Red blood cell production - Health Video: MedlinePlus Medical Encyclopedia Red blood cell production URL of this page: //medlineplus.gov/ency/anatomyvideos/000104.htm Red blood cell production What's this? Play health video with audio description: //medlineplus.gov/ency/videos/mov/200028_eng_ad.mp4 Overview Blood carries various substances that must be brought to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body’s tissues in exchange for carbon dioxide, which is carried to and eliminated by the lungs. Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow called hemocytoblasts give rise to all of the formed elements in blood. If a hemocytoblast commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell. The formation of a red blood cell from hemocytoblast takes about 2 days. The body makes about two million red blood cells every second. Blood is made up of both cellular and liquid components.

If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other. Blood consists of 45% red blood cells, less than 1% white blood cells and platelets, and 55% plasma. Review Date 1/31/2016 Updated by: Laura J. Martin, MD, MPH, ABIM Board Certified in Internal Medicine and Hospice and Palliative Medicine, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team. Related MedlinePlus Health Topics White Blood Cells - How Blood Works | HowStuffWorks White Blood Cells Prev Next White blood cells (WBCs), or leukocytes, are a part of the immune system and help our bodies fight infection. They circulate in the blood so that they can be transported to an area where an infection has developed. In a normal adult body there are 4,000 to 10,000 (average 7,000) WBCs per microliter of blood.

When the number of WBCs in your blood increases, this is a sign of an infection somewhere in your body. Here are the six main types of WBCs and the average percentage of each type in the blood: Neutrophils - 58 percent Monocytes - 4 percent Lymphocytes - 4 percent Most WBCs (neutrophils, eosinophils, basophils and monocytes) are formed in the bone marrow. Neutrophils, eosinophils and basophils are also called granulocytes because they have granules in their cells that contain digestive enzymes. Basophils have purple granules, eosinophils have orange-red granules and neutrophils have a faint blue-pink color. When a granulocyte is released into the blood, it stays there for an average of four to eight hours and then goes into the tissues of the body, where it lasts for an average of four to five days. During a severe infection, these times are often shorter. Neutrophils are the one of the body's main defenses against bacteria. They kill bacteria by actually ingesting them (this is called phagocytosis).

Neutrophils can phagocytize five to 20 bacteria in their lifetime. Neutrophils have a multi-lobed, segmented or polymorphonuclear nucleus and so are also called PMNs, polys or segs. Bands are immature neutrophils that are seen in the blood. When a bacterial infection is present, an increase of neutrophils and bands are seen. Eosinophils kill parasites and have a role in allergic reactions . Basophils are not well understood, but they function in allergic reactions. They release histamine (which causes blood vessels to leak and attracts WBCs) and heparin (which prevents clotting in the infected area so that the WBCs can reach the bacteria). Monocytes enter the tissue, where they become larger and turn into macrophages. There they can phagocytize bacteria (up to 100 in their lifetime) throughout the body. These cells also destroy old, damaged and dead cells in the body. Macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine. The system of macrophages scattered throughout the body is called the reticuloendothelial system.

Monocytes stay in the blood for an average of 10 to 20 hours and then go into the tissues, where they become tissue macrophages and can live for months to years. Neutrophils and monocytes use several mechanisms to get to and kill invading organisms. They can squeeze through openings in blood vessels by a process called diapedesis. They move around using ameboid motion. They are attracted to certain chemicals produced by the immune system or by bacteria and migrate toward areas of higher concentrations of these chemicals. This is called chemotaxis. They kill bacteria by a process called phagocytosis, in which they completely surround the bacteria and digest them with digestive enzymes. In the next section, we'll take a closer look at lymphocytes and platelets. Up Next Blood Basics Blood Basics Home For Patients Blood Basics Blood is a specialized body fluid. It has four main components: plasma, red blood cells, white blood cells, and platelets.

