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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 ). You can learn more about blood and its components by watching the animation "What Is Blood?" at this link: http://www.apan.net/meetings/busan03/materials/ws/education/demo-los/blood-rlo/whatisblood.swf .

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.
  • White blood cells are generally larger than red blood cells but far fewer in number. They defend the body in various ways. For example, white blood cells called phagocytes swallow and destroy microorganisms and debris in the blood.
  • 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.

An overview of red blood cells can be viewed at http://www.youtube.com/watch?v=fLKOBQ6cZHA (16:30).

Hemoglobin is discussed in detail at http://www.youtube.com/watch?v=LWtXthfG9_M (14:34).

Blood Type

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. See http://www.youtube.com/watch?v=zj8ziUBMQhU for more information on blood types.


  • 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.


Use this resource to answer the questions that follow.

  1. What is plasma?
  2. Discuss the components of plasma.
  3. Discuss the role of fibrinogen and fibrin in the plasma.
  4. Why is blood red?
  5. What is the role of red blood cells?
  6. What are white blood cells?
  7. What are platelets? What is their role?
  8. Describe how blood cells are replaced throughout life.


1. Identify three types of blood cells and their functions.

2. 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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