Structure of Small Blood Vessels

Study Notes

Structure of Small Vessels

Small blood vessels, including arterioles, capillaries, and postcapillary venules, are crucial for blood flow and exchange of material.
Arterioles have three layers (tunicae): intima (inner layer with endothelium only), media (smooth muscle, one or two layers), and adventitia (outer layer of loose connective tissue)
Arterioles control blood flow to capillaries through contraction of smooth muscle.
Metarterioles are smaller branches of arterioles, with smooth muscle cells acting as precapillary sphincters regulating blood flow.
A thoroughfare channel is the distal part of the metarteriole, lacking smooth muscle and connecting to the postcapillary venule.
Capillaries are the smallest blood vessels, often smaller than red blood cells, with a single layer of endothelial cells and basal lamina.
Capillaries have a single layer of endothelium for fluid exchange.
In many capillaries, the lumen is narrow, requiring red blood cells to fold for passage.
Pericytes, undifferentiated mesenchymal stem cells, are associated with capillaries.
Capillaries are classified as continuous, fenestrated, or discontinuous/sinusoidal, based on structural characteristics (e.g., openings, gaps).
Postcapillary venules are the smallest veins, with pericytes instead of a tunica media and more permeable walls than capillaries or venules. They are responsible for exchanging materials.
Some venules act as 'leaky' vessels due to vasodilator substances like serotonin and histamine increasing intercellular distances.
Leukocytes preferentially leave the vascular system at postcapillary venules, entering connective tissue spaces via diapedesis.
In certain tissues, arterioles bypass capillaries and connect directly to venules (arteriovenous shunts) to accommodate specialized functions such as thermoregulation.
Arteriovenous shunts have thicker media and adventitia layers and are richly innervated by sympathetic and parasympathetic nerve fibers.