Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Structure of the Human Body
Tissues
Blood and Its Functions

Blood is a type of Connective Tissue with a liquid Extracellular matrix—plasma—which contains cellular elements: erythrocytes and other Cells (Fig. 8). The function of blood is to transport oxygen and nutrients to Organs and Tissues and to remove Metabolic waste products from them.

Blood Plasma is the liquid remaining after the removal of formed elements. Blood plasma contains 90—93% Water, 7—8% of various Proteins (albumins, globulins, Lipoproteins), 0,9% salts, and 0,1% glucose. Blood plasma also contains Enzymes, Hormones, Vitamins, and other substances essential to the body.

Plasma Proteins participate in blood clotting, maintain a constant pH, contain IMMUNOGLOBULINS involved in the body's defense reactions, provide blood viscosity and stable blood pressure in the vessels, and prevent erythrocyte sedimentation.

In a healthy individual, the Blood Glucose Level is 80—120 mg % (4,44—6,66 mмоль/л). A sharp decrease in blood glucose (down to 2,22 mмоль/л) leads to a dramatic increase in the excitability of Brain cells, which can cause convulsions. A further decline in blood glucose levels leads to respiratory and Circulatory Disorders, loss of consciousness, and even death.

The Mineral Substances of blood plasma include NaCl, KCl, СаСl₂, NaHCO₂, NaH₂PO₄ and other salts, as well as Na⁺,Ca²⁺,K⁺ ions. The constant ionic composition of blood ensures stable osmotic pressure and maintains fluid volume in the blood and body cells.

Bleeding and loss of salts are dangerous for the body and its cells. Therefore, in medical practice, an isotonic saline solution is used, which has the same osmotic pressure as blood plasma (0,9% NaCl solution). More complex solutions containing a set of salts essential for the body are called not only isotonic but also isoionic. Blood-substituting solutions containing not only salts but also proteins and glucose are also used.

If erythrocytes are placed in a hypotonic solution with a low salt concentration and low osmotic pressure, water enters the erythrocytes. The erythrocytes swell, their cytolemma ruptures, and Hemoglobin is released into the blood plasma, coloring it. This red-colored plasma is known as laked blood. In a hypertonic solution with 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 non-nucleated cells incapable of division. The erythrocyte count in 1 μl of blood in adult men ranges from 3,9 to 5,5 million (5,0*10¹²/л), and in women from 3,7 to 4,9 million (4,5 X 101²/л). In certain diseases, as well as during severe blood loss, the number of erythrocytes decreases. Consequently, the hemoglobin content in the blood drops. This condition is called anemia.

In a healthy individual, the lifespan of erythrocytes is up to 120 days, after which they die and are destroyed in the Spleen. Approximately 10—15 million erythrocytes die every second. They are replaced by new, young cells formed in the Cytology/practical/86.html">Red Bone Marrow from its stem cells.

Each erythrocyte is shaped like a biconcave disk with a diameter of 7—8 μm and a thickness of 1—2 μm. Externally, erythrocytes are covered by a membrane—the Plasmalemma—through which gases, water, and other elements selectively penetrate. The Cytoplasm of erythrocytes lacks Organelles; 34% of its volume is occupied by the pigment hemoglobin, whose function is to transport oxygen (О₂) and carbon dioxide (СО₂).

Hemoglobin consists of the protein globin and a non-protein heme group containing iron. A single erythrocyte contains up to 400 million hemoglobin molecules. Hemoglobin transports oxygen from the Lungs to organs and tissues. Hemoglobin bound to oxygen (О₂) has a bright red color and is called oxyhemoglobin. Oxygen molecules bind to hemoglobin due to the high partial pressure of О₂ in the lungs. At low oxygen pressure in the tissues, oxygen dissociates from hemoglobin and diffuses from the blood capillaries into the surrounding Cells and Tissues. After releasing oxygen, the blood becomes saturated with carbon dioxide, the pressure of which is higher in the tissues than in the blood. Hemoglobin combined with carbon dioxide (СО₂) is called carbaminohemoglobin. In the lungs, carbon dioxide leaves the blood, and its hemoglobin is re-oxygenated.

