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Introduction to Circulatory System
Veins carry blood to the heart. After the cells use up the oxygen from the hemoglobin, the veins take the deoxygenated blood to the heart. Veins do not direct the blood to the heart first because the heart does not pump blood with wastes. Veins carry deoxygenated blood to the lungs for more oxygen and to remove the carbon dioxide.
Veins are also composed of three major layers.
But these three layers are thinner and less muscular than the layers of the artery because the veins do not need to withstand constant pressure from the heart. Veins carry blood at a lower blood pressure. Veins can also dilate and contract. In order to bring the blood back to the heart, the veins contain valves to prevent the backward flow of blood.
Veins and arteries together play an important role in maintaining a constant temperature within the body. The veins and arteries can exchange heat depending on the outside temperature.
Capillaries are the tiny vessels that branch out through the whole body.
Only one blood cell can fit through a capillary at a time. All cells have a close proximity to a capillary. Several capillaries packed together are called a capillary bed. The cells dump their waste and collect their gases through the wall of the capillary. The capillaries also transport the deoxygenated blood to the veins. The capillary walls are composed of endothelium.
The blood is the vital element of the circulatory system. The average human has four to six liters of blood.
The blood is composed mainly of red blood cells, plasma, white blood cells, and platelets. The red blood cells and plasma are crucial for the transportation of wastes, gases, and nutrients. But how does the oxygen get delivered to the cells when the blood cannot leave the vessels? The oxygen and nutrients leave the blood and travel themselves into the cells in a process called diffusion. The white blood cells are important for bodily defense. The platelets, along with other clotting factors, close up small injuries.