NOTES: The Digestive System (UNIT 8, part 1) Digestion is the process of mechanically and chemically breaking down foods and absorbing the breakdown products. The digestive system consists of an alimentary canal and several accessory organs.

Alimentary canal includes: ● mouth ● pharynx ● esophagus ● stomach ● small intestine ● large intestine ● rectum ● anus

Accessory organs are: ● salivary glands ● liver ● gallbladder ● pancreas

GENERAL CHARACTERISTICS OF THE ALIMENTARY CANAL *The alimentary canal is a muscular tube that passes through the body’s ventral cavity. Different regions carry out different functions, but certain structural characteristics are similar throughout its length.

Structure of the alimentary wall: 4 DISTINCT LAYERS: • MUCOSA (or mucous membrane) -surface epithelium, underlying connective tissue, and a small amount of smooth muscle -in some regions, folds and tiny projections increase the surface area (for absorption) • SUBMUCOSA -loose connective tissue, glands, blood vessels, lymphatic vessels, and nerves

4 layers of alimentary wall… ● MUSCULAR LAYER -consists of two coats of smooth muscle tissue and some nerves; produces movement of the tube ● SEROSA (or serous layer) -outer covering of the tube (also called the visceral peritoneum) -protects underlying tissue -cells here secrete serous fluid, which moistens and lubricates tube’s outer surface

Movements of the tube: *motor functions include: MIXING MOVEMENTS and PROPELLING MOVMENTS (include a wavelike motion called PERISTALSIS)

MOUTH (Oral Cavity) *receives food and begins digestion  Tongue: -rough projections (PAPILLAE) on surface of tongue provide friction to handle food ( turns food into a BOLUS) -papillae also contain TASTE BUDS -covering the root of the tongue are masses of lymphatic tissue: LINGUAL TONSILS

MOUTH (Oral Cavity)  Palate: -forms the roof of the oral cavity -hard palate (anterior) and soft palate (posterior) -soft palate extends posteriorly and downward as a projection called the UVULA

Uvula and soft palate closes off nasopharynx while swallowing.

MOUTH (Oral Cavity)  Teeth: -primary teeth (“baby teeth”) are replaced by the secondary teeth (permanent teeth) -teeth are used to mechanically break food into smaller pieces (this increases the surface area of food allowing digestive enzymes to react more effectively with the food molecules)

SALIVARY GLANDS *salivary glands secrete saliva, which: moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, and helps cleanse the mouth.

SALIVARY GLANDS  Serous cells: secrete a watery fluid that contains the digestive enzyme AMYLASE (starts to break down starch and glycogen into smaller disaccharides)  Mucous cells: secrete a thick fluid called mucus, which binds food particles and lubricates during swallowing

SALIVARY GLANDS There are 3 pairs of salivary glands: Parotid glands (largest); contains only serous cells 2) Submandibular glands; contains both serous and mucous cells (Gleeker gland!) 3) Sublingual glands (underneath the tongue); secretions are mostly mucus…thick and stringy!)

PHARYNX and ESOPHAGUS (important passageways!)  PHARYNX: connects the nasal and oral cavities with the larynx and esophagus

ESOPHAGUS ESOPHAGUS: a food passageway from the pharynx to the stomach; passes through the diaphragm -at distal end, circular smooth muscle fibers thicken and close the entrance to the stomach (prevents regurgitation);  esophageal sphincter -if some of the acidic stomach contents are regurgitated (reflux) into the esophagus, may cause HEARTBURN

STOMACH *the stomach receives food, mixes it with gastric juice, initiates protein digestion, carries on a limited amount of absorption, and moves food into the small intestine.

STOMACH  Structure / Parts: -J-shaped, pouch-like organ that sits inferior to diaphragm in upper left of abdominal cavity -has a capacity of about 1 L -thick folds (rugae) in the inner lining disappear when the stomach wall stretches -separated from the small intestine by a powerful circular muscle: PYLORIC SPHINCTER

*together, these secretions form STOMACH  Gastric Secretions - produced by GASTRIC GLANDS, which have 3 types of cells: ● mucous (goblet) cells: secrete mucus ● chief cells: secretes inactive digestive enzyme (pepsinogen) ● parietal cells: secrete hydrochloric acid (HCl) *together, these secretions form GASTRIC JUICE

STOMACH  PEPSIN: most important digestive enzyme in gastric juice -secreted by chief cells in an inactive form: PEPSINOGEN -when it comes into contact with hydrochloric acid, it becomes the active PEPSIN

STOMACH *a layer of thick mucus is produced by cells in the stomach’s inner lining…this protective coating prevents the stomach from digesting itself.

STOMACH  Mixing and emptying actions: CHYME = a mixture of food particles and gastric juice produced by mixing -chyme is emptied into the small intestine where chemical digestion continues -the rate of emptying depends on the fluidity of chyme and the type of food