CAPILLARY DYNAMICS

Capillary dynamics

•Structure•Mechanism •Siginificance of altered capillary flow

Structure of capillary

Structure of capillary

Mechanism of capillary flow

Forces determining the flow

1. Capillary hydrostatic pressure 2. Interstitial hydrostatic pressure3. Oncotic pressure of plasma4. Oncotic pressure of interstitium

•30 – 40 mm Hg at the arterial end •10 – 15 mm Hg at the venous end

Capillary pressure

Forces determining flowCapillary pressure

•Usually around – 3 mm Hg •Sometimes even positive •Close relationship to lymph flow

Interstitial pressure

Forces determining flowCapillary pressure

Interstitial pressure

•28 mm Hg - due to plasma proteins

•Albumins, globulins, fibrinogen

Capillary oncotic pressure

Forces determining flowCapillary pressure

Capillary oncotic

pressureInterstitial pressure

• 8 mm Hg

Interstitial oncotic pressure

Forces determining flowCapillary pressure

Capillary oncotic

pressure

Interstitial oncotic

pressure

Interstitial pressure

Arterial end (filtration)

Forces favoring filtration:• Capillary pressure 30 mm Hg • Interstitial pressure -3 mm Hg • IS Oncotic pressure 8 mm Hg

(35 mm Hg)Forces opposing filtration: • Capillary oncotic pressure 25 mm Hg

Total flow: 35 – 28 = 10 mm Hg outside

Venous end (reabsorption)

Forces favoring filtration:• Capillary pressure 15 mm Hg • Interstitial pressure -3 mm Hg • IS Oncotic pressure 8 mm Hg

(20 mm Hg)Forces opposing filtration:• Capillary oncotic pressure 28 mm Hg

Total flow: 28 – 20 = 8 mm Hg inside

Whenever hydrostatic pressure is greater than oncotic pressure, fluid will leave the capillaries…

Whenever the oncotic pressure is greater than the hydrostatic pressure fluid will enter the capillaries…

Changes in the flow

• Vasodilatation • Shock (vasoconstriction)• Cardiac failure • Low plasma protein levels