Lecture 21: The Composition and Features of Blood

The Circulatory System

Connection between Heart, Lungs, and Tissue:
- The heart distributes oxygenated blood to tissues.
- Respiration is coupled to oxidative phosphorylation where $O_2$ is the final electron acceptor in the mitochondrial electron transport chain, producing ATP.
- Average adult blood volume is approximately 5L. About 14,000L circulates every 24 hours.
- Large vessels near the heart have high volume and low flow, while small capillaries in tissues have low volume and high flow.
- Arteries:
  - Thick muscular walls.
  - Carry blood away from the heart.
  - Provide constant back-pressure to ensure even blood flow through tissues.
- Veins:
  - Thinner walls.
  - Return blood to the heart.
  - Low pressure with valves to prevent backflow.
Blood Pressure:
- Average blood pressure is considered to be 120/80, but there's a lot of debate around this number.
- 120 (systole): Maximum arterial pressure when the left ventricle (LV) contracts.
- 80 (diastole): Minimum arterial pressure when the heart is in between beats.
- Hypotension (too low):
 - Blood doesn't flow properly through capillaries.
 - Leads to low $O2$ and high $CO2$ .
 - Causes weakness, dizziness, and tiredness.
- Hypertension (too high):
 - Can cause capillary rupture, bleeding, abnormal clotting, and stroke.
- Breathing is regulated by the sympathetic/parasympathetic nervous system via sensing $O2$ / $CO2$ levels in the blood.
- Organ-specific blood distribution:
 - Heart 3%, Bone 5%, Skin 6%, Brain 14%, Skeletal muscle 15%, Kidneys 22%, GI and Spleen 27%, Liver 8%.
- Pulmonary circulation includes the pulmonary artery (to lungs) and pulmonary vein (from lungs).
- Systemic circulation includes the aorta and vena cava.
Oxygen/Carbon Dioxide Exchange:
- Lungs provide a vast surface area for efficient exchange of $O2$ and $CO2$ .
- Oxyhaemoglobin (bright red) carries oxygen from the lungs to the tissues via arteries.
- In tissues, $CO2$ replaces $O2$ .
- Carbaminohaemoglobin (dark red) carries $CO_2$ back to the lungs.
- In the lungs, $O2$ replaces $CO2$ .
Role of Haemoglobin:
- Haemoglobin transports oxygen and carbon dioxide.
- The association and dissociation of $O2$ from haemoglobin depend on the partial pressure of $O2$ .
- $O_2$ readily associates in the lungs and dissociates in the tissues.
- $CO_2$ exchange is vice versa.
- Other molecules like cyanide (cherry red) and carbon monoxide (pinkish) can readily displace $O_2$ from haem.
- Partial Pressure:
 - Air: p $O2$ = 160 mm Hg, p $CO2$ = 0.3 mm Hg
 - Lung alveoli: p $O2$ = 100 mm Hg, p $CO2$ = 35 mm Hg
 - Arterial blood: p $O2$ = 80-100 mm Hg, p $CO2$ = 40 mm Hg
 - Venous blood: p $O2$ = 20-40 mm Hg, p $CO2$ = 50 mm Hg

Major Components of Blood

Cells (originate in bone marrow from a single stem cell):
- Erythroid: Carries oxygen via haemoglobin.
- Myeloid: White cells providing innate immunity.
- Lymphoid: White cells providing adaptive immunity.
- Platelets: Blood clotting and tissue repair.
Proteins:
- Albumin: ~50% of total blood protein. Prevents changes in pH and osmolarity.
- Haemoglobin: In red blood cells. Contains iron. Binds and transports oxygen and carbon dioxide.
- Fibrinogen: Essential for coagulation.
- Immunoglobulins: Antibodies providing specific immunity.
- Hundreds of other proteins at lower amounts.
Lipids:
- Bound in lipoproteins: High, Low, and Very low density (HDL (Good), LDL (Bad), VLDL).
Electrolytes, salts, and minerals:
- $HCO_3^-$ , $Na^+$ , $Cl^-$ , $Ca^{++}$ , $Mg^{++}$ , $K^+$ , creatine, creatinine.
Vitamins and hormones
Glucose:
- Energy source for glycolysis and the Krebs cycle.

