Erythrocyte

SEM image of human erythrocytes (size approximately 6–8 μm)

Erythrocytes or erythrocytes ( singular of the erythrocyte ; from ancient Greek ἐρυθρός erythrós "red" and ancient Greek κύτος kýtos "cavity", "vessel", "envelope"), also called red blood cells , are the most common cells in the blood of vertebrates . As the only known exception among the vertebrates, the crocodile ice fish found in the Southern Ocean do not have any erythrocytes. Among other things, they serve to transport oxygen from the lungs or gills to the various body tissues. Erythrocytes, and thus blood cells , were first described by Jan Swammerdam in 1658 as flat oval particles in frog serum.

Mature erythrocytes of mammals appear under the microscope as pale, round disks of approximately the same size, which are slightly indented (biconcave) in the middle on both sides and have no nucleus . Other organelles such as mitochondria and ribosomes are also missing. The erythrocytes of other vertebrate groups usually have cell nuclei, otherwise they are only missing in some fish and amphibian species. The size can vary by two hundred times between different types.

Figures on human erythrocytes

Diameter: on average 7.5 µm
Thickness: 2 µm at the edge and 1 µm in the center
Volume : about 90 fl (i.e. 90 µm³)
Average concentration in the blood: in men 4.6–5.9 · 10 ⁶ / µl and in women 4.0–5.2 · 10 ⁶ / µl. (10 ⁶ / µl correspond to 10 ¹² / l and 1 / pl. In clinical practice, 10 ¹² / l are also abbreviated as Tera / l.)
Total blood count: 24-30 trillion = 24-30 · 10 ¹²
Development time: about 7 days
Average lifespan: around 120 days or 4 months
New production of erythrocytes: about 1% / day = about 200 billion / day = about 2 million / second
Total surface of all erythrocytes: 4000–4500 m²
Osmotic resistance up to 180 mosmol / l
Membrane potential: −10 mV

Structure and features

Normal (a and b) and deformed erythrocytes (c: spherocytes , d: echinocytes )

Membrane and normal form

There are various glycoproteins (e.g. blood group antigens) on the cell surface . a. Define the serologically detectable genetic characteristics of the erythrocytes.

Erythrocytes owe their disc shape to their membrane skeleton. This consists of a flat network of spectrin filaments, which are held together by short actin filaments. The network is anchored to integral proteins of the plasma membrane through the mediation of adapter proteins such as ankyrin . The integral proteins of the plasma membrane include the anion exchanger 1 and glycophorin .

The biconcave shape enables the erythrocytes to absorb oxygen more quickly, as the diffusion path from the cell membrane into the interior of the cell is shortened and the surface- to-volume ratio is as large as possible (see Figure a and b). Very important for the function of the erythrocytes is their great deformability, which allows them to cross even the smallest capillaries . The close contact between erythrocytes and the endothelium of the vessels means that gas exchange in the capillaries is particularly effective.

Special forms

sickle-shaped erythrocytes in sickle cell anemia

The sickle cell anemia is characterized by crescent-shaped red blood cells. The shape goes back to a hereditary abnormal hemoglobin ( sickle cell hemoglobin , HbS ), which tends to crystallize out when there is a lack of oxygen and therefore takes up less space, whereby the volume of the entire cell shrinks. However, heterozygous people are protected from the severe forms of malaria , as these cells are less vulnerable to plasmodia .

Under special conditions, erythrocytes can take on a spherical, thorn-apple or cup shape, but most often they have a plate-like shape. Spherocytes are spherically deformed erythrocytes, echinocytes or thorn apple cells ( marked with d in the drawing ) also have numerous (10–30) blunt processes and cup-shaped erythrocytes ( c ) are referred to as stomatocytes . These forms can result from dehydration or other influences. Are echinocytes but undetectable despite repeated careful processing, this can result in a Pyruvatkinaseinsuffizienz , a renal failure a, vitamin E deficiency ( hypovitaminosis ) or poisoning be justified.

A fragmentocyte or schistocyte is a damaged or degraded erythrocyte or a fragment of an erythrocyte.

