1. 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.

2. 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).

3. 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.

4. Vessel Structure and Function
3 Types of capillaries in the body

5. 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).

6. Vessel Structure and Function

7. 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.

8. 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

9. 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.

10. Fluid Exchange - Starling Forces