Blood has many different functions, including: transporting oxygen and nutrients to the lungs and tissues forming blood clots to prevent excess blood loss carrying cells and antibodies that fight infection bringing waste products to the kidneys and liver, which filter and clean the blood regulating body temperature The blood that runs through the veins, arteries, and capillaries is known as whole blood, a mixture of about 55 percent plasma and 45 percent blood cells. About 7 to 8 percent of your total body weight is blood. An average-sized man has about 12 pints of blood in his body, and an average-sized woman has about 9 pints. Jump To: The Components of Blood and Their Importance Many people have undergone blood tests or donated blood , but hematology - the study of blood - encompasses much more than this. Doctors who specialize in hematology (hematologists) are leading the many advances being made in the treatment and prevention of blood diseases. If you or someone you care about is diagnosed with a blood disorder, your primary care physician may refer you to a hematologist for further testing and treatment. back to top Plasma The liquid component of blood is called plasma, a mixture of water, sugar, fat, protein, and salts.

The main job of the plasma is to transport blood cells throughout your body along with nutrients, waste products, antibodies, clotting proteins, chemical messengers such as hormones, and proteins that help maintain the body's fluid balance. back to top Red Blood Cells (also called erythrocytes or RBCs) Known for their bright red color, red cells are the most abundant cell in the blood, accounting for about 40-45 percent of its volume. The shape of a red blood cell is a biconcave disk with a flattened center - in other words, both faces of the disc have shallow bowl-like indentations (a red blood cell looks like a donut). Production of red blood cells is controlled by erythropoietin, a hormone produced primarily by the kidneys. Red blood cells start as immature cells in the bone marrow and after approximately seven days of maturation are released into the bloodstream. Unlike many other cells, red blood cells have no nucleus and can easily change shape, helping them fit through the various blood vessels in your body.

However, while the lack of a nucleus makes a red blood cell more flexible, it also limits the life of the cell as it travels through the smallest blood vessels, damaging the cell's membranes and depleting its energy supplies. The red blood cell survives on average only 120 days. Red cells contain a special protein called hemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their color from the hemoglobin. The percentage of whole blood volume that is made up of red blood cells is called the hematocrit and is a common measure of red blood cell levels. back to top Where Do Blood Cells Come From? Blood cells develop from hematopoietic stem cells and are formed in the bone marrow through the highly regulated process of hematopoiesis. Hematopoietic stem cells are capable of transforming into red blood cells, white blood cells, and platelets. These stem cells can be found circulating in the blood and bone marrow in people of all ages, as well as in the umbilical cords of newborn babies.

Stem cells from all three sources may be used to treat a variety of diseases, including leukemia, lymphoma, bone marrow failure, and various immune disorders. back to top Where Can I Find More Information? If you are interested in learning more about blood diseases and disorders, here are a few other resources that may be of some help: Platelets Platelets In this section we introduce platelets themselves. Platelets are the smallest of the three major types of blood cells. Platelets are only about 20% of the diameter of red blood cells. The normal platelet count is 150,000-350,000 per microliter of blood, but since platelets are so small, they make up just a tiny fraction of the blood volume. The principal function of platelets is to prevent bleeding. Red blood cells are the most numerous blood cell, about 5,000,000 per microliter. Red blood cells make up about 40% of our total blood volume, a measure called the hematocrit. Their color is caused by hemoglobin, which accounts for nearly all of the red cell volume.

Hemoglobin is the critical protein that transports oxygen from our lungs to the tissues. Red cells are normally shaped as round, biconcave discs. With microscopic examination, they look like a red or orange tire with a thin, almost transparent center. White blood cells are the largest of the blood cells but also the fewest. There are only 5,000 to 10,000 white blood cells per microliter. There are several different types of white cells but all are related to immunity and fighting infection. Platelet Production Platelets are produced in the bone marrow, the same as the red cells and most of the white blood cells. Platelets are produced from very large bone marrow cells called megakaryocytes. As megakaryocytes develop into giant cells, they undergo a process of fragmentation that results in the release of over 1,000 platelets per megakaryocyte. The dominant hormone controlling megakaryocyte development is thrombopoietin (often abbreviated as TPO). Platelet Structure Platelets are actually not true cells but merely circulating fragments of cells. But even though platelets are merely cell fragments, they contain many structures that are critical to stop bleeding.

