CB (042)microcirculation and tissue fluid exchange (2)

Microcirculation and Tissue Fluid Exchange

Page 1: Introduction

• Overview of microcirculation and tissue fluid exchange, crucial for maintaining homeostasis and nutrient delivery at the cellular level.

• Context: NGU, School of Medicine

• Year: 2024/2025

• Presenter: Physiology Team

Page 2: Lecture Objectives

• Functional Anatomy of the Local Circulatory System

  • Understanding the structural components involved in microcirculation, including arteries, arterioles, capillaries, venules, and veins, as well as their roles in blood flow regulation.

• Transcapillary Exchange Mechanisms

  • Diffusion: Passive movement of molecules from areas of higher concentration to lower concentration, essential for gas exchange (O2 and CO2) and nutrient delivery.

  • Filtration: The movement of fluid across capillary walls influenced by hydrostatic and osmotic forces; key in understanding volume regulation and edema formation.

  • Transcytosis: Mechanism by which macromolecules are transported across endothelial cells via vesicles, vital for the movement of large substances such as antibodies.

• Lymphatic Circulation: Essential role in fluid homeostasis, immune response, and the return of excess interstitial fluid to the bloodstream.

Page 3: Cardiovascular System Layout

• Microcirculation Pathways

  • Blood flow pathways in low O2 (hypoxic) areas versus high O2 (hyperoxic) areas, highlighting the importance in tissue perfusion and oxygen delivery.

• Circulatory Circuits

  • Pulmonary Circulation: Route of blood from the right ventricle to the lungs for oxygenation, involving pulmonary capillaries.

  • Systemic Circulation: Path from the left ventricle to systemic tissues, ensuring nutrients and oxygen reach all body cells.

Page 4: Microcirculatory System Anatomy

• Average Capillary Blood Flow Speed: Approximately 1 mm/s; this slow flow enhances time for nutrient and gas exchange.

• Structures Involved:

  • Arteriole: Smallest branches of arteries, which regulate blood flow into capillaries through vasoconstriction and vasodilation.

  • Venule: Smallest veins that collect blood from capillaries and return it to larger veins.

  • Metarteriole: A short vessel that links arterioles to capillaries, acting as a bypass for blood during times of high demand.

  • Precapillary Sphincter: Ring of smooth muscle that controls blood flow into capillary beds, influenced by local tissue metabolic needs.

  • True Capillaries: Main sites for exchange of substances with surrounding tissues.

  • Nutritional Flow: Direct blood supply providing vital nutrients to tissue cells.

  • Shunt Flow: Pathway allowing blood to bypass capillary networks directly to venules during high metabolic demand or in cold conditions.

Page 5: Vasomotion

• Definition: The dynamic process of intermittent opening and closing of capillaries, facilitating optimal blood flow according to tissue needs.

• Mechanism: Rhythmic contraction and relaxation of metarterioles and precapillary sphincters are finely tuned to local tissue conditions and metabolic signals.

• Control: Primarily influenced by local tissue oxygen levels and metabolites, steering blood flow towards areas with higher demand.

Page 6: Types of Capillaries

• Continuous Capillaries:

  • Found in skin, muscle, lungs, and central nervous system, characterized by tight junctions that limit permeability, important for maintaining the blood-brain barrier.

• Fenestrated Capillaries:

  • Present in exocrine glands, renal glomeruli, intestinal mucosa, liver, spleen, and bone marrow; feature small openings (fenestrae) that increase permeability to larger molecules, essential in filtration processes.

• Discontinuous Capillaries:

  • Characterized by large gaps, found in the liver and spleen, facilitating the passage of larger molecules and cells such as red blood cells, important in filtering blood.

Page 7: Factors Affecting Capillary Pressure

• Pre-capillary Factors

  1. Precapillary Resistance: Impact of arteriolar tone on blood flow into capillaries.

  2. Arterial Blood Pressure: Overall pressure in the arterial system influencing capillary perfusion.

• Post-capillary Factors

  1. Postcapillary Resistance: Resistance offered by venules which can affect the flow dynamics.

  2. Venous Pressure: Pressure in the venous system affecting return flow and pressure gradients across capillaries.

Page 8: Capillary Fragility

• Observation: While capillaries are delicate, their design and structural integrity allow for resilience under physiological conditions.

