Circulatory System
The circulatory system consists of the heart, blood, and blood vessels. There are three types of blood vessels: arteries, veins and capillaries.

The heart is an organ made of cardiac muscle and it acts like a pump. The muscles of the heart contract and pump blood away from the heart (through the aorta) and to the tissues of the body. Diffusion occurs at the tissues, allowing for nutrients and oxygen to enter the cells, and for carbon dioxide and other wastes to be removes from the cells.

Your heart beats when the pacemaker (sinoatrial node located in the right atrium) sends an impulse that causes muscle contractions to occur. The heartbeat is partly controlled by neurotransmitters released by the nervous system.

When your heart pumps, it produces a pressure in your arteries called blood pressure. Your body helps regulate your blood pressure by communicating with the brain. The brain releases neurotransmitters that can raise or lower the blood pressure by relaxing or contracting the smooth muscles of the blood vessels. Your kidneys also play a role in regulating blood pressure by removing water from the blood and releasing it as urine.

Cardiovascular Diseases
High blood pressure can lead to many problems. High blood pressure makes the heart work harder. This can lead to damage or weakening of the blood vessels and heart muscle. Also, if you have high blood pressure, you are more susceptible to heart attack, stroke, coronary artery disease, and other circulatory system diseases. 

Atherosclerosis occurs when fatty plaque deposits build up on the walls of blood vessels. If this condition occurs in the coronary arteries, it can block the flow of blood that supplies the heart with oxygen and nutrients. The part of the heart that isn't getting the supply of oxygen it needs can start to die, causing a heart attack. If the plaque buildup causes a blood clot and the clot breaks free, it can get stuck in other blood vessels. If that happens in the brain, it can cause a stroke.

Cardiovascular diseases can be prevented through the avoidance of smoking, regular exercise and balanced diets.

Blood is composed of 55% plasma and 45% red blood cells (RBCs), white blood cells (WBCs) and platelets. Plasma is made up primarily of water, but also contains salts, nutrients, hormones, and many other substances.

Red blood cells transport oxygen. Hemoglobin is a protein located on RBCs that binds to oxygen in the lungs and transports it to the tissues where the oxygen gets released.

White blood cells are produced from stem cells in bone marrow. The number of WBCs in the blood is much less than that of RBCs. When you are sick, the number of WBCs int he blood increases to help fight infection. There are many types of WBCs and they each have a different job (ex: engulfing microorganisms that cause disease, releasing chemicals, etc.). Some WBCs make antibodies to help destroy pathogens ("bad guys") that provides you with immunity to several diseases. WBCs leak out of the circulatory system into the lymphatic system where they are able to provide defense within the tissues of the body.

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um can you keep it a little simple?