As far as I know, just leg bones produce red blood cells. So, how people who lost their both legs produce red blood cells?
Red blood cells are produced in the red marrow which…
"is found mainly in the flat bones, such as the pelvis, sternum, cranium, ribs, vertebrae and scapulae, and in the cancellous ("spongy") material at the epiphyseal ends of long bones such as the femur and humerus." - Wikipedia
So you are partly right; the femur is associated with red blood cell production, or Erythropoiesis to give it it's technical name, but there are other bones within the human body that also do this job. The process of erythropoiesis is stimulated when the kidneys detect low levels of oxygen in the blood stream and stimulate production of the hormone erythropoietin. Further, the role of the tibia and femur in erythropoiesis also decreases with age whereas…
"the vertebrae, sternum, pelvis and ribs, and cranial bones continue to produce red blood cells throughout life." - again from the wiki page
So I'd suggest it is unlikely that loss of the legs would have a major impact on the production of red blood cells in adults. I imagine that with the loss of legs comes some reduction in functionality of erythropoiesis but also a lower requirement of red blood cell production (less blood capacity = less blood cells needed = less blood cells need to be produced). I can't find any studies which explore the ability or needs of amputees and non-amputees with regards to red blood cell production.
Red blood cells are produced in the red marrow, and white blood cells are produced in the yellow marrow. The marrow is found in the flat bones (i.e.long bones) which is the pelvis, sternum, ribs, and vertebrae. If the long bones are no longer attached the the pelvis and the ribs with have to work harder in order to produce the blood cells needed.
Red blood cell
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Red blood cell, also called erythrocyte, cellular component of blood, millions of which in the circulation of vertebrates give the blood its characteristic colour and carry oxygen from the lungs to the tissues. The mature human red blood cell is small, round, and biconcave it appears dumbbell-shaped in profile. The cell is flexible and assumes a bell shape as it passes through extremely small blood vessels. It is covered with a membrane composed of lipids and proteins, lacks a nucleus, and contains hemoglobin—a red iron-rich protein that binds oxygen.
The function of the red cell and its hemoglobin is to carry oxygen from the lungs or gills to all the body tissues and to carry carbon dioxide, a waste product of metabolism, to the lungs, where it is excreted. In invertebrates, oxygen-carrying pigment is carried free in the plasma its concentration in red cells in vertebrates, so that oxygen and carbon dioxide are exchanged as gases, is more efficient and represents an important evolutionary development. The mammalian red cell is further adapted by lacking a nucleus—the amount of oxygen required by the cell for its own metabolism is thus very low, and most oxygen carried can be freed into the tissues. The biconcave shape of the cell allows oxygen exchange at a constant rate over the largest possible area.
The red cell develops in bone marrow in several stages: from a hemocytoblast, a multipotential cell in the mesenchyme, it becomes an erythroblast (normoblast) during two to five days of development, the erythroblast gradually fills with hemoglobin, and its nucleus and mitochondria (particles in the cytoplasm that provide energy for the cell) disappear. In a late stage the cell is called a reticulocyte, which ultimately becomes a fully mature red cell. The average red cell in humans lives 100–120 days there are some 5.2 million red cells per cubic millimetre of blood in the adult human.
Bill Detrich is a professor of Biochemistry and Marine Biology at the Northeastern University Marine Science Center in Boston, Massachusetts.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’ll be talking about the mind-boggling details of the explosion of the Chernobyl nuclear reactor. But first, let’s talk about blood. In nearly every vertebrate on Earth– parakeets, dogs, lions, sharks, us– blood is red. Distinctively so. You all know that. And there’s a reason for this. The hemoglobin in red blood cells binds oxygen molecules and helps them get to our cells. Without those red blood cells, we’d be anemic, have far lower capacity to use the oxygen we breathe.
But venture to Antarctica and you will find a biological marvel. The world’s only white-blooded fish– the icefish. They’ve evolved translucent blood, free of red blood cells and hemoglobin. And are somehow doing just fine in the cold waters of the Southern Ocean. How do they do it? Researchers writing in the journal, Nature Ecology and Evolution, this week, have clues from the icefish genome. And here to tell us more is Dr. Bill Detrich, Professor of Biochemistry and Marine Biology at Northwestern University’s Marine Science Center in Boston. Welcome Dr. Detrich.
