Anatomy and Physiology

1. Anatomy and Physiology Chapter 15

2. 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.

5. 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.

7. 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.

8. 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)

12. 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.

15. 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.

17. 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

19. 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.

21. 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.

23. 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.

25. The End