Blood PDF

Blood It is a dark, viscous and slightly alkaline shiny liquid that circulates within the circulatory system. It is a specialized connective tissue composed of blood cells (erythrocytes, leukocytes and platelets) and a liquid matrix, plasma. Since it is circulating through the body, it serves as...

Blood It is a dark, viscous and slightly alkaline shiny liquid that circulates within the circulatory system. It is a specialized connective tissue composed of blood cells (erythrocytes, leukocytes and platelets) and a liquid matrix, plasma. Since it is circulating through the body, it serves as a vehicle for the transport of substances. • It transports nutrients from the digestive system to the whole body and then carries the waste products to other organs for disposal. • It also transports hormones and other signaling molecules between cells, as well as respiratory gases (O2 and CO2) and immune cells. • In addition, it helps regulate body temperature and osmotic balance of tissues. 54% plasma 1% platelets + leukocytes 45 % erithrocytes Plasma is a yellowish liquid in which cells, platelets, organic compounds and electrolytes are dissolved or suspended. It is composed approximately 90% by water. Another 9% are proteins and the remaining 1% are nutrients and respiratory gases. The plasma proteins help maintain the osmotic pressure of the blood and thus maintain normal blood volume. The plasma leaves the blood capillaries and passes to the connective tissue as tissue fluid, which will have a composition similar to plasma. The passage of fluid from the blood is limited due to a greater concentration of proteins in the blood compared to the tissue; therefore, blood volume is maintained. Erythrocytes (red blood cells) They are the most abundant formed elements of the blood. They have no nucleus or organelles, but there are numerous enzymes that will participate in obtaining ATP to cover the energy needs of the erythrocyte. They have a lifespan of about 120 days, then they are destroyed in the liver, spleen or bone marrow. They have a biconcave disc shape, which gives them a high surface-volume ratio: • Provides a large surface to facilitate gas exchange. • Adaptation to pass through narrow capillaries without breaking. This shape is maintained and adapted thanks to actin filaments of the cytoskeleton, which will spend ATP after contraction, hence the important energy needs of these cells. Transport of O2 and CO2 The erythrocytes are full of hemoglobin, a protein formed by 4 chains, each linked to a heme group, which contains iron. This is what gives red color to erythrocytes and blood in general. This heme group protects iron from being oxidized, while allowing this molecule to bind oxygen to transport it through the blood. In the lungs, where there will be abundant O2, the hemoglobin will bind 4 molecules of O2, forming oxyhemoglobin. When it reaches the tissues, where there is little O2 and abundant CO2, the O2 is released and the hemoglobin will bind CO2, forming carbaminhemoglobin. Heme group Leukocytes (white blood cells) Leukocytes are larger than erythrocytes, much less numerous, have nuclei and organelles and are not functional in the blood, but they use it as a means of transport to reach the tissues where they will perform their immune function. These cells are classified into two large groups: - Granulocytes or polymorphonuclear cells: present specific granules in their cytoplasm. • Neutrophils (55-70%) • Eosinophils (1-4%) • Basophils (0.2-1.2%) - Agranulocytes: do not present specific granules. • Monocytes (2-8%) • Lymphocytes (17-45%) Neutrophils They are the most numerous leukocytes. They have a multilobed nucleus and specific granules that do not stain with usual dyes. These granules contain enzymes that help the neutrophil in its antibacterial functions. They also contain nonspecific granules that are enzymes. They are attracted by chemotactic substances to the places of infection, where they will bind to selectins to leave the blood vessel where they circulate. Once in the tissue, these cells phagocytize bacteria and release hydrolytic enzymes and leukotrienes, which will initiate the inflammatory process. Once bacteria are eliminated, neutrophils die since they have little capacity to regenerate its enzymes and lysosomes. The accumulation of leukocytes and dead bacteria with tissue fluid form pus. NEUTROPHILS Phagocytosed Bacteria (cocos) Eosinophils They have a bilobed nucleus and specific granules that are stained with acid dyes (eosin). In its membrane they have receptors for IgG, IgE and for the complement. Its specific granules contain the major basic protein, effective to fight parasites and its nonspecific granules are lysosomes with hydrolytic enzymes. Histamine, leukotrienes or eosinophil chemotactic factor attract them to the site of the allergic, inflammatory or parasitic infection. There they pour their major basic protein that attacks parasitic worms. In addition, eosinophils degrade antigen-antibody complexes by moderating the inflammatory or allergic response. Basophils They have an S-shaped nucleus that is usually masked by specific granules that are stained with basic dyes, as well as the nucleus. These granules have heparin, histamine, eosinophil chemotactic factor, neutrophil chemotactic