How Much Blood Is in the Human Body and How Much Can Be Lost?

Most adults have between 4.3 and 6 liters (9-12 pints) of blood, with variations largely dependent on body size and sex. Understanding blood volume aids in making informed decisions about issues like blood donation and blood loss.

Blood Volume Differences By Age and Sex

Blood volume varies from person to person. Factors like your weight, age, and sex determine your adequate blood volume.

Generally, typical amounts of blood are as follows:

• An adult male weighing about 200 pounds and standing about 6 feet tall has an estimated 5.7 liters (5,700 milliliters), or 12 pints, of blood volume (an average of 75 milliliters per kilogram).
• An adult female weighing about 165 pounds and about 5 feet 5 inches tall has an estimated 4.3 liters (4,300 milliliters), or 9 pints, of blood volume (an average of 65 milliliters per kilogram).
• An infant has about 1.2 fluid ounces of blood volume for every pound of body weight, or 75 to 85 milliliters of blood per kilogram.
• A child has about 1 to 1.2 fluid ounces of circulating blood for every pound of body weight, or 70 to 75 milliliters of blood per kilogram.

Men tend to have more blood volume than women because they typically have more lean body mass, while women generally have more adipose (fat) tissue.

What Is the Limit for Safe Blood Loss?

The amount of blood you can lose without serious health consequences depends on factors like age, body size, and the rate of loss. Generally, a healthy adult can safely donate about one pint (500 ml) of blood without risks.

For minimal blood loss, the body can usually replace the blood volume without much impact. However, larger losses of blood volume can be a medical emergency and require lifesaving interventions such as a blood transfusion.

Given the importance of blood volume in transporting oxygen and nutrients throughout the body, regulating temperature, and protecting the body, any type of blood loss should be evaluated by a healthcare provider.

Classification of Blood Loss

The American College of Surgeons developed criteria to classify blood volume loss. This classification was developed based on an otherwise healthy 70-kilogram, or 154-pound, individual.

The various classes of blood loss also include signs and symptoms that are likely to be present:

• Class 1: Class 1 occurs when 15% or less (approximately 750 milliliters) of blood volume is lost. Heart rate might increase slightly, and there is often no change in blood pressure or breathing rate.
• Class 2: Class 2 happens when blood loss is more than 15% but less than 30% (approximately 750 to 1,500 milliliters). Often, heart rate and breathing rate are elevated, and blood pressure might start to decrease.
• Class III: Blood volume loss greater than 30% but less than 40% (approximately 1,500 to 2,000 milliliters) usually causes large decreases in blood pressure, and a person’s mental status, such as alertness and ability to converse, starts to decline. Heart rate and breathing rate are much higher than normal ranges as well. Capillary refill times increase, too. (It takes longer for the smallest blood vessels to refill with blood.)
• Class IV: When blood volume loss is over 40%, or more than 2,000 milliliters, blood pressure becomes very low, heart and breathing rates are very high, and a person’s mental status becomes confused, irritable, and possibly unconscious. Capillary refill times are very long as well.

Methods for Assessing Blood Volume Loss

Determining blood volume status can be an important part of a healthcare provider’s ability to make a diagnosis and determine the most effective treatments. Blood volume can be important in various diseases, such as shock due to blood loss or decompensated heart failure.

Blood volume assessment can involve:

• Physical examination: This involves assessment of factors like skin turgor, dryness of mucous membrane, dizziness when standing, low blood pressure, or high pulse rate. This method is non-invasive but lacks the accuracy of specific blood volume levels.
• Hemoglobin and hematocrit: These two blood tests can be rapidly performed by the lab or within an emergency room. Hematocrit is the percentage of red cells in the blood.

Lab Tests

Testing a blood sample in the laboratory is often done to help determine the extent of blood loss. The primary laboratory value utilized to determine the amount of blood volume loss is hemoglobin. Hemoglobin is an important protein carried by the blood’s red blood cells and transports oxygen throughout the body.

Hemoglobin levels vary based on sex:

• Male: For an adult male, a normal hemoglobin range is 13.5 to 17.5 grams per deciliter.
• Female: For an adult female, a normal hemoglobin range is 12.0 to 15.5 grams per deciliter.

The Association for the Advancement of Blood & Biotherapies (AABB) worked with a panel of experts to recommend that if hemoglobin levels drop below the acceptable range and the individual with a low hemoglobin is having symptoms as described above, a healthcare provider can consider many treatment options, including a blood transfusion.

Other Methods

More specialized methods for measuring blood volume include:

• Internal monitoring of the blood vessels: A pulmonary artery catheter, also called a Swan-Ganz catheter, is inserted into large blood vessels leading into and out of the heart and lungs. This catheter can measure different pressures in the blood vessels and determine if blood volume is too low or too high.
• Nuclear medicine blood volume analysis: This is a specialized method of directly assessing blood volume using a nuclear medicine machine. The technique is minimally invasive but does have small amounts of radiation associated with the testing process. This method of blood volume analysis used to take four to six hours, but newer techniques can complete testing within 90 minutes or less.

How Does the Body Make More Blood?

The bone marrow makes blood cells, a process called hematopoiesis. Stem cells in the bone marrow differentiate to produce the cellular components of blood:

• Red blood cells: Red blood cells carry hemoglobin, a protein that delivers oxygen to cells and removes carbon dioxide waste.
• White blood cells: White blood cells help protect the body from infections from bacteria, viruses, or other foreign invaders.
• Platelets: Platelets are an important part of the body’s clotting process, which is needed to help prevent blood loss.

Peritubular cells in the kidneys are sensitive to oxygen levels in the blood. If the cells sense a decrease (such as due to blood loss), they secrete the hormone erythropoietin. This signals the bone marrow to make more red blood cells.

Iron is needed to produce hemoglobin in red blood cells, so the body taps into iron stores and increases the amount it absorbs from the diet.

Other chemical messengers in the body signal the bone marrow to produce more platelets and white blood cells when they are low.

However, it takes weeks for the marrow to replace any cellular component. If blood loss is too rapid, a transfusion may be needed.

Plasma is the liquid part of blood. When blood is lost, plasma is replenished from the body’s store of liquid. If blood volume is lost, fluids need to be replaced (orally or intravenously) to restore plasma levels. The kidneys also respond to reduce the loss of fluids and maintain plasma volume.

Plasma is replaced relatively rapidly in the body. For example, a person can donate plasma twice weekly, as long as there are 48 hours between donations.