Blood

CONTROL OF BLOOD CLOTTING

Blood clotting is controlled by several different mechanisms, involving various regulatory substances: Vitamin K Prostacyclin Anticoagulants Thrombosis Embolism

BLOOD GROUPS

Blood is classified according to the presence of different genetically determined glycoprotein and glycolipid antigens on the outer surface of red blood cell plasma membranes. These glycoprotein antigens are called agglutinogens, of which there are various types. The type of agglutinogen exhibited on the surface of red blood cells determines which blood group they belong to. The two major blood groups are the ABO blood group and Rh blood group. The transfusion of an incompatible blood type can be life threatening.

Nitric oxide

Nitric oxide (NO) is a gaseous signaling molecule synthesized by an enzyme within the plasma membrane of the endothelial cells of blood vessel walls. It binds to hemoglobin and is released in response to changes in blood flow and blood pressure. NO released from hemoglobin triggers local vasodilation, which improves blood flow and increases oxygen supply to nearby cells.

Rh BLOOD GROUPS

The Rh blood group is based determined on the presence or absence of a specific Rh agglutinogen. This Rh agglutinogen (historically known as Rhesus factor) is known as Rh factor or the type D antigen.

Lymphoid stem cells

also derived from multipotent stem cells, begin development in the red bone marrow, proliferating and differentiating into lymphoblasts, but migrate to lymphatic tissue before completing their differentiation into lymphocytes.

Leukocytes

also known as white blood cells, are larger than red blood cells and less abundant, with their numbers increasing during infection. They are also produced in the bone marrow. In a healthy adult, the total white blood cell count ranges between 5000-10000 cells/μl of blood. There are two main types of white blood cells: granulocytes or granular leukocytes, and agranulocytes or agranular leukocytes

Myeloid stem cells

are derived from multipotent stem cells and begin their development in red bone marrow, proliferating and differentiating into erythroblasts.

Myeloid stem cells

are derived from multipotent stem cells and begin their development in red bone marrow, proliferating and differentiating into myeloblasts and monoblasts, and ultimately giving rise to neutrophils, eosinophils, basophils, and monocytes, along with red blood cells and platelets.

ABO BLOOD GROUPS

ABO blood groups are based on the presence or absence of type A and type B agglutinogens.

Agranular leukocytes (agranulocytes)

Agranular leukocytes are a type of white blood cell containing large nuclei and small cytoplasmic granules not visible under a light microscope.

multipotent stem cells

All red blood cells arise, also known as hemocytoblasts

multipotent stem cells (hemocytoblasts)

All white blood cells arise from

Anticoagulants

Anticoagulants are substances that inhibit blood clotting. These include antithrombin, which inhibits clotting factors XII, X, and II, heparin, which inhibits thrombin, and activated protein C, which inactivates other major clotting factors and stimulates plasminogen activators.

Granular leukocytes (granulocytes)

Granular leukocytes are a type of white blood cell containing large nuclei and specialized enzyme-filled granules within their cytoplasm.

HEMOSTASIS

Hemostasis is a vital physiological response that prevents localized hemorrhage, or mass blood loss, after tissue injury. There are 3 stages of hemostasis: vascular spasm, platelet plug formation, and coagulation (blood clotting):

Type O

Red blood cell plasma membranes display neither type A nor type B agglutinogens.

Type A

Red blood cell plasma membranes display only type A agglutinogens.

Type Rh-

The remaining 15% of the population lack Rh agglutinogen and are thus as Rh negative (Rh-).

Thrombopoietin

Thrombopoietin is synthesized and secreted by cells in the liver. It targets red bone marrow, stimulating the development of megakaryocytes into platelets

Thrombosis

Thrombosis is the clotting of a vessel that has not ruptured. A clot, or thrombus, may form within a blood vessel due to damaged lining endothelial cells caused by trauma, infection, or atherosclerosis, or due to the accumulation of clotting factors in slow flowing blood.

Type AB

Type AB Red blood cell plasma membranes display type A and type B agglutinogens.

Type B

Type B Red blood cell plasma membranes display only type B agglutinogens.

