Blood volume is a measurement of the volume, or amount of space, that the blood takes up in a given person. This includes both red blood cells and plasma; it is not limited to one particular part of blood. Maintaining a normal volume of blood is very important as it carries oxygen and essential nutrients throughout the body. If a person loses too much blood because of a bleeding wound or because of inadequate blood cell synthesis, dangerously low blood pressure can result and may cause vital organs to receive inadequate amounts of oxygen and nutrients.
There are many different factors that affect blood volume from person to person. Females, on average, have less blood than males do, and the bodies of children tend to contain less than those of grown men or women. Most people have roughly 1.2 gallons (4.7 liters) of blood in their bodies. Blood makes up approximately 1/11 of human body weight. People who live at high altitudes tend to have a higher volume because there is less oxygen in the air; the extra blood is needed to carry additional oxygen throughout the body.
Blood volume is regulated by the excretory system, particularly by the action of the kidneys. When the amount of blood in the body increases to a certain point, blood pressure increases and, through mechanisms involving nerves and hormones, a signal is sent to the kidneys. The kidneys then reabsorb less fluid, causing more water and other substances necessary to blood production to be lost through urination, decreasing the volume of blood. The opposite happens when there is too little blood; the kidneys reabsorb more water and other substances necessary for blood, causing less to be lost through urination. Through this and other mechanisms, blood pressure and volume are regulated and maintained at healthy levels.
The volume of blood in a pregnant woman increases drastically over the normal amount of blood in the body, and it can actually increase by 25 to 50% because the developing fetus and the woman's enlarged reproductive organs both need more blood. To supply this blood, the heart must beat harder and faster than usual and some blood vessels grow larger. In most cases, the volume of the plasma component of blood increases more than the volume of red blood cells. This can lead to cases of anemia in pregnant women, which tends to cause dizziness and fatigue.