Blood Composition and Function

Plasma: The Fluid Matrix of Blood

Plasma is the extracellular matrix of blood, serving as the fluid component that suspends cellular elements. It is composed primarily of water, proteins, ions, organic molecules, gases, trace elements, and vitamins. Plasma is nearly identical to interstitial fluid but contains unique plasma proteins.

• Water: Makes up about 92% of plasma, providing a solvent for transport.

• Proteins: Constitute about 7% and include albumins (osmotic regulation), globulins (immune function), fibrinogen (clotting), and transferrin (iron transport).

• Ions: Such as Na+, K+, Cl-, Ca2+, HCO3-, and H+.

• Organic Molecules: Includes glucose, amino acids, lipids, and nitrogenous waste.

• Gases: Oxygen (O2) and carbon dioxide (CO2).

• Trace Elements & Vitamins: Essential for cellular metabolism and function.

Formed Elements: Cellular Components of Blood

The formed elements include red blood cells (RBCs), white blood cells (WBCs), and platelets. Each plays a distinct role in physiological processes.

• Red Blood Cells (Erythrocytes): Specialized for oxygen transport.

• White Blood Cells (Leukocytes): Include lymphocytes, monocytes, neutrophils, eosinophils, and basophils, each with unique immune functions.

• Platelets: Cell fragments essential for blood clotting and hemostasis.

Hematopoiesis: Blood Cell Production

Sites and Regulation of Hematopoiesis

Blood cells are produced in the bone marrow through a process called hematopoiesis. Red bone marrow is active and contains hemoglobin, while yellow bone marrow is inactive and stores fat. Hematopoiesis is regulated by cytokines such as interleukins, colony-stimulating factors (CSF), thrombopoietin (TPO), and erythropoietin (EPO).

• Interleukins (ILs) & CSF: Regulate leukopoiesis (WBC production).

• Thrombopoietin (TPO): Controls platelet production.

• Erythropoietin (EPO): Stimulates RBC production.

Bone Marrow Structure and Function

Bone marrow is a highly vascular tissue found within bones, responsible for the development of all blood cells. It contains a stroma (supportive matrix) and various cells at different stages of development.

• Red Bone Marrow: Active site of hematopoiesis.

• Yellow Bone Marrow: Inactive, stores adipose tissue.

• Stroma: Provides structural support and microenvironment for stem cells.

Red Blood Cells (Erythrocytes)

Structure and Function

RBCs are biconcave, flexible cells specialized for oxygen transport. Their shape increases surface area for gas exchange and allows passage through capillaries.

• Hemoglobin: The oxygen-carrying protein, composed of four globin chains and four heme groups, each with an iron atom.

• Life Span: Approximately 120 days; old RBCs are removed by the spleen.

Hemoglobin Synthesis and Iron Metabolism

Hemoglobin synthesis requires iron, which is transported by transferrin and stored in the liver as ferritin. Iron is essential for heme production and oxygen binding.

• Iron Source: Dietary intake.

• Transport: Transferrin in plasma.

• Storage: Ferritin in liver.

RBC Disorders

Disorders affecting RBCs can decrease oxygen transport and include anemia (low hemoglobin), hemolytic anemia, sickle cell disease, iron-deficiency anemia, and polycythemia vera.

• Anemia: Reduced oxygen-carrying capacity due to low hemoglobin or RBC count.

• Hemolytic Anemia: Increased RBC destruction.

• Sickle Cell Disease: Abnormal hemoglobin causes RBCs to deform.

• Iron-Deficiency Anemia: Insufficient iron for heme synthesis.

Causes of Anemia

Platelets and Hemostasis

Platelet Structure and Function

Platelets are cell fragments derived from megakaryocytes, essential for blood clotting, immunity, and inflammation. They contain mitochondria, smooth ER, and granules with cytokines and growth factors. Platelets have a lifespan of about 10 days.

Hemostasis: Prevention of Blood Loss

Hemostasis is the process that prevents blood loss from damaged vessels. It involves vasoconstriction, formation of a platelet plug, and activation of the coagulation cascade to form a stable clot.

• Vasoconstriction: Reduces blood flow to the injury site.

• Platelet Plug: Platelets adhere to exposed collagen and release factors to recruit more platelets.

• Coagulation Cascade: Series of enzymatic reactions leading to fibrin formation.

Coagulation Pathways

Coagulation converts a platelet plug into a clot via two pathways: intrinsic (contact activation) and extrinsic (cell injury). Both converge on the common pathway, where thrombin converts fibrinogen to fibrin, forming a stable clot.

• Intrinsic Pathway: Initiated by exposure to collagen.

• Extrinsic Pathway: Initiated by tissue factor from damaged cells.

• Common Pathway: Thrombin formation and fibrin polymerization.

Factors Involved in Coagulation

Multiple chemical factors are involved in coagulation, including collagen, von Willebrand factor, kininogen, kallikrein, tissue factor, prothrombin, thrombin, fibrinogen, fibrin, factor XIII, Ca2+, and vitamin K.

Fibrinolysis: Clot Breakdown

Fibrinolysis is the process of breaking down clots. Plasminogen is activated to plasmin by tissue plasminogen activator (tPA), which dissolves fibrin and fibrinogen.

Anticoagulants and Disorders of Coagulation

Anticoagulants

Anticoagulants prevent excessive clotting by inhibiting platelet adhesion and the coagulation cascade. Endogenous anticoagulants include heparin, antithrombin III, and protein C.

• Heparin: Enhances antithrombin activity.

• Antithrombin III: Blocks several coagulation factors.

• Protein C: Inhibits clotting factors.

Hemophilia and Coagulation Disorders

Hemophilia is a group of disorders characterized by ineffective coagulation due to defective or missing factors in the cascade. This results in prolonged bleeding and difficulty forming stable clots.

Summary of Hemostasis and Coagulation

Hemostasis involves a coordinated response to vessel injury, including vasoconstriction, platelet plug formation, coagulation, and fibrinolysis. Proper regulation is essential to prevent excessive bleeding or clotting.

Complete Blood Count (CBC) and Normal Ranges

Laboratory Values and Interpretation

A CBC provides quantitative information about blood components, including hematocrit, hemoglobin, RBC count, WBC count, differential WBC count, and platelet count. These values are essential for diagnosing blood disorders.

Key Equations

Hematocrit Calculation

The hematocrit is calculated as:

Mean Corpuscular Volume (MCV)

Mean Corpuscular Hemoglobin (MCH)

Mean Corpuscular Hemoglobin Concentration (MCHC)

Summary Table: Blood Components

Summary Table: Coagulation Factors

Additional info: Academic context was added to clarify the function and regulation of blood components, hematopoiesis, and hemostasis, as well as to provide self-contained explanations suitable for exam preparation.