Microcirculation and Lymphatic System Notes

Microcirculation

• Main function: Transportation of nutrients to tissues and removal of waste.
• Small arterioles control blood flow to tissues.
• Local conditions in tissue control the diameter of arterioles.
• Capillary walls are thin, made of a single layer of highly permeable endothelial cells, allowing rapid exchange of water, nutrients, and waste between tissues and blood.
• Approximately 10 billion capillaries in the body.
• Total surface area: 500-700 square meters.
• Every cell is within 20-30 micrometers of a capillary.

Structure of the Microcirculation

• Nutrient artery enters an organ or tissue.
• Arterioles are highly muscular.
• Metarterioles are terminal arterioles with intermittent smooth muscle fibers encircle the tube.
• Capillaries originate from metarterioles.
• Precapillary sphincters encircle the entrance to capillaries.
• Venules are larger than arterioles but have weaker muscle walls, though they can still contract considerably.

Organization of Microcirculation

• Microcirculation is specifically organized to serve the needs of each organ.
• Each artery branches 6-8 times before becoming arterioles (10-15 μm diameter).
• Arterioles branch 2-5 times (5-9 μm diameter) before reaching capillaries.
• Arterioles are highly muscular.
• Precapillary sphincters are located at the junction between arterioles and capillaries.

Structure of Capillary Wall

• Composed of unicellular endothelial cells.
• Surrounded by a thin basement membrane.
• Total thickness of the capillary wall is about 0.5 μm.
• Internal diameter is 4-9 μm, just large enough for red blood cells to pass through.

Types of Capillaries

• Continuous capillaries:
  • Location: muscle, connective tissue, neural tissue.
• Fenestrated capillaries:
  • Location: kidney & intestines.

Pores in Capillaries

• Tight junctions: Allow only extremely small molecules (water, oxygen, carbon dioxide) to pass through (e.g., brain).
• Liver capillaries: Wide-open clefts between endothelial cells allow almost all dissolved substances of plasma to pass from blood to liver tissues.
• Gastrointestinal capillary membranes: Intermediate permeability between muscle and liver capillaries.
• Glomerular capillaries of kidneys: Numerous small oval windows called fenestrae allow small molecular and ionic substances to pass through.

Blood Flow

• Blood flow in capillaries is not continuous; it turns on and off every few seconds (vasomotion).
• Vasomotion is due to intermittent contraction of precapillary sphincters.
• Regulation of vasomotion primarily depends on the concentration of oxygen in tissues.
• High oxygen usage leads to more frequent and longer-lasting intermittent blood flow.
• This allows more oxygen and nutrients to be carried to tissues.

Diffusion Through Capillary Membrane

• Diffusion is the most important mechanism for substance exchange.
• Blood flows along the capillary lumen.
• Water molecules and dissolved particles diffuse back and forth, providing continuous mixing.
• Diffusion results from the thermal motion of water molecules and dissolved substances in the fluid.

Capillary Exchange

• Lipid-soluble substances (O2, CO2) diffuse directly through the cell membrane of the capillary.
• Lipid-insoluble substances (H2O, Na, Cl, glucose) cross via intercellular clefts.
• The space between cells is called the interstitium; the fluid in this space is interstitial fluid.

Effect of Molecular Size on Passage

• The width of capillary intercellular cleft pores (6-7 nm) is about 20 times the diameter of water molecules.
• The diameter of plasma protein molecules is slightly greater than the width of the pores.
• Capillaries in various tissues have extremely different permeabilities.

Effect of Concentration Difference

• The net rate of diffusion through any membrane is proportional to the concentration difference of the substance between the two sides of the membrane.
• The concentration of oxygen in capillary blood is greater than in interstitial fluid; therefore, large quantities of oxygen move from blood to tissue.
• The concentration of carbon dioxide is greater in tissues than in blood, resulting in carbon dioxide moving from tissues to blood.

Interstitium and Interstitial Fluid

• One-sixth of the total body volume consists of spaces between cells (interstitium).
• The fluid found in this space is called interstitial fluid.
• Interstitial fluid is a gel almost identical to plasma, but with fewer proteins (due to difficulty passing through pores).
• Diffusion from capillaries into cells must occur through this gel.
• Diffusion allows for rapid transport through the interstitium.

Hydrostatic and Colloid Osmotic Forces

• Starling forces determine fluid movement through the capillary membrane:
  • Capillary pressure (Pc): tends to force fluid outward through the capillary membrane.
  • Interstitial fluid pressure (Pif): tends to force fluid inward when positive, outward when negative.
  • Capillary plasma colloid osmotic pressure (\pip): tends to cause osmosis of fluid inward through the capillary membrane.
  • Interstitial fluid colloid osmotic pressure (\piif): tends to cause osmosis of fluid outward through the capillary membrane.
• Net filtration pressure: If the sum of these forces is positive, there will be fluid filtration across capillaries, and vice versa.

Bulk Flow

• Definition: the mass movement of fluid between blood and interstitial fluid.
• Filtration: The bulk flow is outward from the capillary.
• Absorption: The bulk flow is inward toward the capillary.

Calculation of Bulk Flow

• Net pressure equation: P{cap} + \pi{IF} – \pi{cap} – P{IF}
  • Hydrostatic pressure (P) is the "pushing force" of water pressure out of the capillary (cap) or interstitial fluid (IF).
  • Colloidal osmotic pressure (\pi) is the "pulling force" of protein's ability to draw water into the capillary (cap) or interstitial fluid (IF).
• If the calculation yields a positive number, there is filtration.
• If the calculation yields a negative number, there is absorption.

Lymph System

• An accessory route through which fluid can flow from interstitial spaces into the blood.
• Carries proteins and large particulate matter away from tissue spaces, which cannot be removed by absorption directly into blood capillaries.
• Return of proteins to the blood from the interstitial space is essential; without it, we would die within 24 hours.
• Almost all tissues have special lymph channels to drain excess fluid from interstitial spaces.

Lymph System Functions

• Return fluid and proteins to the circulation.
• It’s a one-way system (like a vacuum cleaner).
• Picks up fat from the small intestine and transfers it to the systemic circulation.
• Filters, captures, and destroys foreign pathogens.
• Right side of the head & neck and the right arm enter the right subclavian vein via the right lymphatic duct.
• The left side of the head & neck, the left arm, and the rest of the body enter the thoracic duct.

Terminal Lymphatic Capillaries

• About 1/10th of the fluid from the arterial ends of capillaries enters lymphatic capillaries.
• Returns to the blood through the lymphatic system instead of venous capillaries.
• Fluid return from lymph is important because substances of high molecular weight cannot be absorbed by tissues in any other way.
• Fluid leaving lymph tissue is prevented from re-entry due to backpressure on endothelial cell “flaps” acting as valves.

Edema

• Increased fluid in the interstitial space.
• Causes:
  • Inadequate drainage: Obstruction of the lymph system, particularly at the nodes (e.g., parasites, cancer, fibrotic tissue growth following radiation therapy or surgical removal of the lymph nodes).
  • Capillary filtration that greatly exceeds capillary absorption:
    • Elevated capillary hydrostatic pressure (e.g., right-sided heart failure/congestive heart disease).
    • Decreased capillary colloidal osmotic pressure (e.g., any condition that decreases albumin in the blood).
    • Elevated interstitial protein concentration.
    • Damage to the capillary wall, allowing proteins to leak out (trauma).