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• At the transition zone between arterioles and venules, the smooth muscle fibers
either forms a cushion that protrudes into the lumen or is circularly arranged.
They are innervated by a dense vasomotor nerve supply.
• Several arteriovenous anastomoses may surround by a thick connective tissue
capsules forming a structure known as glomus. Numerous nerve endings
supply the glomus.
Functions
• When A-V shunts open, the blood is shunted directly into the venous
system bypassing the capillary bed. Therefore, A-V shunts play an
important role in:
• Body temperature conservation.
• Regulate blood flow to the organs with intermittent activity such as
intestine, erectile tissue (penis), liver and thyroid glands.
• Regulate the venous return.
Lymph vessels
• The principal function of the lymph vessels is to drain excess fluid called
lymph from the extracellular spaces and returns it to the blood.
• Blind-end lymph capillaries that converge to form progressively larger
lymphatic vessels drain the excess fluid (lymph).
• They are similar to veins. They have numerous delicate valves similar in their
structures to those of the venous valves.
Lymph capillaries
• They have the same structure as blood capillaries except:
? Lymphatics do not contain RBCs.
? They have discontinuous basal lamina.
? They are not fenestrated.
? They have a wider lumen and irregular diameter.
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