Blood Composition and Function

Blood Composition

• Blood consists of cellular components and a liquid part (plasma or serum).
• Cellular components include erythrocytes, leukocytes, and platelets.
• Blood is 45% cellular components and 55% plasma/serum.

Functions of Blood

• Transports hemoglobin
• Provides nutrition
• Facilitates excretion
• Regulates body temperature
• Maintains salt and water balance
• Maintains acid-base balance
• Provides coagulation factors
• Provides immune defense
• Transports hormones to target organs

Blood Characteristics

• Red blood is due to iron-containing hemoglobin in erythrocytes.
• Hemoglobin's combination with oxygen affects blood color.
• 1 gram of hemoglobin contains 1.34-1.39 ml of oxygen.
• Arterial blood is light red because it's rich in oxygen.
• Venous blood is dark red due to less oxygen.
• Blood reaction is slightly alkaline, with pH kept constant between 7.35-7.40 at 37°C.

Erythrocytes (Red Blood Cells)

• Majority of blood cells, making up 44-48% in men and 38-42% in women (hematocrit).
• Transports oxygen-conducting hemoglobin from lungs to tissues.
• Red blood cells are destroyed and removed from circulation after about 120 days.

Composition of Erythrocytes

• 60-64% water
• 36-34% organic and inorganic substances
• Mature erythrocytes lack nuclei, ribosomes, and mitochondria, so protein synthesis and cell division aren't observed.

Erythrocyte Sizes

• 7-8 μm: normocyte
• Less than 6 μm: microcyte
• More than 8 μm: macrocyte

Erythrocyte Colors

• Normokrom: normal hemoglobin amount
• Hyperchromia: increased hemoglobin (dark)
• Hypochromia: decreased hemoglobin (light)

Erythrocyte Dimensions

• The diameter of erythrocytes is approximately 7-8 μm under normal conditions.
• At its thickest point, an erythrocyte's thickness measures 2.5 μm
• Erythrocytes can go down to 1 μm.
• The average volume of erythrocytes is 90-95 μm3.

Erythrocyte Count

• Men have 4.5-5.5 million /mm3.
• Women have 3.5-4.5 million /mm3.
• Normocythemia is a normal number of erythrocytes.
• Polycythemia is an increased number of erythrocytes.
• Oligocythemia is a decreased number of erythrocytes.

Hemoglobin (Hb)

• A red pigment carrying O2 in vertebrates, Hb consists of 4 subunits.
• Each unit contains 1 heme (carrying Fe+2) and 1 globin polypeptide chain.
• Hb consists of 4 heme and 4 globin chains.
• Adult globin chains are 2 alpha + 2 beta (HbA).

Hemoglobin Types in Humans

• HbA1 (2 alpha + 2 beta)
• HbA2 (2 alpha + 2 delta)
• HbF (2 alpha + 2 gamma)

Hemoglobin Composition

• Four heme groups
• Four iron atoms
• Capable of carrying four oxygen molecules

Hemoglobin Properties

• Connection with oxygen is loose and reversible
• Hb's oxygen affinity depends on PO2
• Binding is easy and fast when PO2 is high in the lungs.
• Arterial blood is 97% saturated with oxygen.
• Oxygen separates from hemoglobin in tissues due to low PO2.

Hb-O2 Saturation Curve

• Illustrates relationship between Hb% saturation and partial O2 pressure
• Hemoglobin's curve is sigmoidal
• Lowering pH, increasing CO2 or temperature shifts the curve to the right
• Increasing pH, reducing CO2 or heat shifts the curve to the left
• Increased DPG shifts the curve to the right
• Decreased DPG shifts the curve to the left
• Easier for Hb to separate from oxygen when shifting to the right. (Oxygen transfer to tissues)
• Easier for Hb to bind oxygen when shifting to the left. (Oxygen passage from the lungs to the erythrocytes)
• This is also known as the Bohr effect.

