The Circulatory System
Lesson 8, Page 5 of 12

The correct answer is all of the above.

The Blood

The blood transports life-supporting food and oxygen to every cell of the body and removes their waste products. It also helps to maintain body temperature, transports hormones, and fights infections. The brain cells in particular are very dependent on a constant supply of oxygen. If the circulation to the brain is stopped, death shortly follows.

Blood has two main constituents. The cells, or corpuscles, comprise about 45 percent, and the liquid portion, or plasma, in which the cells are suspended comprises 55 percent. The blood cells comprise three main types: red blood cells, or erythrocytes; white blood cells, or leukocytes, which in turn are of many different types; and platelets, or thrombocytes. Each type of cell has its own individual functions in the body. The plasma is a complex colorless solution, about 90 percent water, that carries different ions and molecules including proteins, enzymes, hormones, nutrients, waste materials such as urea, and fibrinogen, the protein that aids in clotting.

Red Blood Cells

The red blood cells are tiny, round, biconcave disks, averaging about 7.5 microns (0.003 in) in diameter. A normal-sized man has about 5 l (5.3 qt) of blood in his body, containing more than 25 trillion red cells. Because the normal life span of red cells in the circulation is only about 120 days, more than 200 billion cells are normally destroyed each day by the spleen and must be replaced. Red blood cells, as well as most white cells and platelets, are made by the bone marrow.

The main function of the red blood cells is to transport oxygen from the lungs to the tissues and to transport carbon dioxide, one of the chief waste products, it to the lungs for release from the body.

The substance in the red blood cells that is largely responsible for their ability to carry oxygen and carbon dioxide is hemoglobin, the material that gives the cells their red color. It is a protein complex comprising many linked amino acids, and occupies almost the entire volume of a red blood cell. Essential to its structure and function is the mineral iron.

The blood cell type responsible for the transport of oxygen and carbon dioxide is the ________.

white blood cell
red blood cell
all of the above

(Select the best answer and click on the "Continue" button.)


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