Microcirculation and Blood Flow

Microcirculation and Blood Flow Notes

Lecture Overview

  • Topics Covered:
  • Microcirculation
  • Transport mechanisms across capillaries
  • Starling Equilibrium
  • Distribution of cardiac output
  • Local blood flow regulation
  • Long-term perfusion regulation

Aims of Lecture

  • Microcirculation Components: Understand the types of vessels involved:
  • Arteries, arterioles, capillaries, venules, veins
  • Substance Transport: Describe transport types across capillaries:
  • Lipid-soluble and water-soluble substances
  • Starling Equilibrium: Explain this concept and its significance in blood fluid regulation.
  • Physiological Response Predictions: Analyze effects of changes in plasma protein, blood pressure, and vaso-tones on plasma and interstitial volumes.
  • Colloidal Osmotic Pressure: Define and understand its role in fluid dynamics.
  • Lymphatic System: Describe its functional importance in fluid regulation.

Microcirculation Components

  • Major Components:
  • Arteries: Highly elastic; transport oxygenated blood.
  • Arterioles: Control blood flow into capillaries; variable radius.
  • Capillaries: Site of exchange between blood and tissues.
  • Venules and Veins: Return deoxygenated blood to the heart.
  • Metarterioles and Pre-capillary Sphincters: Regulate blood flow within capillary beds.

Capillary Transport Mechanisms

  • Transport Modes:
  • Types:
    • Passive Transport: Movement of substances without energy (e.g., diffusion).
    • Active Transport: Requires energy to move substances against a concentration gradient.
  • Bulk Flow vs. Diffusion: Distinction between the mechanisms.
  • Specific Substances:
  • Lipid-Soluble: O2, CO2 pass through cell membranes.
  • Water-Soluble: Na+, Cl-, glucose pass through intercellular clefts.
  • Large Molecules: Moved via pinocytosis (e.g., lipoproteins).
  • Plasma Proteins: Typically do not cross capillary walls.

Starling Equilibrium

  • Basic Concept:
  • Balance between hydrostatic and oncotic pressures determines fluid movement across capillary walls.
  • Hydrostatic Pressure: Forces fluid out of the capillaries.
  • Osmotic (Oncotic) Pressure: Pulls fluid back into capillaries.
  • Pressures Defined:
  • PC: Capillary hydrostatic pressure
  • PIF: Interstitial fluid pressure
  • πp: Oncotic pressure of plasma
  • πIF: Interstitial fluid oncotic pressure
  • Fluid Dynamics: Net flow direction determined by differences in these pressures.

Plasma Filtration and Absorption

  • Increased Filtration Causes:
  • Oedema due to vasodilation, hypertension, and protein deficiency.
  • Increased Absorption:
  • Results from vasoconstriction and dehydration.

Lymphatic System and Flow

  • Lymph Transport:
  • Achieved via smooth muscle contractions in lymph vessels; valves prevent backflow.
  • Oedema Factors:
  • Conditions such as elephantiasis result from lymphatic blockage.

Distribution of Cardiac Output

  • Flow Regulation:
  • Total perfusion is equivalent to cardiac output; local arteriolar resistance affects distribution.
  • Tissue Demand: Each organ receives a fraction of cardiac output based on its metabolic needs.

Control of Blood Flow

  • Cardiac Output Impact:
  • Adjusts based on tissue perfusion requirements.
  • Efficient blood supply prevents necrosis.
  • Metabolic Control:
  • Hyperaemia: Increased blood flow due to heightened metabolic activity.
  • Reactive Hyperaemia: Increased blood flow post-ischemia.

Metabolic Regulators

  • Mediators:
  • Vasoconstrictors: O2, glucose (in specific vessels).
  • Vasodilators: K+, CO2, adenosine, H+, PO43-.

Long-Term Regulation of Flow

  • Vasculature Remodeling:
  • Changes in vessel diameter and angiogenesis respond to tissue perfusion changes.
  • Factors like oxygen levels promote remodeling and new vessel formation.

Summary of Key Points

  • Capillary system crucial for substance exchange; blood pressure facilitates fluid movement.

  • Colloidal osmotic pressure pulls fluid back into circulation, maintaining balance.

  • Local control via metabolic signals; long-term adjustments involve structural vascular changes.

  • Understanding microcirculation is essential for insights into cardiovascular health and disease treatment strategies.