Introduction

Blood is a complex mixture of plasma (the liquid component), white blood cells, red blood cells, and platelets. The body contains about 5 to 6 quarts of blood. Once blood is pumped out of the heart, it takes 20 to 30 seconds to make a complete trip through the circulation and return to the heart.

Blood performs a variety of essential functions as it circulates through the body. It delivers oxygen and essential nutrients (such as fats, sugars, minerals, and vitamins) to the body's tissues. It carries carbon dioxide to the lungs and other waste products to the kidneys for elimination from the body. It transports hormones (chemical messengers) to allow various parts of the body to communicate with each other. Also, it carries components that fight infection and stop bleeding.