Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 16

To Read Previous Chapter 14🔗:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-14-overview-of-circulation.html

Capillary Fluid Exchange Mechanics

• Arterial capillary pressure is higher than venous pressure.
• Fluid filters out at arterial capillary ends.
• Fluid is reabsorbed at venous capillary ends.
• Fluid flows through tissue toward venous ends.

Forces Causing Arterial Filtration

• Capillary pressure provides 30 mm Hg outward force.
• Interstitial fluid pressure provides 3 mm Hg outward force.
• Interstitial osmotic pressure provides 8 mm Hg outward force.
• Total outward force equals 41 mm Hg.
• Plasma osmotic pressure provides 28 mm Hg inward force.
• Total inward force equals 28 mm Hg.
• Net outward filtration pressure equals 13 mm Hg.
• One-200th of passing plasma filters into interstitium.

Forces Causing Venous Reabsorption

• Capillary pressure drops to 10 mm Hg outward.
• Interstitial fluid pressure provides 3 mm Hg outward force.
• Interstitial osmotic pressure provides 8 mm Hg outward force.
• Total outward force equals 21 mm Hg.
• Plasma osmotic pressure provides 28 mm Hg inward force.
• Total inward force equals 28 mm Hg.
• Net inward reabsorption pressure equals 7 mm Hg.
• Venous capillaries are more numerous and permeable.
• Nine-tenths of filtered fluid gets reabsorbed.
• One-tenth of filtered fluid enters lymphatics.

Starling Equilibrium for Capillary Exchange

• Arterial filtration almost equals venous fluid reabsorption.
• Mean functional capillary pressure equals 17.3 mm Hg.
• Mean total outward force equals 28.3 mm Hg.
• Mean total inward force equals 28.0 mm Hg.
• Net outward filtration force equals 0.3 mm Hg.
• Systemic net filtration rate is 2 ml/min.

Whole-Body Capillary Filtration Coefficient

• Whole-body filtration rate is 6.67 ml/min/mm Hg.
• Average tissue filtration coefficient equals 0.01 ml/min/mm Hg/100g.
• Capillary permeability varies over 100-fold between tissues.
• Brain and muscle have very low filtration coefficients.
• Liver and renal glomerulus have highest filtration coefficients.
• Muscle interstitial protein concentration is 1.5 g/dl.
• Subcutaneous interstitial protein concentration is 2 g/dl.
• Intestinal interstitial protein concentration is 4 g/dl.
• Liver interstitial protein concentration is 6 g/dl.

Effects of Force Imbalances

• Increased capillary pressure increases fluid filtration.
• Pressure rise of 20 mm Hg increases filtration 68-fold.
• Excess filtration overwhelms lymphatics and causes edema.
• Markedly decreased capillary pressure causes net fluid absorption.

Lymphatic System Structure and Channels

• Lymphatics return excess fluid, proteins, and particles.
• Protein return prevents death within 24 hours.
• Brain, bones, and skin surface lack lymph vessels.
• Prelymphatic channels drain non-lymphatic tissue spaces.
• Lower body lymph drains into thoracic duct.
• Thoracic duct empties into left subclavian vein.
• Right lymph duct drains upper right body.

Terminal Lymphatic Capillaries

• One-tenth of filtered fluid enters lymphatic capillaries.
• Daily lymph flow equals 2 to 3 liters.
• Endothelial cell edges form inward flap valves.
• Anchoring filaments connect endothelial cells to tissues.
• Flap valves prevent backward flow of lymph.

Formation and Composition of Lymph

• Lymph originates directly from interstitial fluid.
• Average tissue lymph protein concentration is 2 g/dl.
• Liver lymph protein concentration reaches 6 g/dl.
• Intestinal lymph protein concentration is 3 to 4 g/dl.
• Thoracic duct lymph protein averages 3 to 5 g/dl.
• Lymphatics absorb dietary fats up to 2 percent.
• Lymph nodes filter and destroy entering bacteria.

Rate of Lymph Flow

• Thoracic duct lymph flow is 100 ml/hr.
• Total resting lymph flow equals 120 ml/hr.
• Pressure rising to 0 mm Hg increases flow 20-fold.
• High capillary pressure increases lymph flow.
• Low plasma osmotic pressure increases lymph flow.
• High interstitial osmotic pressure increases lymph flow.
• Increased capillary permeability increases lymph flow.
• Interstitial pressures above 0 mm Hg cap maximum flow.

Lymphatic Pump Mechanics

• Stretched lymph vessel smooth muscle automatically contracts.
• Vessel segments between valves act as automatic pumps.
• Thoracic duct pump generates pressures up to 100 mm Hg.
• Skeletal muscle contractions compress nearby lymphatics.
• Arterial pulsations compress adjacent lymph vessels.
• Exercise increases lymph flow 10- to 30-fold.
• Tissue swelling pulls anchoring filaments to open capillaries.
• Endothelial actomyosin filaments cause rhythmic capillary contractions.
• Lymph flow equals interstitial pressure times pump activity.

Control of Interstitial Fluid and Pressure

• Leaked proteins accumulate inside interstitial fluid.
• Protein accumulation increases interstitial colloid osmotic pressure.
• Higher osmotic pressure draws water into tissue spaces.
• Increased fluid volume elevates interstitial fluid pressure.
• Elevated interstitial pressure accelerates lymph flow.
• Lymphatics remove excess protein and fluid to achieve equilibrium.
• Negative interstitial pressure holds body tissues together.

Status: Chapter 16 Completed & Simplified. 

Prepared By: Razaullah Khattak (DPT Medical Professional)

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.