Erythrocytes consist mainly of hemoglobin, a complex metalloprotein containing heme groups whose iron atoms temporarily bind to oxygen molecules (O2) in the lungs or gills and release them throughout the body. Oxygen can easily diffuse through the red blood cell's cell membrane. Hemoglobin in the erythrocytes also carries some of the waste product carbon dioxide back from the tissues; most waste carbon dioxide, however, is transported back to the pulmonary capillaries of the lungs asbicarbonate (HCO3?) dissolved in the blood plasma. Myoglobin, a compound related to hemoglobin, acts to store oxygen in muscle cells.
The color of erythrocytes is due to the heme group of hemoglobin. The blood plasma alone is straw-colored, but the red blood cells change color depending on the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet, and when oxygen has been released the resulting deoxyhemoglobin is of a dark red burgundy color. However, blood can appear bluish when seen through the vessel wall and skin. Pulse oximetry takes advantage of the hemoglobin color change to directly measure the arterial blood oxygen saturation using colorimetric techniques. Hemoglobin also has a very high affinity for carbon monoxide, forming carboxyhemoglobin which is a very bright red in color. Flushed, confused patients with a saturation reading of 100% on pulse oximetry are sometimes found to be suffering from carbon monoxide poisoning.
The sequestration of oxygen-carrying proteins inside specialized cells (as opposed to oxygen carriers being dissolved in body fluid) was an important step in the evolution of vertebrates as it allows for less viscous blood, higher concentrations of oxygen, and better diffusion of oxygen from the blood to the tissues. The size of erythrocytes varies widely among vertebrate species; erythrocyte width is on average about 25% larger than capillary diameter, and it has been hypothesized that this improves the oxygen transfer from erythrocytes to tissues.
The only known vertebrates without erythrocytes are the crocodile icefishes (family Channichthyidae); they live in very oxygen-rich cold water and transport oxygen freely dissolved in their blood. While they do not use hemoglobin anymore, remnants of hemoglobin genes can be found in their genome.
Erythrocytes in mammals are anucleate when mature, meaning that they lack a cell nucleus. In comparison, the erythrocytes of other vertebrates have nuclei; the only known exceptions are salamanders of the Batrachoseps genus and fish of the Maurolicus genus with closely related species.
The elimination of the nucleus in vertebrate erythrocytes has been offered as an explanation for the subsequent accumulation of non-coding DNA in the genome.The argument runs as follows: Efficient gas transport requires erythrocytes to pass through very narrow capillaries, and this constrains their size. In the absence of nuclear elimination, the accumulation of repeat sequences is constrained by the volume occupied by the nucleus, which increases with genome size.
When erythrocytes undergo shear stress in constricted vessels, they release ATP, which causes the vessel walls to relax and dilate so as to promote normal blood flow.
When their hemoglobin molecules are deoxygenated, erythrocytes release S-nitrosothiols, which also act to dilate blood vessels, thus directing more blood to areas of the body depleted of oxygen.
Erythrocytes can also synthesize nitric oxide enzymatically, using L-arginine as substrate, as do endothelial cells. Exposure of erythrocytes to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide, which may contribute to the regulation of vascular tonus.
Erythrocytes can also produce hydrogen sulfide, a signalling gas that acts to relax vessel walls. It is believed that the cardioprotective effects of garlic are due to erythrocytes converting its sulfur compounds into hydrogen sulfide.
Erythrocytes also play a part in the body's immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals, which break down the pathogen's cell wall and membrane, killing it.
Mammalian erythrocytes are unique among the vertebrates as they are non-nucleated cells in their mature form. These cells have nuclei during early phases of erythropoiesis, but extrude them during development as they mature in order to provide more space for hemoglobin. The enucleated erythrocytes, called reticulocytes, go on to lose all other cellular organelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum.
As a result of not containing mitochondria, these cells use none of the oxygen they transport; instead they produce the energy carrier ATP by theglycolysis of glucose and lactic acid fermentation on the resulting pyruvate.
Because of the lack of nuclei and organelles, mature red blood cells do not contain DNA and cannot synthesize any RNA, and consequently cannot divide and have limited repair capabilities. The inability to carry out protein synthesis means that no virus can evolve to target mammalian red blood cells. However, infection with parvoviruses (such as human parvovirus B19) can affect erythroid precursors, as recognized by the presence of giantpronormoblasts with viral particles and inclusion bodies, thus temporarily depleting the blood of reticulocytes and causing anemia.
