The human heart is a muscular organ that acts as a pump to circulate blood around the body, feeding the cells oxygen and nutrients and removing carbon dioxide and waste products. The heart, other muscles and the blood vessels are together referred to as the 'circulatory system' and the process known as 'circulation'. The blood vessels that carry the blood away from the heart are termed arteries, and the vessels that carry blood back to the heart are known as veins.
The heart is comprised of four chambers: the right and left atria and the right and left ventricles. Blood returns from the body into the right atrium via the superior vena cava and the inferior vena cava (translating from the latin as upper hollow vein and lower hollow vein), the former returning blood from the upper body and the latter from the lower body. This blood, which is low in oxygen and high in carbon dioxide, is pumped down into the right ventricle where it exits the heart and heads towards the lungs via the pulmonary artery. In the lungs gaseous exchange occurs, whereby the waste product gas carbon dioxide is exchanged for the oxygen which is essential for respiration in the cells. The blood returns to the heart from the lungs into the left atrium via the pulmonary vein. It is then pumped down into the left ventricle where it then exits the heart to the rest of the body via the aorta. The blood thus travels in one direction through the heart chambers. Backflow, or regurgitation, is prevented by valves between the chambers which close regularly with the contractions of the heart.
The veins - the blood vessels that carry the blood back to the heart - are under less pressure than the arteries (that carry the blood from the heart) since the blood is pumped out from the heart at high pressure, but is pulled in at a lower pressure. To withstand this higher blood pressure, the arteries have thicker walls and more muscular, elastic walls. Similarly to the heart chambers, veins have valves to prevent backflow.
Blood is returned to the heart from the body via the superior vena cava and the inferior vena cava.
Blood Flow Explanation
All blood goes into the right side of your heart through two main veins: The superior vena cava (SVC) and the inferior vena cava (IVC). The SVC gets blood from the upper part of the body. The IVC gets blood from the lower part of the body. Blood leaves the SVC and the IVC and enters the right atrium (RA). When the RA squeezes, the blood goes into the tricuspid valve (4) and into the right ventricle (RV). When the RV squeezes, blood is sent through the pulmonary valve, and into the pulmonary artery (PA) and into the lungs where it gathers up oxygen. The right side of the heart sends blood to the lungs to pick up oxygen before going to the left side of the heart. There it is returned to the body full of oxygen. Blood now goes back to the heart from the lungs by way of the pulmonary veins and flows into the left atrium (LA). When the LA squeezes, blood travels through the mitral valve and into the left ventricle (LV). The LV is very important and that pumps blood through the aortic valve into the aorta. The aorta is the big artery of the body.
The average heartbeat is around 72 beats per minute. In one day it beats more than 100,000 times. In a year your heart beats almost 38 million times, and by the time you are 70 years old, on average, it's had 2.5 billion beats.
Blood is returned to the right side of the heart through the inferior and superior vena cava. This is due to the blood being oxygen-poor as it has gone through the body delivering oxygen and picking up other gases such as carbon dioxide. The blood needs to be replenished with oxygen to once again deliver throughout the body since oxygen is needed through the body's systems and organs. In order to replace the carbon dioxide with oxygen the blood must be filtered through the lungs. It must first go through the heart, out into the lungs, and back into the heart to then leave to the rest of the body once again, except this time it is now oxygen-rich. Once it has delivered oxygen and picked up other gases it will once again return to the heart for more oxygen. This cycle is a continuous process.
I left a link to a video to show the pathway of blood flow through the heart below.
Blood is returned to the right side of the heart by pair of superior and inferior vena cava.
Blood entering in the right side of the heart is deoxygenated and it further travels to the lungs by the pumping action of heart. The oxygenation process in the lungs convert the blood in to oxygenated form which enters the left side of the heart by pulmonary veins, that undergoes pumping by the heart to supply the major systems of the body. Like nervous system, GIT system, respiratory system, circulatory system and reproductive system.
The heart is split into four chambers: the atria, the two upper chambers, and the ventricles, the lower chambers. There are two major veins that are connected to the heart. The pulmonary veins carry blood from the lungs to the left side of the heart, which is where it is pumped to the body. The superior and inferior vena cavae are the veins that carry the blood from the body back to the heart.