetition frequency of 60 to 80 beats a minute is considered normal. Pulse repetition frequency however considerably depends on age and fitness, and can vary significantly.
A pulse is obtained by arterial palpation with a trained finger. Electrocardiography, is the method of graphic tracing (Electrocardiogram) of the electric current generated by the cardiac cell and conducting fiber during a heartbeat. ECG is divided into P wave, PR interval, QRS complex, ST segment and T wave. The pulse represents the QRS segment's left ventricle contraction.
Source: Life In The Fast Lane | https://lifeinthefastlane.com/ecg-library/basics/p-wave/