• Reason: Low tension in walls due to small radius (Law of Laplace) contributes to stability despite high surface area for exchange.

Page 9: Transport Mechanisms Across Capillary Walls

• Transcapillary Exchange:

  • Essential for maintaining homeostasis; involves various substances including gases, solutes, water, and macromolecules.

• Mechanisms:

  • Diffusion: Driven by concentration gradients, crucial for gas exchange and solute transport.

  • Filtration/Reabsorption: Governed by pressure differences (osmotic and hydrostatic), playing a key role in fluid balance.

  • Pinocytosis: Endocytic process for the uptake of fluids and solutes by endothelial cells, particularly important for larger molecules.

Page 10: Tissue Fluid Formation and Drainage

• Components of Transcapillary Exchange:

  • Water: Molecules less than 3 nm pass freely across capillary membranes.

  • Solutes: Larger than 3 nm, such as proteins, are selectively permeable based on capillary type and context.

  • Driven by random movement and pressure differences, stabilizing fluid distribution in tissues.

• Fluid Dynamics:

  • Formation rates can reach approximately 3 L/min/kg; precise regulation is vital for maintaining fluid homeostasis and preventing edema.

Page 11: Factors Determining Filtration in Capillaries

• Starling Forces

  • Balance of hydrostatic and colloid osmotic forces that dictate fluid movement across capillary walls.

  • Implications of plasma colloid osmotic pressure and interstitial fluid pressures on overall fluid dynamics.

• Starling's Hypothesis

  • States that net filtration is determined by the balance of outward (hydrostatic) and inward (osmotic) forces, key to understanding fluid shifts in pathophysiological conditions.

  • Key Equation: Qf = k [(Рc + Пi) − (Pi + Пc)], illustrating fluid movement calculations.

Page 12: Pressure Balance Across Capillary Wall

• Measurement:

  • Values indicating pressures (Pc, Пс, Pi) are critical for assessing capillary function and fluid exchange.

  • Net pressure gradient analysis reveals predominance of outward pressure during filtration phases in homeostasis.

Page 13: Variation in Starling Forces by Organ

• Different organs exhibit variability in capillary pressure and the filtration/reabsorption balance, reflecting functional demands (e.g., kidney vs. skin).

Page 14: Filtration in Pulmonary Capillaries

• Observation: Predominantly reabsorption occurs to prevent pulmonary edema, highlighting the capillary’s role in maintaining lung function and gas exchange.

Page 15: Filtration in Kidney Glomerulus

• Function: Exclusive filtration is noted due to specific pressure gradients, underscoring the kidney’s crucial role in waste removal and homeostatic fluid balance.

Page 16: Lymph Circulation

• Quantitative Details:

  • Blood circulation metrics contrast with lymphatic drainage, emphasizing the importance of lymphatic vessels for immune function and excess fluid return.

Page 17: Edema

• Definition: Pathological accumulation of fluid in interstitial spaces, often indicating underlying dysfunction in vascular or lymphatic systems that can result in tissue swelling.

Page 18: Causes of Edema: Inflammation

• Mechanism: Release of inflammatory mediators (e.g., thrombin, bradykinin, histamine) can lead to increased vascular permeability, resulting in arteriolar vasodilation and higher filtration rates into tissues.

Page 19: Causes of Edema: Increased Venous Pressure

• Factors:

  • Conditions such as cardiac insufficiency and thrombosis can significantly elevate venous pressure, affecting filtration processes and contributing to fluid buildup.

Page 20: Causes of Edema: Hypoproteinemia

• Conditions:

  • Medical conditions like nephrotic syndrome, liver cirrhosis, and malnutrition reduce plasma protein levels, leading to generalized edema through decreased oncotic pressure.

Page 21: Causes of Edema: Reduced Lymph Drainage

• Pathophysiology: Understanding how impaired lymphatic drainage alters the net pressure gradients and results in fluid accumulation is crucial for treating related conditions.

Page 22: Conclusion

• Emphasis on the importance of understanding microcirculation and tissue fluid dynamics for innovative and unrestricted thought processes in medical physiology and clinical practice, particularly in treating related disorders.