Main Blood Cell Types and Their Origin

Separation of Blood:
- Centrifugation (with anticoagulant) separates blood into:
  - Plasma (55%).
  - Buffy coat (white blood cells and platelets).
  - Red blood cells (45%).
Abundance of Cell Types:
- Erythrocytes (Red blood cells):
  - Oxygen transport.
  - ~5-6 million/ml.
- Leukocytes (White blood cells):
  - Immune defense.
  - ~10,000/ml.
- Platelets:
  - Coagulation and tissue repair.
  - ~400,000/ml.
Types of Leukocytes:
- Innate Immunity:
  - Neutrophils (most abundant, phagocytic).
  - Macrophages (phagocytic).
- Adaptive Immunity:
  - B cells (antibodies).
  - T cells (cellular immunity).
Hematopoiesis:
- $CD34^+$ stem cells re-populate people who have had ablation therapy for leukemia.
- Stem cells differentiate into myeloid (innate immunity) and lymphoid (adaptive immunity) lineages.
What does $CD34^+$ mean?
- Every cell development stage is characterized by a unique set of cell surface molecules.
- Monoclonal antibodies (mAbs) can selectively bind to specific molecules like $CD34$ .
- Fluorescent or magnetic mAbs are used to immunophenotype.
- $CD$ stands for "Cluster of Differentiation", an international protocol that defines individual cell surface markers of cell differentiation.
- Immunophenotyping quantifies and isolates cell populations using mAb reagents (MACS or FACS).
- Used diagnostically to detect blood diseases, e.g., AIDS (low $CD4$ count) or leukemia (high leukocyte count).

The Main Features of the Coagulation Cascade

Coagulation Overview:
- Coagulation is the cleavage of fibrinogen to fibrin.
- Plasma: Viscous liquid fraction after centrifugation with an anti-coagulant (e.g., heparin). Contains fibrinogen.
- Serum: Clear viscous yellow liquid remaining after coagulation and separation of the fibrin clot.
Two Main Activation Pathways:
- Intrinsic pathway (contact).
- Extrinsic pathway (tissue damage).
- Both pathways lead to the common pathway.
Steps:
- Intrinsic Pathway: Factors XII, XI, IX, VIII
- Extrinsic Pathway: Tissue Factor, VII, V
- Common Pathway: Factor Xa activates prothrombin to thrombin, which cleaves fibrinogen to fibrin, forming a clot.
Regulation and Breakdown:
- Plasminogen is converted to plasmin, which degrades fibrin.
- Tissue plasminogen activator (TPA) is used to treat stroke, myocardial infarct (MI), and pulmonary embolism (PE).
Anticoagulants:
- Hirudin (from leeches).
- Heparin.
Hemophilia:
- Genetic condition resulting in a defective coagulation factor.
- Most common form is X-linked defect for factor VIII.
Key Learning Points - Coagulation
- Coagulation is a proteolytic activation cascade.
- Two pathways for activation:
  - Intrinsic: contact with surfaces.
  - Extrinsic: tissue damage.
- Factor X (10) is the key enzyme common to both pathways.
- Calcium is essential at a number of steps. Remove calcium and blood does not clot.
- Thrombin is the key enzyme that cleaves fibrinogen to fibrin which cross-links to form a clot.
- Hemophilia is a bleeding disorder caused by a defective clotting factor.
- Many parasites and other microbes that rely on blood produce powerful anti-coagulants that typically target the thrombin step.
- Plasminogen is converted to active plasmin which dissolves the fibrin clot (thrombolysis).
- TPA (Tissue Plasminogen Activator) or Streptokinase are used widely in medicine for the treatment of thromboses –myocardial, PE, DVT, brain etc.

The Main Features of the Complement System and Opsonization

Complement Overview:
- First line of defense against invading pathogens.
- Neutrophils sense bacteria via chemicals released by complement proteins (opsonization).
Activation of Complement Proteins:
- Nine major complement proteins attach to the surface of the bacterium in a proteolytic cascade.
- They form stable enzyme complexes called "convertases" on the bacterial surface.
- Ultimately forms a lytic pore called the Membrane Attack Complex (MAC).
- Complement C3 is the most abundant in blood.
Key Learning Points for Complement
- Complement is a proteolytic activation cascade that is essential in innate immunity.
- C3 is the most abundant complement component in serum.
- Complement can be activated by 3 different pathways. The classical pathway is mediated by antibodies IgM or IgG binding to a microbe surface which is then bound by complement C1.
- The lectin pathway involves complement components that recognise unique sugars on bacteria
- The alternate pathway is probably the most important and involves direct activation of C3 when in close contacts with the surface of a bacteria.
- Deposition of complement on microbes is essential for phagocytosis - called opsonisation.
- Deposited complexes are called convertases. These activate more complement in an amplification loop (i.e. the alternate pathway).
- Convertases are irreversibly bound through a covalent bond.
- Cleavage of C3, C4 and especially C5 produce small fragments (C3a, C4a and C5a) that are powerful chemoattractants called anaphylatoxins that attract and activate phagocytes.
- The end stage of complement (C5 onwards) forms a lytic pore that cause some bacteria to lyse. This is the Membrane Attack Complex or MAC.
- People with deficiencies in a complement component are susceptible to chronic infections.
- Many microbes produce proteins called virulence factors that inhibit the complement cascade.