Study Notes on Hemodynamics: Capillary Exchange

Human Anatomy & Physiology II (PSIO202) Study Notes

Hemodynamics: Capillary Exchange

Overview of Capillary Functionality
  • Blood Flow Dynamics: The flow of blood transitions from arterioles into capillaries and subsequently enters venules, establishing a cycle for nutrient and waste exchange.
  • Significant Forces Involved:
    • BHP (Blood Hydrostatic Pressure): Pressure exerted by blood within capillaries.
    • BCOP (Blood Colloid Osmotic Pressure): Osmotic pressure caused by proteins in plasma.
    • IFOP (Interstitial Fluid Osmotic Pressure): Osmotic pressure from proteins in interstitial fluid.
    • IFHP (Interstitial Fluid Hydrostatic Pressure): Pressure exerted by interstitial fluid.
Objectives of Study
  1. Describe and illustrate the general features of a capillary network.
  2. Compare the anatomical and physiological features of each type of capillary.
  3. Explain the mechanisms by which fluid and solutes traverse the capillary wall.
  4. Discuss the forces governing fluid movement into and out of capillaries.
Capillary Overview
  • Nutrient and Gas Exchange:
    • Oxygen and Nutrient Transfer: Move from blood into interstitial fluid.
    • Waste Transfer: Carbon dioxide and metabolic waste products flow from interstitial fluid into blood.
  • Capillary Network Length: If all capillaries were stretched end-to-end, their total length would be approximately 60,000 miles with a cross-sectional area of 5,000 cm².
Characteristics of Capillaries
  • Average Size:
    • Length: ~1 mm
    • Lumen Diameter: ~8 µm
    • Blood Flow Velocity: ~0.1 cm/s
  • Density of Capillaries: The concentration correlates with the metabolic activity of the respective tissue.
Capillary Bed
  • Definition: A capillary bed is defined as a network of capillaries that allows for exchange of materials with nearby tissue cells.
  • Cell Proximity: Each tissue cell is usually within 1-3 cell diameters from the nearest capillary, making diffusion efficient.
    • Note on Diffusion: Efficient only over short distances. Refer to Fick's law of diffusion for details.
Regulation of Blood Flow Through Capillaries
  • Precapillary Sphincters: Muscular rings that regulate blood flow into capillary beds.
    • Sphincters Open: Blood flows through the capillary bed.
    • Sphincters Closed: Blood is directed through a thoroughfare channel, bypassing capillaries.
Mechanisms of Capillary Exchange
  • Capillary Exchange Routes:
    A. Intercellular Clefts or Pores: Allow water and most small substances to diffuse.
    B. Endothelial Cell Membranes: Some small molecules and gases are transported across by diffusion or active transport.
    C. Fenestrations: Present in certain capillaries, allowing larger molecules to move freely.
Types of Capillaries
  • Continuous Capillaries: Found in lungs, skeletal muscle, and connective tissue; formed by tightly joined endothelial cells with pinocytotic vesicles. Diameter of intercellular clefts typically very small.
  • Fenestrated Capillaries: Characterized by pores (diameter 70-100 nm), found in kidneys, endocrine glands, and small intestine; enhance permeability for larger molecules.
  • Sinusoidal Capillaries: Have large openings for cells and proteins, found in the liver, spleen, and bone marrow; facilitate the passage of larger cellular components like erythrocytes and macrophages.
Mechanisms of Fluid Exchange
  • Passive Processes:
    • Diffusion: Movement from high concentration to low concentration until equilibrium.
    • Bulk Flow: Movement of fluid from high pressure to low pressure areas.
  • Active Process:
    • Transcytosis: Involves substances being engulfed in pinocytotic vesicles, traversing endothelial cells, and being released via exocytosis.
Fluid Exchange Dynamics

  • Bulk Flow Mechanics:

    1. Forces Pushing Fluid Out of Capillaries:
    • Blood Hydrostatic Pressure (BHP): Creates pressure that pushes fluid out into interstitial space.
    • Interstitial Fluid Osmotic Pressure (IFOP): A smaller force that pulls water out of capillaries, typically about 1 mmHg.
    1. Forces Pulling Fluid Into Capillaries:
    • Blood Colloid Osmotic Pressure (BCOP): Results from protein concentration differences, typically around 26 mmHg, drawing fluid back into the capillaries.
    • Interstitial Fluid Hydrostatic Pressure (IFHP): Generally negligible but contributes to reabsorption, often about 0 mmHg.
Net Filtration Pressure (NFP)

  • Formula:
    NFP=(BHP+IFOP)−(BCOP+IFHP)NFP = (BHP + IFOP) - (BCOP + IFHP)

    • Represents the net movement of fluid:
    • Positive values indicate net filtration (fluid leaving capillaries).
    • Negative values indicate net reabsorption (fluid entering capillaries).

  • Example Calculations:

    • At the arterial end, NFP=(35+1)−(26+0)=10extmmHgNFP = (35 + 1) - (26 + 0) = 10 ext{ mmHg}
    • At the venous end, NFP=(16+1)−(26+0)=−9extmmHgNFP = (16 + 1) - (26 + 0) = -9 ext{ mmHg}
Conclusion of Fluid Dynamics
  • Constant Filtration: At the arterial end, approximately 20 liters/day of fluid is pushed out into the interstitial fluid.
  • Reabsorption: At the venous end, fluid drawn back into capillaries is approximately 17 liters/day.
  • Total Fluid Loss: About 3 liters/day remains in interstitial fluid, recycled through the lymphatic system, maintaining homeostasis.
Summary on Fluid Dynamics and Capillary Exchange
  • Filtration leads to nutrient and oxygen delivery at the arterial end of capillaries, doing so crucially for tissue health, while reabsorption at the venous end aids in waste transport back to circulation.
  • Understanding Net Filtration Pressure is crucial for comprehending fluid exchange dynamics, emphasizing importance in physiological homeostasis and fluid regulation processes.