Circular muscle, sphincters and narrowing a lumen

Skeletal muscles can be grouped by how their fibres (the long muscle cells, also called myocytes) are arranged. In a circular muscle they run in concentric rings, that is, rings within rings, around an opening. When the ring tightens, the opening gets smaller, and a strong enough contraction can close it completely.

Circular muscle forms the sphincters of the body, rings of muscle that guard an opening. Examples are the orbicularis oris (the muscle around the mouth) and the external anal sphincter.

The sphincter pupillae in the eye is the classic example. The coloured ring of tissue called the iris surrounds the pupil, which is the dark opening that lets light in. The sphincter pupillae, a ring of smooth muscle, encircles the pupil, so when it contracts in bright light the pupil narrows. Its antagonist (opposing muscle), the dilator pupillae, widens the pupil.

Rings of muscle also sit in the wall of hollow organs. The smooth muscle of the small intestine has an inner circular layer and an outer longitudinal layer, whose fibres run along the length of the tube. Contracting the circular layer narrows the lumen and squeezes the contents into less space, so the intraluminal pressure (the pressure of the food, fluid and gas inside the lumen) rises. In experiments on isolated small intestine, the highest pressures came when the contraction closed the lumen completely. Longitudinal contraction shortens the tube instead, and in those experiments it did not close the lumen.