If the blood flow rate is very low, individual erythrocytes can adhere to one another and form chains, which is then referred to as roll formation ( marked with b in the drawing ) or agglomeration. The latter can lead to thrombosis . If these occur in the coronary arteries , a heart attack can occur as a result of the resulting lack of oxygen ; a stroke in the brain . Remedies against it are acetylsalicylic acid (ASA), clopidogrel and phenprocoumon (Marcumar).

ingredients

Erythrocytes consist of 90% of the dry mass of the oxygen-binding protein hemoglobin . This corresponds to approx. 35% of the mass of an erythrocyte or 120–160 g / l of whole blood in women and 140–180 g / l in men. The heme part of this protein gives the erythrocytes and thus also the blood their red color.

Nuclear red blood cells of mammals

Mammalian erythrocytes , in contrast to other vertebrate classes , eject their cell nucleus and organelles from the erythroblast as they mature . Since these erythrocytes do not have a nucleus , the DNA is also missing . However, mRNA is found in small amounts in the cell . The breakdown of the organelles creates additional space for hemoglobin. These erythrocytes also have no mitochondria ; energy is provided by glycolysis with subsequent lactic acid fermentation . The glucose uptake of the erythrocytes is not regulated by insulin because the corresponding receptor is missing. Instead, glucose is absorbed via other glucose transporters (here: GLUT-1 ).

The different red blood cells and their organelles, or lack thereof in the various vertebrate - classes .

Function and life cycle

The task of the erythrocytes is to transport oxygen in the blood vessel system . They take up the oxygen in the lungs capillaries or gills and transport it through the arterial circulation to the arterial capillaries of the tissues and organs, from where the oxygen is released to the cells . The hemoglobin inside the erythrocytes is responsible for binding and transporting oxygen. Hemoglobin also transports some of the carbon dioxide back out of the tissues.

The process by which the red blood cells are created is called erythropoiesis . In the embryo , the liver is the main production site for erythrocytes. Later these are continuously produced in the red marrow of larger bones . There they arise from divisible stem cells that initially produce nucleated erythroblasts , from which the reticulocytes and then the actual erythrocytes emerge, initially by expelling the cell nucleus . The maturation of erythrocytes, which takes place in the bone marrow, is supported by macrophages , a group of leukocytes (white blood cells). (English Here, the immature erythrocytes, in so-called "islands" organize Islands ) to a single on macrophages, which supplies the cells, and also receives the precipitated cell bodies and digested. This process, which was already observed in the early 1940s, requires the retinoblastoma (Rb) protein in a way that is not yet known . The development of an erythrocyte takes about 7 days. Their average lifespan is around 120 days or 4 months. The aging cells gradually lose their malleability and are then broken down by phagocytes in the liver, spleen and bone marrow (RES = reticuloendothelial system) and processed into bile .

In a healthy, adult human there are around 25 trillion erythrocytes, with a total surface area of 4000 m². The body renews around one percent (200 billion) of this every day, which corresponds to 2,000,000 every second. The spleen also serves as a reservoir for erythrocytes, although this effect is limited in humans. In other mammals, such as the dog or horse , the spleen contains a large number of erythrocytes, which are excreted into the bloodstream when stressed and improve the oxygen transport capacity.

The production of erythrocytes is stimulated or controlled by the hormone erythropoietin (EPO), which is constantly being re-formed by the kidneys because the body cannot store this hormone. When the oxygen content of the blood is reduced, the biosynthesis of EPO is started . As a result, the body adapts, for example, to the lower oxygen uptake when staying over 1500 m above sea level, for example during high-altitude training for competitive athletes. Synthetic EPO can also be used as a doping agent .

The relocation of the oxygen-binding protein, in this case hemoglobin, into cells instead of dissolving it directly in the body fluid was an important step in the evolution of vertebrates . It enables thinner blood and longer transport routes for oxygen. This is the only way to ensure that a larger organism (including the blue whale ) is supplied with oxygen. Other oxygen-binding proteins from other animals such as haemocyanin , haemythrin or erythrocruorin can only approximately compensate for these properties through their molecular size.

Diseases

With anemia or anemia, the number of erythrocytes is usually reduced. There are many causes of anemia, with iron deficiency being the most common cause in the western world. Heme synthesis is inhibited by iron deficiency. As a result, the erythrocytes are hypochromic (especially faintly red in the center) and microcytic (smaller than normal). In a group of metabolic diseases , the porphyria , enzymes involved in the synthesis of heme groups partially fail and thereby reduce the amount of hemoglobin in the erythrocytes. Existing iron cannot be completely incorporated into the heme precursors ( porphyrins ), which accumulate in the tissue and cause various symptoms ( sensitivity to light of the skin, severe abdominal pain, etc.).