factor, protease and peroxidase. In their membrane they have IgE receptors. In the first exposure to an antigen, IgE specific for that antigen is synthesized and bound to the basophil membrane. In a second exposure, the antigen binds directly to the IgE of the basophil membrane, which will release the content of its granules. An inflammatory response is initiated, neutrophils and eosinophils are attracted, histamine produces vasodilation, increased capillary permeability and bronchoconstriction, and heparin makes blood flow better. Leukocytes They are classified into two large groups: - Granulocytes or polymorphonuclear cells: present specific granules in their cytoplasm. • Neutrophils (55-70%) • Eosinophils (1-4%) • Basophils (0.2-1.2%) - Agranulocytes: do not present specific granules. • Monocytes (2-8%) • Lymphocytes (17-45%) Both cell types have nonspecific granules that are lysosomes. Monocytes They are agranulocytes. They are the largest blood cells. Its nucleus is large, eccentric and kidney-shaped. They remain in the blood for a few days and then migrate to tissues where they differentiate into macrophages. Lymphocytes They are agranulocytes and are the smallest leukocytes, slightly larger than erythrocytes. They are rounded, with a round nucleus and scarce cytoplasm. There are three types of lymphocytes: T, B and NK cells, which are morphologically equal but with different functions and surface markers. T lymphocytes are formed in the bone marrow, then migrate to the thymus to mature and acquire immunocompetence. B lymphocytes and NK cells are formed in the bone marrow and go directly to the tissues where they will act. Upon contact with a specific antigen, T and B lymphocytes proliferate and give rise to two populations: - Memory lymphocytes: do not participate in the immune response but remain in the body as memory cells; upon a second exposure to that specific antigen, they can generate a response quickly. - Effector cells: those that perform the immune response. When activated, B lymphocytes differentiate into plasma cells that secrete antibodies (Ig) against the specific antigen. T lymphocytes can be cytotoxic, which attack foreign cells or cells altered by viruses and destroy them, or they can be helpers, which activate B lymphocytes. The NK lymphocytes attack foreign cells, or cells altered by viruses, and destroy them. Platelets They are the smallest formed elements of the blood. They are disc-shaped fragments of another larger cell formed in the bone marrow (megakaryocyte). They have no nucleus, but many granules. The peripheral zone of the platelet is lighter and is called hyalomere, whereas the central one, that is where the granules are, is darker and is called granulomere. In the hyalomer there are microtubules arranged in parallel forming a ring and actin and myosin that can be quickly assembled to form a contractile apparatus. The granules of the platelets are of 3 types: – α (alpha) granules: more abundant, they contain fibrinogen, coagulation factors and thrombospondin. – δ or dense granules: they contain Ca2+, serotonin, ATP and ADP. – λ granules: lysosomes. Platelets participate in hemostasis, which is a set of physiological processes that aim to prevent extravasation of blood, repair damaged vessels, keep the blood fluid and restore circulation once the vessel has been repaired. The endothelium of the vessels secretes prostacyclines and NO, which inhibit platelet aggregation, and thrombomodulin that blocks coagulation. When the endothelium suffers a lesion, these substances cease to occur and von Willebrand factor (vWF) and a vasoconstrictor are released to reduce blood flow in the area. Platelets adhere to vWF and this binds to subendothelial collagen (platelet adhesion). When adhering platelets also release the content of their granules, Ca2+, ADP and thrombospondin causes more platelets to aggregate and also release the content of their granules (platelet aggregation). The added platelets form a plug that blocks the bleeding. The coagulation factors will then activate each other in what is known as the coagulation cascade, whose final step activates fibrinogen (protein circulating in the plasma) giving rise to fibrin, which binds to the platelets forming a network and making that contracts, approaches the edges of the wound and forms a denser and more stable clot that also traps erythrocytes and leukocytes. When the vessel is repaired, the endothelial cells release enzymes that initiate thrombus removal. Hematopoiesis It is the physiological process of blood cells formation, since all of them have a limited life and must be renewed continuously. This process begins in the second week of embryonic development and will first occur in the liver and spleen. From the sixth month of development, when the skeletal system develops, it will occur in the bone marrow. After birth, the liver and spleen do not have hematopoietic activity, but they could recover it if necessary. All blood cells originate from pluripotent stem cells. These cells will give rise to multipotential stem cells that will be either common lymphoid progenitors, which give rise to all types of lymphocytes, or common myeloid progenitors, which give rise to erythrocytes, granulocytes, monocytes and megakaryocytes (platelet precursor).