BLOOD

Elements of Blood

Blood is a specialized connective tissue composed of plasma and formed elements. It performs vital functions in transport, regulation, and protection.

• Plasma: Liquid extracellular matrix (~55% of blood volume).

• Buffy coat: Layer containing leukocytes (white blood cells) and platelets (~1%).

• Formed elements: Cells and cell fragments in the plasma.

  • Erythrocytes: Red blood cells (~44% of blood volume).

Functions of Blood

Blood serves multiple essential functions in the body:

• Gas Exchange: Transports oxygen from lungs to tissues via erythrocytes.

• Solute Distribution: Distributes solutes such as nutrients, hormones, and wastes.

• Immune Functions: Transports immune cells and antibodies.

• Body Temperature Regulation: Absorbs and distributes heat produced by cellular metabolism.

• Blood Clotting: Seals damaged vessels to prevent blood loss.

• Acid-Base Homeostasis: Maintains pH between 7.35–7.45; acts as a buffer.

• Stabilization of Blood Pressure: Maintains blood volume and pressure at a constant level.

Plasma

Plasma is the liquid component of blood, consisting mostly of water (about 90%). It determines the viscosity of blood and contains various solutes:

• Albumin: Most abundant plasma protein; maintains osmotic pressure.

• Globulins: Include antibodies and transport proteins.

• Fibrinogen: Involved in blood clotting.

• Other solutes: Glucose, amino acids, nitrogenous wastes, ions.

Erythrocytes (Red Blood Cells)

Erythrocytes are specialized for oxygen transport and make up about 44% of blood volume (hematocrit). They are biconcave discs, which increases surface area for gas exchange, and lack nuclei and most organelles.

• Higher in men than women due to larger body size and greater muscle mass.

• No nucleus or mitochondria: Most erythrocytes do not synthesize proteins or use oxygen for metabolism.

• Contain hemoglobin: Protein that binds and transports oxygen.

Production and Regulation

• Erythropoiesis: Formation of red blood cells in the bone marrow, stimulated by erythropoietin (EPO) from the kidneys.

• Negative feedback: Maintains hematocrit. Low oxygen (hypoxia) increases EPO, stimulating RBC production.

Death and Recycling

• Mature erythrocytes are removed by macrophages in the spleen and liver.

• Hemoglobin is broken down into amino acids, iron (recycled), and heme (converted to bilirubin and excreted).

Hemoglobin

Hemoglobin is a large protein with four polypeptide subunits (two alpha, two beta chains), each containing a heme group that binds oxygen.

• Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen.

• Deoxyhemoglobin: Hemoglobin not bound to oxygen.

• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide.

• Carboxyhemoglobin: Hemoglobin bound to carbon monoxide (CO); CO binds with high affinity and can be fatal.

Anemia

Anemia is a condition characterized by decreased oxygen-carrying capacity of the blood. Symptoms include pallor, weakness, fatigue, and shortness of breath.

• Decreased hemoglobin: Iron deficiency or blood loss.

• Decreased hematocrit: Blood loss or decreased RBC production (e.g., B12 deficiency, bone marrow disorders).

• Abnormal hemoglobin: Sickle cell disease (mutation in hemoglobin S gene).

Polycythemia

Polycythemia is an increased hematocrit, which can be primary (bone marrow disorder) or secondary (response to low oxygen, such as at high altitude).

Leukocytes (White Blood Cells)

Leukocytes are larger than erythrocytes, have nuclei, and use blood as a transport vehicle to reach body tissues. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

Granulocytes

• Neutrophils: Most common; multi-lobed nucleus; phagocytize bacteria and release chemicals to attract other WBCs (chemotaxis).

• Eosinophils: Bilobed nucleus; involved in immune response to parasites and allergies.

• Basophils: Least common; release histamine and other chemicals that mediate inflammation.

Agranulocytes

• Lymphocytes: Large, spherical nucleus; include B-lymphocytes (produce antibodies) and T-lymphocytes (cell-mediated immunity).

• Monocytes: U-shaped nucleus; mature into macrophages that ingest dead cells, debris, and pathogens.

Leukocyte Formation

Leukocytes are formed from hematopoietic stem cells (HSCs) in the bone marrow. Myeloid and lymphoid cell lines give rise to different types of leukocytes.

• Myeloid line: Produces granulocytes and monocytes.

• Lymphoid line: Produces lymphocytes (B and T cells).

Platelets

Platelets are small, anucleate cell fragments involved in hemostasis (blood clotting). They are formed from megakaryocytes in the bone marrow.

• Contain granules: Store clotting factors and enzymes.

• Formation: Megakaryocytes extend processes into blood vessels, which fragment into platelets.

Hemostasis

Hemostasis is the process that stops blood loss from injured vessels. It involves several steps:

1. Vascular spasm: Vasoconstriction reduces blood flow.

2. Platelet plug formation: Platelets adhere to exposed collagen and aggregate.

3. Platelet activation: Platelets release granules containing ADP, serotonin, and clotting factors.

4. Coagulation: Fibrinogen is converted to fibrin, forming a mesh that stabilizes the platelet plug.

5. Retraction of clot: Platelets contract, pulling wound edges together.

6. Thrombolysis: Clot is dissolved after healing (plasmin degrades fibrin).

Clotting Regulation

Clotting is regulated by endothelial cells and plasma proteins:

• Prostacyclin: Inhibits platelet aggregation.

• Nitric oxide: Causes vasodilation.

• Antithrombin-III: Inhibits thrombin and factor X.

• Protein C: Inactivates clotting factors.

Clotting Disorders

• Bleeding disorders: Inability to clot (e.g., hemophilia A and B).

• Hypercoagulable disorders: Excessive clotting (e.g., deep vein thrombosis, pulmonary embolism, myocardial infarction).

Coagulation Cascade

The coagulation cascade is a series of enzymatic reactions that lead to the formation of a fibrin clot. It involves two main pathways:

1. Intrinsic pathway: Triggered by damage inside the vessel; involves factors XII, XI, IX, and VIII.

2. Extrinsic pathway: Triggered by external trauma; involves tissue factor (factor III) and factor VII.

3. Common pathway: Both pathways converge to activate factor X, which, with factor V and calcium, converts prothrombin to thrombin. Thrombin then converts fibrinogen to fibrin, forming the clot.

Key Equations:

• Hematocrit calculation:

• Oxygen transport:

Table: Comparison of Blood Cell Types

Example: In sickle cell anemia, a mutation in the beta-globin gene causes hemoglobin to polymerize under low oxygen, distorting erythrocyte shape and leading to anemia and vascular complications.

Additional info: These notes expand on the original content by providing definitions, context, and a summary table for clarity and exam preparation.