Anatomy and Physiology Chapter 15

Introduction Digestion refers to the mechanical and chemical breakdown of foods so that nutrients can be absorbed by cells. The digestive system carries out the process of digestion. The digestive system consists of the alimentary canal, leading from mouth to anus, and several accessory organs whose secretions aid the processes of digestion. The alimentary canal is a muscular tube about 9 meters long that passes through the body's ventral cavity.

The wall of the alimentary canal consists of four layers: The inner layer is the mucosa, which protects tissues of the canal and carries on secretion and absorption. The next layer is the submucosa, which nourishes the surrounding layers of the canal. The muscular layer propels food through the canal. The outer layer, or serosa, protects underlying tissues and secretes serous fluid.

The alimentary canal propels food from the mouth to the anus through rhythmic contractions called peristalsis. It also provides for the mixing of food with digestive enzymes.

The alimentary canal consists of several main parts and some accessory organs. Mouth It functions to receive food and begins mechanical digestion by mastication. The lips contain sensory receptors that help to judge the temperature and texture of food. The soft palate and uvula function to close off the nasal cavity during swallowing. Tonsils and adenoids help to protect the body from infection. The salivary glands secrete saliva, which moistens and dissolves food particles, allows tasting, and begins carbohydrate digestion. Amylase breaks down complex sugars into disaccharides. The pharynx is a cavity lying behind the mouth The esophagus is a muscular tube leading to the stomach. There are two types of teeth – primary (baby) and secondary (adult)

Stomach The stomach is a J-shaped muscular organ that mixes food with digestive juices, and propels food to the small intestine. The esophageal sphincter helps to prevent regurgitation. The pyloric sphincter controls release of food from the stomach into the small intestine. Gastric glands are basically pits in the mucosa of the stomach and contain three types of cells Mucous cells produce mucus that protects the stomach lining. Chief cells secrete pepsin (to digest protein) as inactive pepsinogen, which is activated when it comes in contact with hydrochloric acid. Parietal cells secrete hydrochloric acid. The stomach absorbs only small quantities of water and certain salts, alcohol, and some lipid-soluble drugs. The stomach churns the food into an acidic Chyme which then enters the small intestine.

Pancreas The pancreas produces pancreatic juice which contains enzymes that digest carbohydrates, fats, proteins, and nucleic acids. Pancreatic enzymes include: pancreatic amylase (for starch) pancreatic lipase (for fats) Trypsin, chymotrypsin and carboxypeptidase (for proteins) Protein-digesting enzymes are released in an inactive form and are activated upon reaching the small intestine. Two nucleases (for nucleic acids). The juice is collected in the pancreatic duct which leads into the small intestine along with the bile duct.

Liver The body’s largest internal organ The liver is divided into right and left lobes, and is enclosed by a fibrous capsule. The liver is important in the metabolism of fats, carbohydrates and proteins. It is also important for the storage of glycogen, and Vitamins A, D, and B12. It filters the blood, removing toxins Major contribution to digestion is the secretion of bile. Bile salts helps emulsify and digest fatty acids and cholesterol. Bile is stored in the gallbladder Too much cholesterol can lead to gallstones

Small Intestine The small intestine has many jobs: Receives secretions from the pancreas and liver Completes digestion of the nutrients in chyme Absorbs the products of digestion Transports the remaining residues to the large intestine. The small intestine consists of the duodenum (short part right after stomach), jejunum (middle portion), and ileum (part right before large intestine). The small intestine is suspended by a double-layered fold of peritoneum called mesentery.

All over the inner lining of the small intestine are villi – small fingerlike projections that increase surface area. Each villus has its own blood supply to aid in absorption of materials. Goblet cells secrete mucous that lines the small intestine. The small intestine is the major site of absorption within the alimentary canal. Monosaccharides and amino acids are absorbed through active transport or facilitated diffusion. Fatty acids dissolve directly into the cells lining the villi. Water is absorbed by osmosis.

Large Intestine The large intestine absorbs water and electrolytes and forms and stores feces. The large intestine consists of several parts: The cecum (pouch at the beginning of the large intestine) Colon (ascending, transverse, descending, and sigmoid regions) The rectum (pouch at the end of the intestine) The anal canal (where feces exits). Feces are composed of undigested material, water, electrolytes, mucus, and bacteria. Both the color of feces and its odor is due to the action of bacteria. The large intestine contains important bacteria which synthesize vitamins and use cellulose.

The End