Blood

Blood is a type of connective tissue. Its intercellular substance is liquid - it is blood plasma. The blood plasma contains ("floats") its cellular elements: erythrocytes, leukocytes, and thrombocytes (platelets). A person weighing 70 kg has an average of 5.0-5.5 liters of blood (this is 5-9% of the total body weight). Blood performs the following functions: transporting oxygen and nutrients to organs and tissues and removing metabolic products from them.

Blood consists of plasma, which is the liquid remaining after the formed elements - cells - have been removed from it. It contains 90-93% water, 7-8% of various protein substances (albumins, globulins, lipoproteins, fibrinogen), 0.9% salts, 0.1% glucose. Blood plasma also contains enzymes, hormones, vitamins and other substances necessary for the body. Plasma proteins participate in the blood clotting process, ensure the constancy of its reaction (pH 7.36), pressure in the vessels, make the blood more viscous, and prevent the sedimentation of erythrocytes. Plasma contains immunoglobulins (antibodies) that participate in the body's defense reactions.

The glucose content of a healthy person is 80-120 mg% (4.44-6.66 mmol/l). A sharp decrease in the amount of glucose (to 2.22 mmol/l) leads to a sharp increase in the excitability of brain cells. A further decrease in the glucose content in the blood leads to impaired breathing, circulation, consciousness and can be fatal for a person.

Blood also contains minerals such as: NaCI, KCI, CaCl2, NaHCO2, NaH2PO and other salts, as well as Na+, Ca2+, K+ ions. The constancy of the ionic composition of the blood ensures the stability of osmotic pressure and the preservation of the volume of fluid in the blood and body cells.

Blood also consists of formed elements (cells): erythrocytes, leukocytes, platelets.

Erythrocytes (red blood cells) are cells without a nucleus, incapable of division. In an adult man, 1 µl of blood contains 3.9-5.5 million (on average 5.0x10'ul), in women - 3.7-4.9 million (on average 4.5x1012/l) and depends on age, physical (muscular) or emotional stress, hormones that enter the blood. With severe blood loss (and some diseases), the content of bodies decreases, while the hemoglobin level decreases. This condition is called anemia.

Each erythrocyte has the shape of a biconcave disk with a diameter of 7-8 μm and a thickness of about 1 μm in the center, and up to 2-2.5 μm in the marginal zone. The surface area of one corpuscle is approximately 125 μm2. The total surface area of all erythrocytes, if the blood is 5.5 liters, reaches 3500-3700 m2. On the outside, they are covered with a semipermeable membrane (shell) - cytolemma, through which water, gases and other elements selectively penetrate. There are no organelles in the cytoplasm: 34% of its volume is the pigment hemoglobin, the function of which is to transfer oxygen (O2) and carbon dioxide (CO2).

Hemoglobin consists of the protein globin and a non-protein group - heme, which contains iron. One erythrocyte contains up to 400 million hemoglobin molecules. Hemoglobin carries oxygen from the lungs to the organs and tissues, and carbon dioxide from the organs and tissues to the lungs. Oxygen molecules, due to its high partial pressure in the lungs, attach to hemoglobin. Hemoglobin with oxygen attached to it has a bright red color and is called oxyhemoglobin. At low oxygen pressure in the tissues, oxygen detaches from hemoglobin and leaves the blood capillaries into the surrounding cells and tissues. Having given up oxygen, the blood is saturated with carbon dioxide, the pressure of which in the tissues is higher than that of the blood. Hemoglobin combined with carbon dioxide is called carbohemoglobin. In the lungs, carbon dioxide leaves the blood, the hemoglobin of which is again saturated with oxygen.

Hemoglobin easily combines with carbon monoxide (CO), forming carboxyhemoglobin. The addition of carbon monoxide to hemoglobin occurs 300 times easier than the addition of oxygen. Therefore, even a small amount of carbon monoxide in the air is enough for it to combine with hemoglobin and block the flow of oxygen into the blood. As a result of oxygen deficiency in the body, oxygen starvation (carbon monoxide poisoning) occurs and headaches, vomiting, dizziness, loss of consciousness and even death occur.

Leukocytes (white blood cells) are highly mobile, but have different morphological features. In an adult, 1 liter of blood contains from 3.8-109 to 9.0-109 leukocytes. According to outdated ideas, this number also includes lymphocytes, which have a common origin with leukocytes (from bone marrow stem cells), but are related to the immune system. Lymphocytes make up 20-35% of the total number of "white" cells in the blood (not erythrocytes).

Leukocytes in tissues actively move towards various chemical factors, among which metabolic products play an important role. When leukocytes move, the shape of the cell and nucleus changes.

All leukocytes are divided into two groups based on the presence or absence of granules in their cytoplasm: granular and non-granular leukocytes. The larger group is granular leukocytes (granulocytes), which have granularity in the form of small granules and a more or less segmented nucleus in their cytoplasm. The cells of the second group do not have granularity in the cytoplasm, their nuclei are non-segmented. Such cells are called non-granular leukocytes (agranulocytes).

Granular leukocytes show granularity when stained with both acidic and basic dyes. These are neutrophilic (neutral) granulocytes (neutrophils). Other granulocytes have an affinity for acidic dyes. They are called eosinophilic granulocytes (eosinophils). Other granulocytes are stained with basic dyes. These are basophilic granulocytes (basophils). All granulocytes contain two types of granules: primary and secondary - specific.