Mammalian erythrocytes are typically shaped as biconcave disks: flattened and depressed in the center, with a dumbbell-shaped cross section, and atorus-shaped rim on the edge of the disk. This distinctive biconcave shape optimises the ?ow properties of blood in the large vessels, such as maximization of laminar flow and minimization of platelet scatter, which suppresses their atherogenic activity in those large vessels. However, there are some exceptions concerning shape in the artiodactyl order (even-toed ungulates including cattle, deer, and their relatives), which displays a wide variety of bizarre erythrocyte morphologies: small and highly ovaloid cells in llamas and camels (family Camelidae), tiny spherical cells in mouse deer (family Tragulidae), and cells which assume fusiform, lanceolate, crescentic, and irregularly polygonal and other angular forms in red deer and wapiti (family Cervidae). Members of this order have clearly evolved a mode of red blood cell development substantially different from the mammalian norm. Overall, mammalian erythrocytes are remarkably flexible and deformable so as to squeeze through tiny capillaries, as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load.
In large blood vessels, red blood cells sometimes occur as a stack, flat side next to flat side. This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation.
The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.
A typical human erythrocyte has a disk diameter of approximately 6.2–8.2 µm and a thickness at the thickest point of 2–2.5 µm and a minimum thickness in the centre of 0.8–1 µm, being much smaller than most other human cells. These cells have an average volume of about 90 fL with a surface of about 136 ?m2, and can swell up to a sphere shape containing 150 fL, without membrane distension.
Adult humans have roughly 20–30 × 1012 (20–30 trillion) red blood cells at any given time, comprising approximately one quarter of the total human body cell number (women have about 4 to 5 million erythrocytes per microliter (cubic millimeter) of blood and men about 5 to 6 million; people living at high altitudes with low oxygen tension will have more). Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets in each microliter of human blood.
Human red blood cells take on average 20 seconds to complete one cycle of circulation.
As red blood cells contain no nucleus, protein biosynthesis is currently assumed to be absent in these cells, although a recent study indicates the presence of all the necessary biomachinery in the cells to do so.
The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin. Each human red blood cell contains approximately 270 million of thesehemoglobin biomolecules, each carrying four heme groups; hemoglobin comprises about a third of the total cell volume. This protein is responsible for the transport of more than 98% of the oxygen (the remaining oxygen is carried dissolved in the blood plasma). The red blood cells of an average adult human male store collectively about 2.5 grams of iron, representing about 65% of the total iron contained in the body. (See Human iron metabolism.)
Human erythrocytes are produced through a process named erythropoiesis, developing from committed stem cells to mature erythrocytes in about 7 days. When matured, in a healthy individual these cells live in blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant). At the end of their lifespan, they become senescent, and are removed from circulation. In many chronic diseases, the lifespan of the erythrocytes is markedly reduced (e.g. patients requiringhaemodialysis).
Erythropoiesis is the development process by which new erythrocytes are produced; it lasts about 7 days. Through this process erythrocytes are continuously produced in the red bone marrow of large bones, at a rate of about 2 million per second in a healthy adult. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), synthesised by the kidney. Just before and after leaving the bone marrow, the developing cells are known as reticulocytes; these comprise about 1% of circulating red blood cells.
The functional lifetime of an erythrocyte is about 100–120 days, during which time the erythrocytes are continually moved by the blood flow push (in arteries), pull (inveins) and a combination of the two as they squeeze through microvessels such as capillaries.
The aging erythrocyte undergoes changes in its plasma membrane, making it susceptible to selective recognition by macrophages and subsequent phagocytosis in themononuclear phagocyte system (spleen, liver and lymph nodes), thus removing old and defective cells and continually purging the blood. This process is termed eryptosis, erythrocyte programmed cell death. This process normally occurs at the same rate of production by erythropoiesis, balancing the total circulating red blood cell count. Eryptosis is increased in a wide variety of diseases includingsepsis, haemolytic uremic syndrome, malaria, sickle cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase deficiency, phosphate depletion, iron deficiency and Wilson's disease. Eryptosis can be elicited by osmotic shock, oxidative stress, energy depletion as well as a wide variety of endogenous mediators and xenobiotics. Excessive eryptosis is observed in erythrocytes lacking the cGMP-dependent protein kinase type I or the AMP-activated protein kinase AMPK. Inhibitors of eryptosis include erythropoietin, nitric oxide, catecholamines and high concentrations of urea.
Much of the resulting breakdown products are recirculated in the body. The heme constituent of hemoglobin are broken down into Fe3+ and biliverdin. The biliverdin is reduced to bilirubin, which is released into the plasma and recirculated to the liver bound to albumin. The iron is released into the plasma to be recirculated by a carrier protein called transferrin. Almost all erythrocytes are removed in this manner from the circulation before they are old enough to hemolyze. Hemolyzed hemoglobin is bound to a protein in plasma called haptoglobin, which is not excreted by the kidney.