Reticulocytes

• Reticulocytes are the last cells before erythrocytes, containing basophilic material, mitochondria, and other cytoplasmic organelle extracts.
• Basophilic material disappears in 1-2 days as the cell matures into an erythrocyte.
• Reticulocytes make up less than 1% of red blood cells in normal blood due to their short lifespan.
• A reticulocyte count from peripheral blood with special staining gives information about bone marrow activity or hematopoiesis.

Hematopoiesis

• Blood cells are produced in different places in the fetus, child, and adult.
• Blood cells appear approximately 18 days after fertilization.
• Primitive, nucleated red blood cells are produced in the vitellus sac in the early weeks of embryonic life.
• Mesoplastic period is the name for this time which lasts 3 months.
• Stem cells migrate to the liver, spleen, thymus, lymph nodes, and bone marrow with vascular system development.
• Making blood foci as stem cells transform into erythrocytic, granulocytic, and megakaryocytic host cells and initiate diffuse hematopoiesis.
• After the 6th week of pregnancy, the hematopoiesis in the vitellus sac regresses and the liver takes over the role of hematopoiesis (hepatic period).
• Fetal Hb (HbF) synthesis begins in this period; in infants, HbF can remain for a maximum of 6 months, after then it makes up only 2% of total Hb.
• From the 20th week of intrauterine life, stem cells, having previously migrated to the bone marrow begin to produce blood, making now a myeloid period.
• A change in Hb synthesis happens along with blood cells in the myeloid period.
• HbF gradually decreases and is replaced by adult Hb (HbA); globin part of HbA chains consists 22 chains.
• Through the age of 5, all bones actively engage in hematopoiesis; red bone marrow diminishes as the child ages, turning to yellow marrow. This is from fat cell infiltration.
• After age 20, hematopoiesis occurs vertebrae, sternum, and ribs, and lasts lifelong in the head and hip bones.

Regulation of Erythropoiesis

• It has to be normal for erythropoiesis to be normal.
• The bone marrow's anatomical structure, local tissue temperature, cell nutrition, and endocrine activity must all be normal.
• Regulation by erythropoietin hormone (EPO).
• EPO is synthesized and released in kidney cells when needed and not stored.

Erythropoietin Synthesis Location

• Fetus and newborns: 85% liver, 15% kidneys
• Adults: 85% kidneys, 15% others

Necessary Materials for Erythrocyte Development

• B12
• Folic acid
• B6
• Ascorbic acid
• Vitamin E
• Minerals

Blood Groups

• Glycoprotein structures that extend out of the erythrocyte membrane differ between individuals.
• These structures have antigenic properties can cause immune responses during blood transfusions.
• The ABO and Rh types need to be known for blood transfusions.

ABO System

• Erythrocyte membranes contain agglutinogens with antigenic properties.
• A and B antigens are found in the majority of the general population.
• Antigens may be present, individually, together, or not at all.
• A person's blood type is determined by the type of the agglutinogen on erythrocyte membranes.
• Antibodies against the agglutinogens are found in plasma (agglutinin).

ABO Blood Type Antigens and Antibodies

• A group: A antigens, Anti-B antibody
• B group: B antigens, Anti-A antibody
• AB group: A and B antigens, No antibody
• O group: No antigens, Anti-A and Anti-B antibodies

Agglutination in Transfusion Reactions

• Antibodies bind to erythrocytes when blood containing A or B agglutinogens is transfused into individuals with anti-A or anti-B in their plasma.
• Because of their multiple binding sites, antibodies bind many erythrocytes at the same time, causing agglutination.
• White blood cells break down agglutinated cells (hemolysis), clog capillaries, release hemoglobin.

Rh System

• Another system with clinical significance besides the ABO system.
• Individuals have to encounter the Rh antigen strongly to have antibodies against it.
• The most common condition due to Rh incompatibility in the clinic is erythroblastosis fetalis disease.