Neutrophils are round, their diameter is 7-9 µm. Neutrophils make up 65-75% of the total number of "white" cells (including lymphocytes). The nucleus of neutrophils is segmented, consists of 2-3 lobes or more with thin bridges between them. Some neutrophils have a nucleus in the form of a curved rod (band neutrophils). The bean-shaped nucleus is found in young (adult) neutrophils. The number of such neutrophils is small - about 0.5%.

The cytoplasm of neutrophils contains granularity, the size of the granules is from 0.1 to 0.8 μm. Some granules - primary (large azurophilic) - contain hydrolytic enzymes characteristic of lysosomes: acidic protease and phosphatase, beta-hyaluronidases, etc. Other, smaller neutrophil granules (secondary) have a diameter of 0.1-0.4 μm, contain alkaline phosphatase, phagocytins, aminopeptidases, cationic proteins. The cytoplasm of neutrophils contains glycogen and lipids.

Neutrophilic granulocytes, being mobile cells, have quite high phagocytic activity. They capture bacteria and other particles, which are destroyed (digested) by hydrolytic enzymes. Neutrophilic granulocytes live up to 8 days. They are in the bloodstream for 8-12 hours, and then go into the connective tissue, where they perform their functions.

Eosinophils are also called ascitophilic leukocytes due to the ability of their granules to be stained with acidic dyes. The diameter of eosinophils is about 9-10 μm (up to 14 μm). Blood in 1 liter contains 1-5% of the total number of "white" cells. The nucleus of eosinophils usually consists of two or, less often, three segments connected by a thin bridge. There are also band and juvenile forms of eosinophils. In the cytoplasm of eosinophils there are two types of granules: small, 0.1-0.5 μm in size, containing hydrolytic enzymes, and large granules (specific) - 0.5-1.5 μm in size, containing peroxidase, acid phosphatase, histaminase, etc. Eosinophils are less mobile than neutrophils, but they also leave the blood for tissues to the foci of inflammation. Eosinophils enter the blood and remain there for 3-8 hours. The number of eosinophils depends on the level of secretion of glucocorticoid hormones. Eosinophils are able to inactivate histamine due to histaminase, and also inhibit the release of histamine by mast cells.

Basophils entering the blood have a diameter of 9 µm. The number of these cells is 0.5-1%. The nucleus of basophils is lobed or spherical. In the cytoplasm there are granules from 0.5 to 1.2 µm in size, containing heparin, histamine, acid phosphatase, peroxidase, serotonin. Basophils participate in the metabolism of heparin and histamine, affect the permeability of blood capillaries, make the blood more viscous.

Non-granular leukocytes, or agranulocytes, include monocytes and leukocytes. Monocytes enter the blood and make up 6-8% of the total number of leukocytes and lymphocytes in the blood. The diameter of monocytes is 9-12 µm (18-20 µm in smears that contain blood). The shape of the nucleus in monocytes varies - from bean-shaped to lobed. The cytoplasm is weakly basophilic, it contains small lysosomes and pinocytotic vesicles. Monocytes originating from bone marrow stem cells belong to the so-called mononuclear phagocytic system (MPS). Monocytes enter the blood and circulate from 36 to 104 hours, then go into the tissues, where they turn into macrophages.

Thrombocytes (platelets) entering the blood are colorless round or spindle-shaped plates with a diameter of 2-3 μm. Thrombocytes are formed by separation from megakaryocytes - giant cells of the bone marrow. Blood (1 l) contains from 200-109 to 300-109 platelets. Each platelet has a hyalomer and a granulomere located in it in the form of grains about 0.2 μm in size. The hyalomer contains thin filaments, and among the accumulation of granulomere grains are mitochondria and glycogen granules. Due to the ability to break down and stick together, platelets make the blood more viscous. The lifespan of platelets is 5-8 days.

Blood also contains lymphoid cells (lymphocytes), which are structural elements of the immune system. At the same time, in scientific and educational literature, these cells are still considered non-granular leukocytes, which is clearly incorrect.

Blood contains a large number of lymphocytes (1000-4000 in 1 mm3), which predominate in the lymph and are responsible for immunity. In the body of an adult, their number reaches 6-1012. Most lymphocytes constantly circulate and enter the blood and tissues, which helps them perform the function of immune defense of the body. All lymphocytes are spherical, but differ from each other in their sizes. The diameter of most lymphocytes is about 8 μm (small lymphocytes). About 10% of cells have a diameter of about 12 μm (medium lymphocytes). In the organs of the immune system, there are also large lymphocytes (lymphoblasts) with a diameter of about 18 μm. The latter do not normally enter the circulating blood. These are young cells that are found in the organs of the immune system. The cytolemma of lymphocytes forms short microvilli. The round nucleus, filled mainly with condensed chromatin, occupies most of the cell. The surrounding narrow rim of basophilic cytoplasm contains many free ribosomes, and 10% of the cells contain a small number of azurophilic granules - lysosomes. Elements of the granular endoplasmic reticulum and mitochondria are few in number, the Golgi complex is poorly developed, the centrioles are small.

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