14 Capillary Exchange

Body Systems: The Cardiovascular System

Introduction

  • Lecture conducted by Dr. Nick Stafford, Division of Diabetes, Endocrinology & Gastroenterology

  • Overview of capillary exchange, focusing on essential processes and their implications in the human body.

Objectives

  • Key Learning Outcomes:

    • Describe the processes through which substances are exchanged at the capillary.

    • Understand the dynamics of capillary exchange and determinants of net filtration pressure.

    • Describe circulation in specific organs with special considerations (heart, lungs, brain).

Content Overview

  1. Introduction to Capillary Exchange

  2. Capillary Structure

  3. Transport Mechanisms in Capillary Exchange

  4. Net Filtration Pressure

    • Net Hydrostatic Pressure

    • Net Osmotic Pressure

  5. Dynamics of Capillary Exchange

  6. Special Circumstances for Capillary Exchange and Blood Flow

    • Pulmonary Circulation

    • Coronary Circulation

    • Cerebral Circulation

Introduction to Capillary Exchange

  • Exchange Requirement:

    • Minimum capillary hydrostatic pressure is necessary for substance/fluids exchange.

  • Key Functions of Capillaries:

    • Blood flow regulation is influenced by the sympathetic nervous system (SNS) and local metabolic factors.

    • Vital for providing oxygen and nutrients, as well as removing metabolic wastes (e.g., CO2).

    • Primary Communication: Capillaries are the exclusive site for blood and interstitial fluid exchanges.

Capillary Structure

  • Capillaries are designed for efficient two-way substance exchange due to:

    • Short diffusion distances resulting from:

      • Thin walls (1µm thickness)

      • Small diameter (8µm)

      • Close proximity to various cells

  • Blood Flow Characteristics:

    • Slow blood flow due to a large network area enhances exchange potential.

    • There are approximately 10+ billion capillaries, providing about 600m² of surface area for exchange.

Capillary Types

  • Continuous Capillaries:

    • Most typical type; prevent large particle passage.

  • Fenestrated Capillaries:

    • Found in endocrine organs, intestines, and kidneys; features pores for faster exchange of water and solutes.

  • Sinusoidal Capillaries:

    • Present in organs like the liver and spleen; larger gaps between endothelial cells allow free exchange of larger solutes (e.g., plasma proteins).

Transport Mechanisms in Capillary Exchange

  • Molecular Movement: Substances migrate based on:

    • Concentration gradience (Diffusion)

    • Pressure gradience (Bulk Flow)

    • Vesicular transport (Transcytosis)

  • Mechanisms Specifics:

    • Diffusion: Movement of lipid-soluble gases (O2, CO2) and water-soluble ions (Na+, K+, Ca2+, Cl-) occurs through channels and membranes.

    • Transcytosis: Involves the transport of larger molecules (e.g., glycoproteins) via vesicles.

    • Bulk Flow: Includes filtration and reabsorption through clefts and pores, primarily facilitating the exchange of water and nutrients.

Net Filtration Pressure

  • Definition: Determines the fluid movement between capillaries and the interstitial fluid.

  • Influencing Forces:

    • Blood Colloid Osmotic Pressure: Osmotic force pulling fluid toward the blood.

    • Interstitial Fluid Colloid Osmotic Pressure: Osmotic force pulling fluid out of the blood.

    • Capillary Hydrostatic Pressure: Exerted pressure pushing fluid out of the capillaries.

    • Interstitial Fluid Hydrostatic Pressure: Affects whether fluid is driven out of or into capillaries.

Dynamics of Capillary Exchange

  • Capillary dynamics involve the relationship between different pressures:

    • NFP determined through the equation: NFP = Net Hydrostatic Pressure - Net Osmotic Pressure (CHP - IHP - BCOP - ICOP).

  • Filtration vs. Reabsorption:

    • When CHP exceeds BCOP, net filtration occurs. In contrast, the opposite leads to reabsorption.

  • Key Dynamics in Vascular Conditions:

    • Hypertension leads to increased filtration and potential edema.

    • Hemorrhage results in higher reabsorption as fluid enters the bloodstream from tissues.

    • Dehydration and tissue damage can also significantly modify filtration dynamics.

Special Circumstances in Capillary Exchange

  • Pulmonary Circulation:

    • Enhances O2 absorption by having lower capillary hydrostatic pressure (average 10 mmHg) compared to systemic circulation.

    • Needs to maintain low pulmonary resistance to ensure effective gas exchange.

    • Risks of pulmonary edema exist if CHP exceeds 25 mmHg.

  • Coronary Circulation:

    • Influenced by SNS and local control mechanisms which promote vasodilation.

    • Blood flow peaks during diastole due to arterial elastic recoil and is restricted during systole.

    • Cardiac tissue is adapted to high oxygen extraction due to vessel density and reserves.

  • Cerebral Circulation:

    • The brain consumes a significant amount of cardiac output relative to its mass (12% CO for 2% body mass).

    • Blood flow is sustained through four main arteries that interconnect, ensuring consistent nutrient delivery even amid disruptions.