Lecture Notes: Capillary Exchange and Circulatory Pathways

Introduction

• Focus on Part 2 of Chapter 20, covering blood vessel physiology, specifically capillary exchange.
• Key processes: Filtration and reabsorption.
• Associated sections: 20.3 and 20.4 in reading material.

Variations in Circulatory Pathways

• Simplest Pathway (A):

  • Single capillary bed from heart to systemic circulation and back.
  • Blood path: Left ventricle → aortic valve → aorta → arteries → arterioles → capillary bed → venules → veins → right atrium.

• Portal System (B):

  • Blood flows through two consecutive capillary networks before returning to the heart.
  • Examples: Brain (hypothalamus and pituitary gland), kidneys, intestines, and liver (hepatic portal system).

• Anastomoses (C, D, E):

  • Arteriovenous anastomosis: shunt bypassing capillary beds.
  • Venous and arterial anastomosis: interconnections that provide alternate pathways (detours) for blood flow in case of blockages.

Capillary Exchange

• Capillaries are crucial for exchanging gases, nutrients, and wastes between blood and tissues.
• Structure and Features:
  • Thin-walled with intercellular clefts and fenestrations.
  • Sinusoidal capillaries have larger openings.
• Exchange Processes:
  • Filtration: Movement of water and solutes out of blood.
  • Reabsorption: Movement of water and solutes into blood.

Transport Mechanisms

• Diffusion:

  • Passive transport of substances down concentration gradient (e.g., oxygen, carbon dioxide).
  • Lipid-soluble substances pass through easily; water-soluble substances require gaps or pores.

• Transcytosis:

  • Endothelial cells transport materials across cell, involving endocytosis and exocytosis.
  • Used for transporting larger molecules like proteins and fatty acids.

• Bulk Flow: Filtration and Reabsorption

  • Filtration: Outward movement at the arterial end due to higher blood hydrostatic pressure (BHP).
  • Reabsorption: Inward movement at the venous end driven by blood colloid osmotic pressure (BCOP).
  • Key Forces:
    • BHP: Pressure exerted by blood plasma, promoting filtration.
    • IFOP: Interstitial fluid osmotic pressure, minor influence on filtration.
    • BCOP: Osmotic pressure from plasma proteins, promoting reabsorption.
    • IFHP: Interstitial fluid hydrostatic pressure, minor influence on reabsorption.

Calculating Net Filtration Pressure (NFP)

• Arterial End: Net pressure = 36 mmHg (out) - 26 mmHg (in) = 10 mmHg (net filtration).
• Venous End: Net pressure = 17 mmHg (out) - 26 mmHg (in) = -9 mmHg (net reabsorption).

Starling's Hypothesis

• Balance between hydrostatic and osmotic pressures determines fluid movement.
• Importance of non-reabsorbed fluid entering lymphatic vessels.

Clinical Application

• Deficit in plasma proteins (e.g., due to liver failure) results in lower BCOP, causing decreased reabsorption and potential edema.

Summary Terminology

• BHP: Blood hydrostatic pressure, driving filtration.
• IFOP: Interstitial fluid osmotic pressure, minor filtration force.
• NFP: Net filtration pressure, balance of filtration and reabsorption.
• BCOP: Blood colloid osmotic pressure, major reabsorption force.
• IFHP: Interstitial fluid hydrostatic pressure, minor reabsorption force.

Conclusion

• Overview of capillary exchange in relation to Starling's hypothesis and clinical implications.