Type Rh+

Up to 85% of the population display Rh agglutinogen on their red blood cell plasma membranes and are thus as Rh positive (Rh+).

ERYTHROPOIESIS

is the process by which red blood cells (erythrocytes) are produced. In adults, new red blood cells are produced in the bone marrow of the sternum, vertebrae, ribs, base of the skull, and the proximal ends of the long bones.

LEUKOPOIESIS

is the process by which white blood cells (leukocytes) are developed from stem cells in red bone marrow. Once mature, they are released into the bloodstream to defend the body from disease-causing micro-organisms, such as bacteria and viruses, and to respond to any tissue damage or foreign objects.

Embolism

A thrombus may break away from the side of a vessel and travel in the bloodstream as an embolus. Embolus is the term given to any piece of debris traveling in the blood, such as pieces of bone, fat, tissue, or even bubbles of air. If an embolus becomes lodged in a blood vessel, it has the potential to block the flow of blood and cause an infarction.

Anti-A and Anti-B agglutinins

Agglutinins are antibodies present in the blood plasma that the body produces against type A and type B agglutinogens. The body produces anti-A antibodies that bind to type A agglutinogens and anti-B antibodies that bind to type B agglutinogens. The body produces agglutinins against agglutinogens that are not expressed on the surface of its red blood cells ABO blood groups are based on the presence or absence of type A and type B agglutinogens. Type A Type A Red blood cell plasma membranes display only type A agglutinogens. Type B Type B Red blood cell plasma membranes display only type B agglutinogens. Type AB Type AB Red blood cell plasma membranes display type A and type B agglutinogens. Type O Type O Red blood cell plasma membranes display neither type A nor type B agglutinogens. Anti-A and Anti-B agglutinins Anti-A and Anti-B agglutinins Agglutinins are antibodies present in the blood plasma that the body produces against type A and type B agglutinogens. The body produces anti-A antibodies that bind to type A agglutinogens and anti-B antibodies that bind to type B agglutinogens. The body produces agglutinins against agglutinogens that are not expressed on the surface of its red blood cells. So for instance, an individual with red blood cells that contain B agglutinogens would produce anti-A antibodies that bind to type A agglutinogens. An individual of blood type O would produce anti-A and anti-B antibodies. If blood with red blood cells expressing a different type of agglutinogen is transferred into an individual, a reaction is triggered. This is because the individual's plasma will contain antibodies that bind to the 'alien' agglutinogens.

Orthochromatic erythroblast

As further differentiation occurs and more hemoglobin is produced, polychromatic erythroblasts develop into orthochromatic erythroblasts (meaning of the same color).

3. COAGULATION (BLOOD CLOTTING)

As liquid blood leaks from the body via a damaged vessel, it forms a gel-like substance called a clot: blood cells trapped within a mesh of protein fibrin threads The formation of a clot, also known as clotting or coagulation, is stimulated and regulated by coagulation or clotting factors, such as calcium ions, enzymes synthesized by liver cells, and chemicals associated with platelets and damaged tissue. Each clotting factor activates another, producing a cascade of reactions and ultimately, producing the clotting protein fibrin. There are multiple stages in the clotting cascade, which can be activated via two different pathways, each involving a series of enzyme steps:

Basophils

Basophils are the least abundant of all white blood cells, making up less than 1% of white blood cells. A healthy adult has approximately 0 - 200 cells/µl of blood. Each has a bi- or tri-lobed nucleus and round granules varying in size. Both the nucleus and the basophilic granules stain deep blue-purple with basic dyes. Basophil granules contain heparin, histamine, and serotonin, which are inflammatory chemical messengers released at sites of inflammation. These substances enhance inflammation and elicit allergic reactions.

Anti-Rh agglutinins

Blood plasma does not normally contain anti-Rh agglutinins, however if blood of type Rh+ is transferred into an individual with type Rh- blood, the individual will begin to produce anti-Rh antibodies. A reaction will not be triggered during the first transfusion but the anti-Rh antibodies will remain in the blood plasma and will trigger a reaction if a second Rh+ transfusion is given

Cytokines

Cytokines are small glycoproteins, acting as local hormones. Examples include interleukins. Cytokines are synthesized and secreted by red bone marrow cells, endothelial cells, leukocytes, macrophages, and fibroblasts. Cytokines target red bone marrow, stimulating proliferation and differentiation of progenitor cells. They also target phagocytes, B cells, and T cells, regulating activities such as phagocytosis and controlling immune responses.

