![]() The advantage of nucleated red blood cells is that these cells can undergo mitosis. Only mammals have anucleated red blood cells, and some mammals (camels, for instance) even have nucleated red blood cells. ![]() In mammals, the lack of organelles in erythrocytes leaves more room for the hemoglobin molecules, and the lack of mitochondria also prevents use of the oxygen for metabolic respiration. There are approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to 25 sextillion (25 × 10 21) molecules of oxygen in the body at any time. Each hemoglobin molecule binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. Hemoglobin is packed into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. The principal job of this protein is to carry oxygen, but it also transports carbon dioxide as well. The red coloring of blood comes from the iron-containing protein hemoglobin, illustrated in Figure a. ![]() In birds and non-avian reptiles, a nucleus is still maintained in red blood cells. In mammals, red blood cells are small biconcave cells that at maturity do not contain a nucleus or mitochondria and are only 7–8 µm in size. Red blood cells, or erythrocytes (erythro- = “red” -cyte = “cell”), are specialized cells that circulate through the body delivering oxygen to cells they are formed from stem cells in the bone marrow. Blood is 20 percent of a person’s extracellular fluid and eight percent of weight. In humans, cellular components make up approximately 45 percent of the blood and the liquid plasma 55 percent. Interstitial fluid that surrounds cells is separate from the blood, but in hemolymph, they are combined. The platelets are responsible for blood clotting. The cells are responsible for carrying the gases (red cells) and immune the response (white). Blood plasma is actually the dominant component of blood and contains the water, proteins, electrolytes, lipids, and glucose. Blood is actually a term used to describe the liquid that moves through the vessels and includes plasma (the liquid portion, which contains water, proteins, salts, lipids, and glucose) and the cells (red and white cells) and cell fragments called platelets. The blood is more than the proteins, though. Hemoglobin is responsible for distributing oxygen, and to a lesser extent, carbon dioxide, throughout the circulatory systems of humans, vertebrates, and many invertebrates.
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