They contain proteins on their surface that allow them to stick to breaks in the blood vessel wall and also to stick to each other. They contain granules that can secrete other proteins required for creating a firm plug to seal blood vessel breaks. Also platelets contain proteins similar to muscle proteins that allow them to change shape when they become sticky. The pictures above show normal platelets on the left. They are shaped like a plate, therefore their name. When platelets are stimulated by a break in the blood vessel wall they change shape as shown in the other three pictures. They become round and extend long filaments. They may even look like an octopus, with long tentacles reaching out to make contact with the broken blood vessel wall or with other platelets. With these long filaments, platelets then form a plug to seal the broken blood vessel. Platelet Function In addition to being the smallest blood cell, platelets are also the lightest. Therefore they are pushed out from the center of flowing blood to the wall of the blood vessel. There they roll along the surface of the vessel wall, which is lined by cells called endothelium. The endothelium is a very special surface, like Teflon, that prevents anything from sticking to it.

However when there is an injury or cut, and the endothelial layer is broken, the tough fibers that surround a blood vessel are exposed to the liquid flowing blood. It is the platelets that react first to injury. The tough fibers surrounding the vessel wall, like an envelop, attract platelets like a magnet, stimulate the shape change that is shown in the pictures above, and platelets then clump onto these fibers, providing the initial seal to prevent bleeding, the leak of red blood cells and plasma through the vessel injury. The color photograph is a microscopic picture of a drop of blood spread out onto a glass slide. The magnification is not as high as the pictures above, so the platelets seem very small. It can be seen that as the platelets touch the glass, they begin to stick together forming a long string. This illustrates the basic function of platelets, to stick to any foreign surface and then to stick together. The red blood cells in this picture are normal, with their round shape and their thin center. Disorders of Platelet Function The most common disorder of platelet function is caused by aspirin. Aspirin blocks one of the steps required for platelets to stick together.

This effect of aspirin is what makes it an effective treatment for patients who have disorders of blood clotting or thrombosis. For example, a person coming to an emergency room with severe chest pain and a suspected heart attack is immediately given aspirin. This prevents some of the platelet clumping which can obstruct the blood flow to the heart. Aspirin is an effective medicine to prevent these clots, but it does not totally paralyze the platelet. Therefore many people take daily aspirin and have no problems with bleeding. However aspirin can be potentially dangerous in patients who already have a risk for bleeding, such as boys with hemophilia or patients who have too few platelets and depend on each platelet having full function. Disorders of Platelets Number: Too Many Platelets Rare conditions result in the bone marrow producing too many platelets, sometimes as many as one million or two million per microliter. In some of these patients, there are increased risks for blood clots, but many patients with these disorders have no problems. Disorders of Platelet Number: Too Few Platelets Disorders with low platelet counts are called thrombocytopenias, a term derived from an old name for platelets, “thrombocytes”.

This name describes the platelets as the cells (“cytes” is a word for cell) that contribute to thrombosis, or blood clotting. The last part of the word, “-penia”, refers to too few cells. Thrombocytopenia is the principal focus of this website. Thrombocytopenia can be caused by failure of the bone marrow to produce normal numbers of platelets. Bone marrow failure has multiple causes. These are not discussed on this website. Thrombocytopenia can also be caused by increased destruction of platelets once they are produced and released into the circulating blood. These disorders are the focus of this website. They are described briefly here and in more detail in their specific sections on this website. Immune Thrombocytopenia (ITP) - This disorder, described in its own section on this website, is caused both by increased platelet destruction and also decreased bone marrow platelet production. These problems are caused by autoantibodies. Antibodies are proteins normally made by a type of white blood cells to react with and defend against foreign materials. For example, antibodies are normally formed to bacteria and viruses, and help with the healing process.

Antibodies are normally stimulated by immunization and vaccination, to prevent infections. Antibodies normally react with organ transplants and attempt to reject these transplants. Autoantibodies are abnormal. These occur when antibody-producing cells receive mixed signals, and identify a normal body tissue as foreign and try to reject it. The term, autoantibody, means an antibody that reacts with the person herself, not foreign material. In ITP, platelets are recognized by the immune system as “foreign” cells and autoantibodies are made against platelets. The autoantibodies destroy the platelets rapidly after they are produced. The autoantibodies also react with the bone marrow megakaryocytes and inhibit platelet production. Drug-induced thrombocytopenia (DITP) - This disorder is described in its own section on this website. Almost all medications can cause bad allergic reactions in sensitive people, but these reactions are rare. Most commonly, drugs may cause a red rash in sensitive people. Drugs can also cause serious reactions with blood platelets.

