Reviewed by: BD Editors

Blood Definition

Blood is the body fluid in humans and other animals that delivers the essential materials for life to the body’s cells. It has sometimes been called a fluid “tissue,” because like solid tissues it contains several types of cells which perform complex functions for the human body.

The components of blood are produced mainly in the bone marrow, where special cells produce red cells, white cells, and platelets. So-called “blood cancers” such as leukemia are actually cancers of the bone marrow. As cancerous tissue replaces healthy bone marrow tissue, healthy red blood cells, white blood cells, and platelets cannot be made.

Despite looking like a simple red fluid, blood is as complex as any tissue in the body. Here we will discuss its functions, its components, and some clinically important characteristics of blood.

Function of Blood

Important functions of the blood include:

Bringing Vital Substances to Cells

Complex multicellular organisms need complex circulatory systems; that’s because we have many cells, and these cells have high metabolisms.

Without highly efficient means of delivering vital substances like oxygen, water, and nutrients, complex and active life forms like ourselves could not exist.

Some of the vital substances which blood delivers to our cells include:

  • Oxygen – Near-constant supply needed for cellular respiration.
  • Water – Correct balance needed for enzymatic activity to proceed smoothly.
  • Nutrients – Fuel for cellular respiration, and necessary materials for cellular maintenance.
  • Biological building blocks – Molecules out of which replacement parts and daughter cells can be made.
  • Chemical messages from other cells – Allows body cells to alter their activity appropriately in response to environmental changes.

Blood also performs other important functions for our bodies, including…

Removing Dangerous Wastes

Most living things produce waste products that, at a certain concentration, become toxic to their own cells. Multicellular organisms with high metabolisms like us have had to find a way to deal with all those waste products in order to allow many cells to live together in a single organism.

We have our liver and kidneys, which break down toxic substances into harmless substances and expel them from the body in the form of urine. Our blood carries toxins from all of our tissues to these organs, where they are processed and removed.

Our blood also releases unwanted gases in the lungs, where they are exchanged for fresh oxygen.

The blood keeps our cells safe by carrying all of these waste products out of our tissues and to the correct processing and elimination organs. Some waste products our blood helps us get rid of include:

  • Carbon dioxide gas – Byproduct of cellular respiration, stops cellular respiration and causes acidification of blood if not removed.
  • Excess water, salt, and other substances – Too much of a good thing can be a bad thing.
  • Debris from dead cells – Cells die regularly, especially red blood cells which are not made to live longer than two weeks. Dead cells release toxic substances as they break down.
  • Toxic waste products of metabolism – Some forms of routine cellular metabolism produce highly toxic substances that can be safely swept away by the blood to the liver and kidneys.
  • Toxins we ingest in our food and water – Our liver and kidneys aren’t fool-proof, but they can handle some toxins we might encounter in the environment.

In addition to transporting substances to and from cells in other organs, the blood also contains its own cells and performs its own unique functions. These include:

Contains and Transports Immune Factors

Our blood contains antibodies and white blood cells which fight viruses, bacteria, and other invaders. Without these vital cells, we would quickly succumb to infections and die.

Our white blood cells even fight cancers that originate within our own bodies. It’s thought that most people develop cancerous cells at some point in their lives – but in most healthy people, the immune system destroys them before they are noticed. That’s why people with immune disorders have a higher chance of developing certain cancers than those with healthy immune systems.

The lymphatic system is also important for the movement, storage, and creation of immune factors.

Contains and Transports Clotting Factors

One of the most serious risks of injury to our body is the risk of blood loss. Because all of our organs, including our brains, rely on constant blood flow to stay alive, loss of large amounts of blood can be devastating. This is the most common cause of death from trauma.

Fortunately, our blood has a response system in place for when we are injured. A combination of cell fragments called platelets, chemicals called clotting factors, and other components of the blood work together to form blood into solid clots and scabs to stop bleeding.

Our blood clotting system cannot save us from large injuries, such as those that result in ruptures to our arteries. But their power can be seen in cases of people who do not have properly working blood clotting system.

People with certain cancers, vitamin deficiencies, and other diseases have blood that does not clot normally. These people can bruise and bleed with no apparent cause, and sometimes die from minor injuries or apparently spontaneous bleeding.

This happens because their clotting systems are not functioning properly. We can be thankful that most of our clotting systems do!

Components of Blood

There are several major components of the seemingly uniform liquid that is our blood. When centrifuged, the components of different densities separate to look something like this:

Blood centrifugation scheme

Here we will discuss the most vital components of blood, including serum, white blood cells or “leukocytes,” red blood cells, and platelets.


Plasma is the liquid which carries the red blood cells, white blood cells, platelets, and other substances found in blood. More than half the volume of our blood is composed of this fluid.

Our blood plasma is mostly water, but it also contains salts proteins, and other substances, which can make it appear thick and syrupy even when the red and white cells have been filtered out.

One important protein, albumin, exists in part to keep the blood thick and syrupy. This ensures that the blood does not leak out of our vessels and into tissues, and slows bleeding when we are injured.

Other substances that can be found in the plasma include:

  • Antibodies, which are proteins that attack invading pathogens
  • Clotting factors, which prevent bleeding
  • Hormones, which are chemical messages sent between different tissues in the body
  • Electrolytes such as salt
  • Nutrients such as sugar, vitamins, and minerals
  • Lipids including cholesterol

So even this seemingly simple fluid is a veritable stew of the ingredients for life! But it could not do its job without…

Red Blood Cells

Red blood cells can be thought of as the cargo ships of the body. They are small, numerous cells which are specifically designed to carry oxygen from the lungs to cells, and carry carbon dioxide back to the lungs to be expelled when we exhale.

