Specific Effects
Although the structure and function is basically the same in smooth muscle cells in different organs, their specific effects or end-functions differ.
Smooth muscle forms precapillary sphincters in blood vessels in metarterioles which regulates the blood flow in capillary beds of various organs and tissues. The contractile function of vascular smooth muscle also regulates the lumenal diameter of the small arteries-arterioles called resistance vessels, thereby contributing significantly to setting the level of blood pressure. Smooth muscle contracts slowly and may maintain the contraction (tonically) for prolonged periods in blood vessels, bronchioles, and some sphincters. Activating arteriole smooth muscle can decrease the lumenal diameter 1/3 of resting so it drastically alters blood flow and resistance. Activation of aortic smooth muscle doesn't significantly alter the lumenal diameter but serves to increase the viscoelasticity of the vascular wall.
In the digestive tract, smooth muscle contracts in a rhythmic peristaltic fashion, rhythmically forcing foodstuffs through the digestive tract as the result of phasic contraction.
A non-contractile function is seen in specialized smooth muscle within the afferent arteriole of the juxtaglomerular apparatus, which secretes renin in response to osmotic and pressure changes, and also it is believed to secrete ATP in tubuloglomerular regulation of glomerular filtration rate. Renin in turn activates the renin-angiotensin system to regulate blood pressure.
Read more about this topic: Smooth Muscle Tissue
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