BILL DETRICH: Well, thank you very much, Ira. It’s a pleasure to be on your show.
IRA FLATOW: It’s a pleasure to have you. Thank you. Paint us a picture of the icefish for us. So we’re on the radio, what does it look like?
BILL DETRICH: Well, imagine a fairly large fish about half a meter in length. Weighing 1.2 to 2 kilograms. Has a very large crocodilian-like head and a rather small body. It’s skin is scaleless and very ghostly pale. And although you can’t see this, it has antifreeze running through its white blood.
IRA FLATOW: And so more than just one fish, it’s actually a group of species. Correct?
BILL DETRICH: Correct. It’s a group of 16 species, none of which produce red blood cells. They’re all profoundly anemic.
IRA FLATOW: Who first discovered this?
BILL DETRICH: Well, this was first recognized by whalers around the turn of the 1900s, when they were in the Southern Ocean and they would catch fish, and open them up and even eat them. But the fish, they noticed, didn’t have red blood. In 1929, a Norwegian zoologist named Ditlef Rustad, actually was able to capture one of these fishes and he communicated this finding to some of his colleagues, who later followed up on it.
IRA FLATOW: So OK, what is the magic the fish does that we can’t do? How does it survive without red blood cells?
BILL DETRICH: Well, that’s a very interesting question, of course. It’s something I’ve been looking into quite a bit. So what do they do? They’ve given up on red blood cells, but it was not a lethal condition for them. On the other hand, it wasn’t absolutely positive, because we’ve seen that they’ve evolved very large vasculature and a much expanded heart. They don’t have scales on their skin, so they can breathe through their skin. There are a number of compensations that allow them to do very well in this cold oxygen-rich ocean that they live in.
IRA FLATOW: And your team sequenced the genome of one species of these icefish. Any clues to how it got this white blood? Because I’ve watched videos of this, it’s just like a little serum, right? Colorless.
BILL DETRICH: That’s right. We have been able to establish that virtually all of the hemoglobin genes are absent in this genome. We are also following up to try to find out the chicken versus the egg question. Did these fish lose their red blood cells first, and therefore, didn’t have a cell to express hemoglobin in? Or alternatively, did they lose their hemoglobin genes and then the red cells withered? So that’s a question that we have to follow up on.
IRA FLATOW: So you say that there is enough oxygen circulating in this serum instead of the red blood cells. Is that because they live in such cold water in Antarctica that there’s plenty of oxygen in the water that can suffuse also through their skin and keep them going?
BILL DETRICH: Yes, the Southern Ocean is essentially saturated in oxygen, among other reasons, because it’s very stormy there and that mixes of the water column.
IRA FLATOW: And so I guess in the long-term, global warming would be a threat to these fish.
BILL DETRICH: These fish, in fact, compared to their red-blooded relatives are much more sensitive to temperature. And they, in fact, are likely to be canaries in the coal mine if we see fish beginning to drop out as the Southern Ocean warms.
IRA FLATOW: So these fish basically have had to find a way to adapt to not having the red blood cells.
BILL DETRICH: That’s right. And I think the key clue to that is that you can take one of the red-blooded fishes that are very closely related, and you can expose that fish to carbon monoxide so that all of the hemoglobin is poisoned, no longer able to carry oxygen, and yet, this red-blooded species doesn’t die either. So what that’s telling us is that even the red-blooded fishes in the Southern Ocean are relying on their red cells more as a reserve capacity for oxygen. And the red-blooded fishes are also living on dissolved oxygen like the ice fishes are.
IRA FLATOW: We’ve talked about how cold adaptation in animals might help us with biomedical research like freezing organs for transplant, for example. Is there anything the icefish and its blood could teach us about ourselves and help us?
BILL DETRICH: Yes, I think that the answer there is that we still don’t know all of the genes that are involved in making red blood cells. And these fish that have lost the capacity to do so, that’s going to leave a genomic signature behind. And that genomic signature is likely to reveal new genes for us to investigate and potential new targets for therapy for anemias.