Hemoglobin readily binds with carbon monoxide (CO) to form 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 with blood hemoglobin and block Oxygen transport. As a result of oxygen deficiency, the body experiences oxygen deprivation (Carbon monoxide poisoning), leading to headaches, vomiting, dizziness, loss of consciousness, and even death.

Leukocytes ("white blood cells"), like erythrocytes, are formed in the bone marrow from stem cells. Leukocytes range in size from 6 to 25 μm and are characterized by a variety of shapes, motility, and Functions. Capable of migrating from Blood Vessels into tissues and back, leukocytes participate in the body's defense reactions; they can engulf and ingest foreign particles, Cell debris, and microorganisms, digesting them. In a healthy person, 1 μl of blood contains between 3,500 and 9,000 leukocytes (3.5–9)x10⁹/l. The leukocyte count fluctuates throughout the day, increasing after meals, during physical exertion, and under intense emotions. In the morning, the number of leukocytes in the blood is reduced.

Based on cytoplasm composition and nuclear shape, leukocytes are classified into granular leukocytes (granulocytes) and nongranular leukocytes (agranulocytes). Granular leukocytes contain A large number of small granules in their cytoplasm that stain with various Dyes. Depending on how the granules react to dyes, 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 (azure); and neutrophilic leukocytes (neutrophils), which contain violet-pink granules.

Nongranular leukocytes include monocytes, which have a diameter of up to 18–20 μm. These are large cells containing nuclei of various shapes: Kidney-shaped, lobulated, or horseshoe-shaped. The cytoplasm of monocytes stains a bluish-gray color. Monocytes, which originate in the bone marrow, are precursors of tissue macrophages. The Circulation time of monocytes in the blood ranges from 36 to 104 hours.

The leukocyte group of blood cells also includes the effector cells of The Immune System—lymphocytes (see "Immune System").

In a healthy person, the 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. In Inflammatory Diseases, the leukocyte count (including lymphocytes) increases. This phenomenon is known as leukocytosis. Allergic diseases lead to an increase in the number of eosinophils, while certain other conditions cause an increase in neutrophils or basophils. When bone marrow function is suppressed—for example, by radiation, high doses of X-rays, or toxic substances—the number of leukocytes in the blood decreases. This condition is called leukemia.

Platelets (thrombocytes), measuring 2–3 μm, are present in 1 μl of blood in amounts of 250,000–350,000 (300x10⁹/l). Physical exertion and food intake increase the platelet count in the blood. Platelets lack a Nucleus. These are spherical plates capable of adhering to foreign surfaces and sticking to one another. In doing so, platelets release substances that promote blood clotting. The lifespan of platelets is up to 5–8 days.

Protective Functions of blood

Blood Coagulation. Blood flowing through undamaged blood vessels remains liquid. When a vessel is damaged, the escaping blood clots relatively quickly (within 3–4 minutes) and turns into a solid clot after 5–6 minutes. This crucial property of blood coagulation protects the body from blood loss. Clotting is associated with The conversion of the soluble protein fibrinogen, found in blood plasma, into insoluble fibrin. The fibrin protein precipitates as a network of fine threads, trapping blood cells within its mesh. This is how a clot (thrombus) is formed.

