A sphincter is a muscle that forms a circle around a
tube, natural opening, or duct in the body. A duct is a
tube that allows fluids and/or substances to pass
through it, especially those produced by organs in the
body known as glands. A gland is an organ in the body
made of special cells that form and release materials
such as fluid.

The sphincter forms a circle around the above-
mentioned structures in such a way that when the
muscle tightens and becomes narrower, the opening or
space inside of the tubes becomes narrower as well,
which can eventually cause them to close. There are
over 50 different sphincters in the human body, most of
which are part of the digestive system.
The lower sphincter of the
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Some sphincters are visible to the naked eye and some of which can only be seen with
a microscope. The most well known visible sphincter is the anal sphincter, which
controls the release of feces from the body. There is an external anal sphincter which
one has voluntary control over but there is also an internal anal sphincter which one
has no control over. Another well known sphincter is the urethral sphincter. The urethral
sphincter controls the passage of urine from the from the urinary bladder to the urethra.
The urethra is a tube shaped structure that drains urine from the body.

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Like the anal sphincter, there is an external and internal urethral
sphincter and the former is under voluntary control unlike the latter.
External spincters are under external control because they are
made of skeletal muscle. Internal sphincters are made of smooth
muscle, which are not under voluntary control.

The most well know microscopic sphincters are the precapillary
sphincters, which are bands of smooth muscle that adjust the flow of
blood in each capillary. Capillaries are small types of blood vessels
that act as an exchange system connecting the smallest veins
(known as venules) with the smallest arteries (known as arterioles).
Veins are blood vessels that carry blood to the heart. Arteries are blood vessels that carry blood away
from the heart to the rest of the body. Sphincter is also known as the sphincter muscle or musculus
sphincter. Sphincter comes from the Greek word "sphingein" meaning "to bind."