The deoxygenated blood collects in two large veins: the superior vena cava (which flows blood from the head and upper limbs) and the inferior vena cava (which flows blood from the abdomen and lower limbs) which empty their contents into the right atrium .
The right atrium is the larger of the two atria because it must be able to contain a greater quantity of blood such as that coming from the body. Blood is then pumped through the tricuspid valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary semilunar valve into the pulmonary trunk. Deoxygenated blood leaves the heart from the pulmonary arteries and travels through the lungs, where it is oxygenated, and from there passes into the pulmonary vein.
The oxygenated blood then enters the left atrium from which it then travels through the bicuspid or mitral valve into the left ventricle. The left ventricle is thicker and more muscular than the right ventricle, because it pumps blood throughout the body (systemic circulation). From the left ventricle, blood is pumped through the semilunar valve into the aorta. Once the blood passes through the systemic circulation, the deoxygenated blood will again be collected within the vena cava and the process will repeat.
The systemic circulation is that part of the cardiovascular system that carries oxygenated blood from the heart to the body, and returns deoxygenated blood to the heart. The oxygenated blood from the lungs leaves the left heart through the aorta, from where it is distributed to organs and tissues, which absorb the oxygen, through a complex network of arteries, arterioles and capillaries. The deoxygenated blood is then collected by venules, from where it flows first into the veins, and then into the inferior and superior vena cava, and from there back to the right heart, completing the cycle. The blood is then reoxygenated through the pulmonary circulation before returning to the systemic circulation.
The pulmonary circulation is that part of the cardiovascular system that transports oxygen-depleted blood from the heart to the lungs, and returns oxygenated blood to the heart.
Deoxygenated blood exits the right heart through the pulmonary arteries, which carry it to the lungs, where red blood cells release carbon dioxide and pick up oxygen as they breathe. The oxygenated blood then leaves the lungs through the pulmonary veins, which return it to the left heart, completing the pulmonary cycle.
The blood is then distributed to the body through the systemic circulation before returning to the pulmonary circulation.
The pulmonary circulation was discovered and published by Ibn Nafis in 1242. It was then published by Michael Servetus in Christianismi restitutio (1553). Since it was a work of theology condemned by most Christian factions of his time, the discovery remained mostly unknown until William Harvey's dissections in 1616.
Heart valves were discovered by a doctor of the school of Hippocrates around the 4th century. B.C. However, their function was not well understood. Since after death it was noted that the veins were full of blood and the arteries were empty, ancient anatomists assumed that the latter had been filled with air and that they served for the transport of air.
Later Herophilus illustrated the arteries, but thinking that the pulse was a property of the arteries themselves. Erasistratus then observed that the arteries that were cut at the waist were bleeding. He attributed this to the phenomenon that air escaping from an artery is replaced with blood that has entered through very small vessels between veins and arteries. So he apparently assumed capillaries but with a reversed flow of blood.
The 2nd century AD the Greek doctor Galen noticed that blood vessels carried blood and identified the dark red one as venous, and the 'brighter and thinner' one as arterial, each with distinct and separate functions. Growth and energy according to him came from the venous blood created in the liver by the chyle, while the arterial blood gave vitality thanks to its content of air, and originated from the heart. Blood flowed from both organs and there was no return of blood to the heart or liver. The heart did not function as a pump, the movement of the heart sucked blood during diastole and the blood was moved by the pulsations of the arteries themselves.
Galen believed that arterial blood was created from venous blood as blood passed from the left ventricle to the right through 'pores' located in the interventricular septum, and air passed from the lungs through the pulmonary artery to the left side of the heart.
Ibn Nafis in 1242 was the first to precisely describe the process of blood circulation in the human body. Contemporary drawings of this process have been found. In 1552 Servetus described the same and Realdo Colombo demonstrated the concept. However, all these findings have not been universally accepted.
Finally William Harvey, a student of Hieronymus Fabricius (who had previously described the valves of the veins without recognizing their function), performed a series of experiments and announced in 1628 the discovery of the human circulatory system as its own system and published a book about this topic. This work with its fundamentally correct exposition slowly convinced us that the capillary system connected the arteries and veins. These were later described by Marcello Malpighi.