In these reactions, drugs stick to the platelet surface, and this combination of the drug bound to the platelet can be recognized by the body as a foreign substance and the body then makes an antibody to the drug-coated platelets, and all platelets can be destroyed. When the drug is stopped, platelet destruction is stopped, and the platelet count returns to normal. But these drug-dependent antibodies can persist for many years, the same way that protective antibodies persist for many years after immunizations. So if the patient takes the drug again, the platelets are coated, the antibody reacts with the platelets and again the platelet count falls immediately to low levels. Therefore recognition of a drug as the cause for thrombocytopenia is critical to avoid any further exposure to that drug. Low platelet counts that occur during pregnancy - This is sometimes called “Gestational thrombocytopenia”. This is described in its own section on this website. It is not a “disorder”. The cause is not known, but the consistent observation is that a few women with uncomplicated pregnancies, perhaps 5%, may have a low platelet count. The platelet count is not very low.

Commonly it is only 100,000-150,000/µL, or just below the lower limit of normal. Our opinion is that this is merely a normal adjustment of the platelet count during pregnancy, that the platelet counts of pregnant women, particularly near the end of pregnancy and at the time of delivery, all shift down a little bit. A reason for this is that the plasma volume is increased during pregnancy, and therefore the platelets are simply diluted in the larger volume of plasma. This is the explanation for why the blood hemoglobin concentration is also a little lower during pregnancy. Our opinion is that this is not a health problem and it does not require any additional testing or care. Thrombotic microangiopathies (TMA) - These disorders are the result of abnormal blood clotting in the smallest blood vessels (arterioles, capillaries) throughout all of the body. There are multiple causes of these syndromes. All are uncommon.

The most important of these syndromes, thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, drug-induced TMA, and complement-mediated TMA, are described in their own sections on this website. Thrombotic thrombocytopenic purpura (TTP) - In this disorder, described in its own section in this website, platelets are consumed in tiny clots in small blood vessels throughout the body. The cause of TTP is a deficiency of a blood enzyme that helps to prevent platelet clumping and the resulting formation of the tiny blood clots. The enzyme is known by its initials, ADAMTS13. A minor deficiency of ADAMTS13 can occur in many different illnesses and appears to be harmless. A severe deficiency of ADAMTS13 can predispose a person to the development of TTP. A severe deficiency of ADAMTS13 doesn’t itself actually cause TTP because patients can have a complete deficiency and have no health problems for many years. But then, when another condition occurs that increases the risk of blood clotting, such as an infection, surgery, or pregnancy, TTP can become a sudden and severe illness.

ADAMTS13 deficiency can be inherited because of a gene mutation, causing lifetime deficiency. Much more commonly ADAMTS13 deficiency can be acquired as a result of an autoantibody. Drug-induced TMA (DITMA)- Just like DITP, drugs can cause TMA by the formation of drug dependent antibodies. Quinine is the most common cause of DITMA related to drug-dependent antibodies. Quinine-induced TMA is typically a very sudden and severe illness with severe kidney injury. DITMA can also be caused by direct drug toxicity. This may also be sudden and severe, as with an illegal drug is used. An example of this is the intravenous injection of the narcotic, Opana (oxymorphone). Other examples of toxic DITMA are from cancer drugs and immunosuppressive drugs used for patients with an organ transplant. Hemolytic uremic syndrome (HUS). This is a TMA syndrome caused by an intestinal infection with a bacteria that secretes a very potent toxin, named Shiga toxin. In the US and Europe, the most common bacteria causing these infections are certain types of E. coli, and the most common type of E. coli is called E.

