Circulatory System
A simple definition for the circulatory system would be that it is the main transportation and cooling system for the body. Red Blood Cells have an important job in this system in that they carry all sorts of packages that are needed by all the cells in the body. Red blood cells carry oxygen and nutrients to the cells, which is needed in order for cells to survive. Besides red blood cells there are also White Blood Cells moving in the circulatory system. White Blood Cells can be considered as the paramedics, police and street cleaners of the circulatory system. Anytime we have a cold, a cut, or an infection the white blood cells go to work. From research I also came to find out that white blood cells also help cuts or wounds to heal quicker. Something I did not know was that puss which appears during an infection are actually white blood cells that are attacking the infection.

The highway system of the Circulatory System consists off a lot of one-way streets. The main routes used by the circulatory system are the veins and arteries. Veins are used to carry blood to the heart. Arteries then carry blood away from the heart. Most of the time, blood in the veins is blood where most of the oxygen and nutrients have already been delivered to the cells. This blood is called deoxygenated and is very dark red. Most of the time blood in the arteries is loaded with oxygen and nutrients and the color is very bright red. There is one artery that carries deoxygenated blood and there are some veins that carry oxygenated blood. In order to get more research on this I had to look up some information on the heart and lungs.

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The heart is not hard to imagine in ones mind because it is simply a two sided, four chambered pump. The heart is mostly made of muscle and is very unique because it is the only muscle that does not become tired like other muscles. . Imagine what would happen if every 15 minutes or so ones heart becomes tired and decides to take a little break! If this were to happen it basically means that in a few moments ones circulatory system will shut down since the heart is the powerhouse of this system. Since it never rest, the heart muscle is always expanding and contracting, usually at between 60 and 100 beats per minute.
Since the heart is divided into two sections this means that each section has a specific job. The right side of the heart is the low-pressure side. Its main job is to push the red blood cells up to the lungs so that they can get recharged with oxygen. Blood enters the right side of the heart through a chamber called the Right Atrium. Atrium is another word for an 'entry room.' Since the right atrium is located above the Right Ventricle, a combination of gravity and an easy squeeze pushes the blood though the Tricuspid Valve into the right ventricle. The tricuspid valve is a valve made up of three 'leaflets' that allows blood to go from top to bottom in the heart but closes to prevent the blood from backing up into the right atrium when the right ventricle squeezes. After the blood is in the right ventricle, the right ventricle begins its contraction to push the blood out toward the lungs. The blood then leaves the right ventricle and enters the pulmonary artery. This artery and its two branches are the only arteries in the body to carry deoxygenated blood.
When the blood leaves the pulmonary arteries it enters capillaries in the lungs. Capillaries are very, very small blood vessels that act as the connectors between veins and arteries. The capillaries in the lungs are very special because they are located against the alveoli or air sacks. When blood in the capillaries goes past the air sacks, red blood cells pick up oxygen. Capillaries are so small, in some places, that only one red blood cell at a time can get through. When the blood has picked up its oxygen, it enters some blood vessels known as the cardiac veins. This is fully oxygenated blood and it is now in veins. The cardiac veins empty into the left atrium. The left side of the heart is the high-pressure side, its job is to push the blood out to the body.
The left atrium sits on top of the left ventricle and is separated from it by the mitral valve. The mitral valve is named this because it resembles, to some people, a Bishop's Mitered Hat. This valve has the same function as the tricuspid valve, it prevents blood from being pushed from the left ventricle back up to the left atrium. The left ventricle is a very high-pressure pump. Its main job is to produce enough pressure to push the blood out of the heart and into the body's circulation. When the blood leaves the left ventricle it enters the Aorta. There are valves located at the opening of the Aorta that prevent the blood from backing up into the ventricle. As soon as the blood is in the aorta, there are arteries called coronary arteries that take some of the blood and use it to nourish the heart muscle
The aorta leaves the heart and heads toward to where else but the head. We have to keep our brains well nourished so we can make good grades in biology lab! The arteries that take the blood to the head are located on something called the aortic arch. After the blood passes through the aortic arch it is then distributed to the rest of the body. The descending aorta goes behind the heart and down the center of the body. From the aorta, blood is sent off to many other arteries and arterioles (very small arteries) where it gives oxygen and nutrition to every cell in the body. At the end of the arterioles are, guess what, capillaries. The blood gives up its cargo as it passes through the capillaries and enters the venous system.
The venous system carries the blood back to the heart. The blood flows from the capillaries, to venues (very small veins), to veins. The two largest veins in the body are the superior and inferior vena cavas. The superior vena cava carries the blood from the upper part of the body to the heart. The inferior vena cava carries the blood from the lower body to the heart. . Many people believe that the blood in the veins is blue but in actuality it is not. Venous blood is really dark red or maroon in color. Veins do have a bluish appearance and this may be why people think venous blood is blue. Both the superior and inferior vena cava ends in the right atrium. The superior vena cava enters from the top and the inferior vena cava enters from the bottom.