Blood makes up about 7% to 8% of the human body weight. For an average adult, this amounts to about 4.5 to 5.5 liters (1.2 to 1.5 gallons) of blood.
Blood is composed of several components, including red blood cells, white blood cells, platelets, and plasma. Red blood cells are the most abundant, making up about 40% to 45% of the total blood volume.
Blood is responsible for transporting oxygen, nutrients, hormones, and waste products throughout the body. It also helps regulate body temperature and pH balance.
Blood is actually a type of connective tissue. It contains cells suspended in a liquid extracellular matrix called plasma.