The amount of blood in a person’s body will depend on their age and size. Losing a certain amount of blood will not cause any harm to the body.

According to an older review article in Critical Care, blood accounts for:

  • approximately 7–8% of an adult’s body weight
  • approximately 8–9% of a child’s body weight
  • approximately 9–10% of an infant’s body weight

In this article, learn more about the average volume of blood in adults and children. Learn, too, what causes blood loss, how it affects the body, and what to do if it happens.

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The amount of blood in a person’s body can vary according to their age and size.

According to a 2020 article, there are around 10.5 pints (5 liters) of blood in the average human adult body, although this will vary depending on various factors. During pregnancy, a woman may have up to 50% more blood.

The average quantities of blood are::

  • about 9 pints (4.3 liters) of blood in an average-sized female (5 feet 5 inches tall and weighing 165 pounds)
  • about 12.2 pints (5.7 l) in an average-sized male (6 feet in height and weighing 200 pounds)
  • in an infant, about 1.2 fluid ounces (fl oz) for every pound of body weight (75 – 80 milliliters (ml) of blood per kilogram).
  • in a child, about 1–1.2 fl oz for every pound of body weight (70–75 ml of blood per kg)

For clarity, Medical News Today has converted these figures from the formula given in Open Anesthesia.

Every 2 seconds, someone in the United States needs blood, but supplies are low due to COVID-19. To find out more about blood donation and how you can help, please visit our dedicated hub.

According to an older article in the Journal of Nuclear Medicine Technology, a blood volume test can measure the amount of blood in a person’s body.

A doctor may use this test to assess a variety of conditions, such as:

There are different ways of testing, but a blood volume test usually involves injecting a small amount of a tracer substance into the body. A healthcare professional will then use imaging technology to track blood moving around the body.

According to the American Red Cross, the standard amount of blood a person will give during a blood donation is 1 pint. This is around 10% of the blood in the body and a safe amount of blood to lose.

A person may feel a little faint after donating blood, and so donation centers ask donors to rest for 10–15 minutes and take some refreshments before leaving.

If a person has an illness or accident, they may lose more blood. This can lead to shock, and it can be life threatening.

Donating blood can save lives, but how does it affect the donor?

Severe bleeding can be dangerous. In medical terms, shock means that not enough oxygen is reaching tissues in the body. Low oxygen levels can cause damage to the brain and other organs.

If someone is losing blood, the body will start to direct blood toward the vital organs and away from the skin, fingers, and toes. A person may begin to look pale or feel numbness in their extremities.

According to a 2019 article, when a person loses around 15% of their blood volume, they can start to experience shock, although their blood pressure and other signs will likely be normal at this point.

After losing 20–40%, the person’s blood pressure will start to fall, and they will begin to feel anxious. If they lose more blood, they will start to feel confused. Their blood pressure may rise to around 120 beats per minute (bpm), as the body tries to maintain blood supply to the vital organs.

When blood loss is 40% or more, the person will be in severe shock. Their pulse rate will rise over 120 bpm. They will feel lethargic and may lose consciousness.

Causes of bleeding and shock

Bleeding can be external or internal, but both types can lead to shock.

External bleeding: Head wounds or a deep wound or a cut on or near a vein, such as on the wrist or neck, can result in severe blood loss.

Internal bleeding: An internal injury, such as a blow to the abdomen, can lead to a sudden and significant loss of blood, but this may not be visible from the outside. The clinical review in Critical Care indicates that medical conditions, such as a perforated ulcer, lung cancer, or a ruptured ovarian cyst, can also cause internal bleeding.

Depending on where internal bleeding occurs, bruising may start to appear. There may be a loss of blood through the mouth, nose, or other orifices.

Learn the difference between an artery and a vein here.

Getting help

A person with severe bleeding will need medical attention.

For external bleeding, the person should:

  • sit or lie down
  • raise the injured part, if possible
  • apply pressure to the wound to slow the bleeding or ask someone else to do this

Someone should call 911 if:

  • bleeding is severe
  • bleeding does not stop or slow down on applying pressure
  • severe bruising appears on the body or the head
  • there is a change in consciousness or difficulty breathing

Transfusions

A blood transfusion is a medical procedure to donate blood to someone who needs it.

Possible reasons include:

  • losing a lot of blood
  • having an illness that affects the blood, such as cancer or anemia

Blood transfusions can be a life-saving procedure. People can also receive other parts of blood, such as plasma and platelets, for various treatment purposes.

The body makes around 2 million red blood cells per second. Blood cells develop from stem cells in the bone marrow. Stem cells are a type of cell that can create other cells. This process happens continually throughout a person’s life.

Blood consists of different parts:

  • Red blood cells carry oxygen and carbon dioxide.
  • White blood cells help defend the body against disease and infection.
  • Platelets help stop bleeding.
  • Plasma carries blood cells, platelets, and other components and supports the immune system. The American Red Cross state that it makes up 55% of blood and is 92% water.

The Red Cross also state that body takes around 24 hours to replace lost plasma, but 4–6 weeks to replace red blood cells.

Red blood cells get their color from hemoglobin, which contains iron. It can take several months for iron levels to return to normal after losing or donating blood. The Office of Dietary Supplements note that frequent donors may have low levels of iron in their blood.

People who have experienced blood loss due to donation or another reason may benefit from:

  • drinking plenty of fluids, especially water
  • consuming iron-rich foods, such as beef liver and fortified foods

Many people now donate plasma. Are there any risks?

The circulatory or cardiovascular system is responsible for moving blood around the body. Within this system, the heart pumps the blood to the blood vessels, which deliver blood to the body’s organs. There, the blood delivers oxygen and other nutrients.

Others systems and organs that play a crucial role are:

  • the kidneys, which regulate the fluid balance in the body
  • the skeletal system, as bone marrow produces blood cells
  • the nervous system, which enables the other systems to fulfill their tasks

A problem with any of these systems can affect blood flow and blood volume, the delivery of oxygen, and a person’s ability to survive.

What are the different blood types, and why does it matter?

Around 7–8% of an adult’s body weight is blood. The body can easily replace a small amount of lost blood, which makes blood donation possible.

If a person loses around 15% or more of their blood, there may be a risk of shock. Anyone who has signs of significant internal or external bleeding should seek immediate medical help.