Chapter 3 - Digestion, Absorption, and Metabolism

Gastrointestinal (GI) tract

mouth, esophagus, stomach, duodenum, jejunum, ileum, colon, rectum, anus, accessary digestive

• organs: salivary glands, liver, pancreas, gallbladder

Mouth

• salivary glands

• functions of saliva

• functions of teeth and tongue

• three phases of swallowing

Salivary Glands

• parotid gland: watery saliva and enzymes

• submandibular gland: mucus and enzymes

• sublingual glands (the smallest): mucus

Functions of Saliva

• moisten food

• amylase in saliva breaks down starch (chemical digestion)

Functions of Teeth and Tongue

• teeth mechanically digestion

• formation and movement of bolus

• tongue begins the pharyngeal phase of swallowing

3 Phases of Swallowing

• oral phase: chewing/bolus

• pharyngeal phase: bolus moved to back of mouth, nasopharynx closes

• esophageal phase: peristalsis moves bolus through esophagus

Esophagus

• a muscular tube through which bolus travels from mouth to stomach

• peristalsis

• segmentation: back and forth movement of the tract

• esophageal/cardiac sphincter: muscular band that opens and closes to stomach, triggered by swallowing, prevents GERD/heartburn

Stomach/Gastric

• Gastric acids: fundus, body, pylorus, pyloric sphincter

• Functions: holding food for partial digestion, producing gastric juices, providing muscular action that combined with gastric juice, and mixes and tears food into smaller pieces (mechanical digestion)

3 Phases of Gastric Secretion

• cephalic (psychic phase): gastrin stimulated

• gastric: gastrin increases release of gastric juices when food present

• intestinal: chyme (semi-liquid)

• aid digestion: hydrochloric acid (HCL), pepsin, intrinsic factor, gastrin (breaks proteins and fats)

Small Intestine

• consists of duodenum, jejunum, and ileum

• passage through takes about 5 hours

• major organ of digestion and most absorption

• contains villi and microvilli

• function of ileocecal valve (sphincter) prevents backflow

Duodenum - Small Intestine Functions

• main role: food from the stomach is mixed with enzymes from the pancreas and bile from the gallbladder

• the enzymes and bile help break down food further in the duodenum

• water, iron, some minerals are absorbed

Jejunum

• inside walls absorb the foods nutrients

• inside walls of the jejunum have many circular folds which make its surface area large enough to absorb all of the nutrients that the body needs

Ileum

absorbs bile acids which are returned to the liver to be made into more bile, then stored in the gallbladder for future use in the duodenum and absorbs vitamin B12, which the body uses to make nerve cells and red blood cells

Hormones and Enzymes

Secretions from small intestine, liver, and pancreas include:

• secretin: tells pancreas to send bicarbonate to duodenum

• bile-emulsifier of fats: coats and allows pancreatic lipase to work

• cholecystokinin (CCK): signals for pancreatic secretions

Absorption

• process by which substances pass through the intestinal mucosa into the blood or lymph

• transport processes: passive diffusion/osmosis (when pressure is greater on one side of the membrane the substance moves from area of greater pressure to less pressure through capillaries)

• facilitated diffusion

• energy-dependent active transport

The Large Intestine

• consists of cecum, colon, rectum, appendix (attached to LI)

• passage through takes about 9 to 16 hours

• site of final absorption of water and some minerals

• bacteria in colon produce several vitamins

• formation of feces and excretion from color through anus (sphincter)

• appendix: infection fighting

Elimination

• expulsion of feces or body waste products is called defecation

• residue may include undigested materials such as cellulous and other dietary fibers, undigested fats may combine with dietary minerals such as calcium and magnesium, and additional residue may include water, bacteria, pigments, and mucus

Duration of Digestion

• mouth: chewing depends on texture and quantity

• esophagus: 5-7 seconds

• stomach: 2-6 hours

• small intestine: approx. 5 hours

• large intestine: 9-16 hour

• total: 16-27 hours ingestion to elimination

Metabolism

set of processes through which absorbed nutrients are used by the body for energy and to form and maintain body structures and functions

Catabolism

break down

• glycogenolysis: break down of glycogen

Anabolism

building

• lipogenesis: fatty acid/triglyceride synthesis (growth, repair)

Function of nutrients (metabolism)

• form new cell structures

• create new substances such as hormones and enzymes

• assists in use of other nutrients in cell

• act as catalysts or coenzymes in transforming and using of carbohydrates, proteins, and lipids

• provides energy

Metabolism Waste Products

• excreted through lungs, kidneys, or large intestine

• liver detoxifies some substances

Metabolism Across the Life Span

• lowered metabolic rates

• lifestyle behaviors affect functioning and health of GI tract

• muscle burns more energy, fat not so much