Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Structure of the Human Body
Tissues, Organs, Organ Systems, and Apparatuses
Connective Tissue - Blood and Its Functions

Blood is a type of Connective Tissue featuring a fluid intercellular matrix that contains cellular elements such as erythrocytes and other Cells (Fig. 11). The function of blood is to transport oxygen and nutrients to Organs and Tissues, and to remove Metabolic waste products from them. Blood consists of primary components: plasma (the liquid intercellular matrix) and the cells suspended within it.

Blood Plasma is the liquid remaining after cellular elements are removed. Blood plasma contains 90–93% Water, 7–8% various protein substances (albumins, globulins, Lipoproteins), 0.9% salts, and 0.1% glucose. Blood plasma also contains Enzymes, Hormones, Vitamins, and other substances essential for the body.

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Fig. 11. Blood Cells:

1 — basophilic granulocyte; 2 — acidophilic granulocyte; 3 — segmented neutrophilic granulocyte; 4 — erythrocyte; 5 — monocyte; 6 — platelets; 7 — lymphocyte

Blood Plasma Proteins maintain the constancy of the blood composition (pH), ensure blood viscosity and a specific level of vascular pressure, prevent erythrocyte sedimentation, and contain IMMUNOGLOBULINS involved in the body's immune responses.

The Blood Glucose Level in a healthy person ranges from 80–120 mg % (4.44–6.66 mmol/L). A sharp decrease in blood glucose (down to 2.22 mmol/L) leads to increased excitability of Brain cells and the onset of convulsions. Further reduction in blood glucose levels leads to impaired Respiration and Circulation, loss of consciousness, and even death.

The mineral components of blood plasma include NaCl, KCl, CaCl, NaHCO₂, NaH₂PO₄, and other salts, as well as Na⁺, Ca²⁺, and K⁺ ions. The constancy of the ionic composition of blood ensures the stability of osmotic pressure and maintains fluid volume within the blood and body cells.

Hemorrhage and salt loss pose serious dangers to the body and its cells. Therefore, in medical practice, blood loss is treated using an isotonic saline solution that has the same osmotic pressure as blood plasma (0.9% NaCl solution). More complex solutions containing a set of essential salts and ions are referred to not only as isotonic but also as isoionic. Replacement fluids containing proteins and glucose In addition to salts are also widely used. It is well established that placing erythrocytes into a hypotonic solution with a low salt concentration and low osmotic pressure causes water to enter the cells. As a result, the erythrocytes swell, their Cell/33.html">Plasma Membrane ruptures, and Hemoglobin is released into the blood plasma, staining it red. This red-tinted plasma is known as laked blood. In a hypertonic solution featuring a high salt concentration and high osmotic pressure, water leaves the erythrocytes, causing them to shrink.

The formed elements (cells) of blood include erythrocytes, leukocytes, and Blood Platelets (thrombocytes).

Erythrocytes (red blood cells) are anucleate cells incapable of division. The erythrocyte count per 1 µL of blood in adult males ranges from 3.9 to 5.5 million (5.0 ∙ 10¹²/L), and in females from 3.7 to 4.9 million (4.5 ∙ 10¹²/L). In certain diseases and cases of severe blood loss, the erythrocyte count decreases, leading to a drop in blood hemoglobin levels. This condition is known as anemia.

In a healthy person, the lifespan of an erythrocyte reaches up to 120 days. Subsequently, erythrocytes perish and are destroyed in the Spleen. They are replaced by new, young cells generated in the Cytology/practical/86.html">Red Bone Marrow from its stem cells.

Each erythrocyte has the shape of a biconcave disk with a diameter of 7–8 µm and a central thickness of 1–2 µm. The erythrocyte is enveloped in a plasma membrane that selectively permits the passage of gases, water, and other substances. The erythrocyte Cytoplasm lacks Organelles; 34% of its volume consists of the pigment hemoglobin, whose function is to transport oxygen (O₂) and carbon dioxide (CO₂). Hemoglobin consists of the protein globin and a non-protein iron-containing heme group. A single erythrocyte contains up to 400 million hemoglobin molecules. Hemoglobin transports oxygen from the Lungs to organs and tissues. Hemoglobin bound to oxygen (O₂) is bright red and called oxyhemoglobin. Oxygen molecules bind to hemoglobin due to its high partial pressure in the lungs. At low oxygen pressure in tissues, oxygen dissociates from hemoglobin and diffuses from blood capillaries into the surrounding Cells and Tissues. Having released oxygen, the blood becomes saturated with carbon dioxide, the pressure of which is higher in tissues than in blood. Hemoglobin bound to carbon dioxide (CO₂) is called carbaminohemoglobin. In the lungs, carbon dioxide leaves the blood, and hemoglobin is re-oxygenated.