The membrane of the red blood cell plays many roles that aid in regulating their surface deformability, flexibility, adhesion to other cells and immune recognition. These functions are highly dependent on its composition, which defines its properties. The red blood cell membrane is composed of 3 layers: the glycocalyx on the exterior, which is rich in carbohydrates; the lipid bilayer which contains manytransmembrane proteins, besides its lipidic main constituents; and the membrane skeleton, a structural network of proteins located on the inner surface of the lipid bilayer. Half of the membrane mass in human and most mammalian erythrocytes are proteins. The other half are lipids, namely phospholipids and cholesterol.
The erythrocyte cell membrane comprises a typical lipid bilayer, similar to what can be found in virtually all human cells. Simply put, this lipid bilayer is composed of cholesterol and phospholipids in equal proportions by weight. The lipid composition is important as it defines many physical properties such as membrane permeability and fluidity. Additionally, the activity of many membrane proteins is regulated by interactions with lipids in the bilayer.
Unlike cholesterol, which is evenly distributed between the inner and outer leaflets, the 5 major phospholipids are asymmetrically disposed, as shown below:
This asymmetric phospholipid distribution among the bilayer is the result of the function of several energy-dependent and energy-independent phospholipidtransport proteins. Proteins called “Flippases” move phospholipids from the outer to the inner monolayer, while others called “floppases” do the opposite operation, against a concentration gradient in an energy dependent manner. Additionally, there are also “scramblase” proteins that move phospholipids in both directions at the same time, down their concentration gradients in an energy independent manner. There is still considerable debate ongoing regarding the identity of these membrane maintenance proteins in the red cell membrane.
The maintenance of an asymmetric phospholipid distribution in the bilayer (such as an exclusive localization of PS and PIs in the inner monolayer) is critical for the cell integrity and function due to several reasons:
- Macrophages recognize and phagocytose red cells that expose PS at their outer surface. Thus the confinement of PS in the inner monolayer is essential if the cell is to survive its frequent encounters with macrophages of the reticuloendothelial system, especially in the spleen.
- Premature destruction of thallassemic and sickle red cells has been linked to disruptions of lipid asymmetry leading to exposure of PS on the outer monolayer.
- An exposure of PS can potentiate adhesion of red cells to vascular endothelial cells, effectively preventing normal transit through the microvasculature. Thus it is important that PS is maintained only in the inner leaflet of the bilayer to ensure normal blood flow in microcirculation.
- Both PS and phosphatidylinositol-4,5-bisphosphate (PIP2) can regulate membrane mechanical function, due to their interactions with skeletal proteins such as spectrin and protein 4.1R. Recent studies have shown that binding of spectrin to PS promotes membrane mechanical stability. PIP2 enhances the binding of protein band 4.1R to glycophorin C but decreases its interaction with protein band 3, and thereby may modulate the linkage of the bilayer to the membrane skeleton.
The presence of specialized structures named "lipid rafts" in the erythrocyte membrane have been described by recent studies. These are structures enriched in cholesterol and sphingolipids associated with specific membrane proteins, namely flotillins, stomatins (band 7), G-proteins, and ?-adrenergic receptors. Lipid rafts that have been implicated in cell signaling events in nonerythroid cells have been shown in erythroid cells to mediate ?2-adregenic receptor signaling and increase cAMP levels, and thus regulating entry of malarial parasites into normal red cells.
The proteins of the membrane skeleton are responsible for the deformability, flexibility and durability of the red blood cell, enabling it to squeeze through capillaries less than half the diameter of the erythrocyte (7–8 ?m) and recovering the discoid shape as soon as these cells stop receiving compressive forces, in a similar fashion to an object made of rubber.
There are currently more than 50 known membrane proteins, which can exist in a few hundred up to a million copies per erythrocyte. Approximately 25 of these membrane proteins carry the various blood group antigens, such as the A, B and Rh antigens, among many others. These membrane proteins can perform a wide diversity of functions, such as transporting ions and molecules across the red cell membrane, adhesion and interaction with other cells such as endothelial cells, as signaling receptors, as well as other currently unknown functions. The blood types of humans are due to variations in surface glycoproteins of erythrocytes. Disorders of the proteins in these membranes are associated with many disorders, such as hereditary spherocytosis, hereditary elliptocytosis, hereditary stomatocytosis, and paroxysmal nocturnal hemoglobinuria.
