Page 109 - Headlines Histology2024_Neat

Metarterioles and precapillary sphincter

Metarterioles
• They are the smallest arterioles with a diameter of less than 40 ?m.
• The tunica media is formed of isolated bundles of smooth muscle cells.
• They are divided into two segments; a proximal segment from which many

capillaries may arise, and a distal segment known as thoroughfare channels that
connect directly with venules.
Precapillary sphincter
• It is formed of few smooth muscle cells that encircle each capillary at the site
of its origin from arterioles.
• Contraction of the smooth muscle within the wall of the arterioles,
metarterioles and precapillary sphincter regulates the blood flow through the
capillaries.

Capillaries

• They are thin-walled vessels with a diameter of 4-10 ?m.
• The capillary wall is composed of endothelial cells, a basal lamina and

pericytes.
• The pericytes are relatively undifferentiated cells of mesenchymal origin that

may differentiate into fibroblasts or smooth muscle when the needs arise.
• A network of capillaries known as capillary beds usually extends throughout

all tissues and organs.
• The capillaries are primarily concerned with exchange of gases (O? and CO?),

Fluids, nutrients and metabolic waste products between blood and tissues.

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