IRA FLATOW: Have you caught one of these fish yourself?
BILL DETRICH: Yes, I have. Yes, quite often.
IRA FLATOW: In a net? With a hook? How easy are they to catch?
BILL DETRICH: We typically use a small scientific net that we trawl behind the Laurence M. Gould, our research icebreaker in the Antarctic Peninsula.
IRA FLATOW: Years ago when I was in Antarctica, I was watching scientists catch what they called the Antarctic cod fish at those times. They were living at the bottom of the continental shelf 1,000 feet down. They’d bring them up, they’d drain the blood because they were looking at the antifreeze. They were studying the antifreeze in the blood. And then they would smoke the fish and it was so delicious, I remember. Because they didn’t need the fish, they just needed the blood. Have you tasted this fish at all?
BILL DETRICH: Yes. And, in fact, the ice fish, the genome we sequenced, that species is very, very good.
IRA FLATOW: Aha. And do these fish have the same kind of antifreeze that other fish have in their regular blood system, so that they don’t freeze?
BILL DETRICH: Right. They have the same antifreeze, in fact, as the cod fish that you experienced.
IRA FLATOW: Mm-hmm. So what do you want to know now? What’s your next step in this research?
BILL DETRICH: Well, the next step in the research is to try to figure out the chicken and egg question. Whether red blood cells were lost first, or whether hemoglobin was lost first and subsequently the red cell disappeared from the blood profile?
IRA FLATOW: And how do you do that?
BILL DETRICH: Well, we’re going to need to sequence other genomes, other icefish genomes and genomes of the red-blooded relatives. And by applying phylogeny we should be able to tease out which of the events occurred first. That’s a long-term goal, but I think it’s doable.
IRA FLATOW: What’s your suspicion? If you were–
BILL DETRICH: If I was a betting man?
IRA FLATOW: If you were a betting man, yeah.
BILL DETRICH: If I was a betting man, I think that the hemoglobin genes went first.
IRA FLATOW: That does make sense, doesn’t it?
BILL DETRICH: It does make sense. And partly, I’d base that on prior research that we’ve done across the 16 species where we see that there are just a couple of different gene variants in terms of the globin genes that have been lost. So I guess based on that, I would be willing to bet that the hemoglobin genes went first.
IRA FLATOW: And why would that be an advantage to them to survive?
BILL DETRICH: Well, not so much the loss of hemoglobin, but the loss of the red cell. Imagine if we take a unit of human blood and cool it down to refrigerator temperatures, it becomes rather viscus. And viscus fluid is harder to pump through circulation than one that isn’t. So potentially, the ice fishes, by giving up their red blood cells, have reduced the energy that they need to pump their blood fluid, if you will.
IRA FLATOW: So it’s easier for them that way. Now, the icefish seem to be the only vertebrates with no red blood cells whatsoever. To me, that’s kind of unheard of. An exception like that in biology to lose something that’s so conserved in a whole group of organisms.
BILL DETRICH: Yes. It’s quite remarkable. Darwin actually had a term for creatures such as the icefish. He called them wrecks of ancient life because they’d lost important traits that were present in their ancestors. And in the case of the icefish we’re talking about it’s loss of red blood cells and also the loss of dense bones.
IRA FLATOW: Oh, so are they more cartilaginous?
BILL DETRICH: Yeah, they are more cartilaginous. And they’re actually rather floppy animals, because they don’t have– their bones are not firm.
IRA FLATOW: So I guess the term icefish would be because they live among where the ice is, but not that they’re frozen or anything. This reminds me of cave fish losing their eyes and not needing them, because they don’t need them, they get along fine without them.
BILL DETRICH: That’s right. They don’t see any photons of light. They actually– cave fish start out making an eye as they are developing, and then that primitive eye regresses and they’re blind.
IRA FLATOW: Fascinating. Thank you, Dr. Detrich. Thank you. We’re much more informed now and thank you for taking time to be with us today.
