CAPILLARIES

Introduction:

• Arteriole opens into rich capillaries network.

Diameter – 

• • Very small.

Number – 

• Enormous.

Cross-section area:

• Large (highest among all vessels).

Volume of blood:

• About 5% of total blood volume.

Velocity of blood flow:

• Very low – Due to enormous surface area.

Direction of blood flow:

• Blood flows from higher pressure to a lower pressure.
• Capillary blood flow requires certain minimum pressure gradient.
• The critical pressure below which capillaries close – Is referred to as “critical closing pressure”.

Structure:

• No smooth muscles in their walls.
• Lined by only single endothelial cell layer.
• Proximal end of capillary surrounded by single smooth muscle cell layer.
• Constitutes precapillary sphincter.

Nerve supply:

• Have very little sympathetic innervation.
• Muscle fibers are controlled entirely by local metabolites – CO2, pH etc.

Functions:

• Exchange of O2 and CO2 between blood & tissue fluid takes place only in capillaries.
• Helps achieve metabolic requirement of tissues – By ensuring adequate capillary exchange.

Fluid exchange across capillaries:

• According to Starling’s hypothesis.
• Fluid transport is governed by hydrostatic & osmotic pressures across capillaries.

Forces favoring fluid movement from capillaries into interstitial space (filtration):

• Increased capillary hydrostatic force.
• Decreased capillary osmotic pressure.
• Increased interstitial osmotic pressure.
• Decreased interstitial hydrostatic pressure.
• Decreased plasma osmotic pressure.
• Net force at arterial end of capillaries.

Forces favoring fluid movement in opposite direction (reabsorption):

• Hydrostatic pressure in interstitial space.
• Osmotic pressure of plasma.
• Net force at venous end of capillaries.

Net movement:

• Determined by algebraic sum of these opposing forces.
• Interstitial space factors (both osmotic & hydrostatic pressure) – Small & insignificant.
• Fluid exchange mainly dependent on capillary hydrostatic pressure & osmotic pressure.
• Fluid lost at arterial end is restored to capillaries.
• This phenomenon is known as starling’s hypothesis.