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What is blood?

Blood is a specialized body fluid. It has four main components: 


The liquid component of blood is called plasma, a mixture of water, sugar, fat, protein, and salts. The main job of the plasma is to transport blood cells throughout your body along with nutrients, waste products, antibodies, clotting proteins, chemical messengers such as hormones, and proteins that help maintain the body's fluid balance. 

Red cells

Known for their bright red colour, red cells are the most abundant cell in the blood, accounting for about 40 to 45 percent of its volume. The shape of a red blood cell is a biconcave disk with a flattened centre - in other words, a red blood cell looks a bit like a donut.

Red cells contain a special protein called haemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their colour from the haemoglobin. The percentage of whole blood volume that is made up of red blood cells is called the haematocrit and is a common measure of red blood cell levels.

White cells

White blood cells protect the body from infection. They are much fewer in number than red blood cells, accounting for about 1 percent of your blood.

The most common type of white blood cell is the neutrophil, which is the "immediate response" cell and accounts for 55 to 70 percent of the total white blood cell count. Each neutrophil lives less than a day, so your bone marrow must constantly make new neutrophils to maintain protection against infection.


Unlike red and white blood cells, platelets are not actually cells but rather small fragments of cells. Platelets help the blood clotting process by gathering at the site of an injury, sticking to the lining of the injured blood vessel, and forming a platform on which blood coagulation can occur. This results in the formation of a fibrin clot, which covers the wound and prevents blood from leaking out. Fibrin also forms the initial scaffolding upon which new tissue forms, thus promoting healing.

Where do blood cells come from?

Blood cells develop from hematopoietic stem cells and are formed in the bone marrow through the highly regulated process of haematopoiesis. Hematopoietic stem cells are capable of transforming into red blood cells, white blood cells, and platelets. These stem cells can be found circulating in the blood and bone marrow in people of all ages, as well as in the umbilical cords of new born babies. Stem cells from all three sources may be used to treat a variety of diseases, including leukaemia, lymphoma, bone marrow failure, and various immune disorders. 

What are the different blood groups?

O positive: 35% (percentage of population)

O negative: 13%

A positive: 30%

A negative: 8%

B positive: 8%

B negative: 2%

AB positive: 2%

AB negative: 1%

What does blood do?

Blood has many different functions, including:

  • Transporting oxygen and nutrients to the lungs and tissues

  • Forming blood clots to prevent excess blood loss

  • Carrying cells and antibodies that fight infection

  • Bringing waste products to the kidneys and liver, which filter and clean the blood

  • Regulating body temperature

How much blood do I have?

The blood that runs through the veins, arteries, and capillaries is known as whole blood, a mixture of about 55 percent plasma and 45 percent blood cells. About 7 to 8 percent of your total body weight is blood. An average-sized man has about 12 pints of blood in his body, and an average-sized woman has about nine pints. 

What happens if I start bleeding?

When your skin is cut, scraped, or punctured, you usually start to bleed. Within minutes or even seconds, blood cells start to clump together and clot, protecting the wound and preventing further blood loss. These clots, which turn into scabs as they dry, are created by a type of blood cell called a platelet. The clot also contains a protein called fibrin, which forms a net to hold the clot in place.

How much blood can you lose without experiencing any side effects?

Most adults can lose up to 15% of their blood without experiencing any major side effects or changes in vital signs. Some, however, may feel lightheaded or dizzy if this amount is lost quickly.

How much blood can you lose before you begin to experience mild side effects?

You’ll start to feel mild side effects, such as nausea, when blood loss reaches 15 to 30% of total blood volume. This amount of loss increases your heart and respiratory rates and your urine output and blood pressure will be decreased. You may feel anxious or uneasy.

Your body will start to compensate for blood loss by constricting the blood vessels in your limbs and extremities; this is your body’s attempt to maintain your blood pressure and blood flow. This subsequently lowers the amount of blood your heart pumps outside the centre of your body. Your skin may become cooler and pale.

How much blood loss can occur before you pass out?

When blood loss nears 30 to 40% of total blood volume, your body will have a traumatic reaction. Your blood pressure will drop down even further, and your heart rate will further increase.

You may show signs of obvious confusion or disorientation. Your breathing will be more rapid and shallow.

As the volume loss climbs, your body may not be able to maintain circulation and adequate blood pressure. At this point, you may pass out and you’ll need help quickly to prevent additional blood loss and greater side effects.

How much blood loss can occur before you go into haemorrhagic shock?

Haemorrhagic, or hypovolemic, shock occurs when you’ve lost 20 percent or more of your total blood volume. Your symptoms will become more severe as the blood loss increases. You may experience:

  • Rapid breathing

  • Weakness or fatigue

  • Confusion

  • Cool, pale skin

  • Sweaty, moist skin

  • Anxiety or unease

  • Low urine output

  • Drowsiness

  • Unconsciousness

Your body can’t compensate for much longer on its own in a blood volume loss of over 40%.  At this stage, your heart can’t properly maintain blood pressure, pumping, or circulation. Your organs may begin to fail without adequate blood and fluid; you’ll likely pass out and slip into a coma.


How much blood loss can occur before you die?

Without treatment, your body will completely lose its ability to pump blood and maintain oxygen delivery once you’ve lost about 50% of your blood volume.

Your heart will stop pumping, other organs will shut down, and you’ll likely be in a coma. Death is extremely likely if aggressive life-saving measures haven’t been taken.

Your body can compensate for a good deal of blood loss however, at a certain point, it shuts down unnecessary components in order to protect your heart.

You’ll likely feel very fatigued in the moments before entering into a coma. If close to death, these feelings may not even be noticed.

How much blood loss can occur before you need a transfusion to recover?

The average haemoglobin level is between 13.5 to 17.5 grams per decilitre for men and 12 to 15.5 grams per decilitre for women. Most doctors won’t consider a transfusion until the haemoglobin levels in your blood reach 7 or 8 grams per decilitre.

This isn’t the only parameter involved in the approach to treating blood volume loss if you’re actively bleeding. However, your haemoglobin level is important for making a red blood cell transfusion decision; the treating doctor would use these and other factors to decide if a transfusion is necessary and if it would be effective for your situation.

The most important thing the treating doctor will try and do is to replace the lost fluids so there is sufficient volume in your circulatory system to pump around the remaining blood. This can usually be achieved with the use of sterile transfusion fluids and then a transfusion of red cells.

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