Which organelles do red blood cells have
from ancient Greek: ἐρυθρός ("erythrós") - red
Synonyms: red blood cell, red blood cell, erythrocyte, normocyte
English: erythrocyte, red blood cell
As Erythrocytes This is the name given to the cellular elements of human blood that contain the blood pigment hemoglobin.
2.1 Shape and surface
Erythrocytes are cells without organelles that are no longer capable of cell division due to the lack of a nucleus. The early erythrocytes, which occasionally still contain residues of nuclear material, are called reticulocytes.
The cells have a biconcave, flattened shape with a diameter of around 7.5 µm and a thickness of around 2.5 µm (normal distribution according to the Price-Jones curve). They do not contain an endoplasmic reticulum, ribosomes or mitochondria. The energy is gained through anaerobic glycolysis. The main component of the erythrocytes is the protein hemoglobin, which gives them their characteristic red color and is responsible for the transport of oxygen. The amount of hemoglobin in the cytosol of a single erythrocyte is between 28 and 36 picograms (MCH). Each erythrocyte thus contains around 280 million hemoglobin molecules.
The special shape of the erythrocytes increases the surface area of the cells and thereby improves gas exchange. In addition, the flexibility is improved, which allows the erythrocytes to migrate through capillaries that are smaller in diameter than their own. If narrow capillaries are passed, erythrocytes can deform or, as part of pseudoagglutination, assemble into so-called rouleaux in order to pass through the vessels.
A dense structure-giving network of filaments, the erythocyte cytoskeleton, located under the cell membrane and radiating into it, enables the biconcave shape to be maintained. Some proteins, such as spectrin or ankyrin, are essential for the structure.
2.2 Form variants
In addition to the typical biconcave shape, erythrocytes can also be present in a different shape. Some shape variants are listed below:
2.3 Other morphological peculiarities
Erythrocytes are responsible for transporting oxygen in the blood. The cells take up oxygen from the alveoli in the pulmonary capillaries and bind it to the hemoglobin. The process of oxygen uptake takes place very quickly. The contact time between the erythrocyte and the alveolar space is only about 0.3 seconds.
The oxygen bound to hemoglobin is transported by the erythrocytes into the capillaries on the periphery of the body. There the oxygen is released back into the tissue, which is known as deoxygenation of the blood.
To a lesser extent, erythrocytes are also responsible for transporting carbon dioxide. In the lungs, they emit carbon dioxide, either in the form of bicarbonate or bound to hemoglobin.
Erythrocytes promote the formation of bicarbonate in the blood because they contain α-carbon anhydrases, which cause the reaction of carbon dioxide and water to carbonic acid, which takes place very slowly in the plasma, by around the 10th century7- accelerate times.
3.2 Life cycle
Erythrocytes are formed in the bone marrow in the course of erythropoiesis from hematopoietic stem cells (CFU-E), pass through various intermediate stages (e.g. proerythroblast, macroblast, basophilic erythroblast) up to reticulocytes. This still contains residues of rRNA, which are visible under a light microscope as fine lattice-like structures (substantia granulofilamentosa). In the peripheral blood, the final maturation to nucleated and organelle-free erythrocytes takes place.
The hormone erythropoietin produced by the kidneys is necessary for their development. Erythrocytes are consumption cells. After a lifespan of around 120 days, they are broken down by macrophages in the liver, spleen and bone marrow. When they are exposed to stressors, their lifespan is shortened. The erythrocytes then go into eryptosis (apoptosis of the erythrocytes) and are then broken down. The iron contained in them is temporarily stored by the macrophages in the form of hemosiderin and reused.
Since erythrocytes do not contain mitochondria, they have to generate the energy necessary for their metabolism in other ways. Energy is therefore generated using anaerobic glycolysis. The pyruvate formed from glucose is reduced to lactate. Erythrocytes contain a special isoenzyme of pyruvate kinase, which is known as PKR. It catalyzes a strongly exergonic reaction in which phosphoenolpyruvate is converted into pyruvate. If this enzyme is deficient, it leads to serious functional disorders that lead to hemolysis.