Erythroblastosis Fetalis (Hemolytic Disease of Newborn)

• If a Rh(-) mother carries a Rh(+) baby, the baby's Rh antigens pass to the mother through the placenta causing production of anti-Rh antibodies.
• The antibodies administered during the first pregnancy will not harm the baby.
• The likelihood of the disease will increase with each subsequent pregnancy.
• Antibodies crossing the placenta agglutinate the erythrocytes of the fetus.
• Hemolysis, hemoglobin release and increased bilirubin occurs.
• Increasing bilirubin in the blood damages nerve cells (Kernikterus).

Leukocytes (White Blood Cells)

• Mobile cells of the defense system
• Sent to areas they will be used, after forming in the bone marrow
• The number of leukocytes is 4000-10000 / mm3
• Have distinct morphology and functions

Leukocyte Counts and Percentages

• Total: 4000-10000
• Neutrophils: 3000-6000 (50-65%)
• Eosinophils: 150-300 (1-4%)
• Basophils: 0-100 (0.4%)
• Lymphocytes: 1500-4000 (20-40%)
• Monocytes: 300-600 (2-6%)

Neutrophils

• Contains 2-5 segments (average 3)
• The core has coarse dense chromatin that is dark and purple
• The cytoplasm contains light pink-blue granules
• The vesicles inside cause the granular appearance
• It makes up 50-70 percent of total leukocytes
• Has a life time of 1-2 days

Eosinophils

• Usually has 2 segments
• Contains large and dark pink granules
• Makes up 2% - 6% of the leukocytes
• Their numbers increase due to parasitic infections
• They accumulate in tissues and skin caused by allergic reactions

Basophils

• Usually contain 2 segments
• Granules are large, dark blue-black and fill the whole cytoplasm
• 0.5% of all leukocytes
• Secretes heparin, histamine, bradykinin, serotonin
• Plays a role in allergic reactions

Monocytes

• Large and round cells that are 16-20 μm in diameter
• The core is kidney-like, oval or even round
• They are phagocytic cells that develop in the bone marrow
• Precursors to tissue macrophages

Lymphocytes

• Typically 10 μm in diameter, round or oval, and the nucleus shows similar characteristics
• Narrow cytoplasm, and the nucleus contains a small notch
• Not phagocytic
• Pass into the blood from lymph nodes and other lymph tissues then to the blood and back to the tissues by diapedesis

Platelets

• Round or oval, and small with a diameter of 1-4 μm
• It is made from megakaryocytes in the bone marrow.
• Lacks nuclei
• The normal range is 150 000-400 000/mm3

Thrombopoiesis

• Formation of megakaryocytes in the bone marrow takes ~5 days
• Megakaryocytes feature a large cytoplasm with a lobed nucleus
• Large cells with a diameter of 30-100 μm

Thrombocytes (platelets)

• live in the peripheral blood for 8-10 days as a consequence of cytoplasmic fragmentation
• Platelets have to be destroyed in the spleen at the end of its life span
• Thrombocytopenia: Number <150 000 / mm3
• Thrombocytosis: Number > 400 000 / mm3

Hemostasis

• Is a series of physiological reactions that occur by the regular operation of thrombocyte, endothelial and coagulation systems and the mechanisms that regulate them.
• Can be described as stopping the bleeding.

Hemostasis Mechanisms

• Narrowing of the injured vessel (constriction)
• Platelet accumulation in the lesion area
• Coagulation activation
• Activation of fibrinolysis

Formation of the Platelet Plug

• If there is a tear on the vessel wall the thrombocytes begin to change structurally, swelling, taking on a irregular appearance, the pseudopods contract by elongating and discharge their granules out.
• After vascular damage, the stickiness of platelets increases and an increase in the activation of platelets happens (ADP and Thromboxane A2 release increases).

Activator substances and proteins

• Originating from the damaged vessel wall and platelets start the coagulation process.
• 3-6 minutes after the rupture of the vein, the ruptured end of the vein is filled with a clot.
• The clot retracts (shrinks) after 20 minutes to 1 hour.
• Over 50 substances affect coagulation.

