Blood

Blood Functions

• Blood transports vital substances.
• Blood maintains the stability of interstitial fluid.
• Blood distributes heat.

Blood Cells

• 45% of blood is formed elements (mostly cells).
• Formed elements are primarily created in red bone marrow.
• Types of formed elements:
  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (cell fragments)

Blood Volume

• Approximately 8% of body weight.
• Varies with body size.
• Varies with fluid and electrolyte concentration.
• Varies with the amount of adipose tissue.
• Average adult blood volume: ~5 liters.

Blood Composition

• 45% of blood is formed elements (cells).
  • 99% of formed elements are red blood cells.
  • Remainder is white blood cells & platelets.
• 55% of blood is plasma.
  • Plasma components: water, amino acids, proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, wastes.
  • The formed elements and plasma percentages can vary.

Hematocrit

• Hematocrit, or packed cell volume, is the percentage of red blood cells in blood.

Origin of Blood Cells

• Blood cells originate from hematopoietic stem cells in red bone marrow.
• Erythropoietin and thrombopoietin are proteins vital for blood cell formation.

Red Blood Cells (Erythrocytes)

• Erythrocytes are small, biconcave cells.
• Contain hemoglobin for oxygen transport.
• Can readily squeeze through capillaries due to shape.
• Mature red blood cells lack a nucleus.
• They have a lifespan of approximately 120 days.
• They are removed by the liver and spleen.
• Energy production is by glycolysis, as they lack mitochondria.

Hemoglobin

• Hemoglobin is a protein in red blood cells that carries oxygen.
• When oxygen is bound to hemoglobin, the blood is bright red (oxyhemoglobin).
• When oxygen is released, the blood is dark red/blue (deoxyhemoglobin).

Red Blood Cell Production (Erythropoiesis)

• Erythrocytes are produced in red bone marrow.
• Erythropoietin is a hormone that regulates red blood cell production.
• Erythropoiesis operates through a negative feedback mechanism, responding to low oxygen levels.
• Low blood oxygen stimulates the kidneys to release erythropoietin.

Dietary Factors

• Nutrients like vitamin B12, folic acid, and iron are crucial for red blood cell production.
• Intrinsic factor, a protein produced by the stomach, facilitates B12 absorption.
• Vitamin C enhances iron absorption.

Anemia

• Anemia is a condition resulting from decreased red blood cell count, hemoglobin deficiency, or both.
• Types of anemia include iron-deficiency anemia, pernicious anemia, aplastic anemia, and hemolytic anemia.
• Sickle-cell anemia and thalassemia are genetic disorders causing abnormal hemoglobin production.

Destruction of Red Blood Cells

• Worn-out or damaged red blood cells are removed by macrophages in the liver and spleen.
• Components of hemoglobin are recycled.
  • Globin (proteins) are broken down into amino acids.
  • Iron is conserved and reused.
  • Heme is broken down into bilirubin, which is excreted in bile.

White Blood Cells (Leukocytes)

• Leukocytes, or white blood cells, protect against disease.
• Types of leukocytes:
  • Granulocytes (contain granules):
    • Neutrophils, Eosinophils, Basophils
  • Agranulocytes (lack granules):
    • Lymphocytes, Monocytes

Neutrophils

• Most abundant leukocyte (54-62% of leukocytes).
• Phagocytize bacteria, fungi, some viruses.
• Elevated in bacterial infections.

Eosinophils

• Bilobed nucleus, coarse granules.
• Defend against parasitic worm infestations and allergic reactions.
• 1-3% of leukocytes.

Basophils

• Deep blue granules, bilobed nucleus.
• Release histamine, promoting inflammation; release heparin, inhibiting blood clotting.
• Less than 1% of leukocytes.

Monocytes

• Largest leukocytes, kidney-shaped nucleus.
• Leave the bloodstream to become macrophages and phagocytize foreign material.
• 3-9% of leukocytes.

Lymphocytes

• Large spherical nucleus, thin cytoplasm.
• Important in immunity, produce antibodies.
• 25-33% of leukocytes.

White Blood Cell Functions (WBC)

• Phagocytize bacteria.
• Produce proteins to destroy foreign particles.
• Leukocytes squeeze through capillary walls (diapedesis) to enter tissues to fight infections.

