Blood: Components, Functions, and Physiology (Chapter 19 Study Guide)

Components & Functions of Blood

Overview

Blood is a specialized connective tissue essential for transporting substances, regulating physiological parameters, and defending the body. It consists of plasma and formed elements, each with distinct roles.

• Functions: Transport of gases (O2, CO2), nutrients, wastes; regulation of pH and temperature; defense against pathogens; clotting.

• Characteristics:

  • Temperature: 38°C (100.4°F)

  • pH: 7.35–7.45 (slightly alkaline)

  • Volume: ~7% of body weight (~5 L in average adult)

• Composition:

  • Plasma (~55%): mostly water (90%), proteins, solutes

  • Formed elements (~45%): red blood cells (RBCs), white blood cells (WBCs), platelets

Additional info: Plasma is the liquid matrix, while formed elements are the cellular components.

Plasma Proteins

Types and Functions

Plasma proteins are crucial for maintaining osmotic pressure, immune responses, and blood clotting. Most are synthesized in the liver.

• Albumins (60%): Maintain osmotic pressure, transport substances.

• Globulins (35%): Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen (4%): Involved in clotting; converts to fibrin during coagulation.

• Others (1%): Enzymes, hormones, regulatory proteins.

Additional info: Albumins and globulins are most clinically significant for exams.

Red Blood Cells (RBCs)

Structure and Function

RBCs are the most abundant formed elements, specialized for oxygen transport.

• Percentage: ~99.9% of formed elements

• Hematocrit: ~46% (♂), ~42% (♀)

• Structure: Biconcave discs, increase surface area, flexible, stack in rouleaux

• Lifespan: ~120 days; lack nucleus and mitochondria

• Hemoglobin:

  • Normal: 14–18 g/dL (♂), 12–16 g/dL (♀)

  • Structure: 4 subunits (2α, 2β), each with heme + iron

  • O2 binds to oxyhemoglobin; CO2 binds to carbaminohemoglobin

Key Fact: 280 million Hb per RBC; each Hb carries 1 billion O2 molecules.

RBC Formation (Erythropoiesis)

• Occurs in red bone marrow

• Stem cell progression: Hemocytoblast → Proerythroblast → Erythroblast → Reticulocyte → Mature RBC

• EPO (Erythropoietin): Hormone from kidneys (in response to hypoxia); stimulates RBC production

• Requires amino acids, iron, folic acid, vitamins B12, B6

• Lack of B12 → Pernicious anemia

Additional info: EPO is clinically relevant in anemia and blood doping.

RBC Recycling

• Macrophages in spleen/liver/bone marrow recycle RBCs

• Heme → biliverdin → bilirubin (excreted in bile)

• Excess bilirubin → jaundice

Blood Types

Antigens and Compatibility

Blood types are determined by the presence of specific antigens on RBCs. Compatibility is crucial for safe transfusions.

• ABO System:

  • Type A: A antigen, anti-B antibody in plasma

  • Type B: B antigen, anti-A antibody in plasma

  • Type AB: A and B antigens, no antibodies

  • Type O: No antigens, both anti-A and anti-B antibodies

• Rh System: Rh+ has antigen, Rh– lacks antigen

• Cross-reaction: Antibodies cause agglutination if incompatible

• Universal donor: O– (but cross-matching still required)

White Blood Cells (WBCs)

Types and Functions

WBCs are immune cells that protect against infection and disease. They are classified by appearance and function.

• Count: 5,000–10,000/μL

• Types:

  • Neutrophils (50–70%): Phagocytic, bacteria killers, short-lived, form pus

  • Eosinophils (2–4%): Attack parasites, modulate allergic reactions, reduce inflammation

  • Basophils (<1%): Release histamine (dilates vessels), heparin (anticoagulant)

  • Lymphocytes (20–40%):

    • T cells: Cell-mediated immunity

    • B cells: Produce antibodies

    • NK cells: Destroy abnormal cells

  • Monocytes (2–8%): Become macrophages, phagocytize pathogens and debris

Additional info: Granulocytes = neutrophils, eosinophils, basophils; Agranulocytes = lymphocytes, monocytes.

Platelets

Structure and Role

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

• Derived from megakaryocytes in bone marrow

• Count: 150,000–500,000/μL

• Functions: Release clotting chemicals, form temporary plugs, reduce vessel breakage

• Regulation: Controlled by TPO (thrombopoietin), IL-6, Multi-CSF

Hemostasis (Stopping Bleeding)

Phases and Mechanisms

Hemostasis is the process that prevents blood loss after vessel injury, involving three main phases.

1. Vascular phase: Vessel spasm, sticky endothelium

2. Platelet phase: Platelet adhesion and aggregation; formation of platelet plug; release of ADP, serotonin, clotting factors, Ca2+

3. Coagulation phase: Cascade of reactions (extrinsic, intrinsic, common pathways)

   • Key reaction: $\text{Factor X} \rightarrow \text{Prothrombin activator} \rightarrow \text{Thrombin} \rightarrow \text{Fibrinogen} \rightarrow \text{Fibrin clot}$

• Control: Anticoagulants (heparin, antithrombin III, prostacyclin)

• Requirements: Ca2+ and vitamin K

• Fibrinolysis: Clot dissolved by plasmin

Additional info: Disorders of hemostasis include hemophilia (clotting deficiency) and thrombosis (excess clotting).