Hemoglobin readily combines with carbon monoxide (CO), forming carboxyhemoglobin. The binding of carbon monoxide to hemoglobin occurs 300 times more easily and rapidly than that of oxygen. Therefore, even a small amount of carbon monoxide in the air is sufficient to bind to blood hemoglobin and block oxygen uptake. As a result of this oxygen deprivation, the body suffers from oxygen starvation (Carbon monoxide poisoning), leading to headaches, vomiting, dizziness, loss of consciousness, and even death.

Leukocytes ("white" blood cells), much like erythrocytes, originate in the bone marrow from stem cells. Leukocytes range in size from 6 to 25 µm and vary significantly in shape, motility, and function. By virtue of their ability to migrate out of Blood Vessels into tissues and return, leukocytes participate in the body's defense mechanisms. They can engulf and absorb foreign particles, cellular debris, and microorganisms, and digest them. A healthy individual has between 3,500 and 9,000 leukocytes per 1 µL of blood [(3.5 — 9) ∙ 10¹²/L]. Leukocyte counts fluctuate throughout the day, increasing after meals, during physical exertion, and under strong emotions, while reaching lower levels in the morning hours.

Based on cytoplasmic composition and nuclear Morphology, leukocytes are divided into granulocytes and agranulocytes. Granulocytes feature numerous small cytoplasmic granules that stain with various Dyes. Based on dye affinity, they are classified into eosinophilic leukocytes (eosinophils), whose granules stain bright pink with eosin; basophilic leukocytes (basophils), whose granules stain dark blue or purple with basic dyes (such as azure); and neutrophilic leukocytes (neutrophils), which contain violet-pink granules.

Agranulocytes include monocytes, which measure up to 18–20 µm in diameter. These are large cells containing a Nucleus of various shapes: Kidney-shaped, lobular, or horseshoe-shaped. The cytoplasm of monocytes stains a bluish-gray color. Of bone marrow origin, monocytes serve as precursors to tissue macrophages. The transit time of monocytes in the circulation ranges from 36 to 104 hours.

Lymphocytes—the active cells of The Immune System—are still traditionally classified within the leukocyte group (see "Immune System").

In a healthy person, blood contains 60–70% neutrophils, 1–4% eosinophils, 0–0.5% basophils, and 6–8% monocytes. Lymphocytes account for 25–30% of all white blood cells. During Inflammatory Diseases, the count of leukocytes (as well as lymphocytes) increases—a condition known as leukocytosis. Allergic conditions trigger an elevation in eosinophils, whereas certain other diseases lead to an increase in neutrophils or basophils. Conversely, suppression of bone marrow function—such as from radiation, high doses of X-rays, or toxic substances—leads to a decrease in leukocyte counts, a condition referred to as leukopenia.

Thrombocytes (blood platelets), measuring 2–3 µm, are present in the blood at a concentration of 250,000 to 350,000 per 1 µL (300 ∙ 10⁹/L). Physical exertion and food intake increase platelet counts. Platelets are anucleate, spherical microplates capable of adhering to foreign surfaces and aggregating with one another. In the process, platelets release substances that promote blood clotting. The lifespan of platelets is 5–8 days.

Functions OF BLOOD. Blood Coagulation. Blood circulating through intact blood vessels remains fluid. When a vessel is injured, the escaping blood coagulates relatively quickly (within 3–4 minutes), turning into a dense clot (thrombus) in 5–6 minutes. This vital clotting property protects the body against blood loss. Coagulation involves The conversion of soluble plasma fibrinogen into insoluble fibrin. The fibrin protein forms a fine-meshed network of delicate threads that trap blood cells, thereby forming a thrombus.

