why do red blood cells contain haemoglobin

One, mammals are younger in natural history than birds. Before mammals, nucleated blood cells were the norm. Mammals evolved enucleated blood cells to use oxygen more efficiently. Two, birds have a different respiratory system that lets them transport oxygen efficiently without needing enucleated blood cells. They have "flow-through" respiration that lets air move continuously through the body. Mammals have alveoli that are like a dead-end for moving air. б
Red blood cells (RBC) or erythrocytes are continually formed in the bone marrow. RBC originate from nucleated stem cells, which mature into nucleated erythroblasts, then differentiate into a-nuclear reticulocytes and finally into RBC. RBC are terminally differentiated cells (they cannot divide anymore) and are shed from the bone marrow into the blood circulation.


They live approximately 120-180 days. In contrast to mammals, RBCs in birds, reptiles and other "lower" vertebrates have a nucleus. The a-nucleated erythrocyte, as it is seen in mammals, is considered more evolutionarily "advanced". (seeб www. ultranet. com/~jkimball/BiologyPages/V/Vertebrates. html). The lower vertebrates (e. g. , birds) are considered earlier on the evolution ladder and have a different circulatory system (seewww. sciencenet. org,www. historyoftheuniverse. com/blood. html,б ). In addition to the differences in the circulatory system, mammals have smaller end-bloodvessels (capillaries of about 3 micron in diameter) than birds. In order to squeeze through these small blood capillaries, RBC which are about 10 micron in diameter, must be very flexible. The presence of a nucleus would prevent big nucleated RBC to squeeze through these small capillaries.


Therefore, during the evolutionary development, nature has found that it was better to get rid of the nucleus and also other cell organelles (e. g. , endoplasmic reticulum for protein synthesis) which were not needed for their actual function as oxygen carrier. Sue Thornquist (www. vet. orst. edu/clinpath/learning/vm736/avianhem. htm) also thinks that the absence of the nucleus in birds is based on evolutionary differences but sheБs not sure whether this theory has been proved. As homeotherms evolved, they had increased oxygen demands due to different metabolic requirements. Birds appear to have adapted to increased oxygen demands by developing a "flow-through" respiratory system (interconnecting tubes for continuous flow, rather than blind-ended alveoli) that's more efficient than mammals'.


Mammals may have diverged here and developed anucleated RBC's with increased oxygen carrying capacity to adapt to the increased oxygen demands Blood is used to transport materials around the body, and to protect against disease. Blood contains plasma Liquid, non-cellular part of the blood, a liquid that contains dissolved substances, cells and cell fragments. These include: Red blood cells, which transport oxygen White blood cells, which protect against disease Platelets, which help the blood to clot The table explains how the structure of a red blood cell is adapted to its function. Plasma transports dissolved substances around the body, including: Nutrients, such as water, glucose, amino acids, minerals and vitamins Waste substances, such as carbon dioxide and urea

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