Lesson 3

In this lesson, we will cover:

• Red Blood Cells
• White Blood Cells
• Platelets

Located in the blood plasma, red blood cells, also called erythrocytes, are specialized for the transport of oxygen. Almost the entire volume of a mature red blood cell is filled with hemoglobin, the protein which is responsible for carrying the oxygen, and contains practically none of the usual cell organelles. In an adult mammal, even the nucleus, endoplasmic reticulum, mitochondria, and ribnosomes are absent, having been extruded from the cell i the course of its development. On average, there are about 5 million red blood cells per cubic meter of blood; in the adult human body there are 25 trillion.

Because red blood cells cannot repair themselves, (as they have no nucleus or other important organelles), their life span is comparatively short, some 120 to 130 days. At any given moment, red blood cells in your body are dying at a rate of about 2 million per second, and to replace them, new ones are formed in the bone marrow at about the same rate.In mammals, new red blood cells are produced chiefly in the bone marrow.

Red blood cells carry oxygen from the lungs to tissues and exchange it for carbon dioxide to carry back to the lungs for exhalation. As oxygen and carbon dioxide carriers, they play important roles in cellular respiration and photosynthesis (in plants and protists), which help provide energy for functions throughout the body. For example, in cellular respiration, basic cells use glucose and oxygen to create carbon dioxide, water and energy. Red blood cells clearly have an important function everywhere in the body, and thus use an efficient circulatory system to travel from head to toe. They are quite pliable and fold so they can enter the tiny capillaries that extend to the far reaches of the body. .

White blood cells, also called leukocytes, are much rarer than red blood cells, as there are only one or two of them for every thosand red blood cells in the human bloodstream. These cells are nearly colorless, are larger than red blood cells, condtain no hemoglobin, and have a nucleus.

The chief function of white blood cells is to defend the body against invaders such as viruses, bacteria, and other foreign substances. While all red blood cells are generally similar, there are many different classes of white blood cells:

All blood cells, including red bloods, develop from pluripotent hemopoietic stem cells, located in the liver for fetuses, and in the bone marrow for adults. The essential lymphocytes are broken into two categories: B lymphocytes and T lymphocytes, sometimes shortened as B cells and T cells. B and T cells both play crucial roles in the immune response. T cells work on cell-mediated immune responses, while B cells help the immune system through humoral antibody responses. The main difference between B and T cells is how they are formed. Both bgin as stem cells in teh bone marrow, but future T cells migrate via the blood to the thymus. While some of these cells die in the thymus, others migrate to the spleen (called the peripheral lymphoid tissue) where they become T cells. On the other hand, in mammals, future B cells travel directly from the bone marrow to the spleen, without passing through the thymus. (In birds, the B cells also travel through the bursa of Fabricius between the bone marrow and spleen.) Because most of the migration of lymphocytes from thymus and bursa occurs early in development, removing either of these organs at the adult stage has relatively little effect, and so thus it was only recently that it was discovered that these organs were involved in the immune response.

Platelets, so called because they look like little plates, are colorless irregularlay shaped disks smaller than red blood cells. Platelets, which are also located in the blood, are not entire cells, but small detached cell fragments from large celles, megakaryocytes, found in the bone marrow. Essentially, they are little bags of chemicals which play crucial roles in controlling the clotting of blood and in repairing breaches in the walls of blood vessels.

That's it! You have made it through lesson 3, the cells in our body. If you want to proceed to the next lesson, first take the lesson 3 quiz.