Lecture 12: Capillaries, Lymphatics, and Veins

Lecture 12: Capillaries, Lymphatics, and Veins

Lecture Objectives

• Anatomy and Function of Capillaries

  • Understand the structure of capillaries.

  • Comprehend their role in nutrient, gas, and waste exchange.

• Leaky Capillaries

  • Identify different levels of "leakiness" of various capillary types.

• Fluid Regulation

  • Discuss the factors and forces regulating fluid leakage from capillaries.

• Lymphatic System

  • Explain how the lymphatic system returns filtered fluid to the cardiovascular system.

• Arteries vs. Veins

  • Differentiate the structural features of arteries and veins and their functions as pressure and volume reservoirs, respectively.

Capillaries: The Microcirculation

• Primary Exchange Site

  • Responsible for the exchange of:

  • Gases

  • Nutrients

  • Water

  • Waste products

• Capillary Density

  • Density is greatest in tissues with high oxygen consumption or exchange requirements:

  • High Density Examples:

    • Heart

    • Skeletal muscle

    • Glands

    • Brain

  • Low Density Examples:

    • Cartilage

    • Subcutaneous tissue

  • Not all capillaries are open at all times: only 20% of those in skeletal muscle are open at rest.

  • Regulation occurs through arterioles and metarterioles; sphincters are controlled locally by metabolites, not innervated.

Types of Capillaries

• Tight Capillaries

  • Most common type.

  • Characterized by well-sealed junctions (10-15 nm wide).

  • Example: Blood-brain barrier containing tight junctions to prevent large molecules from leaking.

• Fenestrated Capillaries

  • Contain fenestrations (membrane-lined holes) which range from 20-100 nm in diameter and can be closed by a diaphragm.

  • Found in:

  • Intestine

  • Glomerulus

  • Exocrine glands

• Sinusoidal Capillaries

  • Have large gaps between cells (100-1000 µm wide).

  • Facilitate cell movements in and out of circulation.

  • Examples: Liver, bone marrow, spleen.

Mechanisms of Transcapillary Exchange

• Transcapillary Exchange Mechanisms

  • Diffusion

  • Main route for gases, water, and small solutes.

  • Filtration

  • Occurs mainly at fenestrations.

  • Vesicular Transport

  • Bi-directional; includes:

    • Transcytosis of Macromolecules

    • Transendothelial Channels: a stack of fused endocytotic vesicles across endothelial cells.

Diffusion and Fick's Law

• Gases

  • Exchange occurs via direct diffusion across the endothelial membrane.

• Small Solutes

  • Allows diffusion through small pores and clefts.

• Polar Molecules

  • Exhibit decreased permeability due to poor lipid solubility.

• Large Molecules

  • Typically, no diffusion occurs for molecules larger than 60 kDa (e.g., albumin, which is 69 kDa).

• Flux Equation: $J = - P \cdot A \cdot ([solute]<em>{out} - [solute]</em>{in})$

  • Where:

    • $J$: Flux (quantity moved per unit time)

    • $P$: Permeability coefficient

    • $A$: Capillary surface area

Capillary Exchange: Starling Forces

• Net Filtration Pressure

  • Calculated as follows:
    $\Delta P = P<em>{capillary} - P</em>{interstitial}$

  • Typically, $P<em>{interstitial}$ is approximately 0; thus $\Delta P \approx P</em>c$, which is essentially the blood pressure.

• Oncotic Pressure

  • Defined as colloid osmotic pressure with the difference:
    $\Delta \pi = \pi<em>{cap} - \pi</em>{interstitial}$

• Equation for Oncotic Pressure:
  $\pi = \sigma R T (C<em>i - C</em>o)$

  • Where:

    • $\sigma$: Colloid reflection coefficient

• Fluid Movement by Convection

  • The net filtration pressure can thus be represented as:
    $\text{Net Filtration Pressure} = \Delta P - \Delta \pi$

• Typical Values

  • Actual net filtration pressure is around 0.3 mmHg, resulting in 2-3 liters of fluid movement daily from blood to interstitial fluid (IF).

Pathological Conditions Affecting Capillary Dynamics

• Increased Blood Volume

  • Causes a rise in plasma proteins, reducing colloid osmotic pressure and increasing fluid filtration.

• Inflammation

  • Increases capillary leakiness and the number of open capillaries.

• Other Disturbances

  • Can lead to:

  • Edema

  • Increased venous pressure

  • Factors related to pregnancy, capillary injury, burns, inflammation, dehydration, lymphatic obstruction.

Lymphatic System

• Overview

  • Lymphatic capillaries have closed ends with valve-like inter-endothelial junctions instead of tight junctions, and fine filaments anchor them to surrounding tissues.

• Lymphatic Flow Dynamics

  • Expansion Phase:

  • Hydrostatic pressure in capillaries >> lymphatic pressure; interendothelial valves allow interstitial fluid to enter lymphatics.

  • Compression Phase:

  • Tissue movement compresses lymphatics, causing lymphatic pressure >> hydrostatic pressure; valves close to force lymph downstream.

• Lymphatic Flow

  • Approximately 2-3 liters per day.

  • Primarily unidirectional, aided by:

  • Increased interstitial pressure

  • Compression from skeletal muscle contractions

  • Myogenic tone of lymphatic smooth muscle (VSM) cells which contract due to stretch.

Pathology of the Lymphatic System

• Elephantiasis

  • Characterized by extreme swelling, primarily in the lower body due to obstruction of lymph flow from:

  • Parasitic infections (e.g., filariasis)

  • Persistent exposure to alkaline soils (e.g. podoconiosis)

• Consequences of Lymph Node Removal

  • Such as post-mastectomy for breast cancer, which can lead to lymphedema in the arm and other nearby regions.

Veins and Their Characteristics

• Structure of Veins

  • Composed of three layers:

  • Tunica Intima: Endothelial cells

  • Tunica Media: VSM cells

  • Tunica Adventitia: Connective tissue with fewer elastin fibers compared to arteries.

• Valves in Veins

  • Present to ensure unidirectional flow of blood back to the heart.

• Compliance and Capacitance

  • Veins exhibit high compliance, allowing for significant volume changes with little change in pressure.

  • Venous pooling occurs as most blood (approximately 70%) resides on the venous side of circulation, enabling adjustment to metabolic demands.

Capacitance and Cardiovascular Control

• Passive vs. Active Capacitance

  • Passive Capacitance: Involves changes in venous volume due to blood flow without active constriction.

  • Active Capacitance: Involves vasoconstriction from sympathetic stimulation to manage blood flow.

  • The relationship between compliance and capacitance is crucial, underpinning both normal physiology and responses to stress (e.g., hemorrhage).

Varicose Veins

• Symptoms

  • Heavy or aching legs

  • Ankle swelling

  • Skin discoloration from metabolite accumulation

• Complications

  • Increased risk of fainting (syncope)

  • Intolerance to prolonged standing

  • Possible development of eczema or thrombophlebitis (leading to serious conditions like pulmonary embolism, deep vein thrombosis, stroke, or myocardial infarction).

• Treatment Options

  • Compression stockings

  • Leg elevation

  • Anti-inflammatory or anticoagulant drugs

  • Surgical options for vein removal.

Study Questions

• Comparative Anatomy

  • Discuss the anatomical differences among arteries, veins, and capillaries, relating these features to their respective functions.

• Capillary Types

  • Provide examples from the body where each type of capillary is found.

• Compliance vs. Capacitance

  • Explain the differences in mechanics and their implications on venous versus arterial function.

• Pathological Conditions

  • Identify two disorders related to lymphatic or venous dysfunction and describe the underlying mechanisms involved.