Introduction
Did you know that blood is a tissue? Blood is a fluid connective tissue that is made up of
red blood cells, white blood cells, platelets, and plasma. It moves around the body through
the blood vessels by the pumping action of the heart. Oxygen rich blood carried in arteries
brings oxygen and nutrient to all the body’s cells. Oxygen-poor blood carries carbon dioxide
and other metabolic wastes away from the cells. As well as the transport of gases, nutrients,
and wastes, blood has many other functions that are important to homeostasis. You will
learn more about these functions in this lesson.
Components of Blood
Bloodisacolloidalsolution. Acolloidalsolutionitismadeupofparticlesthataresuspended
in a fluid. The cells in blood are suspended in plasma, the liquid part of blood. The cells
that make up the blood are shown inFigure18.15. The different components of blood have
different roles. Some of the roles of blood include:
- The defense of the body against infection by microorganisms or parasites.
- The transport of chemical messages, such as hormones and hormone-like substances.
- The control of body temperature.
- The repair of damage to body tissues.
Plasma
If you were to filter out all the cells in blood, plasma is what would be left over.Plasmais
the golden-yellow liquid part of the blood. Plasma is about 90 percent water and about 10
percent dissolved proteins, glucose, ions, hormones, and gases. Blood is made up of mostly
plasma. The blood cells make up the rest of the volume.
Red Blood Cells
Red blood cells (RBCs)are flattened disk-shaped cells that carry oxygen. They are
the most common blood cell in the blood. There are about 4 to 6 million RBCs per cubic
millimeter of blood. Each RBC has 200 million molecules of hemoglobin. Hemoglobin is the
protein that carries oxygen. Hemoglobin also gives the RBCs their red color. Red blood
cells are made in the red marrow of long bones, ribs, skull, and vertebrae. Each red blood
cell lives for only 120 days (about three months). After this time, they are destroyed in liver
and spleen. Red blood cells are shown inFigure18.16. Mature RBCs do not have a nucleus
or other organelles.