Eosinophils

Eosinophils account for 2-4% of white blood cells. A healthy adult has approximately 0-450 cells/µl of blood. Each has a bi-lobed nucleus and large, round granules. Eosinophilic granules stain an orange to red color with acidic dyes. The most commonly used acidic dye is eosin, which is how the cell gets its name Eosinophil granules contain enzymes such as histaminase, which when released, counteracts the effects of histamine during inflammatory reactions. They also have the capacity to phagocytize antigen-antibody complexes, pathogens, and parasites.

RED BLOOD CELLS

Erythrocytes, also known as red blood cells, are the most abundant cells in the blood. They are thin, disc-shaped cells, about 7.5μm in diameter, with a depression in the middle on both sides. This biconcavity increases the surface area to allow efficient diffusion of gases. The cells are small and flexible enough to squeeze through tiny capillaries. A healthy male has a total red blood count between 4.7-6.1 million red blood cells/μl blood, and a healthy female has between 4.2-5.4 million red blood cells/μl blood. Red blood cells do not have a nucleus and are absent of most organelles, leaving space for large amounts of hemoglobin (Hb). Red blood cells bind oxygen in the lungs and carry it to tissues throughout the body where it is exchanged for the waste product carbon dioxide.

Erythropoietin

Erythropoietin is a glycoprotein hormone, synthesized and secreted by the kidneys and liver in response to hypoxia. Hypoxia is caused by reduced oxygen availability, such as at high altitude, or during periods of increased metabolic demand i.e., during exercise) or due to low red blood cell count, following blood loss for example. In addition, erythropoietin is secreted into the blood and targets red bone marrow, stimulating proerythroblasts to proliferate and to differentiate into erythrocytes at a faster rate. This results in an increased reticulocyte blood count, providing the blood with a greater oxygen carrying capacity in order to counteract hypoxic conditions.

Granulocyte (myelocyte) cell lineage

Granulocytes arise from myeloid stem cells, which proliferate and differentiate into myeloblasts: the first identifiable progenitor cells in the granulocyte lineage. As myoblasts undergo further cell divisions, they accumulate lysosomes (protein filled granules) and become promyelocytes. Distinctive granules form in promyelocytes, which then differentiate into myelocytes. Proliferation ceases at this stage but differentiation continues: nuclei become arc-like, and eosinophilic, neutrophilic, and basophilic band cells are formed. Nuclei constrict, nuclear segmentation begins, and mature granulocytes (eosinophils, neutrophils, and basophils) leave the bone marrow and enter the blood. Bone marrow stores mature granulocytes which is why 10-20 times more granulocytes are found in the bone marrow than in the blood.

Hemoglobin

Hemoglobin is a molecule consisting of a protein called globin, made up of four polypeptide chains, and an organic molecule, heme, with an iron at the center, which weakly binds reversibly to oxygen molecules. Each hemoglobin molecule binds four oxygen molecules, one bound to each iron ion; a reaction that is reversed to release oxygen into interstitial fluid for diffusion into cells. In healthy adults, the normal hemoglobin level ranges between 14-18 g/dl in men, and 12-16 g/dl in women. Carbon dioxide is transported in red blood cells as bicarbonate (HCO3-). The enzyme carbonic anhydrase is carried in red blood cells and works as a catalyst in the conversion of carbon dioxide to bicarbonate through the following reactions: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- When hemoglobin combines with oxygen, it turns a characteristically bright red color, but as this oxygen is lost to the tissues of the body, it becomes dark red

Common pathway to thrombin

In the presence of Ca2+, prothrombin is converted into thrombin by the action of prothrombinase.

Common pathway to fibrin

In the presence of Ca2+, thrombin catalyzes the conversion of soluble fibrinogen into insoluble threads of fibrin. Thrombin also activates clotting factor XIII, which enhances the strength and stability of the fibrin threads. Clotting factor XII is also released by platelets in the clot

TRANSFUSION REACTION

It is important that donor and recipient blood types are matched during a transfusion or antibodies in the recipient's plasma may attack agglutinogens (antigens) on donor red blood cells, causing a transfusion reaction.

