An erythrocyte is a blood cell that is capable of transporting oxygen to the tissues due to hemoglobin, and carbon dioxide to the lungs. This is a cell of simple structure, which is of great importance for the life of mammals and other animals. The red blood cell is the most numerous cell type in the body: about a quarter of all cells in the body are red blood cells.
General patterns of the existence of an erythrocyte
Erythrocyte - a cell that originated from a red germ of hematopoiesis. About 2.4 million of these cells are produced per day, they enter the bloodstream and begin to perform their functions. During the experiments, it was determined that in an adult, erythrocytes, the structure of which is significantly simplified compared to other cells of the body, live 100-120 days.
In all vertebrates (with rare exceptions), oxygen is transported from the respiratory organs to the tissues through the hemoglobin of erythrocytes. There are exceptions: all members of the white-blooded fish family exist without hemoglobin, although they can synthesize it. Since, at the temperature of their habitat, oxygen dissolves well in water and blood plasma, these fish do not need its more massive carriers, which are erythrocytes.
Chordata erythrocytes
A cell like an erythrocyte has a different structure depending on the class of chordates. For example, in fish, birds and amphibians, the morphology of these cells is similar. They differ only in size. The shape of red blood cells, volume, size, and the absence of some organelles distinguish mammalian cells from others found in other chordates. There is also a pattern: mammalian erythrocytes do not contain extra organelles and a cell nucleus. They are much smaller, although they have a large contact surface.
Considering the structure of frog and human erythrocytes, common features can be immediately identified. Both cells contain hemoglobin and are involved in oxygen transport. But human cells are smaller, they are oval and have two concave surfaces. Frog erythrocytes (as well as birds, fish and amphibians, except salamander) are spherical, they have a nucleus and cellular organelles that can be activated when necessary.
In human erythrocytes, as in the red blood cells of higher mammals, there are no nuclei and organelles. The size of erythrocytes in a goat is 3-4 microns, in humans - 6.2-8.2 microns. In amphium (tailed amphibian), the cell size is 70 microns. Clearly, size is an important factor here. The human erythrocyte, although smaller, has a greatersurface due to two concavities.
The small size of the cells and their large number made it possible to multiply the ability of the blood to bind oxygen, which is now little dependent on external conditions. And such structural features of human erythrocytes are very important, because they allow you to feel comfortable in a certain habitat. This is a measure of adaptation to life on land, which began to develop even in amphibians and fish (unfortunately, not all fish in the process of evolution were able to populate the land), and reached its peak in higher mammals.
The structure of human erythrocytes
The structure of blood cells depends on the functions assigned to them. It is described from three angles:
- Features of the external structure.
- Component composition of an erythrocyte.
- Internal morphology.
Outwardly, in profile, an erythrocyte looks like a biconcave disk, and in full face - like a round cell. The diameter is normally 6, 2-8, 2 microns.
More often in the blood serum there are cells with small differences in size. With a lack of iron, the run-up decreases, and anisocytosis is recognized in the blood smear (many cells with different sizes and diameters). With a deficiency of folic acid or vitamin B12 the erythrocyte increases to a megaloblast. Its size is approximately 10-12 microns. The volume of a normal cell (normocyte) is 76-110 cubic meters. microns.
The structure of erythrocytes in the blood is not the only feature of these cells. Much more important is their number. The small size allowed to increase their number and, consequently, the area of the contact surface. Oxygen is more actively captured by human erythrocytes than frogs. And most easily it is given in tissues from human erythrocytes.
Quantity really matters. In particular, an adult has 4.5-5.5 million cells per cubic millimeter. A goat has about 13 million red blood cells per milliliter, while reptiles have only 0.5-1.6 million, and fish have 0.09-0.13 million per milliliter. A newborn baby has about 6 million red blood cells per milliliter, while an older child has less than 4 million per milliliter.
RBC functions
Red blood cells - erythrocytes, the number, structure, functions and developmental features of which are described in this publication, are very important for humans. They implement some very important features:
- transport oxygen to tissues;
- carry carbon dioxide from tissues to lungs;
- bind toxic substances (glycated hemoglobin);
- participate in immune reactions (immune to viruses and due to reactive oxygen species can have a detrimental effect on blood infections);
- capable of tolerating some drugs;
- participate in the implementation of hemostasis.