In the case of polyglobules - e.g. B. polycythemia vera , polycythemia rubra hypertonica and polyglobulia of the newborn - erythrocytes occur more frequently. Due to the increased number of platelets or erythrocytes, as in the case of polycythemia, the blood becomes thicker. There is a risk of thrombosis and, as a result, embolism .

With hemolysis (increased breakdown of erythrocytes) jaundice occurs, caused by the hemoglobin breakdown product bilirubin . In addition, gallstones in the form of pigment stones (bilirubin stones ) can form due to the bilirubin overload .

Mutations in the globin chains are associated with various hemoglobinopathies , such as sickle cell anemia and thalassemia . Sickle cell anemia is a disease primarily found in areas affected by malaria . With her, the erythrocytes deform crescent-shaped in the deoxy, i.e. oxygen-poor state. In this form, they can only pass through the small capillaries with difficulty, and the erythrocytes are increasingly destroyed, known as hemolysis. Homozygous carriers of these genes have a significantly shortened life expectancy, but the heterozygous carriers are protected against malaria because the pathogen ( Plasmodium falciparum ) cannot multiply in such deformed erythrocytes.

The spherocytosis is a genetic defect in which spherical erythrocytes, called spherocytes by a disturbed cytoskeleton occur.

In favism , the genetic deficiency of an enzyme ( glucose-6-phosphate dehydrogenase ) causes the erythrocytes to burst after taking certain medications ( acetylsalicylic acid ) or foods ( broad beans ).

Older literature

Ludwig Heilmeyer , Herbert Begemann: blood and blood diseases. In: Ludwig Heilmeyer (ed.): Textbook of internal medicine. Springer-Verlag, Berlin / Göttingen / Heidelberg 1955; 2nd edition ibid. 1961, pp. 376-449, here: pp. 379-382 ( Die Erythrocyten ).

Web links

Commons : Erythrocyte - Collection of pictures, videos and audio files

Wiktionary: Erythrozyt - explanations of meanings, word origins, synonyms, translations

Individual evidence

^ Wilhelm Gemoll: Greek-German School and Handbook , Munich / Vienna 1965.
^ Christopher D. Moyes, Patricia M. Schulte: Tierphysiologie . Pearson Studium, Munich 2008, ISBN 978-3-8273-7270-3 , pp. 426 ( limited preview in the Google book search - English: Principles of Animal Physiology . Translated by Monika Niehaus , Sebastian Vogel).
^ Robert F. Schmidt , Florian Lang, Manfred Heckmann: Physiologie des Menschen. with pathophysiology . 31st edition. SpringerMedizin Verlag, Heidelberg 2010, ISBN 978-3-642-01650-9 .
↑ Hermann Rein and Max Schneider : Introduction to Human Physiology . Springer, Berlin, ¹⁵ 1964, p. 22 on Stw. "Erythrozytenform".
↑ Systematics of the vertebrate classes , overview
↑ Ehud Skutelsky, David Danon: An electron microscopic study of nuclear elimination from the late erythroblast. In: The Journal of Cell Biology. Volume 33, No. 3, 1967, pp. 625-635, doi: 10.1083 / jcb.33.3.625 .

[1] Wilhelm Gemoll: Greek-German School and Handbook , Munich / Vienna 1965.

[Tierphys-2] Christopher D. Moyes, Patricia M. Schulte: Tierphysiologie . Pearson Studium, Munich 2008, ISBN 978-3-8273-7270-3 , pp. 426 ( limited preview in the Google book search - English: Principles of Animal Physiology . Translated by Monika Niehaus , Sebastian Vogel).

[3] Robert F. Schmidt , Florian Lang, Manfred Heckmann: Physiologie des Menschen. with pathophysiology . 31st edition. SpringerMedizin Verlag, Heidelberg 2010, ISBN 978-3-642-01650-9 .

[EPM-4] Hermann Rein and Max Schneider : Introduction to Human Physiology . Springer, Berlin, ¹⁵ 1964, p. 22 on Stw. "Erythrozytenform".

[5] Systematics of the vertebrate classes , overview

[6] Ehud Skutelsky, David Danon: An electron microscopic study of nuclear elimination from the late erythroblast. In: The Journal of Cell Biology. Volume 33, No. 3, 1967, pp. 625-635, doi: 10.1083 / jcb.33.3.625 .