Lecture 1: The Composition and Features of Blood

Key Learning Objectives

The Circulatory System - Understand the connection between heart, lungs, and tissue. - Grasp the concept of blood pressure and its clinical implications. - Understand the critical role of haemoglobin in the transport of oxygen ( $O_2$ ) and carbon dioxide ( $CO_2$ ).
Blood Composition and Origin - Identification of the major components of blood. - Knowledge of the main blood cell types and their origin from hematopoietic stem cells.
Laboratory and Diagnostic Methods - Basic principles of cell separation (centrifugation, $\text{MACS}$ , $\text{FACS}$ ). - Identification of cell surface markers (Cluster of Differentiation or $\text{CD}$ antigens).
Immune and Hemostatic Systems - The main features of the complement system and the process of opsonization (coating pathogens to aid immune response). - The coagulation cascade: sequential activation of blood factors to form clots and stop bleeding.

The Circulatory System: Hemodynamics and Respiration

Blood Volume and Circulation - Average Adult Blood Volume: Approximately $5\,L$ . - Daily Circulation Rate: About $14,000\,L$ of blood is pumped through the body every $24$ hours.
Heart Function and Oxygen Distribution - The heart's primary function is to distribute oxygenated blood to tissues. - Aerobic Respiration: Coupled to oxidative phosphorylation ( $\text{OxPhos}$ ). $O_2$ serves as the final electron acceptor in the mitochondrial electron transport chain ( $\text{ETC}$ ), used to produce high-energy $\text{ATP}$ which powers most cellular processes. - Note: While heart anatomy and specific $\text{OxPhos}$ mechanics are not examinable, they provide the biological necessity for circulation.
Vessel Dynamics - Large Vessels: Located close to the heart; characterized as high volume/low flow. - Small Capillaries: Located in tissue; characterized as low volume/high flow. This system creates sufficient pressure to force blood through miles of microscopic capillaries.
Arteries vs. Veins - Arteries: Thick, muscular, and elastic. They carry blood away from the heart. Their elasticity provides constant back-pressure on the heart to ensure even, unidirectional flow. Arteries are generally located deep (near the bone) for protection. - Veins: Thinner-walled vessels that return blood to the heart. They are often located near the skin. - Valves: Located along the veins to prevent back-flow, ensuring blood does not move backwards or stop.

Blood Pressure: Regulation and Distribution

Standard Measurements - Average Pressure: $120/80\,\text{mm Hg}$ . This is the pressure required to raise a column of mercury to specific millimetric heights. - Systolic Pressure ( $120\,\text{mm Hg}$ ): The maximum arterial pressure attained during left ventricle ( $\text{LV}$ ) contraction. - Diastolic Pressure ( $80\,\text{mm Hg}$ ): The minimum arterial pressure when the heart is between contractions.
Clinical Concerns - Hypotension (Low BP): Blood does not flow effectively through capillaries. Results in low $O_2$ and high $CO_2$ levels. Symptoms include weakness, dizziness, tiredness, and fainting. - Hypertension (High BP): Puts excessive pressure on capillaries. This can cause ruptures, abnormal clotting, and strokes if capillaries in the brain are affected.
Regulation and Monitoring - Breathing and blood flow are regulated via the sympathetic and parasympathetic nervous systems, which sense $O_2$ and $CO_2$ levels. - BP Measurement Technique: A cuff is placed around the arm; the first sound heard via stethoscope indicates systolic pressure, and the cessation of sound indicates diastolic pressure.
Relative Blood Distribution (General Context) - Liver: $27\%$ - Kidneys: $22\%$ - GI and Spleen: $21\%$ - Skeletal Muscle: $15\%$ - Brain: $14\%$ - Skin: $6\%$ - Bone: $5\%$ - Heart Muscle: $3\%$

Oxygen and Carbon Dioxide Exchange

Mechanism of Exchange - The lungs provide a massive surface area for efficient gas exchange in the alveoli. - Arterial Blood: Rich in $O_2$ , carrying Oxyhaemoglobin (bright red). - Venous Blood: Rich in $CO_2$ , carrying Carbaminohaemoglobin (dark red). - Note: Humans can survive with only one lung.
Partial Pressures of Gases ( $\text{mm Hg}$ ) - Air: $p(O_2) = 160$ , $p(CO_2) = 0.3$ - Lung Alveoli: $p(O_2) = 100$ , $p(CO_2) = 35$ - Arterial Blood: $p(O_2) = 80-100$ , $p(CO_2) = 40$ - Venous Blood: $p(O_2) = 20-40$ , $p(CO_2) = 50$
Hemoglobin and Gaseous Affinity - Hemoglobin contains a Haem Porphyrin ring with iron ( $Fe$ ) that binds oxygen. - Association/dissociation of $O_2$ is dependent on the partial pressure of $O_2$ . It associates in high-pressure areas (lungs) and dissociates in low-pressure areas (tissues).
Displacement and Poisoning - Other molecules can displace $O_2$ from haem, with lethal consequences. - Cyanide: Binds to haem; blood and lips appear "cherry red." - Carbon Monoxide ( $CO$ ): Displaces $O_2$ ; blood/lips appear "bright cherry" or "pink." - Hypoxia: General lack of oxygen causes lips to turn blue.

