Molecular & Cell Biology For Dummies
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Blood is the fluid that sustains life. The components of blood include red blood cells, white blood cells, platelets, and plasma. Some blood cells carry oxygen (necessary for metabolic reactions), some blood cells fight off invading substances that could destroy your cells, and other blood cells help to form clots, which keep your body from losing too much blood.

The fluid portion of the blood carries nutrients needed to fuel each cell in the body. It also shuttles wastes that need to be transported to the excretory system to be passed out of the body and carbon dioxide that needs to be transported to the lungs to be exhaled.

Red blood cells

The red blood cells, which are also called erythrocytes (erythro means red; cytes = cells) have the important responsibility of carrying the oxygen throughout the body. Hemoglobin exists in the red blood cells. Hemoglobin not only binds oxygen and transports it to capillaries, but it also helps to transport carbon dioxide from the capillaries back to the lungs to be exhaled.

By transporting oxygen and hemoglobin, blood is an extremely important part of homeostasis — a conglomerate of processes that allows your body to adjust to changes in external temperatures and hormone levels. But, many of the processes that occur to help your body adjust to changes could not happen without the blood transporting certain hormones, nutrients, oxygen, or electrolytes.

If a person has too few red blood cells, as determined by a red blood cell count, or if there is not enough hemoglobin in the red blood cell, he or she is diagnosed with anemia. Because hemoglobin carries oxygen, anemia often causes people to feel fatigued.

Anemia can be caused by any of the following:

  • Dietary deficiencies

  • Metabolic disorders

  • Hereditary conditions

  • 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

The white blood cells, which are also called leukocytes (leuko = white), are involved in functions controlled by the immune system. The immune system is responsible for fighting infections. If a person has a low white blood cell count, it means that the immune system is not functioning properly. If a white blood cell count is too high, it indicates that the person has some type of infection.

There are five important types of white blood cells:

  • Basophils release histamines. Histamines are those annoying little chemical molecules that cause you to swell up with hives, itch like crazy, sneeze, wheeze, and get teary-eyed when you are around something to which you are allergic. All of those reactions cause inflammation, which enlists the help of stronger white blood cells. Sneezing and getting watery eyes are physiologic reactions to help flush the offending allergen from your mucous membranes.

  • Eosinophils “eat” other cells. The technical term for the eating of a cell is phagocytosis, so eosinophils are said to phagocytize complexes formed between antigens (the invading offender) and antibodies (a “home team” defender).

  • Lymphocytes kill cells that contain viruses. Lymphocytes scan the body looking for viruses. There are two types of lymphocytes: B cells and T cells. T cells are the type of virus hunters measured in a person with acquired immunodeficiency syndrome (AIDS). If the T-cell count decreases, it indicates that the human immunodeficiency virus that causes AIDS is winning the fight in that infected person’s body.

  • Monocytes are precursors to macrophages, meaning “big eater.” Macrophages digest bacteria and viruses.

  • Neutrophils are the most abundant white blood cells in the body. These cells phagocytize bacteria, and in doing so keep your system from being overrun by every germ with which it comes into contact.


Platelets are pieces of cells that work to form blood clots. They work to keep your body from losing too much blood when you sustain an injury and help in wound healing.


When blood is put into a test tube and spun in a centrifuge, the blood cells and platelets gravitate to the bottom of the tube, and the plasma is a clear layer on top. Think of the action inside blood vessels; imagine plasma as a river and the blood cells and platelets as leaves floating in it. Plasma is the “stream” in bloodstream. The plasma contains many important proteins, without which you would die.

Two major proteins contained in plasma are:

  • Gamma globulin (also called immunoglobulin): Gamma globulin is a broad term for a class of proteins that make up the different types of antibodies. The production of antibodies, which help to fight infections, is controlled by the immune system.

  • Fibrinogen: Fibrinogen is a protein involved in blood clotting.

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