Unless you’re watching a vampire movie at Halloween, donating blood or suffering from a cut, you probably don’t give much thought to blood on a regular basis. This fluid that flows through our bodies, though, is truly a matter of life and death.

In honor of American Heart Month, let’s learn a little bit more about the blood that our hearts pump through our bodies.

Blood is composed of cells and plasma. The cells include red blood cells, white blood cells and platelets.

The red blood cells carry oxygen from the lungs and give blood its red color. The white blood cells fight infections. The platelets help form clots to stop bleeding in case of a cut.

All these cells float in the liquid plasma, which is mostly water. Plasma also contains nutrients, electrolytes, hormones and protein antibodies to fight infection.

The amount of blood in a human body varies, depending on factors such as age, sex, overall health and even where a person lives. For example, men tend to have more blood than women of comparable size and weight.

Interestingly, people who live at high altitudes may have up to two liters of extra blood compared to those who live at lower altitudes. Because the air at higher altitudes has less oxygen, people who live at high altitudes need extra blood to deliver the right amount of oxygen to their lungs.

Scientists estimate the volume of blood in a human body to be approximately 7 percent of body weight. An average adult body with a weight of 150 to 180 pounds will contain approximately 4.7 to 5.5 liters (1.2 to 1.5 gallons) of blood. An average child with a body weight of 80 pounds will have approximately half the amount of blood as an adult.

Blood carries out many critical functions in the body. It transports nutrients and oxygen to the body’s cells. It also takes away waste from those cells. Blood also moves hormones and chemicals around the body.

You might be surprised to learn that blood also plays a special role in regulating body temperature. When your body heats up, blood helps keep the temperature steady by transferring extra heat to the skin, where it can be released from the body.

As part of the immune system, blood also helps fight disease. If you scrape your knee at the park, the platelets in blood begin to clot to help stop the bleeding. This self-repair function prevents further blood loss, which could be fatal in cases of massive bleeding.