Blood has four main components, with red blood cells being the most abundant of the four. Erythrocytes or red blood cells perform many essential functions in the body. Most notably, red blood cells contain important molecules that allow them to carry oxygen and carbon dioxide through the body. Red blood cells begin as immature cells in the bone marrow and enter the bloodstream as they mature. They are flexible and can change shape easily, allowing them to fit through blood vessels of any size.
Red blood cells possess a biconcave shape with a flat center. In other words, both sides of the cell resemble a shallow bowl, but with plump, torus-like edges. The increased surface area of the biconcave shape facilitates better gas exchange in comparison to cells with more spherical shapes. Because they are flexible and malleable, their size can vary. A typical human red blood cell has a diameter of between 6.2 and 8.2 micrometers. The thickest area of a red blood cell is just over two micrometers while the thinnest may be under a single micrometer thick. For comparison, a single human hair is between 60 and 120 micrometers wide.
Generally, adult humans have anywhere between 20 trillion and 30 trillion red blood cells at any given moment. Men typically have higher red blood cell counts than women. Living at high altitudes tends to increase red blood cell counts in both sexes. A man will typically have around five million cells per microliter of blood while women have around four million. In comparison, there are fewer than 11 thousand white blood cells and 400 thousand platelets per microliter. Red blood cells account for 45 percent of the cells in the blood.
In mammals, red blood cells lack many of the internal components other cells have. They lack mitochondria and therefore rely on anaerobic respiration. This means red blood cells do not use any of the oxygen they collect, allowing more oxygen to travel to areas that need it. Red blood cells also lack endoplasmic reticula so they cannot synthesize proteins. Instead, they contain structural proteins that allow them to change shape and maintain their structure.
Red blood cells have a membrane that enables many functions. The membrane has three layers: the glycocalyx, the lipid bilayer, and the membrane skeleton. The glycocalyx is the most exterior layer and is rich in carbohydrates. The lipid bilayer contains many transmembrane proteins that allow for the exchange of materials through the membrane. Finally, the membrane skeleton consists of a structural network of proteins that sits on the inner area of the lipid bilayer.
In vertebrates, red blood cells consist mostly of a metalloprotein -- hemoglobin. Within hemoglobin are heme groups with iron atoms that can temporarily bind to oxygen molecules. This is what allows the cells to collect oxygen in the lungs and transport it through the body. Hemoglobin is red, and gives red blood cells their signature color. The cells can change color depending on the level of oxygen in the cell, however. Hemoglobin with oxygen, oxyhemoglobin, is scarlet. After the red blood cell releases the oxygen, oxyhemoglobin becomes deoxyhemoglobin and develops a dark red color.
Air sacs in the lungs or alveoli allow red blood cells to absorb oxygen from inhaled air. Then the red blood cells travel through blood vessels such as veins and capillaries until they reach capillary beds. These areas are extremely narrow and slow the flow of cells dramatically, creating a longer amount of time for the release of oxygen into the surrounding areas. Carbon dioxide enters the bloodstream, and some of it binds to the amino acids present in hemoglobin, becoming carbaminohemoglobin. The red blood cells then carry carbon dioxide back to the lungs, where they exchange CO2 for oxygen.
If red blood cells experience shear stress, they release ATP to relax the surrounding blood vessels and promote normal blood flow. In addition, they release organic compounds that direct blood to areas of the body that require oxygen. Red blood cells also play a role in the immune system. By releasing free radicals from their hemoglobin, red blood cells can break down the cell walls and membranes of pathogens. Without their protective walls and membranes, the pathogens die.
Erythropoiesis, the process by which the body produces new red blood cells, takes around seven days. In embryos, this process occurs in the liver. After birth, responsibility transfers to the red bone marrow of large bones. The body produces more than two million red blood cells per second in the bone marrow. Using glucose, lipids, amino acids, iron, copper, zinc, and B vitamins, the bone marrow creates a blood stem cell which may become a red blood cell. Reticulocytes are developing red blood cells that begin to flow from the bone marrow into the blood. Around one percent of circulating red blood cells are reticulocytes.
On average, red blood cells have a lifetime of between 100 and 120 days. Over time, red blood cells undergo changes that allow macrophages from the bone marrow, liver, and spleen to remove them. The body breaks down the red blood cells and recycles their various components. Globin, the protein portion of hemoglobin, breaks down into amino acids, which may travel to the bone marrow for use in the production of new erythrocytes. The body stores iron in the liver or spleen, though it may also send it to the bone marrow for erythropoiesis. Non-iron portions degrade into biliverdin and then bilirubin, which the liver uses to manufacture bile.
Many conditions and diseases can affect the body’s red blood cell count and cause harmful effects as a result. Low red blood cell counts can lead to fatigue, dizziness, weakness, headaches, shortness of breath, and pale skin. High red blood cell counts can cause similar issues, as well as joint pain, itching skin, and tenderness. Physicians may need to order additional testing to understand why a patient’s red blood cell count is above or below normal.
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