The process of blood coagulation occurs with the participation of substances released during The breakdown of platelets and tissue damage. A protein is released from damaged platelets and tissue cells, which, by interacting with Blood Plasma Proteins, is converted into active thromboplastin. The formation of thromboplastin requires the presence of, in particular, an antihemophilic factor in the blood. If this antihemophilic factor is absent or deficient in the blood, blood coagulation is low, and the blood fails to clot. This condition is known as hemophilia. Next, with the participation of the formed thromboplastin, the plasma protein prothrombin is converted into the active enzyme Thrombin. Under the action of the formed thrombin, the soluble plasma protein fibrinogen is converted into insoluble fibrin. Blood cells settle within the network of these fibrin protein fibers. To prevent blood from clotting inside blood vessels, the body has an anticoagulation system. Heparin, a substance produced in The Liver and lungs, prevents blood clotting by inactivating thrombin.

Blood Groups. Blood transfusion. In cases of blood loss due to trauma or during certain surgeries, transfusion of blood from another person (donor blood) to the individual (called the recipient) is practiced. It is crucial that the donor blood is compatible with the recipient's blood. This is because when blood from different individuals is mixed, erythrocytes entering another person's plasma may clump together (agglutinate) and then break down (hemolyze). Hemolysis is the process of destruction of the erythrocyte cytolemma (cell membrane) and the release of hemoglobin into the surrounding blood plasma. Hemolysis of erythrocytes (blood) can occur when incompatible blood groups are mixed, when a hypotonic solution is introduced into the blood, under METABOLISM/18.html">The Influence of toxic chemicals such as ammonia, gasoline, chloroform, and others, or as a result of certain snake venoms.

This is because the blood of every individual contains specific proteins capable of interacting with corresponding proteins in another person's blood. In erythrocytes, these protein substances are called agglutinogens, designated by the capital letters A and B. Blood plasma also contains protein substances called agglutinins α (alpha) and β (beta). Blood clumping (agglutination and hemolysis of erythrocytes) occurs when matching agglutinogens and agglutinins meet (A with α; B with β). Based on the presence of agglutinogens and agglutinins, human blood is classified into four groups (Table 3).

Class="center">Table 3 Classification of human blood groups

As shown in Table 3, the plasma of blood group I contains both agglutinins (α and β), while THE RED BLOOD cells of this group have no agglutinogens at all. Blood group II has agglutinin β in its plasma and agglutinogen A on its red blood cells. Blood group III has agglutinin α in its plasma and agglutinogen B on its red blood cells. Blood group IV has no agglutinins in its plasma at all, while its red blood cells contain both agglutinogens, A and B.

The blood of all four groups is equally functional and differs only in the content of agglutinogens and agglutinins. A person's blood group is constant. It does not change throughout life and is inherited. During blood transfusions, blood group compatibility must be taken into account. It is crucial that the donor's red blood cells do not agglutinate in the recipient's blood as a result of the transfusion.

Considering the presence of agglutinins and agglutinogens in the blood, group I blood can be transfused to individuals with any blood group. Therefore, people with blood group I are called universal Donors. People with blood group IV are called universal recipients; they can receive blood of any other group because there are no agglutinins in their blood plasma.

In addition to agglutinogens A and B, the red blood cells of some individuals may contain an agglutinogen known as the Rhesus factor. This factor was first discovered in the blood of rhesus macaque monkeys. The Rhesus factor is found in the blood of approximately 85% of people. The blood of such individuals is called Rhesus-positive (Rh+). Blood that lacks the Rhesus factor is called Rhesus-negative (Rh^-). The phenomenon of the Rhesus factor is that the blood of these individuals lacks substances known as anti-Rh agglutinins. If a person with Rhesus-negative blood is repeatedly transfused with Rhesus-positive blood, anti-Rh agglutinins and hemolyzing substances will form in the recipient's blood under the Influence of the donor's Rh agglutinogen. This can cause agglutination and hemolysis of red blood cells.

Thus, if a mother has Rhesus-negative blood and the fetus has Rhesus-positive blood inherited from the father, the fetal blood triggers the formation of anti-Rh agglutinins in the mother's Rhesus-negative blood. These agglutinins can cross the Placenta and destroy the fetal red blood cells. In this case, the fetus may die in the womb, or the baby may be born with so-called hemolytic jaundice.