Red blood cells contain hemoglobin – a protein which is beautifully tailored to bind aggressively to oxygen in the lungs, and then release it and pick up carbon dioxide at a slow, steady rate as it passes through the body.

Hemoglobin is a pigment which changes color slightly, depending on whether it is bound to a molecule of oxygen or not. That’s why blood drawn from veins, which carry oxygen-depleted blood back toward the lungs, is a dark red that can appear almost brown. Blood drawn from arteries, which carry oxygen-rich blood from the lungs to the tissues, is a bright red.

White Blood Cells

White blood cells perform both immune and clean-up functions for the body. Like red blood cells, they are made by stem cells in the bone marrow.

There are many types of white blood cells, which play many different roles in immune response to infection and injury. Some types of white blood cells include:

  • Neutrophils – Target bacteria and fungi.
  • Eosinophils – Target larger parasites such as those which cause malaria. Also play a role in allergic inflammatory responses.
  • Basophils – Release chemicals that enhance inflammatory responses.
  • B Lymphocytes – Release antibodies and assist in activating T cell lymphocytes.
  • T Lymphocytes – Different subtypes help the immune system learn to “recognize” new infection so it can target it; help immune system to activate in response to infection, then return to normal after infection has passed; target virus-infected and tumor cells.
  • Natural Killer Lymphocytes – Target virus-infected and tumor cells for destruction.
  • Monocyte – Migrate into tissues and mature into macrophages, literally “big eaters,” which engulf harmful cells and cellular debris and destroy them; some mature into Kupffer cells, which live in the liver and break down and recycle dying red blood cells.


Platelets are cell fragments – bits of membrane-bound cytoplasm – which stop bleeding by clumping together to form clots and scabs seal wounds. Like red and white blood cells, they are made in the bone marrow. Cancer of the bone marrow may prevent production of properly functioning platelets.

Platelets have two states: active platelets, which are prepared to create blood clots, and inactive platelets that do not clot. Under normal circumstances, the endothelial lining of healthy blood vessels produces chemical messages that tells platelets to remain in their inactive form, so that they don’t form clots inside of healthy blood vessels.

Under normal circumstances, platelets are activated when a nearby injury starts a chemical cascade that urges platelets and other nearby clotting factors to activate. These factors then release clot-promoting messages of their own, encouraging more clotting factors to join their growing clot.

Platelets can sometimes be incorrectly activated when endothelial lining is damaged and does not produce the usual inhibitory messages for platelets. This can happen in people with some metabolic disorders and some forms of cardiovascular disease.

Blood Types

In early medical history, it was thought that blood transfusions from one person to another might be impossible. This was because when this was attempted, most test subjects died.

In time, however, scientists discovered the existence of “blood types” – a few basic proteins that coat the surface of our blood cells, helping the immune system to differentiate between our own blood cells and foreign invaders.

Attempting to transfuse blood of an incompatible type into a person can cause a clotting reaction, which may be fatal. Fortunately, today doctors have rapid tests to determine a patient’s blood type, and store blood bags for transfusion sorted by type so that patients are assured of getting a compatible treatment.

The three common blood type protein markers recognized by science are called the A, B, and Rh proteins.

The A/B protein group can give rise to blood types A, B, AB, or O. There is no “O” blood type protein – instead “O” is the blood type used to describe people who have neither A nor B marker proteins.

Each of these blood types can also be positive or negative for the Rh protein, leading to blood types such as “AB+” or “AB-.”

The blood type “O negative” is known as the universal donor. Because it does not have A or B proteins and is negative for the Rh protein, people of any blood type can receive O negative blood without having an adverse immune response to foreign proteins.

Unfortunately, people with “O negative” blood type also have the narrowest selection of possible donors for themselves. O negative people cannot receive any blood that has A, B, or Rh proteins; they can only receive blood from other O negative people.

When platelets are deficient or absent, the results can be seen in an increased probability of catastrophic bleeding events, including spontaneous bleeding and bleeding from minor injuries.


1. Which of the following is NOT a function of blood?
A. To transport oxygen to our cells
B. To transport water to and away from our cells, as needed
C. To break down toxins into harmless substances
D. To fight infections

Answer to Question #1
C is correct. Blood cannot break down toxins; however, it can deliver toxic substances from our cells to the liver and kidneys, which can do this job.

2. Which of the following is NOT likely to be found in a blood sample?
A. Red blood cells
B. Plasma
C. White blood cells
D. Bone marrow cells

Answer to Question #2
D is correct. Although bone marrow cells are the sites of production for red blood cells, white blood cells, and platelets, bone marrow cells themselves are not normally found in the blood.

3. Which of the following is NOT likely to be found in blood plasma?
A. Salt
B. Digestive enzymes
C. Hormones
D. Albumin

Answer to Question #3
B is correct. Digestive enzymes are not normally found in blood plasma. The other items, however, are all common components of plasma!


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  • Haubrich, W. S. (2004). Kupffer of Kupffer cells. Gastroenterology, 127(1), 16. doi:10.1053/j.gastro.2004.05.041
  • Wilson, J. H., & Hunt, T. (2002). Molecular biology of the cell, 4th ed. New York: Garland.
  • American Society of Hematology. (2014, March 29). Retrieved July 11, 2017, from

Cite This Article

MLAAPAChicago Editors. "Blood." Biology Dictionary,, 26 Jul. 2017, Editors. (2017, July 26). Blood. Retrieved from Editors. "Blood." Biology Dictionary., July 26, 2017.

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