The red blood cell membrane proteins organized according to their function:
Structural role – The following membrane proteins establish linkages with skeletal proteins and may play an important role in regulating cohesion between the lipid bilayer and membrane skeleton, likely enabling the red cell to maintain its favorable membrane surface area by preventing the membrane from collapsing (vesiculating).
- Ankyrin-based macromolecular complex – proteins linking the bilayer to the membrane skeleton through the interaction of their cytoplasmic domains with Ankyrin.
- Band 3 – also assembles various glycolytic enzymes, the presumptive CO2 transporter, and carbonic anhydrase into a macromolecular complex termed a "metabolon," which may play a key role in regulating red cell metabolism and ion and gas transport function);
- RhAG – also involved in transport, defines associated unusual blood group phenotype Rhmod.
- Protein 4.1R-based macromolecular complex – proteins interacting with Protein 4.1R.
- Protein 4.1R – weak expression of Gerbich antigens;
- Glycophorin C and D – glycoprotein, defines Gerbich Blood Group;
- XK – defines the Kell Blood Group and the Mcleod unusual phenotype (lack of Kx antigen and greatly reduced expression of Kell antigens);
- RhD/RhCE – defines Rh Blood Group and the associated unusual blood group phenotype Rhnull;
- Duffy protein – has been proposed to be associated with chemokine clearance;
- Adducin – interaction with band 3;
- Dematin- interaction with the Glut1 glucose transporter.
Blood diseases involving the red blood cells include:
- Anemias (or anaemias) are diseases characterized by low oxygen transport capacity of the blood, because of low red cell count or some abnormality of the red blood cells or the hemoglobin.
- Iron deficiency anemia is the most common anemia; it occurs when the dietary intake or absorption of iron is insufficient, and hemoglobin, which contains iron, cannot be formed
- Sickle-cell disease is a genetic disease that results in abnormal hemoglobin molecules. When these release their oxygen load in the tissues, they become insoluble, leading to mis-shaped red blood cells. These sickle shaped red cells are less deformable and viscoelastic meaning that they have become rigid and can cause blood vessel blockage, pain, strokes, and other tissue damage.
- Thalassemia is a genetic disease that results in the production of an abnormal ratio of hemoglobin subunits.
- Hereditary spherocytosis syndromes are a group of inherited disorders characterized by defects in the red blood cell's cell membrane, causing the cells to be small, sphere-shaped, and fragile instead of donut-shaped and flexible. These abnormal red blood cells are destroyed by the spleen. Several other hereditary disorders of the red blood cell membrane are known.
- Hemolysis is the general term for excessive breakdown of red blood cells. It can have several causes and can result in hemolytic anemia.
- The malaria parasite spends part of its life-cycle in red blood cells, feeds on their hemoglobin and then breaks them apart, causing fever. Both sickle-cell disease and thalassemia are more common in malaria areas, because these mutations convey some protection against the parasite.
- Polycythemias (or erythrocytoses) are diseases characterized by a surplus of red blood cells. The increased viscosity of the blood can cause a number of symptoms.
- In polycythemia vera the increased number of red blood cells results from an abnormality in the bone marrow.
Several blood tests involve red blood cells, including the RBC count (the number of red blood cells per volume of blood), the hematocrit (percentage of blood volume occupied by red blood cells), and the erythrocyte sedimentation rate. Many diseases involving red blood cells are diagnosed with a blood film (or peripheral blood smear), where a thin layer of blood is smeared on a microscope slide. The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation.High red blood cell count may be caused by low oxygen levels, kidney disease or other problems.
Low oxygen levels
Your body may increase red blood cell production to compensate for any condition that results in low oxygen levels, including:
- Congenital heart disease in adults
- Heart failure
- A condition present at birth that reduces the oxygen-carrying capacity of red blood cells (hemoglobinopathy)
- High altitudes
- COPD (chronic obstructive pulmonary disease) and other lung diseases
- Pulmonary fibrosis
- Sleep apnea
- Nicotine dependence(smoking)
Certain drugs stimulate the production of red blood cells, including:
- Anabolic steroids
- Blood doping (transfusion)
- Injections of a protein (erythropoietin) that enhances red blood cell production
Increased red blood cell concentration
- Dehydration (If the liquid component of the blood (plasma) is decreased, as in dehydration, the red blood cell count increases. This is due to the red blood cells becoming more concentrated. The actual number of red blood cells stays the same.)
Rarely, in some kidney cancers and sometimes after kidney transplants, the kidneys might produce too much erythropoietin. This enhances red blood cell production.