coli O157:H7. E. coli O157:H7 is a common and normal intestinal bacteria in cattle. Therefore eating undercooked beef or contact with cattle can create a risk for HUS. Most cases of HUS are sporadic, just a single person is infected. Sometimes a large dramatic outbreak can occur, related to a restaurant or a contaminated food or water source. Shiga toxin can cause severe kidney injury. Complement-mediated TMA. This is a recently recognized cause, usually associated with an inherited gene mutation that causes an abnormality of the immune (complement) system. The complement proteins are normal blood proteins that can act as scavengers so our bodies can remove our own damaged cells or destroy foreign cells. If the complement system becomes overactive because of an inherited gene mutation that limits the normal regulation and control, the complement system can damage our own cells. This is what happens in complement-mediated TMA. The most vulnerable target seems to be the kidneys and the most serious problems are kidney injury and failure. In 2011 a new anti-complement drug was approved specifically for this TMA. Its generic name is eculizumab; its trade name is Soliris.

It is extremely expensive – the price is over $600,000 per year of treatment. However it is effective when used for the proper patients. IMPORTANT! The information included in this website is for educational purposes only. The designers and operators of this site take no responsibility for the things you may do with this information. For advice on your unique medical condition, please consult your health care professional. By going further into this website you acknowledge that you have read and understood this disclaimer. Bone Anatomy | ASU - Ask A Biologist Ask A Biologist Bone Anatomy show/hide words to know Bone marrow: the tissue in the middle of bones that creates red and white blood cells ... Compact bone: the heaviest, hardest type of bone in the body. Lining cell: flat cells that make up and cover the outer layer of bone. Marrow: a soft tissue found inside bones. Osteoblast: cells make new bone. Osteoclast: cells in your body that break down bone material in order to reshape it. Osteocyte: a star shaped bone cell with long branching arms that connect it to its neighboring cells.

Osteon: tube shaped structure in bones with an open space for blood vessels, veins, and nerves in the center. Platelet: a small cell fragment without a nucleus that helps stop blood the flow of blood when the body is injured. Spongy bone: bone that has many spaces in it, giving it a spongy appearance Bone Basics Have you ever seen fossil remains of dinosaur and ancient human bones in textbooks, television, or in person at a museum? It's easy to look at these and think of bones as dry, dead sticks in your body, but this couldn't be further from the truth. Bones are made of active, living cells that are busy growing, repairing themselves, and communicating with other parts of the body. Lets take a closer look at what your bones do and how they do it. How many bones are in the human body? The skeleton of an adult human is made up of 206 bones of many different shapes and sizes. Added together, your bones make up about 15% of your body weight. Newborn babies are actually born with many more bones than this (around 300), but many bones grow together, or fuse, as babies become older.

Some bones are long and thick, like your thigh bones. Others are thin, flat, and wide, like your shoulder blades. Support: Like a house is built around a supportive frame, a strong skeleton is required to support the rest of the human body. Without bones, it would be difficult for your body to keep its shape and to stand upright. Protection: Bones form a strong layer around some of the organs in your body, helping to keep them safe when you fall down or get hurt. Your rib cage, for example, acts like a shield around your chest to protect important organs inside such as your lungs and heart. Your brain is another organ that needs a lot of protection. The thick bone layer of your skull protects your brain. For this purpose, being "thick-headed" is a very good thing. Movement: Many of your bones fit together like the pieces of a puzzle. Each bone has a very specific shape which often matches up with neighboring bones. The place where two bones meet to allow your body to bend is called a joint. How many different ways can you move your joints? Some bones, like your elbow, fit together like a hinge that lets you bend your arm in one specific direction.

Other bones fit together like a ball and socket, such as the joint between your shoulder and arm. This type of joint lets you rotate your shoulder in many directions, or swing it all the way around in a circle like softball pitchers do. The movement of our bodies is possible because of both joints and muscles. Muscles often attach to two different bones, so that when the muscle flexes and shortens, the bones move. This allows you to bend your elbows and knees, or pick up objects. A skeleton has plenty of joints, but without muscles, there is nothing to pull the bones in different directions. More than half of the bones in your body are actually located in your hands and feet. These bones are attached to many little muscles that give you very exact control over how you move your fingers and feet. Blood Cell Formation: Did you know that most of the red and white blood cells in your body were created inside of your bones? This is done by a special group of cells called stem cells that are found mostly in the bone marrow, which is the innermost layer of your bones. Storage: Bones are like a warehouse that stores fat and many important minerals so they are available when your body needs them.

