Microcirculation and Blood Flow

Introduction to Microcirculation

  • Microcirculation involves small blood vessels (capillaries, arterioles, venules) facilitating exchange between blood and tissues.
  • Key components: arterioles, capillaries, venules, and lymphatic vessels.

Transport Across Capillaries

  • Transport mechanisms:
  • Between cells: water-filled pores allow selective movement.
  • Transcellular: active or passive transport processes.
  • Lipid soluble substances: pass through endothelial cell membranes (e.g., O2, CO2).
  • Water-soluble substances: diffuse between endothelial cells (e.g., Na+, Cl-, glucose).
  • Large molecules: moved via pinocytosis (e.g., lipoproteins).
  • Plasma proteins: generally do not cross capillary walls.

Starling Equilibrium

  • Governs fluid movement across capillary walls based on:
  • Hydrostatic pressure (pushes fluid out) vs. oncotic (osmotic) pressure (pulls fluid in).
  • Osmotic pressure from proteins (e.g. albumin) mostly accounts for colloidal osmotic pressure (25 mmHg).
  • Starling forces include:
  • Pc: hydrostatic pressure inside the capillary
  • Pif: hydrostatic pressure in the interstitial fluid
  • πp: oncotic pressure of plasma
  • πif: oncotic pressure of interstitial fluid

Plasma Filtration and Osmotic Pressure

  • Fluid filtration occurs more at the arterial end of capillaries due to higher hydrostatic pressure initially.
  • Reabsorption occurs towards the venous end due to increased oncotic pressure.
  • Total osmotic pressure of blood includes all solutes, with a notable difference between plasma (due to proteins) and interstitial fluid.

Lymphatic System Function

  • Transports lymph via smooth muscle contractions.
  • Functions to return excess interstitial fluid to the bloodstream and prevents edema.
  • Blockage of lymphatic vessels (e.g., due to filariasis) can lead to swelling (elephantiasis).

Distribution of Cardiac Output

  • Total perfusion is equal to cardiac output.
  • Arteriolar resistance determines blood flow distribution to various organs:
  • Tissues selectively receive blood according to their metabolic needs to minimize workload on the heart.

Local Control of Blood Flow

  • Hyperaemia: increased blood flow in response to increased metabolism.
  • Reactive hyperaemia: post-ischaemia increased blood flow is proportional to duration of prior flow loss.
  • Local metabolites (e.g., O2, CO2, adenosine, K+, H+) regulate vascular resistance and smooth muscle tone.

Metabolic Mediators

  • Biological mediators that affect vascular response:
  • O2: generally a vasoconstrictor except in pulmonary circulation.
  • CO2: acts as a vasodilator.
  • Adenosine: promotes vasodilation.
  • H+: results in vasodilation.
  • K+: promotes dilation in low doses.

Long-Term Regulation of Blood Flow

  • Remodelling: vessels increase diameter when perfusion decreases or metabolic rate increases.
  • Angiogenesis: formation of new blood vessels occurs due to chemical signals, predominantly in tissues with high metabolic demands or in tumors.
  • Growth factors: include Endothelial cell growth factor (ECGF), Fibroblast growth factor (FGF), promoting vessel growth.

Summary Points

  • Microcirculation involves complex networks of small blood vessels where essential exchange processes occur.
  • Fluid exchange across capillaries is dictated by Starling forces, plasma proteins, and hydrostatic pressures.
  • Local metabolic activities drive changes in blood flow via various control mechanisms involving mediators.
  • Lymphatic systems help manage interstitial fluid balance, contributing to overall homeostasis.
  • Long-term adaptations include structural changes to vessels in response to varying metabolic needs or pressures.