Digestive System

• This tube—called the alimentary canal or gastrointestinal (GI) tract— may be as long as 8 meters.
• Mechanical digestion is the breakdown of large pieces of complex molecules to smaller pieces of complex molecules.
• Chemical digestion is the breakdown of complex molecules to their building blocks so that they can be absorbed.
• The mouth can also be called the oral cavity. The roof of the oral cavity consists of the hard palate, formed by the maxilla and palatine bones, and the soft palate, composed of soft tissue.
• the soft palate ends with the uvula, a posterior projection that directs materials downward to the pharynx so that they do not travel to the nose
• A baby is not born with teeth but will develop two sets of teeth over its lifetime—a deciduous (primary) set and a permanent (secondary) set.
• The baby’s first set of teeth—the deciduous teeth—begin to erupt, or grow in, at approximately 6 months and will be complete by the age of 2 years. This primary set consists of 10 teeth in each jaw.
• This secondary set begins to erupt at 6 years of age and will not be fully complete, with 16 teeth in each jaw, until the individual reaches 17 to 25 years of age.
• a tooth is held in its bony socket (alveolus) in the jaw by periodontal ligaments.
• The tissue surrounding a tooth is the gingiva, commonly called the gum.
• The portion of the tooth emerging from the gingiva is called the crown. The crown is covered by a very hard, smooth, white layer called enamel. The enamel’s function is to protect the underlying layer, the dentin. The root of the tooth, below the gum line, is not covered by enamel. Deep to the dentin is a pulp cavity that contains the blood vessels and nerve for the tooth.
• The tongue is composed of skeletal muscle tissue anchored to the floor of the oral cavity by a medial fold called the lingual frenulum
• The salivary glands, which produce about 1.0 to 1.5 liters (L) of saliva a day, consist of the parotid glands (anterior to the ears), the submandibular glands (inferior to the angle of the mandible on each side), and the sublingual glands (below the tongue).
• The saliva, which is mostly water, also contains the enzymes amylase and lingual lipase, along with mucus, lysozymes, and antibodies.
• A dental caries, commonly called a cavity, is an erosion through the enamel into the dentin. If the erosion continues to the pulp cavity, bacteria may gain access and travel beyond the tooth’s root. This infection is called an abscess.
• The process of chewing, called mastication, uses the masseter and temporalis muscles to move the jaw in a crushing motion, while the tongue, orbicularis oris, and buccinator muscles work to keep the food between the teeth.
• The esophagus is a straight, muscular tube that extends from the laryngopharynx, travels through the mediastinum, penetrates the diaphragm, and connects to the stomach. It is lined by stratified squamous epithelial tissue.
• Swallowing, called deglutition, is a very complex process controlled by the medulla oblongata.
• The stomach is a J-shaped organ found in the upper left quadrant of the abdomen, immediately inferior to the diaphragm.
• The cardiac sphincter (lower esophageal sphincter) controls the opening to the stomach from the esophagus.
• The stomach can be described in terms of the following areas: the lesser curvature on the inside of the J; the greater curvature on the outer side of the J; the fundus, superior to the cardiac sphincter; the body, making up the majority of the stomach; and the pyloric region leading to the smooth muscle pyloric sphincter, which regulates the passage of materials to the duodenum.
• Chronic leakage of gastric juices back to the esophagus is called gastroesophageal reflux disease (GERD).
• Mucous cells secrete a highly alkaline mucus to protect the stomach walls from the hostile environment caused by the acid and digestive enzymes produced in the stomach.
• Endocrine cells secrete many hormones, but we will focus on the hormone gastrin.
• Parietal cells produce and secrete hydrochloric acid and intrinsic factor.
• Chief cells secrete pepsinogen and gastric lipase.
• Regenerative cells are stem cells that divide and differentiate to replace any of the other cells of the gastric pits and gastric glands.
• The liver is a large, reddish-brown organ immediately inferior to the diaphragm on the right side of the abdominal cavity.
• the gallbladder is a pear-shaped sac on the inferior side of the liver
• If the gallbladder concentrates the bile too much, the cholesterol in bile may precipitate (settle out as a solid), forming gallstones (cholelithiasis). When the gallbladder is directed to release its bile, the stones may block the cystic duct, causing pain and inflammation (cholecystitis). Surgery—a cholecystectomy —may be necessary to remove the gallbladder and the gallstones within.
• The small intestine consists of duodenum, jejunum, and ileum
• Segmentation is a stationary constriction of the smooth muscle in ringlike patterns. This type of contraction further churns the chyme, mixing in the bile and digestive enzymes to finish chemical digestion. It also allows for maximum contact between chyme and the villi, facilitating maximum absorption of nutrients.
• The large intestine (colon) is made up of six regions: the cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum
• Functions of the Digestive System: Ingestion, Digestion, Absorption, and Defecation