Coagulation and Anticoagulation

• Some substances provide coagulation (procoagulant), while other prevent coagulation (anticoagulant).
• Dominant anticoagulants under normal circumstances avoid coagulation and clot formation.
• Coagulation starts from activating damaged circulating procoagulants when vessel becomes damaged.

Basic Mechanism of Coagulation

1. Activation of clotting factors, which are inactive in the blood, and their formation of the prothrombin activator complex

2. Prothrombin activator catalyzes the conversion of prothrombin to thrombin

3. Thrombin converts fibrinogen to fibrin threads

Coagulation Initiation

• Prothrombin activator forms from the contact of blood with damaged endothelial cells and collagen which results from tissue damage.
• The prothrombin activator is created in 2 ways that constantly interact with each other; Extrinsic Path and Intrinsic Path

Extrinsic Path

• Tissue damage leads to more release of tissue factor, but starts with it's release.
• Tissue factor works like a cofactor and is found inside endothelial cells that enables Factor VII to activate.
• Tissue factor, Factor VII, Ca2 + together makes tissue thromboplastin.
• Tissue thromboplastin activates Factor X to make Factor Xa.

Intrinsic Path

• Collagen released as a result of damage to the vessel wall activates the intrinsic pathway.
• Factors XII, XI, IX, X activate respectively.
• All reactions include calcium and platelet membrane phospholipids working as cofactors.

Common Path

• Prothrombin activator (PA) forms when Factor Xa, Ca2 +, PL and Factor Va by the activation of the intrinsic and extrinsic pathway join up.
• From Prothrombin, PA provides thrombin formation
• Thrombin allows creation of fibrin monomers originating with fibrinogen
• Fibrin monomers start to polymerize
• Fak XIIIa then has to become activated to then stabilize the fibrin polymers

Clot Retraction

• The clot will begin to shrink within a few minutes.
• Platelets are what causes retraction.
• An indicator of Platelet deficiency is failure of retraction.
• Substances that provide retraction are contractile proteins thrombasthenin, such as actin and myosin where thrombocytes.
• Vessels approach each other at the ends as the clot shrinks.

Feed-Back Regulation of Clot Formation

• The amount of Prothrombin activator formed is directly proportional to tissue damage.
• Triggering the coagulation mechanism causes it to spread through the blood within minutes.
• Clot formation is caused by the clot through feed-back path.

Prevention of Coagulation

• The intrinsic coagulation system is often inactive because the endothelium tissue is smooth
• Platelets separate due to the inner layer of the endothelium and glycocalyx.
• In plasma, Antithrombin III joins with thrombin and Factors IXa, Xa, Xla and XIla in activates them.
• Antithrombin III effectiveness is increased by Heparin, an anticoagulant in plasma.

Fibrinolysis

• Called the Fibrinolytic System it lets the clot disappear by melting it.
• The most important enzyme is Plasmin, which damages fibrin, causing coagulation
• Leads to the formation of Fibrin Degradation Products.

Key substance for Activation of Plasminogen

• Tissue Plasminogen Activator (t-PA) activates tissue plasminogen, which is from injured tissues within 1-2 days.
• Use as a thrombolytic, t-PA is used at the clinic.

Bleeding Disorders

• Bleeding disorders caused by platelets.
• Vitamin K Deficiency related diseases.
• Those caused by coagulation factors deficiency.

Vitamin K Deficiency Disorders

• Thrombin in the liver is necessary for the production of Factor X, IX, VII.
• A deficiency is not noted often because since K vit synthesized from intestine bacteria.
• Bleeding happens occasionally since bacterial flora has not yet been made with a newborn.
• Since K vit deficiency is fat-soluble, it is experienced with conditions like fat absorption.
• Vitamin K won't absorb in the event that the liver doesn't emit excessive bile, but absorption is limited.
• Spontaneous Bleeding happens during the liver caused by limited elements and insufficient digestion K Vit.