BILL DETRICH: Well, thank you very much for having me. And I hope I’ve stimulated some interest in these really unusual creatures.
IRA FLATOW: You certainly have. Dr. Bill Detrich, Professor of Biochemistry and Marine Biology at Northeastern University in Boston.
4 Ways of Increasing Your Red Blood Cells
Red blood cells, also known as erythrocytes, are the most common blood cells in the body. In fact, about a quarter of all cells in the body are red blood cells. Their primary function is to carry oxygen to all tissues of the body, picking up the oxygen from the lungs and releasing it as they enter the capillaries. Over 2.4 million new red blood cells are produced every second, and they survive in the body for up to 120 days.
There are many reasons that your red blood cell count could be too low. The most common reasons include anemia, bone marrow failure, malnutrition, leukemia, hemolysis due to transfusions or injury to the blood vessels, being too hydrated, nutritional deficiencies, or even pregnancy. There are some drugs that can also bring down your red blood cell counts, including several cancer drugs.
Fortunately, there are several ways to increase the red blood cell count in your body.
How to Increase Red Blood Cells with Foods
Eating the right foods can help increase the number of red blood cells in your body:
- Iron. Food rich in iron can help your body rebuild what it has lost. Lentils and legumes are a great way to get the iron you need and they are healthy for you in many other ways, too.
- Copper. This vital mineral can be found in many foods, including shellfish, poultry, liver, whole grains, beans, cherries, chocolate and nuts.
- Folic Acid. Long known as a great help for pregnant and nursing mothers, foods that contain folic acid include lentils, dark green leafy vegetables, blackeyed peas and cereals fortified with folic acid.
- Vitamin A. This very important vitamin can be found in a multitude of fruits, including grapefruit, mango, watermelon, plums, cantaloupe and apricots.
- Vitamin B12. Meat, eggs and fortified cereals are a great way to get plenty of B12 in your diet. Since those on a western diet get plenty of this, a lack of B12 is rare.
- Vitamin B6. This vitamin is found in a wide variety of foods, including meats, whole grains and bran, nuts and seeds, fish, vegetables and legumes.
Supplements to Increase Red Blood Cells
Sometimes diet isn&rsquot enough to increase red blood cells. In that case, turning to supplements can help your body produce the red blood cells it needs. Here are a few options:
- Iron. This is a vital nutrient that your blood cells need to function properly. Women need 18 mg and men need 8 mg of iron per day.
- Vitamin B12. Derived from mostly animal foods, B12 can be lacking in vegetarians. Everyone needs 2.4 mcg per day, and a supplement can provide most of that.
- Vitamin B6. Women need 1.5 mg of this vitamin each day, while men need a bit more at 1.7 mg. A supplement can provide this, and you can boost the intake with baked potatoes, bananas and fish.
- Vitamin E. This vitamin is excellent for good health, including red blood cells. Everyone needs about 15 mg of this per day. However, supplements might provide much more than that, so speak with your doctor about whether that is okay for you.
How to Increase Red Blood Cells with Lifestyle Changes
There are a few lifestyle changes you can try that might keep your red blood cell count going strong. Here are a few that you can try right now:
- Exercise. Good amounts of exercise make the body use more oxygen, which demands more red blood cell production. This is especially effective if you live at a high altitude. But keep in mind that you must have certain vitamins in order to make this work, especially B12 and B6, so make sure to get plenty of them in your diet.
- Cut off Certain Things. Keep in mind that some medications can cause lower red blood cell counts, and so can excessive alcohol consumption. For instance, if you have been diagnosed with thrombocytopenia &ndash low amounts of platelets in the blood &ndash you might want to avoid aspirin and alcohol.
Medical Ways to Increase Red Blood Cells
What if you have tried iron-rich diet, and you have also taken supplements, but your red blood cell count remains low? In that case, it might be time for medical intervention. Keep in mind that this is usually a last resort, and most doctors will only go this route if your deficiencies in red blood cells are significant.
- Medications. Antibiotics for infections, drugs that help fight auto-immune disorders and hormones that regulate menstrual bleeding are a few of the ways medications can help relieve the problem.
