Biology For Dummies
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Blood is the fluid that sustains life in animals with a closed circulatory system — including you. Some blood cells carry oxygen, which is necessary for metabolic reactions; some blood cells fight off invading substances that could destroy your cells; and other blood cells help form clots, which keep your body from losing too much of this precious fluid and assist with wound healing.

The solids found in your essential fluid

Although your blood is a fluid, it contains solid parts called formed elements. These solid parts are your red blood cells, white blood cells, and platelets.

The word solid is used simply to differentiate platelets and blood cells from the liquid portion of blood (the plasma), but platelets and blood cells definitely aren’t hard. They’re small and flexible so they can squeeze through your capillaries.

Red blood cells

Your red blood cells, called erythrocytes, have the important responsibility of carrying oxygen throughout your body. To do this, they carry a special iron-containing protein called hemoglobin. Hemoglobin uses its iron to capture and hold oxygen so that the red blood cells can carry it to your capillaries. Hemoglobin also helps carry carbon dioxide from the capillaries back to the lungs to be exhaled. By transporting oxygen and carbon dioxide, your red blood cells are an extremely important part of homeostasis — how your body tries to constantly achieve and maintain balance.

If a person has too few red blood cells or if her red blood cells don’t have enough hemoglobin, she has anemia. Anemia, which often leads to feelings of fatigue, can be caused by dietary deficiencies, metabolic disorders, hereditary conditions, or damaged bone marrow.

Red blood cells are created in the red bone marrow. They live about 120 days shuttling oxygen and carbon dioxide, and then certain white blood cells destroy them in the liver and spleen. As the red blood cells are destroyed, the iron they contain is recycled back to the red bone marrow to be used in new cells. The rest of the material in the old red blood cells is degraded and transported to the digestive system, where much of it ends up in fecal matter.

White blood cells

Your white blood cells, called leukocytes, help you fight infections. If a person has a low white blood cell count, her immune system isn’t functioning properly. If her white blood cell count is too high, that indicates she has some type of infection.

Following are the five important types of white blood cells you should know:

  • Basophils release histamines, those annoying little chemical molecules that cause you to swell up with hives, itch like crazy, sneeze, wheeze, and get teary-eyed when you’re around something you’re allergic to. All of these reactions are side effects of inflammation, a very important defensive process that helps you clear damaging agents out of your body.
  • Eosinophils help defend the body against invading organisms, particularly parasitic worms.
  • Lymphocytes are key players in your adaptive immune response, the response your body makes to defend you against invading microbes. Two of their important functions are to destroy virally infected cells and to make defensive proteins called antibodies.
  • Monocytes are precursors to macrophages. Macrophages digest bacteria and viruses (macro- means “big,” and phago means “to eat,” so a macrophage is literally a big eater).
  • Neutrophils are the most abundant white blood cells in the body. These cells eat bacteria, helping to keep your body from being overrun by every germ you meet.


Platelets, called thrombocytes, are pieces of cells that work to form blood clots. Platelets form when pieces are torn off of cells called megakaryocytes. Because they’re just cell fragments, platelets are smaller than red or white blood cells. They survive in the blood for about ten days.

The number of platelets in the blood is often determined as part of a complete blood count. Low numbers of platelets can indicate certain cancers and chronic bleeding disorders. Increased numbers of platelets may be a sign of chronic infection or certain blood diseases.

The plasma “stream” in your bloodstream

The liquid portion of your blood is plasma. Your blood cells and platelets flow in your plasma much like leaves float in a stream. In fact, when you think about it, plasma literally puts the “stream” in bloodstream.

Plasma contains many important proteins, without which you’d die. Two major proteins found in plasma are

  • Gamma globulin: Also called immunoglobulin, gamma globulin is a broad term for a class of defensive proteins that make up the different types of antibodies. The production of antibodies, which help to fight infections, is controlled by your immune system.
  • Fibrinogen: This protein is involved in blood clotting.

How blood clots form

When you cut your finger chopping an onion or picking up a piece of broken glass, your body embarks on a mission to form a blood clot (a semisolid plug made of blood cells trapped in a protein mesh) to prevent you from bleeding to death.

First, the injured blood vessel constricts, reducing blood flow to the injured blood vessel, which helps limit blood loss. (Tourniquets help squeeze off blood flow in much the same way when major blood vessels are damaged.) With the injured blood vessel constricted, the platelets present in the blood that’s passing through that vessel start to stick to the collagen fibers that are part of the blood vessel wall. Eventually, a platelet plug forms, and it fills small tears in the blood vessel.

After the platelet plug is formed, enzymes called clotting factors (your body has 12 of them) initiate a chain of reactions to create a clot. The process is rather complex, but you don’t need to know all the nitty-gritty details; just focus on these highlights:

  • After a platelet plug forms, the coagulation phase begins, kicking off a cascade of enzyme activations that ultimately convert inactive prothrombin to active thrombin. (Calcium is required for this reaction to occur.)
  • Thrombin acts as an enzyme and causes fibrinogen — one of the two major plasma proteins — to form long fibrin threads.
  • Fibrin threads entwine the platelet plug, forming a meshlike framework.
  • The fibrin framework traps red blood cells that flow toward it, forming a clot. (Note: Because red blood cells are tangled in the meshwork, clots appear to be red. As the red blood cells trapped on the outside dry out, the color turns a brownish red, and a scab forms.)

About This Article

This article is from the book:

About the book author:

René Fester Kratz, PhD, teaches biology at Everett Community College. Dr. Kratz holds a PhD in Botany from the University of Washington. She works with other scientists and K?12 teachers to develop science curricula that align with national learning standards and the latest research on human learning.

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