Summary

Summary

Digestion mechanically and chemically breaks down foods and absorbs them. The digestive system consists of an alimentary canal and several accessory organs. Regions of the alimentary canal perform specific functions. It consists of four major layers: mucosa, submu-cosa, muscular layer, and serosa. Seven essential steps make up the functions of the digestive system: inges-tion, propulsion, mechanical processing, digestion, secretion, absorption, and excretion. The two basic types of motor functions in the alimentary canal are mixing and propelling movements.

The mouth receives food and begins diges-tion. The tongue mixes food particles with saliva during chewing­. Salivary glands secrete saliva, which ­moistens food, helps bind food particles, begins chem-ical digestion of carbohydrates, makes taste possible, and helps clean the mouth. Humans have a primary and a secondary­ set of teeth that form during their lifetimes. The dentin that makes up a tooth is harder than bone. The pharynx and esophagus are important passageways, allowing food, liquids, and air to pass.

The stomach receives food, mixes it with gastric juice, carries on a limited amount of absorption, and moves food into the small intestine. Pepsin is the most important protein-digesting enzyme produced by the gastric mucosa. The small intestine extends from the pyloric sphincter to the large intestine and is the lon-gest portion of the alimentary canal. It plays the major role in the digestion and absorption of nutrients. The pancreas produces pancreatic juice with enzymes that can split carbohydrates, fats, nucleic acids, and pro-teins. The liver metabolizes these substances, storing some of them, filters the blood, destroys toxins, and secretes bile. The small intestine receives ­secretions from the pancreas and liver, completes nutrient ­digestion, absorbs the products of digestion, and transports­ the residues to the large intestine. The ­gallbladder is mainly a storage organ for bile, which is a fat emulsifier­. Bilirubin is a waste product of the heme of hemoglobin. The large intestine reabsorbs water and electrolytes and forms and stores feces.

Nutrition requires carbohydrates, lipids, and pro-teins in large amounts. Carbohydrates are organic compounds, and include sugars and starches. Lipids include fats, fat-like substances, and oils. Proteins are created from amino acids and include enzymes, plasma proteins, muscle components, hormones, and antibodies. Vitamins are organic compounds required for normal metabolism, and include fat-soluble and water-soluble forms.