Vessel Structure and Function
• The terminal end of an arteriole tapers toward
the capillary junction to form a single
metarteriole.
• At the metarteriole-capillary junction, the distal most
muscle cell forms the
precapillary sphincter
which monitors and
regulates blood flow
into the capillary bed.
Vessel Structure and Function
• Capillaries are different from other vascular
structures in that they are made of only a single
endothelial cell sitting on a very thin basement
membrane - there are no other tunics, layers or
muscle.
• The minimalist nature of capillaries allows them to
be freely permeable to
many substances (gases,
fluids, and small ionic
molecules).
Vessel Structure and Function
• The body contains three types of capillaries:
• Continuous capillaries are the most common with
endothelial cells forming a continuous tube,
interrupted only by small intercellular clefts.
• Fenestrated capillaries (fenestra = windows), found
in the kidneys, villi of small intestines, and endocrine
glands are much more porous.
• Sinusoids form very porous channels through which
blood can percolate, e.g., in the liver and spleen.
Vessel Structure
and Function
3 Types of capillaries in the body
Vessel Structure and Function
• Veins have thinner walls, less muscle and elastic
tissue, and are designed to operate at much
lower pressures.
• Intravenous pressure in venules (16 mmHg) is less
than half that of arterioles (35 mmHg), and drops to
just 1-2 mmHg in some larger veins.
• Because intravenous pressure is so low, veins have
valves to keep blood flowing in only 1 direction.
• When exposed to higher than normal pressures, veins can
become incompetent (varicose veins).
Vessel Structure and Function
Fluid Exchange - Starling Forces
• As blood flows to the tissues of the body,
hydrostatic and osmotic forces at the capillaries
determine how much fluid leaves the arterial
end of the capillary and how much is then
reabsorbed at the venous end. These are called
Starling Forces.
• Filtration is the movement of fluid through the walls
of the capillary into the interstitial fluid.
• Reabsorption is the movement of fluid from the
interstitial fluid back into the capillary.
Fluid Exchange - Starling Forces
• Two pressures promote filtration:
• Blood hydrostatic pressure (BHP) generated by the
pumping action of the heart - decreases from 35 to
16 from the arterial to the venous end of the
capillary
• Interstitial fluid osmotic pressure (IFOP), which is
constant at about 1 mmHg
Fluid Exchange - Starling Forces
• Two pressures promote reabsorption:
– Blood colloid osmotic pressure (BCOP) is due to the
presence of plasma proteins too large to cross the
capillary - averages 36 mmHg on both ends.
– Interstitial fluid hydrostatic pressure (IFHP) is
normally close to zero and becomes a significant
factor only in
states of edema.
Fluid Exchange - Starling Forces