Iris sphincter muscle Iris sphincter muscle The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil. It is found in vertebrates and some cephalopods. Initially, all the myocytes are of the smooth muscle type but, later in life, most cells are of the striated muscle type. Its dimensions are about 0.75 mm wide by 0.15 mm thick. In humans, it functions to constrict the pupil in bright light (pupillary light
Smooth muscle carbon dioxide by producing vasodilation, and responds to oxygen by producing vasoconstriction. Pulmonary blood vessels within the lung are unique as they vasodilate to high oxygen tension and vasoconstrict when it falls. Bronchiole, smooth muscle that line the airways of the lung, respond to high carbon dioxide producing vasodilation and vasoconstrict when carbon dioxide is low. These responses to carbon dioxide and oxygen by pulmonary blood vessels and bronchiole airway smooth muscle aid in matching perfusion and ventilation within the lungs. Further different smooth muscle tissues display extremes of abundant to little sarcoplasmic reticulum so excitation-contraction coupling varies with its