- Surgery. If low red blood cell counts are being caused by physical ailments, surgery might help. Removal of the spleen, removing tumors or treating bleeding ulcers can all help increase your red blood cell count.
- Blood Transfusions. A transfusion of packed red blood cells can help your body carry oxygen, as well as help control bleeding and blood pressure.
Erythropoietin. This hormone stimulates the bone marrow to produce more red blood cells. This is often used for individuals who are experiencing kidney failure or going through chemotherapy treatment.
How to increase red blood cells? These tips and tricks can help you increase red blood cell production however, if none of them seem to work well or you are suffering from symptoms, speak to a doctor about the condition. From there, you can figure out what you need to spur production of red blood cells, as well as rule out any medical conditions that need more intense treatment.
Causes of Burning Calf Pain
Psoriasis causes dry, itchy red patches that can also be painful. Psoriasis is a chronic condition, which sometimes flares up and other times goes into remission, according to the Mayo Clinic 12. Psoriasis is a condition of the immune system. The body's T cells mistakenly attack healthy skin cells rather than infection. Flareups are caused by injury, infection, stress, cold weather, smoking and some medications.cause:
- Flareups are caused by injury
- cold weather
- some medications
- Psoriasis causes dry, itchy red patches that can also be painful.
- Psoriasis is a chronic condition, which sometimes flares up and other times goes into remission, according to the Mayo Clinic 1.
How do people who have lost both of their legs produce red blood cells? - Biology
Today I found out the red juice in raw red meat is not blood. Nearly all blood is removed from meat during slaughter, which is also why you don’t see blood in raw “white meat” only an extremely small amount of blood remains within the muscle tissue when you get it from the store.
So what is that red liquid you are seeing in red meat? Red meats, such as beef, are composed of quite a bit of water. This water, mixed with a protein called myoglobin, ends up comprising most of that red liquid.
In fact, red meat is distinguished from white meat primarily based on the levels of myoglobin in the meat. The more myoglobin, the redder the meat. Thus most animals, such as mammals, with a high amount of myoglobin, are considered “red meat”, while animals with low levels of myoglobin, like most poultry, or no myoglobin, like some sea-life, are considered “white meat”.
Myoglobin is a protein that stores oxygen in muscle cells, very similar to its cousin, hemoglobin, that stores oxygen in red blood cells. This is necessary for muscles which need immediate oxygen for energy during frequent, continual usage. Myoglobin is highly pigmented, specifically red so the more myoglobin, the redder the meat will look and the darker it will get when you cook it.
This darkening effect of the meat when you cook it is also due to the myoglobin or more specifically, the charge of the iron atom in myoglobin. When the meat is cooked, the iron atom moves from a +2 oxidation state to a +3 oxidation state, having lost an electron. The technical details aren’t important here, though if you want them, read the “bonus factoids” section, but the bottom line is that this ends up causing the meat to turn from pinkish-red to brown.
Pro-tip: when searching for non-copyrighted pictures for an article, don’t search for “white meat” or really any variation of that on Google Image Search.
If you liked this article and the Bonus Facts below, you might also enjoy:
- It is possible for meat to remain pinkish-red all through the cooking if it has been exposed to nitrites. It is even possible for packagers, through artificial means, to keep the meat looking pink, even after it has spoiled, by binding a molecule of carbon monoxide to produce metmyoglobin. Consumers associate pink meat with “fresh”, so this increases sales, even though the pink color has little to do with the freshness of meat.
- Pigs are often considered “white meat”, even though their muscles contain a lot more myoglobin than most other white meat animals. This however, is a much lower concentrate of myoglobin than other “red meat”, such as cows, due to the fact that pigs are lazy and mostly just lay around all day. So depending on who you talk to, pigs can be considered white meat or red meat they more or less sit in between the two classifications.
- Chickens and Turkeys are generally considered white meat, however due to the fact that both use their legs extensively, their leg muscles contain a significant amount of myoglobin which causes their meat to turn dark when cooked so in some sense they contain both red and white meat. Wild poultry, which tend to fly a lot more, tend to only contain “dark” meat, which contains a higher amount of myoglobin due to the muscles needing more oxygen from frequent, continual usage.