These minerals are continuously being recycled through your bones--deposited and then taken out and moved through the bloodstream to get to other parts of your body where they are needed. What are your bones made of? Now that you know what bones do, let's take a look at what they're made of and their anatomy. Each bone in your body is made up of three main types of bone material: compact bone, spongy bone, and bone marrow. Compact bone Compact bone is the heaviest, hardest type of bone. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day. About 80% of the bone in your body is compact. It makes up the outer layer of the bone and also helps protect the more fragile layers inside. If you were to look at a piece of compact bone without the help of a microscope, it would seem to be completely solid all the way through. If you looked at it through a microscope, however, you would see that it's actually filled with many very tiny passages, or canals, for nerves and blood vessels. Compact bone is made of special cells called osteocytes. These cells are lined up in rings around the canals.

Together, a canal and the osteocytes that surround it are called osteons. Osteons are like thick tubes all going the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center. Looking at the osteons in bone (A) under a microscope reveals tube-like osteons (B) made up of osteocytes (C). These bone cells have long branching arms (D) which lets them communicate with other cells. Spongy bone Spongy bone is found mostly at the ends of bones and joints. About 20% of the bone in your body is spongy. Unlike compact bone that is mostly solid, spongy bone is full of open sections called pores. If you were to look at it in under a microscope, it would look a lot like your kitchen sponge. Pores are filled with marrow, nerves, and blood vessels that carry cells and nutrients in and out of the bone. Though spongy bone may remind you of a kitchen sponge, this bone is quite solid and hard, and is not squishy at all. Bone marrow The inside of your bones are filled with a soft tissue called marrow.

There are two types of bone marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets are made. Platelets are small pieces of cells that help you stop bleeding when you get a cut. Red bone marrow is found in the center of flat bones such as your shoulder blades and ribs. Yellow marrow is made mostly of fat and is found in the hollow centers of long bones, such as the thigh bones. It does not make blood cells or platelets. Both yellow and red bone marrow have many small and large blood vessels and veins running through them to let nutrients and waste in and out of the bone. When you were born, all of the marrow in your body was red marrow, which made lots and lots of blood cells and platelets to help your body grow bigger. As you got older, more and more of the red marrow was replaced with yellow marrow. The bone marrow of full grown adults is about half red and half yellow. The inside story Bones are made of four main kinds of cells: osteoclasts, osteoblasts, osteocytes, and lining cells. Notice that three of these cell type names start with 'osteo.' This is the Greek word for bone.

When you see 'osteo' as part of a word, it lets you know that the word has something to do with bones. Osteoblasts are responsible for making new bone as your body grows. They also rebuild existing bones when they are broken. The second part of the word, 'blast,' comes from a Greek word that means 'growth.' To make new bone, many osteoblasts come together in one spot then begin making a flexible material called osteoid. Minerals are then added to osteoid, making it strong and hard. When osteoblasts are finished making bone, they become either lining cells or osteocytes. Osteocytes are star shaped bone cells most commonly found in compact bone. They are actually old osteoblasts that have stopped making new bone. As osteoblasts build bone, they pile it up around themselves, then get stuck in the center. At this point, they are called osteocytes. Osteocytes have long, branching arms that connect them to neighboring osteocytes. This lets them exchange minerals and communicate with other cells in the area. Lining cells are very flat bone cells.

These cover the outside surface of all bones and are also formed from osteoblasts that have finished creating bone material. These cells play an important role in controlling the movement of molecules in and out of the bone. Osteoclasts break down and reabsorb existing bone. The second part of the word, 'clast,' comes from the Greek word for 'break,' meaning these cells break down bone material. Osteoclasts are very big and often contain more than one nucleus, which happens when two or more cells get fused together. These cells work as a team with osteoblasts to reshape bones. This might happen for a number of reasons: When a bone breaks, a thick lump of bone called a hard callus forms around the break in the process of healing. The callus is slowly broken down by osteoclasts until the bone is returned to its original shape. When new blood vessels, nerves, and veins are needed in an area, osteoclasts break down bone material to make new passages. Bones that are used more often and need to support more weight, such as the bones of athletes, become thicker and stronger over time.

