Blood

Blood circulates through a system of vessels called arteries and veins. Arteries carry blood away from the heart; veins carry blood back to the heart. The average human body has about 5 quarts of blood. Large people may have slightly more; small people may have slightly less. Every 60 seconds, about 1⁄5 quart of blood flows out of your heart through your coronary arteries. Sixty seconds after that, the blood zips through your entire circulatory system and heads back to your heart.

The life span of one red blood cell is about 120 days for a man and about 14 days less for a woman. Men have more red blood cells — about 4.5 to 6.2 million per cubic microliter of blood compared to 4 to 5.5 million for women. Because males have more red blood cells, they also have higher values of hemoglobin, the pigment in red blood cells that carries oxygen throughout the body. They also have higher levels of iron, an important element in hemoglobin.

White blood cells play a primary role in your immune system as avengers that zero in on invaders, such as bacteria, to chew them up and spit them out. The normal number of white blood cells is exactly the same for men and women — 4,100 to 10,900 per microliter of blood. Blood is a vehicle for nutrients, medications, and other circulating particles such as — what a surprise — the lipoproteins that carry cholesterol.

By the way, the blood for a cholesterol test always comes from a vein, not an artery. Blood from a vein is easier and safer to obtain, and it’s a representative sample of what’s in your body. And yes, clenching your fist does make your vein pop up so it’s easier to puncture.