http://www.ck12.org Chapter 1. Human Biology
FIGURE 1.45
The flattened shape of red blood cells helps them carry more oxygen than
if they were rounded.- Neutrophils are WBCs that can squeeze through capillary walls and swallow particles such as bacteria and
parasites. - Macrophages are large WBCs that can also swallow and destroy old and dying cells, bacteria, or viruses.
Below, a macrophage is attacking and swallowing two particles, possibly disease-causing pathogens (Figure
1.46). Macrophages also release chemical messages that cause the number of WBCs to increase. - Lymphocytes are WBCs that fight infections caused by viruses and bacteria. Some lymphocytes attack and
kill cancer cells. Lymphocytes called B-cells make antibodies.
FIGURE 1.46
A type of white blood cell, called a
macrophage, is attacking a cancer cell.Platelets
Platelets(Figure1.47) are very small, but they are very important in blood clotting. Platelets are not cells. They
are sticky little pieces of larger cells. Platelets bud off large cells that stay in the bone marrow. When a blood vessel
gets cut, platelets stick to the injured areas. They release chemicals called clotting factors, which cause proteins to
form over the wound. This web of proteins catches red blood cells and forms a clot. This clot stops more blood from
leaving the body through the cut blood vessel. The clot also stops bacteria from entering the body. Platelets survive
in the blood for ten days before they are removed by the liver and spleen.