Bones that are used less often, such as those that need to be put into a cast for long periods of time, are broken down, becoming smaller and thinner. It's not completely understood how bone cells in your body are able to work together and stay organized, but pressure and stress on the bone might have something to do with it. Bone Facts The smallest bone in the human body is called the stirrup bone, located deep inside the ear. It's only about 3 millimeters long in an adult. The longest bone in the human is called the femur, or thigh bone. It's the bone in your leg that goes from your hip to your knee. In an average adult, it's about 20 inches long. References: Marieb. E.N. (1989) Human Anatomy and Physiology, CA: Benjamin/Cummings Publishing Company, Inc Heller, H.C., Orians, G.H., Purves, W.K., Sadava, D. (2003) Life: The Science of Biology, 7th Edition. Sunderland, MA: Sinauer Associates, Inc. & W. H.

Freeman and Company Skeleton Image: By Lady of Hats - Mariana Ruiz Villarreal, via Wikimedia Commons. What Are Red Blood Cells? - Health Encyclopedia - University of Rochester Medical Center What Are Red Blood Cells? Red blood cells play an important role in your health by carrying fresh oxygen throughout the body. Click to Enlarge Red blood cells are round with a flattish, indented center, like doughnuts without a hole. Your healthcare 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. Click to Enlarge 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 there are too few 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 enough red blood cells. 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. This causes 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 happens 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 happens 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. Medical Reviewers: Human Anatomy: Blood - Cells, Plasma, Circulation, and More Human Anatomy Next Blood is a constantly circulating fluid providing the body with nutrition, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood "thicker" than pure water.

The average person has about 5 liters (more than a gallon) of blood. A liquid called plasma makes up about half of the content of blood. Plasma contains proteins that help blood to clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients. About half of blood volume is composed of blood cells: • Red blood cells, which carry oxygen to the tissues • White blood cells, which fight infections • Platelets, smaller cells that help blood to clot Blood is conducted through blood vessels (arteries and veins). Blood is prevented from clotting in the blood vessels by their smoothness, and the finely tuned balance of clotting factors. Blood Conditions Hemorrhage ( bleeding ): Blood leaking out of blood vessels may be obvious, as from a wound penetrating the skin. Internal bleeding (such as into the intestines, or after a car accident) may not be immediately apparent. Hematoma : A collection of blood inside the body tissues. Internal bleeding often causes a hematoma. Leukemia: A form of blood cancer, in which white blood cells multiply abnormally and circulate through the blood. The excessive large numbers of white cells deposit in the body's tissues, causing damage.

Multiple myeloma : A form of blood cancer of plasma cells similar to leukemia. Anemia, kidney failure and high blood calcium levels are common in multiple myeloma. Lymphoma : A form of blood cancer, in which white blood cells multiply abnormally inside lymph nodes and other tissues. The enlarging tissues, and disruption of blood's functions, can eventually cause organ failure. Anemia : An abnormally low number of red blood cells in the blood. Fatigue and breathlessness can result, although anemia often causes no noticeable symptoms. Hemolytic anemia : Anemia caused by rapid bursting of large numbers of red blood cells (hemolysis). An immune system malfunction is one cause. Hemochromatosis : A disorder causing excessive levels of iron in the blood. The iron deposits in the liver, pancreas and other organs, causing liver problems and diabetes. Sickle cell disease : A genetic condition in which red blood cells periodically lose their proper shape (appearing like sickles, rather than discs). The deformed blood cells deposit in tissues, causing pain and organ damage. Bacteremia : Bacterial infection of the blood. Blood infections are serious, and often require hospitalization and continuous antibiotic infusion into the veins.