The blood clotting process involves substances released during platelet destruction and tissue injury. Damaged platelets and tissue cells release a protein that interacts with plasma proteins to form active thromboplastin. Thromboplastin formation requires the presence of specific factors in the blood, notably the antihemophilic factor. If this factor is deficient or absent, blood clotting is impaired, leading to a condition known as hemophilia. Next, with the participation of the generated thromboplastin, the plasma protein prothrombin is converted into the active enzyme Thrombin. Thrombin then acts upon soluble plasma fibrinogen, converting it into insoluble fibrin. To prevent intravascular blood clotting, the body maintains an anticoagulant system. The Liver and lungs produce heparin, a substance that inhibits coagulation by converting thrombin into an inactive state.

Blood Groups. Blood Transfusion. In cases of blood loss due to trauma or other medical conditions, transfusing blood from one person (the donor) to another (the recipient) is practiced. It is crucial that the donor blood is compatible with the recipient's blood. When blood from different individuals is mixed, erythrocytes entering another person's plasma may clump together (agglutinate) and subsequently break down (hemolyze). Hemolysis refers to the destruction of the erythrocyte plasma membrane and the release of hemoglobin into the surrounding plasma. Hemolysis of erythrocytes (blood) can occur when incompatible blood groups are mixed, when a hypotonic solution is introduced into the bloodstream, under METABOLISM/18.html">The Influence of chemical toxins such as ammonia, gasoline, or chloroform, or as a result of certain snake venoms. As is well known, human blood contains specific proteins capable of interacting with Homologous proteins from another individual. In erythrocytes, these protein substances are called agglutinogens and are designated by the capital letters A and B. Blood plasma also contains protein substances known as agglutinins: a (alpha) and β (beta). Agglutination and hemolysis occur when corresponding agglutinogens and agglutinins meet (A and a; B and β). Taking into account the presence of specific agglutinins and agglutinogens, human blood is divided into four main groups (Table 4).

Table 4 Classification of human blood groups

As shown in Table 4, the plasma of group I (O) blood contains both agglutinins (α and β), whereas red blood cells of this group lack agglutinogens entirely. Group II (A) blood plasma contains agglutinin β, and its red blood cells carry agglutinogen A. Group III (B) blood plasma contains agglutinin α, and its red blood cells carry agglutinogen B. Group IV (AB) blood plasma contains no agglutinins at all, while its red blood cells contain both agglutinogens, A and B.

Blood of all four groups is equally functional, differing only in the presence of specific agglutinogens and agglutinins. A person's blood type remains constant throughout life and is genetically inherited. Blood compatibility must be strictly considered during transfusions. It is crucial that donor red blood cells do not clump together (agglutinate) in the recipient's bloodstream.

Considering the presence of agglutinins and agglutinogens, individuals with group I (O) blood are referred to as universal Donors. Individuals with group IV (AB) blood are called universal recipients; they can receive blood of any other group because their blood plasma contains no agglutinins.

In addition to agglutinogens A and B, THE RED BLOOD cells of certain individuals may contain an agglutinogen known as the Rhesus factor (Rh). This factor was first discovered in the blood of rhesus macaques. The Rhesus factor is present in approximately 85% of people, whose blood is designated as Rhesus-positive (Rh⁺). Blood lacking the Rhesus factor is called Rhesus-negative (Rh⁻). The Significance of the Rhesus factor lies in the fact that normal blood does not inherently contain anti-Rhesus agglutinins. If a Rhesus-negative individual receives a repeat transfusion of Rhesus-positive blood, the donor's Rhesus agglutinogens stimulate The production of anti-Rhesus agglutinins and hemolysins in the recipient's blood. This can lead to agglutination and hemolysis of red blood cells. For instance, if a mother has Rhesus-negative blood and the fetus has Rhesus-positive blood inherited from the father, the fetal blood can trigger the production of anti-Rhesus agglutinins in the mother's Rhesus-negative circulation. These agglutinins can cross the Placenta and destroy the fetal red blood cells. In such cases, the fetus may die in utero, or the child may be born with hemolytic disease of the newborn.