Bone marrow overproduction
- Polycythemia vera
- Other myeloproliferative disorders
Normal RBC range is:
- Male: 4.7 to 6.1 million cells per microliter (cells/mcL)
- Female: 4.2 to 5.4 million cells/mcL
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
Higher-than-normal numbers of RBCs may be due to:
Your RBC count will increase for several weeks when you move to a higher altitude.
Drugs that can increase the RBC count include:
Lower-than-normal numbers of RBCs may be due to:
- Bone marrow failure (for example, from radiation, toxins, or tumor)
- Deficiency of a hormone called erythropoietin (caused by kidney disease)
- RBC destruction (hemolysis) due to transfusion, blood vessel injury, or other cause
- Bone marrow cancer called multiple myeloma
- Nutrition deficiencies of iron, copper, folic acid, vitamin B6, or vitamin B12
Drugs that can decrease the RBC count include:
- Chemotherapy drugs
- QuinidineRed blood cells: The blood cells that carry oxygen. Red cells containhemoglobin and it is the hemoglobin which permits them to transport oxygen(and carbon dioxide). Hemoglobin, aside from being a transport molecule, is a pigment. It gives the cells their red color (and their name).
The abbreviation for red blood cells is RBCs. Red blood cells are sometime simply called red cells. They are also called erythrocytes or, rarely today, red blood corpuscles.
What Are Red Blood Cells?
Red blood cells play an important role in your health by carrying fresh oxygen throughout the body. The oxygen gives your blood its bright red color.
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Red blood cells are round with a flattish, indented center, like doughnuts without a hole. Your health care provider can check on the size, shape, and health of your red blood cells using tests, such as the complete blood count screening.
Red blood cells at work
Hemoglobin is the protein inside red blood cells that carries oxygen. Red blood cells also remove carbon dioxide from your body, transporting it to the lungs for you to exhale.
Red blood cells are made inside your bones, in the bone marrow. They typically live for about 120 days, and then they die.
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Nutrition and red blood cells
Foods rich in iron help you maintain healthy red blood cells. Vitamins are also necessary to build healthy red blood cells. These include vitamin E, found in foods such as dark green vegetables, nuts and seeds, mango, and avocados; vitamins B2, B12, and B3, found in foods such as eggs, whole grains, and bananas; and folate, available in fortified cereals, dried beans and lentils, orange juice, and green leafy vegetables.
Illnesses of the red blood cells
Most people don't think about their red blood cells unless they have a disease that affects these cells. Problems with red blood cells can be caused by illnesses or a lack of iron or vitamins in your diet. Some diseases of the red blood cells are inherited.
Diseases of the red blood cells include many types of anemia, a condition in which your body can't produce enough normal red blood cells to carry sufficient oxygen throughout the body. People with anemia may have red blood cells that have an unusual shape or that look normal, larger than normal, or smaller than normal.
Symptoms of anemia include tiredness, irregular heartbeats, pale skin, feeling cold, and, in severe cases, heart failure. Children who don't have enough healthy red blood cells grow and develop more slowly than other children. These symptoms demonstrate how important red blood cells are to your daily life.
These are common types of anemia:
Iron-deficiency anemia. If you don't have enough iron in your body, your body won't be able to make the hemoglobin that helps red blood cells carry oxygen. Iron-deficiency anemia is the most common form of anemia. Among the causes of iron deficiency are a diet low in iron, a sudden loss of blood, a chronic loss of blood (such as from heavy menstrual periods), or the inability to absorb enough iron from food.
Sickle cell anemia. In this inherited disease, the red blood cells are shaped like half moons rather than the normal indented circles. This change in shape can make the cells "sticky" and unable to flow smoothly through blood vessels, causing a blockage in blood flow. This blockage may cause acute or chronic pain and can also lead to infection or organ damage. Sickle cells die much more quickly than normal blood cells–in about 10 to 20 days instead of 120 days–causing a shortage of red blood cells.
Normocytic anemia. This type of anemia occurs when your red blood cells are normal in shape and size, but you don't have enough of them to meet your body's needs. Diseases that cause this type of anemia are usually long-term conditions, like kidney disease, cancer, or rheumatoid arthritis.
Hemolytic anemia. This type of anemia occurs when red blood cells are destroyed by an abnormal process in your body before their lifespan is over. As a result, your body doesn't have enough red blood cells to function, and your bone marrow cannot make enough to keep up with demand.
Fanconi anemia. This is a rare inherited disorder in which your bone marrow isn't able to make enough of any of the components of blood, including red blood cells. Children born with this disorder often have serious birth defects because of the problems with their blood and may develop leukemia.