Animal CellsRed Blood CellsRed blood cells (RBCs) are also known as erythrocytes. There are between 4.2 and 6.2 million red blood cells in a cubic millimeter of blood. They specialize in transporting oxygen throughout the body. As a result, red blood cells are small and have a biconcave shape to increase their surface area and optimize the amount of oxygen that diffuses across their cell membrane. Additionally, red blood cells have no organelles other than a cell membrane and a cytoskeleton (in mammalian red blood cells). After oxygen diffuses from the lung's alveoli into red blood cells, it attaches to the main protein of red blood cells, hemoglobin, to form bright red oxyhemoglobin. . The red blood cells then travel around the body in the blood and provide oxygen to other cells in the body. During this time, carbon dioxide attaches to the hemoglobin of red blood cells, forming blue deoxyhemogobin. The red blood cells therefore release carbon dioxide into the lungs, which is then released into the air, and repeat this process. Red blood cells are formed during a process called erythropoiesis. This process occurs in the red bone marrow and constantly produces new red blood cells. Erythropoiesis involves the differentiation of hemocytoblasts into erythroblasts which is stimulated by a hormone produced in the kidney called erythropoietin. This hormone is only released when the kidney detects that there is not enough oxygen in the blood. The erythroblasts then fill with hemoglobin and then extrude their nucleus to form reticulocytes. Although they do not have a nucleus, reticulocytes nevertheless have an endoplasmic reticulum (ER). These cells lose the ER while in circulation and become red blood cells. Red blood cells are needed to replace old red blood cells which have a lifespan of 120 days. Their membranes...... middle of paper ......psychology.about.com/od/biopsychology/ss/neuronanat.htmhttp://psychology.about.com/od/biopsychology/ss/neuronanat_2.htmhttp http://psychology.about.com/od/biopsychology/ss/neuronanat_3.htmhttp://psychology.about.com/od/biopsychology/ss/neuronanat_4.htmhttp://psychology.about.com/od/biopsychology/ ss /neuronanat_5.htmhttp://psychology.about.com/od/biopsychology/ss/neuronanat_6.htmhttp://psychology.about.com/od/biopsychology/f/neuron01.htmhttp://www.ehow.com/ about_4588807_what -is-mesophyll.htmlhttp://www.botany.uwc.ac.za/SCI_Ed/grade10/anatomy/leaves.htmhttp://www.lab.anhb.uwa.edu.au/mb140/corepages/blood/ blood.htmhttp://www.biologyreference.com/Co-Dn/Differentiation-in-Plants.htmlhttp://www.wisegeek.com/what-are-the-Different-types-of-plant-cells.htmhttp: / /biology.about.com/library/weekly/aa022201a.htmhttp://www.wisegeek.com/what-is-a-xylem-cell.htm