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 sphincterwhich monitors and regulates blood flow into the capillary bed.

• 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

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