Lecture 9 Microcirculation and Capillary Networks

Microcirculation Role

• The smallest blood vessels, capillaries, are responsible for the exchange of substances between blood and tissues.
• Capillaries represent only 5% of the total blood volume.
• They are the sites where water, nutrients, waste products, and gases are exchanged with interstitial fluid and tissues.
• Capillaries have a diameter of 7-8 micrometers, allowing red blood cells (7.5 micrometers diameter) to pass through.
• The walls of capillaries are composed of a single layer of endothelial cells, facilitating efficient exchange.
• Capillaries form dense networks, ensuring all cells within 100 micrometers are close to a capillary.

Capillary Network

• The density of capillaries varies depending on the metabolic activity of the tissue.
• Metarterioles, connecting arterioles to capillary networks, possess smooth muscle rings that regulate blood flow.
• In skeletal muscle, all metarterioles open during exercise, maximizing blood supply.
• The small diameter of capillaries results in slow blood flow, facilitating adequate time for exchanges.
• The extensive branching of capillaries reduces resistance to blood flow.

Transcapillary Exchange Mechanisms

• Diffusion: Movement of substances across capillary walls from areas of high concentration to lower concentration.
  • Lipid-soluble substances (oxygen and carbon dioxide) readily diffuse across cell membranes.
  • Water and lipid-insoluble molecules (sodium, chloride, glucose, and amino acids) move through pores or clefts in capillary walls.
  • The size and number of pores vary depending on the tissue.
  • Some capillaries are fenestrated (intestine, liver, kidneys), containing large gaps that allow water and soluble macromolecules to pass.
• Bulk Flow: Mass movement of fluid and dissolved substances driven by pressure gradients.
  • Filtration: Movement out of capillaries into interstitial fluid.
  • Absorption: Movement from interstitial fluid back into capillaries.
  • Bulk flow maintains fluid balance between intravascular and interstitial compartments.
  • Factors influencing bulk flow:
    • Pressure gradients (hydrostatic and osmotic)
    • Permeability of capillary walls
    • Size of the diffusion surface
    • Blood flow

Pressure Gradients & Starling Forces

• Hydrostatic pressure: Pressure generated by the heart that drives filtration out of capillaries.
  • The pressure in the interstitial fluid is negligible.
• Colloid osmotic pressure (oncotic pressure): Pressure generated by the presence of large proteins, primarily albumin, within blood vessels.
  • This pressure opposes hydrostatic pressure due to the tendency of water to move towards areas of higher solute concentration.
• Starling forces represent the balanced forces driving filtration and absorption across capillary walls.
  • Hydrostatic pressure pushes fluid out of capillaries.
  • Osmotic pressure draws fluid into capillaries.
  • In the first portion of the capillary, hydrostatic pressure dominates leading to filtration.
  • Towards the end, hydrostatic pressure decreases, and if it becomes lower than osmotic pressure, net reabsorption occurs.

Fluid Movement & Lymphatic System

• The majority of fluid that enters the interstitial space during filtration is collected by the lymphatic system.
• The lymphatic system drains this fluid back into large veins.
• Fluids that remain in the blood after filtration return to the heart via venules.
• When the lymphatic system cannot handle the excess fluid, edema develops.

Edema Formation: Causes & Mechanisms

• Edema is the abnormal accumulation of fluid within the interstitial space.
• Four main mechanisms contribute to edema formation:
  • Increased hydrostatic pressure:
    • Hypertension (high blood pressure)
    • Increased venous pressure (heart failure, venous obstruction)
  • Increased interstitial protein concentration:
    • Inflammation of capillaries, causing increased leakiness.
  • Decreased oncotic pressure:
    • Loss of plasma proteins (GI disease, liver disease, malnutrition)
  • Obstruction of lymphatic vessels:
    • Lymphatic dysfunction or blockage preventing proper fluid drainage

Edema Example

• "Dependent edema" refers to swelling in areas located below the heart, such as the legs.
• Dependent edema can be caused by right-sided heart failure, leading to increased venous pressure and subsequent filtration pressure in the affected areas.

Microcirculation and Fluid Exchange

• The microcirculation is responsible for the exchange of water, nutrients, waste products, and gases between the blood and the interstitial fluid.
• Capillaries are the smallest blood vessels, with a single layer of endothelial cells and a diameter of 7-8 µm.
• Red blood cells are about 7.5 µm in diameter, meaning they must travel through capillaries in a single file line.
• The density of capillaries within a tissue varies, depending on the metabolic activity of the tissue.
• Metarterioles are small arterioles that directly connect to the capillary network.
• Most capillaries contain pores or clefts that allow the passage of water and small, lipid-insoluble molecules.
• Some capillaries, such as those in the intestines, liver, and kidneys, are fenestrated and contain large gaps allowing for the passage of large molecules.

Transcapillary Exchange

• Transcapillary exchange involves both diffusion and bulk flow.
• Diffusion is the movement of substances across the capillary wall, driven by concentration gradients.
• Lipid-soluble substances, such as oxygen and carbon dioxide, can diffuse freely across capillary membranes.
• Water and lipid-insoluble molecules, such as sodium, chloride, glucose, and amino acids, move through pores or clefts.
• Bulk flow is the mass movement of water and dissolved substances across the capillary wall, driven by pressure gradients.
• Two types of pressure gradients influence bulk flow: hydrostatic pressure and osmotic pressure.

Hydrostatic Pressure

• Hydrostatic pressure is the pressure exerted by a fluid against a surface.
• In the microcirculation, hydrostatic pressure pushes fluid out of the capillaries and into the interstitial space.

Osmotic Pressure

• Oncotic pressure is also known as colloid osmotic pressure.
• Oncotic pressure is caused by proteins, which remain in the blood and attract water.
• This pressure counteracts the hydrostatic pressure, pulling fluid back into the capillaries.

Starling Forces

• The balance between hydrostatic and osmotic pressures determines the net movement of fluid across the capillary wall.
• The net movement of fluid is generally towards the interstitial space, where it is picked up by the lymphatic system before being returned to the blood via large veins.
• If the lymphatic system is unable to keep up with the accumulation of fluid, edema forms.

Edema Formation

• Edema is the abnormal accumulation of fluid in the interstitial space.
• There are four main mechanisms that can cause edema:
  • Increased hydrostatic pressure, such as that caused by hypertension or right-sided heart failure.
  • Increased interstitial protein concentration, due to inflammation or a "leaky" capillary wall.
  • Decreased oncotic pressure, due to protein loss or poor nutrition.
  • Obstruction of lymphatic vessels.

Clinical Example

• Dependent edema is swelling in the lower extremities and other areas of the body that are normally below the heart.
• This is often a sign of right-sided heart failure because the right side of the heart is unable to pump blood effectively, leading to venous congestion.
• Venous congestion increases hydrostatic pressure, resulting in increased fluid filtration into the interstitial space.