The major purpose of the cardiovascular system  is to provide oxygenated blood to our cells and return deoxygenated blood to the heart, where it travels to the lungs to be reoxygenated. First of all, deoxygenated blood comes from the body, from the venous system, and travels via the inferior (from lower body) and superior (from upper body) vena cava to the right atrium. Blood flows from the right atrium to the right ventricle and out to the lungs through the pulmonary artery. The blood then picks up oxygen in the lungs and travels via the pulmonary vein to the left atrium and down to the left ventricle. From the left ventricle, oxygenated blood is pumped out through the aorta to the body.

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The immune system consists of specialized cells and structures that protect the body against invasion by harmful substances. Specifically, the immune system:
1. Defends against infection by protecting the body against invading microorganisms.
2. Maintains homeostasis by removing old cells, primarily by the spleen.
3. Identifies circulating cells and destroys mutant cells.

The primary immune organs are the:

Lymph nodes        Thymus         Spleen        Tonsils          Bone marrow

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