Smooth muscles around the blood vessels regulate their diameter and thus the flow of blood through them. They thus can control blood pressure. By reducing or increasing blood flow to an organ, they indirectly control level of activity of that organ.
Rings of smooth muscle are called sphincters. They are seen in the digestive and urinary systems. Gullet allows food into food pipe i.e. the esophagus. Cardiac sphincter lets the food enter into the stomach. Pyloric sphincter located at the lower end of stomach relaxes to permit food from stomach into the small intestine.
Bile produced by the liver is stored in the gallbladder. When the smooth muscle in the walls of the gallbladder contract bile is ejected into the digestive tract.
The ilio-caecal valve opens to let the food go from small intestine into the large intestine. Anal sphincter gives us - control on bowel movements and thus the dignity.
Sphincter of Oddi ( Please see the author's article "Italians in anatomy" - http://www.suite101.com/article.cfm/our_... ) allows pancreatic juice and bile into the duodenum, the first part of small intestine.
Smooth muscle in the walls of the ureters help to carry urine to the urinary bladder. Gravity helps downward passage of urine when we are in a sitting or standing position.
Urethral sphincter is at the base of urethra near the bladder. It controls the act of micturition that is voiding of urine. The urinary bladder has a distinct layer of smooth muscle - the detrusor muscle. This coat contracts to push the urine out of our body into environment.
Peristalsis is the rhythmic wave of contraction passing through a hollow muscular tube, like the intestines. Smooth muscles in such tubes push the food, waste material, urine etc. Generally in one direction, like food travels away from mouth, (during vomiting it travels in opposite direction i.e. antiperistalsis). smooth muscles of the arrector pili elevate hair, giving us goose bumps.
By and large, the smooth muscle of the stomach and intestines achieves propulsion and mixing of food.
In respiratory system, smooth muscle contraction or relaxation alters the diameters of the respiratory passageways. They change the resistance to airflow.
Layers of smooth muscle help move sperm along the epididymis of reproductive tract in males. Smooth muscle contraction causes the ejection of glandular secretions from the accessory glands, like seminal vesicles, Prostrate gland, into the reproductive tract.
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