- White meat is made up of “fast fibers” that are used for quick bursts of activity. These muscles get energy from glyocogen which, like myoglobin, is stored in the muscles.
- Fish are primarily white meat due to the fact that they don’t ever need their muscles to support themselves and thus need much less myoglobin or sometimes none at all in a few cases they float, so their muscle usage is much less than say a 1000 pound cow who walks around a lot and must deal with gravity. Typically, the only red meat you’ll find on a fish is around their fins and tail, which are used almost constantly.
- Some fish, such as sharks and tuna, have red meat because they are fast swimmers and are migratory and thus almost always moving they use their muscles extensively and so they contain a lot more myoglobin than most other sea-life.
- For contrast, the white meat from chickens is made up of about .05% myoglobin with their thighs having about .2% myoglobin pork and veal contain about .2% myoglobin non-veal beef contains about 1%-2% of myoglobin, depending on age and muscle use.
- The USDA considers all meats obtained from livestock to be “red” because they contain more myoglobin than chicken or fish.
- Beef meat that is vacuum sealed, thus not exposed to oxygen, tends to be more of a purple shade. Once the meat is exposed to oxygen, it will gradually turn red over a span of 10-20 minutes as the myoglobin absorbs the oxygen.
- Beef stored in the refrigerator for more than 5 days will start to turn brown due to chemical changes in the myoglobin. This doesn’t necessarily mean it has gone bad, though with this length of unfrozen storage, it may have. Best to use your nose to tell for sure, not your eyes.
- Before you cook the red meat, the iron atom’s oxidation level is +2 and is bound to a dioxygen molecule (O2) with a red color as you cook it, this iron loses an electron and goes to a +3 oxidation level, and now coordinates with a water molecule (H2O). This process ends up turning the meat brown.
So, next time I’m barbecuing and my friend says, “The bloodier the better!” I’m going to correct him and say, “No. The more myoglobin, the better!”
On second thought, I may get punched!
Nitrite soaking of meat is one of the oldest myoglobin fixes. The procedure was discussed at length in several German chemistry journals as early as the 19th century. The structure of MbNO2 was one of the first bioinorganic structures solved and continues to this day to be a system of great interest. Nitrites and nitrosyls are important biological signaling molecules. While nitrite soaking of meat may improve their sale value, high concentrations of nitrites should be avoided. Foods like packaged pepperoni (while delicious) do contain very high amounts of nitrite. When proteins are exposed to heat, thermal degradation occurs and the free nitrite groups will attach, forming nitrosamines. Nitrosamines have been implicated in pancreatic cancer (among other types). Strangely, nitrite should theoretically kill any organism whose respiration is dependent on Mb/Hb systems, since nitrite is favorable to oxygen. This means that nitrite displaces oxygen from myoglobin and hemoglobin at a very fast rate. The reversal of this process, believed to be mediated by cd1 nitrite reductase is the subject of a great deal of study in biochemistry (on experimental, analytical and theoretical fronts).
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The body stores iron in the form of 2 proteins – ferritin (in men it accounts for about 70% of stored iron, in women 80%) and haemosiderin. The proteins are found in the liver, bone marrow, spleen and muscles. If too much iron is taken out of storage and not replaced through dietary sources, iron stores may become depleted and haemoglobin levels fall.
After a donation, most people's haemoglobin levels are back to normal after 6 to 12 weeks. This is why we ask donors to wait for a minimum of 12 weeks between donations (12 weeks for men and 16 weeks for women) to ensure that we don’t risk lowering your haemoglobin levels over the long term.
Show/hide words to know
Antigen: a molecule that can be recognized by the immune system. more
Cytotoxins: chemicals that kill cells.
Lymph system: the network of vessels, tissues, and organs that immune cells use to move through the body.
Molecule: a chemical structure that has two or more atoms held together by a chemical bond. Water is a molecule of two hydrogen atoms and one oxygen atom (H2O). more
Receptor: a molecule on the surface of a cell that responds to specific molecules and receives chemical signals sent by other cells.
