16.1 Plasma and the Cellular Elements of Blood

Plasma: The Extracellular Matrix of Blood

Blood plasma is the liquid component of blood, serving as its extracellular matrix. It is essential for transporting nutrients, hormones, and waste products throughout the body.

• Volume and Percentage: An average adult (70 kg) has about 5 liters of blood, which is approximately 7-8% of body weight.

• Composition: Plasma consists of water (about 92%), proteins (7%), and dissolved solutes such as electrolytes, nutrients, and waste products.

• Comparison with Interstitial Fluid: Both plasma and interstitial fluid are extracellular fluids, but plasma contains more proteins, especially albumin, globulins, and fibrinogen.

• Site of Plasma Protein Production: Most plasma proteins are synthesized in the liver.

• Main Groups of Plasma Proteins: Albumins, globulins, fibrinogen, and transferrin. Each group has specific functions such as maintaining osmotic pressure, immune defense, and blood clotting.

• Role in Capillary Filtration: Plasma proteins help maintain oncotic pressure, which opposes hydrostatic pressure and prevents excessive fluid loss from capillaries.

Cellular Elements: RBCs, WBCs, and Platelets

The cellular elements of blood include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Each type has distinct functions vital for homeostasis.

• Main Cell Types and Functions:

  • Red Blood Cells (Erythrocytes): Transport oxygen and carbon dioxide.

  • White Blood Cells (Leukocytes): Defend against infection and foreign substances.

  • Platelets (Thrombocytes): Play a key role in blood clotting.

• Formal Names: Erythrocytes (RBCs), Leukocytes (WBCs).

• Parent Cells of Platelets: Platelets are derived from megakaryocytes in the bone marrow.

• Types of WBCs and Functions:

  • Neutrophils: Phagocytosis of bacteria.

  • Lymphocytes: Immune response (B cells, T cells, NK cells).

  • Monocytes: Phagocytosis; become macrophages.

  • Eosinophils: Combat parasites and mediate allergic responses.

  • Basophils: Release histamine; involved in inflammation.

Blood Cell Production: Hematopoiesis

Stem Cells and Differentiation

Blood cells are produced in the bone marrow through a process called hematopoiesis, which begins with pluripotent hematopoietic stem cells.

• Location of Stem Cells: Pluripotent hematopoietic stem cells are found in the bone marrow.

• Differentiation: These stem cells give rise to all specialized blood cells through a series of developmental stages.

• Sites of Hematopoiesis: Occurs in the yolk sac (embryo), liver and spleen (fetus), and bone marrow (children and adults).

• Red vs. Yellow Bone Marrow: Red marrow is active in hematopoiesis; yellow marrow is mostly adipose tissue and less active.

• Cell Fate: Most blood cells produced become RBCs; a smaller percentage become WBCs.

• Life Span Comparison: RBCs live about 120 days; WBCs vary from hours to years depending on type.

Regulation by Cytokines

Cytokines are signaling proteins that regulate hematopoiesis and immune responses.

• Definition: Cytokines are small proteins important in cell signaling, especially in immune responses and blood cell production.

• Key Cytokines in Hematopoiesis:

  • Colony-Stimulating Factors (CSFs): Stimulate production of WBCs.

  • Interleukins: Modulate immune cell development.

  • Erythropoietin (EPO): Stimulates RBC production.

• CSF Production: CSFs are produced in the bone marrow and other tissues.

16.2 Platelets and Thrombopoiesis

Platelet Formation and Function

Platelets are small, anucleate cell fragments essential for blood clotting and vessel repair.

• Formation: Platelets form from megakaryocytes in the bone marrow, which fragment into thousands of platelets.

• Polyploidy: Megakaryocytes become polyploid by replicating DNA without cell division.

• Intracellular Components: Platelets contain granules with clotting factors, enzymes, and signaling molecules.

• Life Span: Platelets typically survive 7-10 days in circulation.

Thrombopoietin and Platelet Production

Thrombopoietin (TPO) is a hormone that regulates platelet production.

• Definition: TPO is produced mainly in the liver and stimulates megakaryocyte development and platelet formation.

