The diameter of a typical human erythrocyte disk is 6–8 µm, much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups.
Adult humans have roughly 2–3 × 1013 red blood cells at any given time (women have about 4 million to 5 million erythrocytes per cubic millimeter (microliter) of blood and men about 5 million 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 a cubic millimeter of human blood. The red blood cells store collectively about 3.5 grams of iron, more than five times the iron stored by all the other tissues combined.
The process by which red blood cells are produced is called erythropoiesis. Erythrocytes are continuously being produced in the red bone marrow of large bones, at a rate of about 2 million per second. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), which is used for doping in sports. Just before and after leaving the bone marrow, they are known as reticulocytes which comprise about 1% of circulating red blood cells. Erythrocytes develop from stem cells through reticuloctyes to mature erythrocytes in about 7 days and live a total of about 120 days. The aging cells swell up to a sphere-like shape and are engulfed by phagocytes, destroyed and their materials are released into the blood. The main sites of destruction are the liver and the spleen. The heme constituent of hemoglobin is eventually excreted as bilirubin.
The blood types of humans are due to variations in surface glycoproteins of erythrocytes.
Red blood cells can be separated from blood plasma by centrifugation. During plasma donation, the red blood cells are pumped back into the body right away, and the plasma is collected. Some athletes have tried to improve their performance by doping their blood: First about 1 liter of their blood is extracted, then the red blood cells are isolated, frozen and stored, to be reinjected shortly before the competition. (Red blood cells can be conserved for 5 weeks at −78 °C.) This practice is hard to detect but may endanger the human cardiovascular system which is not equipped to deal with blood of the resulting higher viscosity.
Showing posts with label Blood. Show all posts
Showing posts with label Blood. Show all posts
Monday, January 8, 2007
Mammalian Erythrocytes
Erythrocytes in mammals are anucleate when mature, meaning that they don't have a cell nucleus and thus no DNA. In comparison, the erythrocytes of nearly all other vertebrates have nuclei; the only known exception is salamanders of the Batrachoseps genus. Mammalian erythrocytes also lose their other organelles including their mitochondria and produce energy by fermentation, via glycolysis of glucose followed by lactic acid production. Like most cell types, red cells do not have an insulin receptor and thus glucose uptake is not regulated by insulin. As a result of the lack of nucleus and organelles, the cells cannot produce new structural or repair proteins or enzymes and their lifespan is limited.
Mammalian erythrocytes are biconcave disks: flattened and depressed in the center, with a dumb-bell shaped cross section. This shape (as well as the loss of organelles and nucleus) optimizes the cell for the exchange of oxygen with its surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. Erythrocytes are circular, except in the camel family Camelidae, where they are oval.
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 that are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.
Mammalian erythrocytes are biconcave disks: flattened and depressed in the center, with a dumb-bell shaped cross section. This shape (as well as the loss of organelles and nucleus) optimizes the cell for the exchange of oxygen with its surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. Erythrocytes are circular, except in the camel family Camelidae, where they are oval.
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 that are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.
Red Blood Cells
Red blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood.
Red blood cells are also known as RBCs or erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte nowadays translated as "cell"). A schistocyte is a red blood cell undergoing fragmentation, or a fragmented part of a red blood cell.
Red blood cells are also known as RBCs or erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte nowadays translated as "cell"). A schistocyte is a red blood cell undergoing fragmentation, or a fragmented part of a red blood cell.
Blood Oxygenation
Blood oxygenation is measured in several ways, but the most important measure is the hemoglobin (Hb) saturation percentage. This is a non-linear (sigmoidal) function of the partial pressure of oxygen. About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at normal pressure is chemically combined with the Hb. Only 1.5% is physically dissolved in the other blood liquids and not connected to Hb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species .
With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.
Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.
Under normal conditions in humans at rest, haemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."
A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 20% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to extract as much oxygen as possible from this sparse supply.
Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes carboxyhemoglobin which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a cigarette (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.
With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.
Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.
Under normal conditions in humans at rest, haemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."
A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 20% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to extract as much oxygen as possible from this sparse supply.
Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes carboxyhemoglobin which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a cigarette (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.
Production and Degradation of Blood
Blood cells are produced in the bone marrow; the process is termed hematopoiesis. The proteinaceous component (including clotting proteins) is produced overwhelmingly in the liver, while hormones are produced by the endocrine glands and the watery fraction regulated by the hypothalamus and maintained by the kidney and indirectly by the gut.
Blood cells are degraded by the spleen and the Kupffer cells in the liver. The liver also clears some proteins, lipids and amino acids. The kidney actively secretes waste products into the urine. Erythrocytes usually live up to 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.
Blood cells are degraded by the spleen and the Kupffer cells in the liver. The liver also clears some proteins, lipids and amino acids. The kidney actively secretes waste products into the urine. Erythrocytes usually live up to 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.
Blood
Blood is a highly specialized circulating tissue consisting of several types of cells suspended in a fluid medium known as plasma. The cellular constituents are: red blood cells, which carry respiratory gases and give it its red color because they contain hemoglobin (an iron containing protein that binds oxygen in the lungs and transports it to tissues in the body), white blood cells (leukocytes), which fight disease, and platelets, cell fragments which play an important part in the clotting of the blood. Medical terms related to blood often begin with hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood." Anatomically, blood is considered a connective tissue from both its origin in the bones and its function.
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