The circulatory system is the complex network responsible for delivering nutrients, hormones, and gasses such as oxygen to the body's cells. This system, also known as the cardiovascular system, works in tandem with other systems in the body to maintain homeostasis -- the body’s ability to maintain stability despite constantly changing -- and is integral to many of the functions that allow the body to thrive. The essential parts of the circulatory system are the blood, blood vessels, and the heart, though many secondary components help keep the system working.
For the sake of explanation, educators often state that the circulatory system "begins" in the right atrium. In actuality, if the body functions properly, the system doesn’t start or end anywhere. Each part of the system is working at the same time. The right atrium in the upper-right portion of the heart receives deoxygenated blood through two large veins. The superior vena cava accepts blood from parts of the body such as the head and the arms. The inferior vena cava accepts blood from the legs and lower abdomen.
In the wall of the right atrium is a group of cells that control the contractions of the heart. This sinoatrial node sends electrical impulse controls the heart and pushes the blood from the right atrium into the right ventricle. Before it enters the right ventricle, it must pass through the tricuspid valve, which prevents backflow. Once the blood enters the right ventricle, the atrioventricular node adjusts the speed at which the blood flows to prevent the ventricle from contracting without a sufficient level of blood. The two nodes working together causes the heart to beat and allows the blood to move throughout the circulatory system.
The deoxygenated blood needs to return to the lungs for more oxygen. During pulmonary circulation, the right ventricle contracts to send blood through the pulmonary valve into the pulmonary artery. The pulmonary valve is responsible for ensuring the blood only flows into the artery and not back into the ventricle. The pulmonary artery connects to a multitude of other, smaller arteries and capillaries to deliver the blood to the pulmonary alveoli in the lungs.
The pulmonary alveoli are small, hollow cavities in the lungs. When we inhale, the alveoli absorb oxygen from the air. Capillaries surrounding these cavities allow gas exchange between the alveoli and the blood. Carbon dioxide exits the blood and enters the alveoli and oxygen travels from the alveoli to the blood. When the lungs exhale, they release the carbon dioxide from the body and the process begins again. This is an example of the circulatory system working together with the respiratory system.
Now that the blood is full of oxygen, it must deliver that oxygen throughout the rest of the body. To do this, the heart contracts to pull the blood from the pulmonary alveoli into four veins, two for each lung. The blood travels through these four pulmonary veins and fills the left atrium of the heart. After this process, the body begins transferring the oxygen to the rest of the body.
The left ventricle is the largest of the four heart chambers. Because of this, it is also capable of providing the most pressure to move the blood around the body. The blood travels through the mitral valve out of the left atrium and begins to fill the left ventricle. When the left ventricle prepares to expel the blood to the rest of the body, the mitral valve closes, due to the difference in pressure. The ventricle then pushes the blood to the aorta.
The aorta is the main artery, located just above the heart. As it travels down the abdomen, it diverges into two separate, smaller arteries. The blood moving from the left ventricle passes through the aortic valve before it enters the aorta. From there, it flows to the smaller arteries and capillaries that spread throughout the human body. However, before the blood can reach the capillaries, it must travel through small blood vessels called arterioles. The arterioles change diameter to adjust the blood pressure and speed.
The adjustments in pressure and speed of the arterioles functions allow for a constant exchange of gasses, nutrients, and other contents from the blood to the cells. The blood transfers oxygen and nutrients into the cells and receives carbon dioxide and other waste materials. After the blood loses its oxygen, it must return to the heart so the process can begin anew. First, it enters blood vessels similar to the arterioles venules. From the venules, the blood drains into the veins themselves.
Veins can be considered the opposite of arteries as they carry blood to the heart rather than away from it, though veins are less muscular. Most veins have valves that prevent backflow of blood. The veins from the arms and the head connect to the superior vena cava while the veins from the legs and abdomen connect to the inferior vena cava. The blood returns to the right atrium, and the circulatory system starts again.
It is important to remember that the circulatory system never actually ends, as long as the body is healthy. All processes happen simultaneously, and the blood never stops flowing. When the system is broken down, its complex parts can be easy to remember and understand.
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