1 (b.a.) Blood versus Nerve Cells
Without red blood cells, your body could not use the oxygen that you breathe in. Red blood cells take up oxygen in the lungs and deliver it to cells elsewhere in the body. Red blood cells, like most blood cells, are produced in bone marrow. Under a microscope, these cells look like disks with pinched-in centers.
A red blood cell is made mostly of hemoglobin (hee muh gloh bin), which is an iron-containing protein that binds chemically to oxygen molecules. When hemoglobin combines with oxygen, the cells become bright red. Without oxygen, the cells are dark red. Thus, blood leaving the heart through the aorta is bright red, whereas blood returning from the body to the heart through veins is dark red.
Every second, about 2 million red blood cells in your body die. Fortunately, your bone marrow produces new red blood cells at the same rate.
In a many-celled organism, the cells are often quite different from each other and are specialized to perform specific functions. Nerve cells are specialized to transmit information from one part of your body to another, and red blood cells carry oxygen throughout your body. The levels of organization in the human body consist of cells, tissues, organs, organ systems and finally the organism. The smallest unit of organization is the cell. The next largest unit is tissue; then organs, then the organ system. Finally the organism, is the largest unit of organization.