Microcirculation and Lymphatics

dr .Irvan

Circulatory System

• Cardiac output is determined by the metabolic needs of the body

• Functional Anatomy– The large arteries serve as blood conduits– Arterioles are the “Stopcocks” of the circulation– Capillaries are the site of exchange of gases, water, and

solutes– The veins serve as storage vessels and also as blood

conduits

Anatomy of Microcirculation

Arterioles are the “Stopcocks” of the circulation

• Arterioles– Thick smooth muscle

– Give rise to metarterioles, then to capillaries

• Metarterioles– Precapillary sphincters regulate flow into the capillaries

– Local conditions in the tissues regulate the opening and closing of the precapillary sphincters

Capillaries

• Thin-walled

• Smallest vessels in the circulation

• Have the greatest cross-sectional area because they are so numerous

• Have the greatest surface area for exchange

Capillaries

• True capillaries are devoid of smooth muscle and are incapable of active constriction

• Capillary distribution varies from tissue to tissue

Why can capillaries withstand high intravascular pressures?

• Laplace’s Law– Wall Tension = Pr

– Wall Stress = Pr/w

• P = transmural pressure

• r = radius of the vessel

• w = wall thickness

LaPlace’s Law

Mechanisms of Exchange Across The Capillary Endothelium

• Diffusion– Oxygen, Carbon Dioxide, Lipid-Soluble Substances

• Bulk flow (via intercellular clefts or “pores”)– Water, Electrolytes, Small Molecules

• Vesicular transport– Proteins

• Active transport– Ions, Glucose, Amino Acids

Diffusion Through the Capillary Membrane

Capillaries

• The permeability of the capillary endothelial membrane is not the same in all body tissues

Body Fluid Compartments

Body Fluid Compartments(% of Total Body Weight)

40%

20%

0%

10%

20%

30%

40%

50%

ICF ECF

Body Fluid Compartments(% of Total Body Weight)

40%

15%

4%1%

0%

10%

20%

30%

40%

50%ICF

Interstitial Fluid

Plasma

Transcellular Fluid

Intracellular Fluid

28 L

281 mOsm/L

Plasma 3 L 282 mOsm/L

Interstitial Fluid 11 L

281 mOsm/L

Cell Membrane

Capillary MembraneLymphaticsExtracellular

Fluid

14 L

Fluid Intake Fluid Losses

Distribution of Various IV Fluids Throughout the Body Fluid Compartments

ICF = 28 LInterstitial Fluid = 11 L Plasma

= 3 L

RBC = 2 L

ECF = 14 L

TBW = 42 L

5 % Dextrose

0.9 % Saline or LR

Colloid

Transcapillary Fluid Exchange

Factors Determining Fluid Movement

Capillary Pressure

Plasma Colloid Osmotic Pressure

Interstitial Fluid Colloid Osmotic Pressure

Interstitial Fluid Pressure

CAPILLARY INTERSTITIAL FLUID

Net Driving Forces

Filtration - Reabsorption

• Hydrostatic Forces– Pc = Capillary hydrostatic pressure

• Higher at the arteriolar end than at the venule end. Tends to force fluid outward through the capillary membrane.

– Pif = Interstitial fluid hydrostatic pressure• Normally interstitial fluid hydrostatic pressure is negative.

This negative pressure is due to the pumping action of the lymphatics. This is usually an outward force for fluid movement.

Filtration-Reabsorption

• Osmotic Forces p = plasma colloid osmotic pressure

• Also known as plasma oncotic pressure. Plasma proteins are the major determinant of oncotic pressure. Albumin, the most abundant plasma protein, generates about 70% of the oncotic pressure. Tends to cause inward (into capillary) movement of fluid.

if = interstitial fluid osmotic pressure• Also know as interstitial fluid oncotic pressure. Caused by the

small amount of plasma proteins that leak into the interstitial space. Tends to cause outward movement of fluid.

OsmosisSemi-Permeable Membrane

Osmosis

Water moves from a region of high

concentration to one that has a lower

concentration of water

Gibbs-Donnan Effect

• Albumin exerts a greater osmotic force than can be accounted for solely on the basis of the number of molecules dissolved in the plasma– Albumin has a negative charge

• Attracts sodium ions

– Albumin binds a small number of chloride ions• Attracts sodium ions

Filtration - Reabsorption

• Filtration – The sum of the hydrostatic and osmotic forces

favors the net movement of water from the capillary to the interstitial space

• Reabsorption– The sum of the hydrostatic and osmotic forces

favors the net movement of water from the interstitial space to the capillary

Starling Forces

ifcifc rcPP

Net Driving Force =

Filtration: NDF>0Reabsortion: NDF<0

Filtration-Reabsorption:Baseline Beginning of Capillary

• Pc = 30 mm Hg• Pi = -3 mm Hg c = 26 mm Hg I = 6 mm Hg • rc = 0.9

• NDF = [30-(-3)]-0.9[(26-6)] = 15 mm Hg

Pc

Capillary Length0

30

10

Filtration-Reabsorption:Baseline End of Capillary

• Pc = 10 mm Hg• Pi = -3 mm Hg c = 26 mm Hg I = 6 mm Hg • rc = 0.9

• NDF = [10-(-3)]-0.9[(26-6)] = -5 mm Hg

Pc

Capillary Length0

30

10

Filtration-Reabsorption:Baseline

Filtration

NDF

0

15

-15

Reabsorption

Clinical Correlates

• Dehydration– Decreased capillary hydrostatic pressure– Increased oncotic pressure

Filtration-Reabsorption:Dehydration Beginning of Capillary

• Pc = 25 mm Hg• Pi = -3 mm Hg c = 31 mm Hg I = 6 mm Hg • rc = 0.9

• NDF = [25-(-3)]-0.9[(31-6)] = 5.5 mm Hg

Pc

Capillary Length0

25

5

Filtration-Reabsorption:Dehydration End of Capillary

• Pc = 5 mm Hg• Pi = -3 mm Hg c = 31 mm Hg I = 6 mm Hg • rc = 0.9

• NDF = [5-(-3)]-0.9[(31-6)] = -14.5 mm Hg

Pc

Capillary Length0

25

5

Filtration-Reabsorption:Dehydration

Reabsorption

NDF

0

15

-15

Filtration

Lymphatics

Lymphatics

• Close-ended network of highly permeable lymph capillaries

• Lacking in tight junctions

• Fine filament anchors to connective tissue

• Muscular contraction:– Distortion and opening of spaces

Lymphatics

• Pumping by the lymphatic system is the basic cause of the negative pressure in the interstitial fluid space

Lymphatics

• Plasma filtrate is returned to the circulation by:– Tissue pressure– Intermittent skeletal muscle activity– Lymphatic vessel contraction– System of one-way valves

• Returns the following to the circulation:– Protein (Albumin)– Bacteria– Fat– Excess fluid

Clinical Correlates

• Edema– Lymphatic obstruction

– Change in capillary permeability

– Reduction in plasma protein

– Increased capillary hydrostatic pressure

THE END