The volume of blood in a human body is estimated to be about seven percent of total body weight. Thus, for an adult of weight between 150-180 pounds, there would be 4.7-5.5 liters (which is equivalent to 1.2 to 1.5 gallons) of blood. (http://wonderopolis.org/wonder/how-much-blood-is-in-your-body/)
As one moves up the troposphere (the atmospheric layer that is closest to Earth), the amount of oxygen decreases. Thus, people who live in high altitude regions require more blood in order to deliver enough oxygen to their lungs. Thus, people who live in high elevations may have up to two additional liters of blood! (http://wonderopolis.org/wonder/how-much-blood-is-in-your-body/)
Blood is composed of red blood cells (45%), white blood cells (<1%), and plasma (55%). Plasma is mostly water, but also contains proteins, electrolytes, nutrients, hormones, water, and gasses. There are approximately 20-30 trillion blood cells within the body.