Red blood cell

Erythrocytes (singular of the erythrocyte, from Ancient Greek ἐρυθρός erythros "red" as well as ancient Greek κύτος kytos " hollow ", " vessel ", "envelope", also called red blood cells ) are the most common cells in the blood of vertebrates. They serve to transport oxygen from the lungs or gills to the various body tissues. Erythrocytes were first described in 1658 by Jan Swammerdam.

Mature erythrocytes of mammals appear under the microscope as approximately equal, pale, round discs that are slightly indented in the middle of both sides ( biconcave ) and have no nucleus. Other organelles such as mitochondria and ribosomes are missing. The red blood cells of other vertebrate groups usually have cell nuclei, they are missing otherwise only found in some fish and amphibians. The size may vary by two hundred times between different species.

Numbers to the erythrocytes in humans

• Diameter: about 7.5 microns ( 7.5 x 10 -6m )
• Thickness: 2 microns at the edge and 1 micron in the center
• Weight: about 3 × 10-11 g
• Volume: about 90 fl (ie 90 μm3 )
• Average concentration in the blood: about 4.5-5.5 Mio/mm3 (ie about 5 × 106/μl = 5 × 106/mm3 = 5 × 1012 / l = 5 Tera / l = 5 · 1015/m3 )
• Number in the blood of men: 4.6 to 6.2 · 106/μl
• Number in the blood in women: 4.0 to 5.4 · 106/μl
• Total number in the blood: 24-30 trillion = 24-30 · 1012
• Development time: about 7 days
• Average lifespan: about 120 days or 4 months
• New production of red blood cells: about 1 % / day = about 200 billion / day = about 2 million / second
• Total surface: 4000-4500 m2
• Osmotic resistance to 180 mosmol / l
• Membrane potential: - 10mV

Construction

Membrane and normal form

On the cell surface glycoproteins are the possibilities (eg blood group antigens ) that define the serologically detectable genetic characteristics of erythrocytes among others.

The disc-shaped erythrocytes owe their membrane skeleton. This consists of a two-dimensional network of spectrin filaments are held together by short actin filaments. Through the mediation of adapter proteins, such as ankyrin and 4.1, the network is anchored to the integral proteins of the plasma membrane. One of the integral proteins of the plasma membrane include the band 3 tetramer and glycophorin.

The bi-concave shape, it enables the erythrocytes absorb oxygen faster, since the diffusion distance of the cell membrane is reduced in the interior of the cell ( see Figure B). Very important for the function of erythrocytes is their strong ductility, which allows them to pass through even the smallest capillaries. The close contact between erythrocytes and the endothelium of the vessels of the gas exchange in the capillaries is particularly effective.

Special shapes

Sickle cell anemia characterized by crescent-shaped red blood cells. The form dates back to a hereditary abnormal hemoglobin (sickle cell hemoglobin, HbS ), which tends to crystallize at oxygen deficiency and therefore takes up less space, reducing the volume of the whole cell shrinks. Heterozygous Affected are, however, protected from the severe forms of malaria, as these cells are less vulnerable to attack by plasmodia.

Under special conditions, can take erythrocytes ball, Datura or cup shape. Spherocytes are spherical - deformed erythrocytes, echinocytes or Datura cells ( in the drawing marked d) also have numerous (10-30 ) blunt projections on and cup-shaped erythrocytes ( c ) are referred to as stomatocytes. These forms can also be caused dehydration or other influences. Are echinocytes However, despite several careful processing detectable, this can be due to a Pyruvatkinaseinsuffizienz, renal insufficiency, vitamin E deficiency ( hypovitaminosis ) or poisoning.

A Fragmentozyt or Schistozyt is damaged or being in erythrocyte degradation or a fragment of an erythrocyte.

At very low flow velocity of the blood individual erythrocytes can adhere and form chains, then one speaks of a rouleaux formation ( in the drawing marked b ) or agglomeration.

Ingredients

Erythrocytes consist of 90 % of the dry mass of the oxygen -binding protein hemoglobin. This corresponds to approximately 35% of the mass of a red blood cell or 120-160 g / l of whole blood in women and 140-180 g / l in men. The heme portion of this protein gives the erythrocytes, and thus the blood red color.

Seedless red blood cells of mammals

The erythrocytes of mammals encounter in the course of their maturation from erythroblasts of their nuclei and organelles from. Since these erythrocytes have no nucleus, the DNA is missing. However, is found in the cell mRNA in small quantities. The removal of organelles additional space for hemoglobin is created. These erythrocytes have no mitochondria, the energy is provided by the glycolysis, followed by lactic acid fermentation. Glucose uptake of the erythrocytes will not be regulated by insulin because the corresponding receptor is missing. Instead, the uptake of glucose by other glucose transporter occurs ( here: GLUT1 ).