There are 24 to 30 trillion erythrocytes (24-30 x 10) circulating in a healthy adult's bloodstream12), which have a total surface area of more than 4,500 m². That corresponds to an area larger than half a soccer field. As part of erythropoiesis, around one percent of the erythrocytes are renewed every day, which corresponds to a formation rate of more than 3,000,000 erythrocytes per second.
On the membrane surface of the erythrocytes there is a dense coat of glycoproteins, which determine the blood groups of a person. These glycoproteins represent strong surface antigens that can be recognized by the immune system as endogenous or foreign. The function of many of these membrane proteins has now been clarified.
The main human blood group, the AB0 system, is based on differences in simple carbohydrate chains. In contrast to platelets, erythrocytes have few or no HLA antigens.
5 laboratory diagnostics
5.1 Red blood cell count
The erythrocyte count is one of the most commonly determined laboratory values for human blood and is determined by automatic counting devices. Their reference value is:
- in men 4.8-5.9 million / µl or 4.8-5.9 / pl
- in women 4.3-5.2 million / µl or 4.3-5.2 / pl
The values may differ for children. Newborns often have physiological polyglobules with an erythrocyte count of 4.5 to 6.5 million / µl.
5.2 Red cell indices
The indices MCH, MCV and MCHC are further important parameters for the characterization of erythrocytes:
If other laboratory values such as hematocrit and hemoglobin concentration are not available anyway, the third can be calculated from two of the values. The following applies here:
5.3 Width of erythrocyte distribution
The erythrocyte distribution width (EVB) describes the range between the smallest and largest erythrocytes. An increased EVB is an indication of anisocytosis.
5.4 Erythrocyte morphology
More precise statements about the erythrocyte morphology can be made microscopically in the so-called "red differential blood count". For this purpose, a blood smear is made.
The graphic representation of the erythrocyte measurement values with the help of a hematology device is called an erythrocytogram. The individual results for erythrocyte volume and hemoglobin content are output as a point cloud.
5.6 Metabolic activity
In the case of certain questions, intracellular parameters in erythrocytes can be examined, e.g. B. folic acid or thiopurine methyltransferase.
Newborns still have fetal erythrocytes whose characteristics differ from those of normal, adult erythrocytes. The erythrocytes of newborns are macrocytic, the MCV is 99-113 fl. The MCH (33.0-38.0 pg) is also higher than that of adults. Because of these characteristics, the life of the erythrocytes in newborns is shortened to 70-80 days because they are less malleable than adult erythrocytes.
The newborn's erythrocytes contain around 80% fetal hemoglobin (HbF) and only around 20% HbA. Due to the fetal hemoglobin, the erythrocytes have an increased affinity for oxygen, i.e. a better O2-Recording, but a worse O2-Delivery. The proportion of HbF is higher in premature infants than in mature newborns. After the 15th day of life, the HbF percentage decreases continuously and at the age of 6 months it almost reaches the values of adults.
A pathologically reduced number of erythrocytes is referred to as erythrocytopenia, an increased number as erythrocytosis or polyglobulia. Metabolic disorders, iron or vitamin deficiencies, myeloproliferative diseases and genetic changes (e.g. sickle cell anemia, thalassemia, spherical cell anemia) can lead to pathological changes in the erythrocytes and / or to erythrocyte deficiency and thus to a lack of hemoglobin. These disorders are summarized in clinical medicine under the term "anemia".
Conversely, myeloproliferative diseases can also lead to an increased formation of erythrocytes - for example in erythremia or polycythemia vera.
In the case of glucose-6-phosphate dehydrogenase deficiency, the intake of certain drugs (e.g. acetylsalicylic acid) or food leads to haemolysis.
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