When a heart contracts and forces blood into the blood vessels, there is a certain path that the blood follows through the body. The blood moves through pulmonary circulation and then continues on through systemic circulation.
Humans and other mammals have two-circuit circulatory system s : one circuit is for pulmonary circulation, and the other circuit is for systemic circulation. As each atrium and ventricle contract, blood is pumped into certain major blood vessels, and from there, continues through the circulatory system.
The part of the circulatory system that transports deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side of the heart.
Blood that is lacking oxygen is said to be deoxygenated. This blood has just exchanged oxygen for carbon dioxide across cell membranes, and now contains mostly carbon dioxide. Deoxygenated blood enters the right atrium through the superior vena cava and the inferior vena cava.
From the right atrium, the deoxygenated blood drains into the right ventricle through the tricuspid valve. When the ventricles contract, the tricuspid valve closes off the opening between the ventricle and the atrium so that blood does not flow back up into the atrium.
As the right ventricle contracts, it forces the deoxygenated blood through the pulmonary semilunar valve and into the pulmonary artery. Note that this is the only artery in the body that contains deoxygenated blood; all other arteries contain oxygenated blood. The semilunar valve keeps blood from flowing back into the right ventricle once it is in the pulmonary artery.
The pulmonary artery carries the blood that is very low in oxygen to the lungs, where it becomes oxygenated.
The aorta leads to smaller arteries, arterioles, and finally capillaries. Waste and carbon dioxide diffuse out of the cell into the blood and oxygen in the blood diffuses out of the blood and into the cell, blood then moves to venious capillaries, and then the venae cavae: the lower inferior vena cava and the upper superior vena cava, through which the blood re-enters the heart at the right atrium. in the liver.
Oxygenated blood enters the systemic circulation when leaving the left ventricle, through the aortic semilunar valve. The first part of the systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and branches into major arteries to the upper body before passing through the diaphragm, where it branches further into arteries which supply the lower parts of the body. After their passage through body tissues, capillaries merge once again into venules, which continue to merge into veins.
The venous system finally coalesces into two major veins: the superior vena cava (roughly speaking draining the areas above the heart) and the inferior vena cava (roughly speaking from areas below the heart). These two great vessels empty into the right atrium of the heart. The general rule is that arteries from the heart branch out into capillaries, which collect into veins leading back to the heart..
Portal veins are a slight exception to this. In humans the only significant example is the hepatic portal vein which combines from capillaries around the gut where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the liver.