Let's continue to consider such a cell as an erythrocyte, its structure is maximally optimized for the implementation of the above functions. It is as light and mobile as possible, has a large contact surface for gas diffusion.and the course of chemical reactions with hemoglobin, as well as rapidly dividing and replenishing losses in peripheral blood. This is a highly specialized cell, the functions of which cannot yet be replaced.
RBC membrane
A cell like an erythrocyte has a very simple structure, which does not apply to its membrane. It is 3 layers. The mass fraction of the membrane is 10% of the cell. It contains 90% proteins and only 10% lipids. This makes erythrocytes special cells in the body, since in almost all other membranes, lipids predominate over proteins.
The volumetric shape of erythrocytes due to the fluidity of the cytoplasmic membrane can change. Outside the membrane itself is a layer of surface proteins with a large number of carbohydrate residues. These are glycopeptides, under which there is a bilayer of lipids, with their hydrophobic ends facing in and out of the erythrocyte. Under the membrane, on the inner surface, there is again a layer of proteins that do not have carbohydrate residues.
Erythrocyte receptor complexes
The function of the membrane is to ensure the deformability of the erythrocyte, which is necessary for capillary passage. At the same time, the structure of human erythrocytes provides additional opportunities - cellular interaction and electrolyte current. Proteins with carbohydrate residues are receptor molecules, thanks to which erythrocytes are not "hunted" by CD8-leukocytes and macrophages of the immune system.
Erythrocytes exist thanks to receptors and are not destroyed by their own immunity. And when, due to repeated pushing through the capillaries or due to mechanical damage, erythrocytes lose some receptors, spleen macrophages "extract" them from the bloodstream and destroy them.
Internal structure of an erythrocyte
What is an erythrocyte? Its structure is no less interesting than its functions. This cell is similar to a bag of hemoglobin bounded by a membrane on which receptors are expressed: clusters of differentiation and various blood groups (according to Landsteiner, rhesus, Duffy and others). But inside the cell is special and very different from other cells in the body.
The differences are as follows: erythrocytes in women and men do not contain a nucleus, they do not have ribosomes and an endoplasmic reticulum. All these organelles were removed after filling the cell cytoplasm with hemoglobin. Then the organelles turned out to be unnecessary, because a cell with a minimum size was required to push through the capillaries. Therefore, inside it contains only hemoglobin and some auxiliary proteins. Their role has not yet been clarified. But due to the lack of an endoplasmic reticulum, ribosomes and a nucleus, it has become light and compact, and most importantly, it can easily deform along with a fluid membrane. And these are the most important structural features of red blood cells.
RBC life cycle
The main features of erythrocytes are their short life. They cannot divide and synthesize protein due to the nucleus removed from the cell, and therefore structuraldamage to their cells accumulates. As a result, erythrocytes tend to age. However, the hemoglobin that is captured by spleen macrophages at the time of RBC death will always be sent to form new oxygen carriers.
The life cycle of a red blood cell begins in the bone marrow. This organ is present in the lamellar substance: in the sternum, in the wings of the ilium, in the bones of the base of the skull, and also in the cavity of the femur. Here, a precursor of myelopoiesis with a code (CFU-GEMM) is formed from a blood stem cell under the action of cytokines. After division, she will give the ancestor of hematopoiesis, denoted by the code (BOE-E). It forms the precursor of erythropoiesis, which is designated by the code (CFU-E).
The same cell is called the colony-forming cell of the red blood germ. It is sensitive to erythropoietin, a hormonal substance secreted by the kidneys. Increasing the amount of erythropoietin (according to the principle of positive feedback in functional systems) accelerates the processes of division and production of red blood cells.
Formation of red blood cells
The sequence of cellular bone marrow transformations of CFU-E is as follows: an erythroblast is formed from it, and from it - a pronormocyte, giving rise to a basophilic normoblast. As the protein accumulates, it becomes a polychromatophilic normoblast and then an oxyphilic normoblast. After the nucleus is removed, it becomes a reticulocyte. The latter enters the bloodstream and differentiates (matures) to a normal erythrocyte.
Destruction of red blood cells
Approximately 100-125 days the cell circulates inblood, constantly carries oxygen and removes metabolic products from tissues. It transports carbon dioxide bound to hemoglobin and sends it back to the lungs, filling its protein molecules with oxygen along the way. And as it gets damaged, it loses phosphatidylserine molecules and receptor molecules. Because of this, the erythrocyte falls "under the sight" of the macrophage and is destroyed by it. And heme, obtained from all the digested hemoglobin, is again sent for the synthesis of new red blood cells.