Lymphocyte cell lineage

Lymphocytes arise from lymphoid stem cells, which proliferate and differentiate into lymphoblasts, the first identifiable progenitor cells in the lymphocyte lineage. Lymphoblasts undergo cell division several times to form prolymphocytes, with a high nuclear to cytoplasmic ratio. Most prolymphocytes leave bone marrow and travel to the thymus (a lymphatic organ) where further differentiation into T lymphocytes occurs. T lymphocytes then migrate to secondary lymphoid tissue. B lymphocytes undergo differentiation entirely within bone marrow before migrating to lymphoid tissue. Bone marrow stromal cells (fibroblasts, adipocytes and macrophages) secrete cytokines which control B lymphocyte development. Thymic stromal cells secrete cytokines which control T lymphocyte development. The life span of lymphocytes varies from a few days to 20 years

Lymphocytes

Lymphocytes constitute about 20-30% of the white blood cell population. A healthy adult has between 1000-4800 cells/µl of blood. They vary in size from about 6-14μm in diameter and each has a large, round nucleus. The fine cytoplasm stains pale blue while the nucleus dark purple. Only a small percentage of lymphocytes are found circulating in the bloodstream, with a larger proportion residing in lymphoid tissue. Three types: T lymphocytes (T cells), B lymphocytes (B cells), and natural killer cells (NK cells). T lymphocytes (T cells) play an important role in cell-mediated immunity, attacking virus infected cells, fungi, cancer cells, and some bacteria. B lymphocytes (B cells) play an important role in the antibody-mediated immune response, producing antibodies against antigens to destroy bacteria. Natural killer cells (NK cells) attack tumor cells and virus infected cells.

Monocytes

Monocytes account for about 2-8 % of white blood cells. A healthy adult has between 0-800 cells/µl of blood. They are large, up to 3 times the size of a red blood cell, with a large often horseshoe-shaped nucleus. The abundant cytoplasm contains fine lysosomal granules and stains pale blue while the nucleus stains dark purple. After circulating in the bloodstream, typically for around 24 hours, monocytes migrate to the tissues where they differentiate into macrophages. They may become either fixed macrophages; residing in a particular tissue, or wandering macrophages; where they roam around and collect at sites of infection and inflammation. Function Engulf and remove cellular debris and microbes by phagocytosis. Play a key role in the body's defense against viruses, parasites and chronic infections. Monocytes also activate lymphocytes, enabling the immune response.

Monocyte cell lineage

Monocytes also arise from myeloid stem cells, which proliferate and differentiate into monoblasts: the first identifiable progenitor cells in the monocyte lineage. Monoblasts proliferate, forming differentiating promonocytes, where production of lysosomes begins. Promonocytes leave bone marrow and travel to lymphoid tissue, where further differentiation occurs. Monocytes populate various tissues as macrophages. Monocytes and macrophages have a life span of a few months.

Neutrophils

Neutrophils are most abundant, accounting for up to 50-70% of white blood cells. A healthy adult has approximately 1800-7800 cells/µl of blood. Each has a multi-lobed nucleus (usually 3-5 lobes). Due to this variability in the shape of their nuclei, they are sometimes known as polymorphonuclear leukocytes. Neutrophils also contain small, evenly distributed granules which take up both acidic and alkali dyes, staining the cytoplasm lilac or lavender Due to their abundance and mobility, neutrophils are often the first cells at the site of an inflammation. They are attracted to pathogens and inflamed tissues by the chemicals they secrete, which include toxins, kinins, and colony-stimulating factors. Neutrophil granules contain digestive chemicals, such as hydrolyzing enzymes and hydrogen peroxide. At sites of pathogen invasion and inflammation, neutrophils engulf pathogens by phagocytosis and release these chemicals to destroy the phagocytized pathogens. Neutrophil granules also release defensins, which are proteins capable of defending the body against fungi and bacteria.