White Blood Cell Counts (WBC Count)

• Leukopenia: low WBC count, indicating potential infections or immune disorders.
• Leukocytosis: high WBC count, indicating infection, inflammation, or certain diseases.

Blood Platelets (Thrombocytes)

• Platelets, or thrombocytes, are small cell fragments derived from megakaryocytes.
• Help control blood loss from broken vessels, forming platelet plugs.

Blood Plasma

• Plasma is the straw-colored liquid portion of blood (55%).
• Primarily composed of water (92%).
• Contains dissolved substances like proteins, electrolytes, nutrients, wastes, and gases.

Plasma Wastes

• Includes non-protein nitrogen substances, such as urea, uric acid, creatinine, and bilirubin.

Plasma Electrolytes

• Electrolytes in plasma include sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate.

Plasma Proteins (3 types)

• Albumins: Most abundant plasma proteins, produced in the liver; maintain osmotic pressure of blood, binding and transporting molecules (e.g., fatty acids, hormones).
• Fibrinogen: Produced in liver, crucial for blood clotting.
• Globulins: Include alpha, beta, and gamma globulins. Alpha and beta globulins are produced in liver, assist in transporting lipids, and fat-soluble vitamins; gamma globulins (antibodies) are made in lymphatic tissues, essential for fighting infections.

Gases and Nutrients

• Blood carries gases like oxygen, carbon dioxide, and nitrogen.
• Blood carries nutrients like amino acids, simple sugars, nucleotides, and lipids.
• Nutrients are often carried as lipoproteins (e.g. LDL, HDL).

Hemostasis

• Hemostasis is the process of stopping bleeding.
• Three mechanisms:
  • Blood vessel spasm (vasoconstriction)
  • Platelet plug formation
  • Blood coagulation (blood clot formation)

Blood Vessel Spasm

• Damaged blood vessels constrict to reduce blood loss.
• Stimulated by nervous system responses and serotonin release from platelets; influenced by pain receptors in injured tissue.

Platelet Plug Formation

• Platelets adhere to exposed collagen in damaged blood vessels.
• Plates adhere to each other to form a plug that helps stop blood loss.

Blood Coagulation

• Formation of a blood clot is triggered by a cascade of reactions involving clotting factors.
• Vitamin K is necessary.
• Conversion of fibrinogen into fibrin, creating a meshwork that traps blood cells and platelets, forming a blood clot.
• Two main pathways: extrinsic and intrinsic.

Extrinsic Pathway

• Initiated by tissue damage releasing tissue factor (thromboplastin).
• Tissue factor activates clotting cascade.

Intrinsic Pathway

• Initiated by exposure of blood to foreign surfaces or damage to the blood vessel itself.
• Cascade of reactions, involving factors from blood itself.

Fate of Blood Cells

• Blood clot retracts, pulling the edges of the damaged vessel together.
• Serum, plasma minus clotting factors, is squeezed out.
• Platelet-derived growth factor stimulates tissue repair.
• Plasminogen is converted to plasmin and digests the blood clot.

Thrombus and Embolism

• Thrombus: abnormal blood clot in a blood vessel.
• Embolus: blood clot or other material carried by blood flow that blocks a vessel.
• Embolism occurs when an embolus blocks blood flow in a vessel.
• Named according to location: e.g., pulmonary embolism.

Blood Grouping and Transfusion

• ABO system: Categorizes blood types based on the presence or absence of A and B antigens and corresponding antibodies.
• Blood types A, B, AB, and O.
• Type O blood is universal donor; Type AB is universal recipient (no antibodies).
• Rh factor: Presence or absence of antigen D.
• Sensitization: When Rh- negative individuals are exposed to Rh+ blood, they develop antibodies, leading to agglutination if they are exposed again.
• Erythroblastosis fetalis is a condition occurring in pregnancies where the mother is Rh-negative and the fetus is Rh-positive, leading to fetal blood cell destruction.

Prevention of Coagulation

• Healthy endothelial cells prevent spontaneous clotting by producing prostacyclin and nitric oxide to inhibit platelet adhesion.
• Antithrombin and heparin inhibit thrombin activation.