Circulatory Systems in Plants and Animals
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The Path of Blood through the Human Body

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. Pulmonary and systemic are the two circuits in the two-circuit system of higher animals with closed circulatory systems.

Humans and other mammals have two-circuit circulatory systems: one circuit is for pulmonary circulation (circulation to the lungs; pulmo = lungs), and the other circuit is for systemic circulation (the rest of the body). As each atrium and ventricle contract, blood is pumped into certain major blood vessels, and from there, continues through the circulatory system.

The intertwined circulatory system pathways: Pulmonary circulation and systemic circulation work to
The intertwined circulatory system pathways: Pulmonary circulation and systemic circulation work together.

Pulmonary circulation

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.

Superior means higher, and inferior means lower, so the superior vena cava is at the top of the right atrium, and the inferior vena cava enters the bottom of the right atrium.

From the right atrium, the deoxygenated blood drains into the right ventricle through the right atrioventricular (AV) valve, which is so named because it is between the atrium and the ventricle. This valve is also referred to as the tricuspid valve because it has three flaps in its structure. When the ventricles contract, the AV 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. Semilunar means half-moon and refers to the shape of the valve. 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.

Systemic circulation

Freshly oxygenated blood returns to the heart via the pulmonary veins. Note that these are the only veins in the body that contain oxygenated blood; all other veins contain deoxygenated blood.

The pulmonary veins enter the left atrium. When the left atrium relaxes, the oxygenated blood drains into the left ventricle through the left AV valve. This valve is also called the bicuspid valve because it has only two flaps in its structure.

Now the heart really squeezes. As the left ventricle contracts, the oxygenated blood is pumped into the main artery of the body — the aorta. To get to the aorta, blood passes through the aortic semilunar valve, which serves to keep blood flowing from the aorta back into the left ventricle.

The aorta branches into other arteries, which then branch into smaller arterioles. The arterioles meet up with capillaries, which are the blood vessels where oxygen is exchanged for carbon dioxide.

Capillary exchange

Capillaries bridge the smallest of the arteries and the smallest of the veins. Near the arterial end, the capillaries allow materials essential for maintaining the health of cells to diffuse out (water, glucose, oxygen, and amino acids).

To maintain the health of cells, it is also necessary for the capillaries to transport wastes and carbon dioxide to places in the body that can dispose of them. The waste products enter near the venous end of the capillary. Water diffuses in and out of capillaries to maintain blood volume, which adjusts to achieve homeostasis.

Capillaries are only as thick as one cell, so the contents within the cells of the capillaries can easily pass out of the capillary by diffusing through the capillary membrane. And, because the capillary membrane abuts the membrane of other cells all over the body, the capillary’s contents can easily continue through the abutting cell’s membrane and get inside the adjoining cell.

The process of capillary exchange is how oxygen leaves red blood cells in the bloodstream and gets into all the other cells of the body. Capillary exchange also allows nutrients to diffuse out of the bloodstream and into other cells. At the same time, the other cells expel waste products that then enter the capillaries, and carbon dioxide diffuses out of the body’s cells and into the capillaries.

How capillary exchange works.
How capillary exchange works.

After the capillaries “pick up” the garbage from other cells, the capillaries carry the wastes and carbon dioxide through the deoxygenated blood to the smallest of the veins, which are called venules. The venules branch into bigger vessels called veins. The veins then carry the deoxygenated blood toward the main vein, which is the vena cava. The two branches of the vena cava enter the right atrium, which is where pulmonary circulation begins.

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