Major Components of Blood

1. Blood Cells (All originate from a single hematopoietic stem cell in the bone marrow): - Erythroid: Red blood cells carrying oxygen via hemoglobin. - Myeloid: White cells providing innate immunity. - Lymphoid: White cells providing adaptive immunity. - Platelets: Involved in clotting and tissue repair.
2. Major Proteins: - Albumin: Constitutes nearly $50\%$ of total blood protein. Functions as a buffer to prevent changes in $pH$ and osmolarity. - Haemoglobin: Found in red cells; contains iron and transports $O_2$ / $CO_2$ . - Fibrinogen: Essential precursor for coagulation. - Immunoglobulins: Antibodies providing highly specific (adaptive) immunity. - Complement: Proteins for rapid defense against pathogens.
3. Lipids: - Carried in lipoproteins: High density ( $\text{HDL}$ - Good), Low density ( $\text{LDL}$ - Bad), and Very Low Density ( $\text{VLDL}$ ).
4. Electrolytes, Salts, Minerals, and Energy: - Ions: $HCO_3^-$ , $Na^+$ , $Cl^-$ , $Ca^{++}$ , $Mg^{++}$ , $K^+$ . - Waste markers: Creatine, creatinine. - Glucose: Energy source for glycolysis and the Krebs cycle. - Vitamins and hormones.

Cell Separation and Abundance

Centrifugation with Anti-coagulant: - Plasma ( $55\%$ ): Top layer. Color is usually clear/yellow but can be milky white if high in fat. - Buffy Coat: Middle layer containing white blood cells and platelets. - Red Blood Cells ( $45\%$ ): Bottom layer.
Abundance of Main Cells: - Erythrocytes: $~5-6 \times 10^6/\text{ml}$ . Transport oxygen; have no nucleus. - Leukocytes: $~10,000/\text{ml}$ . Immune defense. - Platelets: $~400,000/\text{ml}$ . Coagulation and tissue repair.
Types of Leukocytes: - Innate Immunity: - Neutrophils: Most abundant, phagocytic. - Eosinophils and Basophils: Contain secretory granules for parasites and allergic responses (histamine). - Monocytes: Become Macrophages (large phagocytes). - Adaptive Immunity: - B Cells: Produce antibodies. - T Cells: Destroy infected or abnormal cells (cellular immunity).

Hematopoiesis and Immunophenotyping

Stem Cells: - $CD34^+$ stem cells represent $1$ in $10,000$ white cells in the bone marrow. They can re-populate the blood system after ablation therapy for leukemia.
The CD System (Cluster of Differentiation): - An international protocol defining markers on the cell surface. Hundreds of antigens are defined by monoclonal antibodies ( $\text{mAb}$ ). - Examples: - $CD3^+$ : Found on T cells. - $CD19^+$ : Found on B cells. - $CD4$ and $CD8$ : Markers for specific T cell subsets.
Diagnostic Utility: - AIDS: Indicated by very low $CD4^+$ T cell counts. - Neutropenia: Low myeloid count signaling infection or cancer.
Isolation Techniques: - $\text{MACS}$ : Magnetic-activated cell sorting. - $\text{FACS}$ : Fluorescence-activated cell sorting.

The Coagulation Cascade

Process: The proteolytic cleavage of fibrinogen into fibrin.
Definitions: - Plasma: The liquid fraction after centrifugation using an anti-coagulant (e.g., heparin). Still contains fibrinogen. - Serum: The clear yellow liquid remaining after blood has clotted. Does not contain fibrinogen.
Activation Pathways: - Intrinsic Pathway (Contact): Triggered by contact with foreign surfaces (glass tubes, heart valves). Involves Factors $XII, XI, IX, VIII$ . - Extrinsic Pathway (Tissue Damage): Triggered by wounds or infection. Involves Tissue Factor, and Factors $V, VII$ .
Fundamental Points: - Factor $X (10)$ : The key enzyme common to both pathways. - Thrombin: Activated by Factor $Xa$ ; it cleaves fibrinogen into fibrin fibers that cross-link to form a clot. - Calcium ( $Ca^{++}$ ): Essential for multiple steps in the cascade. Removing calcium stops blood from clotting.
Clinical Context: - Haemophilia: Genetic bleeding disorder caused by a defective clotting factor. The most famous is the X-linked Factor $VIII$ defect (Queen Victoria's line). - Anti-coagulants: Used by parasites (e.g., leeches: Hirudin) or in medicine (Heparin). - Thrombolysis (Clot Dissolving): Plasminogen is converted to Plasmin, which degrades fibrin. $\text{TPA}$ (Tissue Plasminogen Activator) or Streptokinase are used to treat strokes, myocardial infarctions ( $\text{MI}$ ), and pulmonary embolisms ( $\text{PE}$ ) if given early.

The Complement System

Innate Defense: First line of defense against pathogens. It is a proteolytic activation cascade.
Components: $9$ major proteins ( $C1$ to $C9$ ). $C3$ is the most abundant in serum.
Activation Pathways: - Classical: Mediated by antibodies ( $IgM$ or $IgG$ ) binding to a microbe, which then binds $C1$ . - Lectin: Recognizes unique sugars on bacteria. - Alternative: Direct activation of $C3$ on the surface of bacteria; includes massive amplification loops.
Core Functions: - Opsonization: Deposition of complement on microbes. This is essential for neutrophils to sense, attract to, and engulf the bacteria (phagocytosis). - Anaphylatoxins: Small fragments ( $C3a, C4a, C5a$ ) released during cleavage that attract and activate phagocytes. - $\text{MAC}$ (Membrane Attack Complex): The end stage ( $C5$ onwards) forms a lytic pore in the bacterial membrane, causing lysis.
Clinical and Biological Features: - Convertases: Stable complexes formed on the bacterial surface, irreversibly bound via covalent bonds. - Deficiencies: Individuals missing complement components are highly susceptible to chronic infections. - Virulence Factors: Microbes produce proteins to inhibit this cascade to survive immune attacks.