Malaria : Infection of red blood cells by Plasmodium, a parasite transmitted by mosquitos. Malaria causes episodic fevers, chills, and potentially organ damage. Thrombocytopenia : Abnormally low numbers of platelets in the blood. Severe thrombocytopenia may lead to bleeding. Leukopenia: Abnormally low numbers of white blood cells in the blood. Leukopenia can result in difficulty fighting infections. Disseminated intravascular coagulation (DIC): An uncontrolled process of simultaneous bleeding and clotting in very small blood vessels. DIC usually results from severe infections or cancer. Hemophilia : An inherited (genetic) deficiency of certain blood clotting proteins. Frequent or uncontrolled bleeding can result from hemophilia. Hypercoaguable state: Numerous conditions can result in the blood being prone to clotting. A heart attack, stroke, or blood clots in the legs or lungs can result. Polycythemia : Abnormally high numbers of red blood cells in the blood. Polycythemia can result from low blood oxygen levels, or may occur as a cancer-like condition.

Deep venous thrombosis (DVT): A blood clot in a deep vein, usually in the leg. DVTs are dangerous because they may become dislodged and travel to the lungs, causing a pulmonary embolism (PE). Myocardial infarction (MI): Commonly called a heart attack, a myocardial infarction occurs when a sudden blood clot develops in one of the coronary arteries, which supply blood to the heart. Continued Blood Tests Complete blood count: An analysis of the concentration of red blood cells, white blood cells, and platelets in the blood. Automated cell counters perform this test. Blood smear: Drops of blood are smeared across a microscope slide, to be examined by an expert in a lab. Leukemia, anemia, malaria, and numerous other blood conditions can be identified with a blood smear. Blood type : A test for compatibility before receiving a blood transfusion. The major blood types (A, B, AB, and O) are determined by the protein markers (antigens) present on the surface of red blood cells. Coombs test : A blood test looking for antibodies that could bind to and destroy red blood cells.

Pregnant women and people with anemia may undergo Coombs testing. Blood culture : A blood test looking for infection present in the bloodstream. If bacteria or other organisms are present, they may multiply in the tested blood, allowing their identification. Mixing study: A blood test to identify the reason for blood being "too thin" (abnormally resistant to clotting). The patient's blood is mixed in a tube with normal blood, and the mixed blood's properties may provide a diagnosis. Bone marrow biopsy : A thick needle is inserted into a large bone (usually in the hip), and bone marrow is drawn out for tests. Bone marrow biopsy can identify blood conditions that simple blood tests cannot. Continued Stem Cell Basics I. | stemcells.nih.gov Stem Cell Basics I. Where can I get more information? I. Introduction: What are stem cells, and why are they important? Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive.

When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell. Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division , sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions. Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: embryonic stem cells and non-embryonic "somatic" or "adult" stem cells . The functions and characteristics of these cells will be explained in this document. Scientists discovered ways to derive embryonic stem cells from early mouse embryos more than 30 years ago, in 1981.

The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and grow the cells in the laboratory. These cells are called human embryonic stem cells . The embryos used in these studies were created for reproductive purposes through in vitro fertilization procedures. When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor. In 2006, researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be "reprogrammed" genetically to assume a stem cell-like state. This new type of stem cell, called induced pluripotent stem cells (iPSCs) , will be discussed in a later section of this document. Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst , the inner cells give rise to the entire body of the organism, including all of the many specialized cell types and organs such as the heart, lungs, skin, sperm, eggs and other tissues.

In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease. Given their unique regenerative abilities, stem cells offer new potentials for treating diseases such as diabetes, and heart disease. However, much work remains to be done in the laboratory and the clinic to understand how to use these cells for cell-based therapies to treat disease, which is also referred to as regenerative or reparative medicine . Laboratory studies of stem cells enable scientists to learn about the cells’ essential properties and what makes them different from specialized cell types. Scientists are already using stem cells in the laboratory to screen new drugs and to develop model systems to study normal growth and identify the causes of birth defects. Research on stem cells continues to advance knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. Stem cell research is one of the most fascinating areas of contemporary biology, but, as with many expanding fields of scientific inquiry, research on stem cells raises scientific questions as rapidly as it generates new discoveries.

Question: In which part of the human body do red blood cells develop?
Answer: Bone marrow