Microcirculation physiology

Starling Forces - The Great Exchange

• Fluid movement across capillaries is governed by Starling forces, balancing hydrostatic (push) and oncotic (pull) pressures. Net movement determines filtration or absorption.
• Net Filtration Pressure Equation: $J_v = K_f [ (P_c - P_i) - \sigma (\pi_c - \pi_i) ]$
  • $P_c$ (Capillary hydrostatic): Pushes fluid out.
  • $\pi_c$ (Capillary oncotic): Pulls fluid in.
  • $P_i$ (Interstitial hydrostatic): Pushes fluid in.
  • $\pi_i$ (Interstitial oncotic): Pulls fluid out.
• 📌 Mnemonic: 'Push out, Pull in'. P for Hydrostatic Pressure Pushes; π for Oncotic Pressure Pulls.

⭐ The reflection coefficient (σ) indicates the effectiveness of the capillary wall in preventing protein passage. A value of 1 implies impermeability, while 0 implies free passage.

Flow Regulation - Local Traffic Control

Autoregulation is the intrinsic ability of an organ to maintain constant blood flow despite changes in perfusion pressure. Key mechanisms:

• Myogenic Mechanism: Smooth muscle contracts in response to ↑ stretch (↑ pressure) and relaxes with ↓ stretch.
• Metabolic Mechanism: Tissue metabolism dictates local blood flow.

Hyperemia:

• Active Hyperemia: Blood flow increases to meet metabolic demand (e.g., exercise).
• Reactive Hyperemia: A transient increase in blood flow following a period of ischemia.

Local Vaso-regulators:

• Vasodilators:
  • Nitric Oxide (NO)
  • Adenosine
  • $K^+$
  • $CO_2$
  • $H^+$
• Vasoconstrictors:
  • Endothelin
  • Myogenic response

⭐ In the heart, adenosine is the most critical local vasodilator linking coronary blood flow to myocardial oxygen demand.

Lymphatics & Edema - The Drainage System

Lymphatic capillaries drain excess interstitial fluid, protein, and macromolecules, returning it to circulation. Failure of this system results in edema-fluid accumulation in the interstitium.

• Pathophysiology (Altered Starling Forces):

  • ↑ $P_c$ (Capillary Hydrostatic Pressure): Pushes excess fluid out (e.g., heart failure, DVT).
  • ↓ $\pi_c$ (Plasma Oncotic Pressure): Fails to pull fluid in (e.g., liver failure, nephrotic syndrome).
  • ↑ $K_f$ (Capillary Permeability): Allows protein and fluid to leak out (e.g., inflammation, burns).
  • Lymphatic Obstruction: Impairs fluid return (e.g., post-surgery, filariasis).

• Types: Pitting edema (low-protein fluid) vs. Non-pitting edema (protein-rich fluid).

⭐ Lymphedema, caused by lymphatic obstruction (e.g., post-mastectomy, filariasis), is a classic cause of non-pitting edema due to the high protein content of the interstitial fluid.

• Starling forces (hydrostatic & oncotic pressures) dictate fluid flux across the capillary wall.
• Capillary hydrostatic pressure (Pc) is the main force driving filtration (fluid out).
• Plasma oncotic pressure (πc) is the main force driving reabsorption (fluid in).
• Arterioles are the primary site of vascular resistance, regulating blood flow into the capillary bed.
• Lymphatics are essential for draining excess interstitial fluid, preventing edema.