Unique: one of a kind.
Red Blood Cell Production
Red blood cell (RBC) production (erythropoiesis) takes place in the bone marrow under the control of the hormone erythropoietin (EPO). Juxtaglomerular cells in the kidney produce erythropoietin in response to decreased oxygen delivery (as in anemia and hypoxia) or increased levels of androgens. In addition to erythropoietin , red blood cell production requires adequate supplies of substrates, mainly iron, vitamin B12, folate, and heme.
RBCs survive about 120 days. They then lose their cell membranes and are then largely cleared from the circulation by the phagocytic cells of the spleen and liver. Hemoglobin is broken down primarily by the heme oxygenase system with conservation (and subsequent reutilization) of iron, degradation of heme to bilirubin through a series of enzymatic steps, and reutilization of amino acids. Maintenance of a steady number of RBCs requires daily renewal of 1/120 of the cells immature RBCs (reticulocytes) are continually released and constitute 0.5 to 1.5% of the peripheral RBC population.
With aging, hemoglobin and hematocrit (Hct) decrease slightly, but not below normal values. In menstruating women, the most common cause of lower RBC levels is iron deficiency due to chronic blood loss resulting from menstruation.
About 95 percent of the dry weight of the red blood cell consists of hemoglobin, the substance necessary for oxygen transport. Hemoglobin is a protein a molecule contains four polypeptide chains (a tetramer), each chain consisting of more than 140 amino acids. To each chain is attached a chemical structure known as a heme group. Heme is composed of a ringlike organic compound known as a porphyrin, to which an iron atom is attached. It is the iron atom that reversibly binds oxygen as the blood travels between the lungs and the tissues. There are four iron atoms in each molecule of hemoglobin, which, accordingly, can bind four atoms of oxygen. The complex porphyrin and protein structure provides the proper environment for the iron atom so that it binds and releases oxygen appropriately under physiological conditions. The affinity of hemoglobin for oxygen is so great that at the oxygen pressure in the lungs about 95 percent of the hemoglobin is saturated with oxygen. As the oxygen tension falls, as it does in the tissues, oxygen dissociates from hemoglobin and is available to move by diffusion through the red cell membrane and the plasma to sites where it is used. The proportion of hemoglobin saturated with oxygen is not directly proportional to the oxygen pressure. As the oxygen pressure declines, hemoglobin gives up its oxygen with disproportionate rapidity, so that the major fraction of the oxygen can be released with a relatively small drop in oxygen tension. The affinity of hemoglobin for oxygen is primarily determined by the structure of hemoglobin, but it is also influenced by other conditions within the red cell, in particular the pH and certain organic phosphate compounds produced during the chemical breakdown of glucose, especially 2,3-diphosphoglycerate (see below Respiration).
Hemoglobin has a much higher affinity for carbon monoxide than for oxygen. Carbon monoxide produces its lethal effects by binding to hemoglobin and preventing oxygen transport. The oxygen-carrying function of hemoglobin can be disturbed in other ways. The iron of hemoglobin is normally in the reduced or ferrous state, in both oxyhemoglobin and deoxyhemoglobin. If the iron itself becomes oxidized to the ferric state, hemoglobin is changed to methemoglobin, a brown pigment incapable of transporting oxygen. The red cells contain enzymes capable of maintaining the iron in its normal state, but under abnormal conditions large amounts of methemoglobin may appear in the blood.
Sickle cell anemia is a serious and often fatal disease characterized by an inherited abnormality of hemoglobin. Persons who have sickle cell anemia are predominantly of African descent. The disease is caused by the mutation of a single gene that determines the structure of the hemoglobin molecule. Sickle hemoglobin differs from normal hemoglobin in that a single amino acid (glutamic acid) in one pair of the polypeptide chains has been replaced by another (valine). This single intramolecular change so alters the properties of the hemoglobin molecule that anemia and other effects are produced. Many other genetically determined abnormalities of hemoglobin have been identified. Some of these also produce diseases of several types. Study of the effects of altered structure of hemoglobin on its properties has greatly broadened knowledge of the structure-function relationships of the hemoglobin molecule.