Two types of stomach occur: mixing waves and peristaltic waves. Both types of movements result from smooth muscle contractions in the stomach wall. The contractions occur about every 20 seconds and proceed from the body of the stomach toward the pyloric sphincter. Relatively weak contractions result in mixing waves, which thoroughly mix ingested food with stomach secretions to from chyme. The more fluid part of the chyme is pushed toward the pyloric sphincter, whereas the more solid center moves back toward the body of the stomach. Stronger contractions result in peristaltic waves, which force the chyme toward and through the pyloric sphincter. The pyloric sphincter usually remains closed because of mild tonic contraction. Each peristaltic contraction in sufficiently strong to pump a few milliliters of chyme through the pyloric opening and into the duodenum. Roughly 20% of the stomach contractions are peristaltic waves, and 80% are mixing waves.