What is blood?
Blood is a mixture of substances in the body mainly comprised of four parts. It has red and white cells, plasma and platelets. Platelets are tiny pieces of cells that enable us not to bleed to death when we are injured. When we get cut, platelets race to the wound and stick there. Plasma is the fluid part of blood which is just a mixture of sugars, proteins, fats and other parts like potassium and magnesium. Red cells have hemoglobin, which is extremely important. Hemoglobin is responsible for carrying oxygen to the cells--without oxygen, the cells will die. White cells are the portion of the blood which contains antibodies. People are constantly bombarded by bacteria and outside infectious agents, from the environment, for example. Without antibodies, we would not be able to fight off disease.
It comes from bone marrow.
Bone marrow is mainly how the body makes blood. It is forms inside the marrow of the body's longest bones, like the arms, legs and back. Cells fill in the tiny arteries inside the bone by a process known as hematopoiesis. Basically, there is genetic material that determines what cells will become and where in the body they will end up. Hematopoiesis happens when the body uses its stem cells to enable a person to create new cells. You need to be able to make new cells to generate enough blood volume to keep organs like the spleen, liver and heart alive.
What's the Heart Got to Do With It?
At any given time, a person has about five liters of blood in their body. This equates to about a gallon or so. After the blood forms in the marrow, it has to be disbursed throughout the body by the circulatory system--to replenish the organs with oxygen and nutrients. The electrical system of the body allows the heart to beat at a certain rhythm. Because of this, the chambers of the heart, also called the atria and ventricles, expand and contract at a regular pace sending blood back and forth through it. One group of chambers pull the blood in, while the other sends the blood out. This process happens more than 100, 000 times a day.