Clot retraction and repair

Once a clot is formed and blood loss is stopped, platelets pull on the fibrin threads, causing them to contract and the clot to retract. The torn edges of the damaged vessel are drawn together, fibroblasts begin to form new connective tissue, and new endothelial cells begin to repair the lining of the ruptured vessel. As the clot blocks the path of tissue healing, once damage is repaired, the clot is dissolved in a process known as fibrinolysis. The inactive enzyme plasminogen, present in blood plasma, accumulates in a clot as it forms. Thrombin, activated clotting factor XII, and tissue plasminogen activator (synthesized by endothelial cells) are all chemicals that stimulate the conversion of inactive plasminogen into its active form plasmin. The active enzyme plasmin digests fibrin threads, dissolving the clot. It also inactivates fibrinogen, clotting factors V and XII, and prothrombin.

PLATELET PLUG FORMATION

Platelets are small cytoplasmic fragments derived from large precursor cells called megakaryocytes. These cells develop from hemopoietic stem cell precursors called megakaryoblasts, which form in the bone marrow. In the sequence of megakaryocyte development, megakaryoblasts differentiate into promegakaryocytes before finally developing into large megakaryocyte cells. Both the production (from multipotent hemocytoblasts within the bone marrow) and the differentiation of megakaryocytes is stimulated by thrombopoietin, a growth factor produced in the liver. Each platelet is a disc-shaped fragment, containing no nucleus, but instead, is filled with granules and surrounded by a plasma membrane. Platelets have a short life span, circulating in the blood for less than 10 days before being recycled. A healthy adult would have between 15 and 45 million platelets/µl blood. There are three stages of the platelet plug formation: a) Platelet adhesion Platelets stick to the exposed collagen and connective tissue of a damaged blood vessel. b) Platelet release reaction Once the platelets start to clump, they become activated, interact with neighboring platelets, and release chemicals, such as ADP, thromboxane A2, and serotonin, which activate passing platelets as well as inducing vasoconstriction, decreasing blood flow through the damaged vessel. c) Platelet aggregation As passing platelets are activated, they stick to the growing mass causing many platelets to accumulate at the site of blood loss. A platelet plug is thus formed.

Polychromatic erythroblast

Polychromatic erythroblasts are smaller than their predecessors, possessing a large nucleus and thick strands of chromatin. Their cytoplasm is even more basophilic, with increased numbers of ribosomes (and so stains a deep purple). However, as hemoglobin synthesis begins and iron molecules start to accumulate, the purple staining ribosomes become masked by the deep pink of hemoglobin, hence the name polychromatic, meaning of many colors.

Positive feedback

Positive feedback of clotting factor V increases prothrombinase synthesis, increasing thrombin production and hence also increasing synthesis of fibrin threads. Positive feedback of thrombin increases platelet activation, which in turn increases platelet aggregation and phospholipid secretion.

Basophilic erythroblast

Proerythroblasts undergo multiple cell divisions, eventually giving rise to early basophilic erythroblasts, where ribosome synthesis begins.

Progenitor cell stage-

Progenitor cell stage Myeloblasts are committed to differentiating into eosinophils, neutrophils, or basophils Monoblasts are committed to differentiating into monocytes (which ultimately become macrophages). Lymphoblasts are committed to differentiating into either T lymphocytes, B lymphocytes (which ultimately become plasma cells), or natural killer cells

Prostacyclin

Prostacyclin is a prostaglandin that is synthesized and released by endothelial cells and white blood cells. It opposes the actions of thromboxane A2, inhibiting platelet adhesion.