Function and life cycle

The role of erythrocytes is to transport oxygen in the blood vascular system. Take in the pulmonary capillaries or gills on the oxygen and transport it via the arterial circulation to the arterial capillaries of the tissues and organs, where the oxygen to the cells is released again. For binding and transporting oxygen, the hemoglobin within the red blood cells are responsible. Hemoglobin carries some cases the carbon dioxide back from the tissues.

The process by which the red blood cells will emerge called erythropoiesis. In the embryo, the liver is the main production site of the erythrocytes. Later, these are continuously produced in the red bone marrow of large bones. They arise here from proliferating stem cells initially produce nucleated erythroblasts, from which first emerge reticulocytes and erythrocytes under the actual expulsion of the nucleus. Taking place in the bone marrow maturation of erythrocytes by macrophages, a group of leukocytes (white blood cells ) are supported. Here, the immature erythrocytes arranged in so-called "islands" (English: Iceland ) on a single macrophages, which supplies the cells and also receives the separated cell bodies and digested. This process is observed in the early 1940s needed to not yet known in detail, the retinoblastoma (Rb ) protein. The development of a red blood cell takes about 7 days. Their average life span is about 120 days, or 4 months. The aging cells gradually lose their deformability and then of phagocytes in the liver, spleen and bone marrow (RES = reticuloendothelial system ) are terminated.

In a healthy adult humans there are about 25 trillion red blood cells, with a total surface of 4000 m2. The body renews it daily about one percent ( $ 200 billion ), equivalent to 2,000,000 in the second. The spleen is also used as a reservoir for erythrocytes, this effect is limited to humans. In other mammals, such as in the dog or horse, the spleen contains a large number of erythrocytes, which are excreted in stress in the blood circulation and to improve the oxygen-carrying capacity.

The production of erythrocytes (EPO ) stimulates or controlled, which is constantly being re- formed by the kidneys because the body can not store this hormone by the hormone erythropoietin. The synthesis takes place in the body by measuring the surface tension of the red cell and the associated oxygen content as it flows through the kidneys. Thus, for example, an adaptation to the lower oxygen content in the air stays of more than 1500 m above sea level ( altitude training of competitive athletes, etc.). EPO can and is also used as a doping agent in synthetic form.

The shift of the oxygen- binding protein, to solve in this case hemoglobin, into cells, instead it directly in the body fluid, an important step in the evolution of vertebrates was. It allows smoother thin blood and longer transport distances of oxygen. Only in this way, the supply of a larger organism ( up to the blue whale ) is guaranteed with oxygen. Other oxygen-binding proteins from other animals such as hemocyanin, hemerythrin or erythrocruorin can compensate for these properties remotely via their molecular size.

Diseases

• With a Anaemia is usually the number of red blood cells is reduced. There are many causes of anemia, with iron deficiency is likely to be the most common reason in the Western world. Iron deficiency, the heme synthesis is inhibited. As a result, the erythrocytes are hypochromic (especially in the center weaker red) and microcytic ( smaller than normal). In a group of metabolic diseases, the porphyrias, fall enzymes of heme synthesis from partial throttle and thus the amount of hemoglobin in the erythrocytes. Iron present can not completely in the heme precursors ( porphyrins ) to be installed, which accumulate in tissues and cause various symptoms ( skin photosensitivity, severe abdominal pain, and others).
• In polycythemia - for example, polycythemia vera, polycythemia rubra hypertonica and the polycythemia of the newborn - erythrocytes occur frequently. Due to the increased number of platelets or erythrocytes, such as in the case of polycythaemia the blood becomes more viscous. There is a risk of thrombosis and in consequence of an embolus.
• In hemolysis (increased breakdown of red blood cells ) occurs jaundice, caused by the hemoglobin metabolite bilirubin. Also, can form by the overload that bilirubin gallstones in the form of pigment stones ( Bilirubinsteinen ).
• Mutations into the globin chains are connected to different hemoglobinopathies, such as sickle cell anemia and thalassemia. In Sickle cell anemia is a mainly occurring in malaria-endemic areas disease. For her, the red cells deform in deoxy-, that is oxygen-poor state sickle-shaped. In this form it can pass through the small capillaries with difficulty and is there an increase in the destruction of red blood cells, called hemolysis. Homozygous carriers of this genetic material have a significantly shortened life expectancy, which are heterozygous carriers, however, protected against malaria disease because the pathogen ( Plasmodium falciparum ) is unable to reproduce in such a deformed erythrocytes.
• The ball -cell anemia is a genetic defect, occurring in the spherical erythrocytes, known as spherocytes, by a disturbed cytoskeleton.
• When Favism the genetic lack of an enzyme ( glucose-6 -phosphate dehydrogenase ) leads to use of certain drugs (acetylsalicylic acid ) or food ( beans ) to burst the red blood cells.