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Blood carries oxygen to your tissues and waste away from them. It also helps you maintain a comfortable temperature.

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What exactly is blood?

All your cells need oxygen, as oxygen is the final electron acceptor during cellular respiration. How do they get this oxygen? From blood. Blood cells flow through the vessels of the human circulatory system. But what exactly is blood? It does transport oxygen, but also has other functions.


Blood is a fluid connective tissue. It circulates throughout the body through blood vessels by the pumping action of the heart. Blood in arteries carries oxygen and nutrients to all the body’s cells. Blood in veins carries carbon dioxide and other wastes away from the cells to be excreted. Blood also defends the body against infection, repairs body tissues, transports hormones, and controls the body’s pH.

Composition of Blood

The fluid part of blood is called plasma. It is a watery golden-yellow liquid that contains many dissolved substances and blood cells. Types of blood cells in plasma include red blood cells, white blood cells, and platelets (see Figure below). 

Components cells of blood

Cells in blood include red blood cells, white blood cells, and platelets.

  • The trillions of red blood cells in blood plasma carry oxygen. Red blood cells contain hemoglobin, a protein with iron that binds with oxygen. Red blood cells are made in the marrow of long bones, rib bones, the skull, and the vertebrae. These cells survive for about 120 days, and then they are destroyed. Mature red blood cells lack a nucleus and other organelles, allowing for more hemoglobin, and therefore more oxygen to be carried by each cell.
  • White blood cells are generally larger than red blood cells but far fewer in number. They defend the body against foreign bacteria, viruses and other pathogens. For example, white blood cells called phagocytes swallow and destroy microorganisms and debris in the blood, neutrophils engulf bacteria and other parasites, and lymphocytes fight infections caused by bacteria and viruses.
  • Platelets are cell fragments involved in blood clotting. They stick to tears in blood vessels and to each other, forming a plug at the site of injury. They also release chemicals that are needed for clotting to occur.

Blood type is a genetic characteristic associated with the presence or absence of certain molecules, called antigens, on the surface of red blood cells. The most commonly known blood types are the ABO and Rhesus blood types.

  • ABO blood type is determined by two common antigens, often referred to simply as antigens A and B. A person may have blood type A (only antigen A), B (only antigen B), AB (both antigens), or O (no antigens).
  • Rhesus blood type is determined by one common antigen. A person may either have the antigen (Rh+) or lack the antigen (Rh-).

Blood type is important for medical reasons. A person who needs a blood transfusion must receive blood that is the same type as his or her own. Otherwise, the transfused blood may cause a potentially life-threatening reaction in the patient’s bloodstream. 

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  • Blood is a fluid connective tissue that contains a liquid component called plasma.
  • Blood also contains dissolved substances and blood cells.
  • Red blood cells carry oxygen, white blood cells defend the body, and platelets help blood clot.


  1. What type of tissue is blood?
  2. Identify three types of blood cells and their functions.
  3. People with type O blood are called “universal donors” because they can donate blood to anyone else, regardless of their ABO blood type. Explain why.

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ABO blood type

Major blood group system characteristic of all humans and many other primates; consists of four principal types: A, B, AB, and O; also known as ABO blood group.


Molecule that the immune system identifies as foreign and responds to by forming antibodies.


Fluid connective tissue that circulates throughout the body through blood vessels.

blood type

Genetic characteristic associated with the presence or absence of antigens on the surface of red blood cells.


Protein of red blood cells that binds and carries oxygen; allows oxygen to be transported in the blood.


White blood cells that swallow and destroy microorganisms and debris in the blood.


Cell fragment in blood that helps blood clot.

red blood cell

Type of cell in blood that contains hemoglobin and carries oxygen .

Rhesus blood type

The second most significant blood-group system in human-blood; an individual either has, or does not have, the Rhesus factor on the surface of their red blood cells.

white blood cell

Type of cell in blood that defends the body against invading microorganisms or other threats in blood or extracellular fluid.

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