RED BLOOD CELL LIFE CYCLE

Red blood cells have relatively short life cycles and do not divide once mature, therefore they must be continually replaced to ensure the body functions efficiently. Red blood cells travel vast distances during their 120 day lifespan and are constantly exposed to severe stresses, such as squeezing in and out of capillaries. Red blood cells are deficient in many cellular organelles to leave space for large amounts of hemoglobin (Hb) and so are not able to repair themselves after damage. Ruptured red blood cells are therefore removed from the circulation as they pass through the spleen and liver, where the products of their breakdown are recycled and re-used during erythropoiesis (formation of new red blood cells). The life cycle of a red blood cell is summarized below 1Circulating red blood cells become damaged and pass through the liver, spleen, and red bone marrow. 2 Here, they are phagocytized and broken down. During this process, hemoglobin is split into heme and globin for recycling later on. 3 The globin part of hemoglobin breaks down further into amino acids, which are released and used to synthesize other proteins. 4 The heme part of hemoglobin is split into iron (Fe3+) and non-iron parts. The iron part is stored for subsequent use, whereas the non-iron part undergoes modification and is excreted. 5 (Iron part) Fe3+ immediately binds to a transporter protein called transferrin (too much free iron is toxic to cells). Fe3+-transferrin then travels to the liver, where Fe3+ detaches from transferrin and attaches to other proteins, ferritin and hemosiderin, which store Fe3+ until it is needed. When required for red blood cell formation (erythropoiesis), stored Fe3+ is released from ferritin and re-attaches to transferrin in order to travel to red bone marrow. In the red bone marrow, red blood cell precursors absorb iron and use it to synthesize new hemoglobin. Once hemoglobin is available, erythropoiesis can take place and new red blood cells are created, which then enter the circulation. 5 (Non-iron part) Meanwhile, the non-iron part of heme is converted to an organic compound called biliverdin. Biliverdin is then further converted into a pigment called bilirubin which is released into the bloodstream and travels to the liver. When bilirubin reaches the liver, it is excreted in bile into the small intestine and from there travels to the large intestine. In the large intestine, bilirubin is converted into pigments called urobilinogen and stercobilinogen by bacteria. In the presence of oxygen, stercobilinogen and urobilinogen can also be converted into urobilin and stercobilin. Urobilin is reabsorbed into the blood and excreted in the urine. Stercobilin is subsequently excreted in the feces.

Erythrocyte

Reticulocytes, filled with hemoglobin, are released into the blood and take around two days to become mature erythrocytes, before which cellular enzymes degrade any remaining ribosomes. By this stage, the mature erythrocytes have no nucleus and thus are no longer true 'cells'. For this reason, they are sometimes referred to as red blood corpuscles. Reticulocytes typically account for about 1% of erythrocytes in the blood and are used as an indicator of rate of red blood cell formation.

HEMOPOIETIC GROWTH FACTORS

The differentiation and proliferation of progenitor cells during hemopoiesis is regulated by hormones called hemopoietic growth factors. Hemopoietic growth factors include erythropoietin, thrombopoietin, and cytokines.

Vitamin K

The fat-soluble vitamin K, absorbed through the lining of the intestine along with lipids, is required by the liver for the synthesis of various clotting factors. Vitamin K deficiency can lead to uncontrollable bleeding due to the absence of clot formation.

Hemolysis

The large, cross-linked complexes, formed through agglutination, activate specific proteins in the plasma membranes of the donated red blood cells. The active plasma proteins cause the red blood cells to rupture and leak hemoglobin into the blood plasma, known as hemolysis, ultimately resulting in kidney damage.

VASCULAR SPASM

Vascular spasm is the contraction of the smooth muscle in the blood vessel wall. It constricts the damaged vessel, immediately slowing blood loss, giving the latter phases of hemostasis time to set in. Vascular spasm is triggered by neural reflexes and locally acting chemicals released from activated platelets

Proerythroblast

When a myeloid stem cell develops into a cell committed to the erythrocyte cell lineage, it is called a proerythroblast: the earliest distinguishable red blood cell precursor

Reticulocyte

When an orthochromatic erythroblast has reached a hemoglobin concentration of around 34%, it rids itself of most of its organelles, including its nucleus, causing it to take on the biconcave shape of a reticulocyte. A reticulocyte is an immature red blood cell, containing scattered clumps of ribosomes and rough endoplasmic reticulum. Its cytoplasm stains orange with a slight hint of gray-blue, and strands of blue can be observed

Agglutination

When blood of an incompatible type/group is transfused into an individual, antibodies present in the recipient's blood plasma bind to the agglutinogens (antigens) expressed on the surface of the donor red blood cells. This causes the red blood cells to clump together, forming a large, cross-linked complex: a process known as agglutination