16.3 Red Blood Cells (Erythrocytes)

RBC Count and Hematocrit

Red blood cells are the most numerous blood cells and are responsible for oxygen transport.

• RBC Count: About 5 million RBCs per microliter (μL) of whole blood.

• Comparison: RBCs far outnumber WBCs and platelets.

• Hematocrit: The percentage of blood volume occupied by RBCs; determined by centrifugation.

• Formula:

Mature RBCs Lack a Nucleus

Mature erythrocytes are biconcave, flexible, and lack nuclei, which limits their lifespan and metabolic capabilities.

• Immature Forms: Reticulocytes and erythroblasts are immature RBCs; reticulocytes still contain some organelles.

• ATP Source: Mature RBCs rely on glycolysis for ATP production (anaerobic metabolism).

• Shape: The biconcave shape is maintained by cytoskeletal proteins (e.g., spectrin).

• Flexibility: Necessary for passage through capillaries.

• Osmotic Effects: In hypertonic solutions, RBCs shrink; in hypotonic solutions, they swell and may lyse.

Hemoglobin Synthesis and Iron Metabolism

Hemoglobin is the oxygen-carrying protein in RBCs, requiring iron for its function.

• Structure: Hemoglobin consists of four globin chains, each with a heme group containing iron.

• Synthesis: Occurs in developing erythrocytes in the bone marrow.

• Destruction: Old RBCs are destroyed in the spleen and liver; hemoglobin is broken down into heme and globin.

• Bilirubin: Heme is converted to bilirubin, which is excreted in bile.

• Jaundice: Accumulation of bilirubin causes yellowing of skin and eyes.

RBC Disorders and Anemia

Disorders affecting RBCs can impair oxygen transport and cause symptoms such as fatigue.

• Major Causes of Anemia: Blood loss, decreased RBC production, increased RBC destruction, and abnormal hemoglobin.

• Iron Deficiency: Leads to microcytic, hypochromic anemia.

• Sickle Cell Disease: Abnormal hemoglobin causes RBCs to become rigid and sickle-shaped.

• Polycythemia: Excess RBC production increases blood viscosity and risk of thrombosis.

16.4 Platelets

Platelet Structure and Function

Platelets are essential for initiating blood clotting and repairing damaged vessels.

• Formation: Platelets are fragments of megakaryocytes.

• Intracellular Components: Contain granules with clotting factors, ADP, serotonin, and calcium.

• Life Span: 7-10 days.

16.5 Hemostasis and Coagulation

Hemostasis: Stopping Blood Loss

Hemostasis is the process that prevents and stops bleeding after vessel injury.

• Major Steps:

  1. Vascular spasm (vasoconstriction)

  2. Platelet plug formation

  3. Coagulation (clot formation)

• Platelet Adhesion vs. Aggregation: Adhesion is attachment to exposed collagen; aggregation is clumping together.

• Initiation: Exposed collagen and tissue factor start the clotting cascade.

Platelet Activation and Clotting

Platelet activation leads to the release of chemicals that promote clotting and vessel repair.

• Chemicals Released: ADP, serotonin, thromboxane A2 (vasoconstrictors).

• Positive Feedback: Platelet activation amplifies the response, forming a plug at the injury site.

Coagulation Pathways

Coagulation involves a cascade of enzymatic reactions that convert fibrinogen to fibrin, stabilizing the clot.

• Intrinsic vs. Extrinsic Pathways: Intrinsic is activated by damage inside the vessel; extrinsic by tissue factor outside the vessel.

• Convergence: Both pathways activate factor X, leading to thrombin formation and fibrin cross-linking.

• Fibrinolysis: The process of breaking down clots, mainly by plasmin.

• Equation:

Anticoagulants and Hemophilia

Anticoagulants prevent clot formation, while hemophilia is a disorder of impaired coagulation.

• Anticoagulants: Substances like heparin (endogenous) and warfarin (exogenous) inhibit clotting factors.

• Hemophilia: Genetic disorder causing deficiency of clotting factors; treated with factor replacement therapy.

Table: Main Blood Cell Types and Functions

Table: Major Plasma Proteins and Functions

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been inferred and constructed based on standard textbook knowledge.