Chapter 14: Functions of the Digestive System What activities occur within the GI Tract?

Essential Activities of the GI Tract Ingestion Propulsion Mechanical Digestion Chemical Digestion Absorption Defacation

Ingestion Ingestion  Food enters the mouth leads to Mechanical and Chemical Digestion Teeth and tongue help to Physically Break it down Saliva:  starts to break down starch (into maltose)  is continuously produced, but in excess with food ingestion  other stimuli can cause production of Saliva (smells, thoughts, nervous response)

Propulsion  Food need to be moved from one digestive organ to another Peristalsis  rhythmic contractions of smooth muscle to propel food forward Segmentation  single segments of the small intestine alternately contract/relax

Propulsion and Swallowing Swallowing (Deglutition) has 2 phases and involves many structures: Tongue, soft palate, pharynx, esophagus Phase 1  Buccal Phase (this is voluntary action in the mouth) Phase 2  Pharyngeal-Esophageal Phase –transport through the pharynx and esophagus Involuntary action controlled by the Autonomic Nervous System

Propulsion At the end of the esophagus  swallowed food presses against the Cardioesophageal sphincter  Chyme approaches the pyloric region of the stomach  Pyloric Sphincter—allows passage into the small intestine

Mechanical Digestion Mechanical Digestion  prepares food for further breakdown by enzymes  Grinding action of the teeth  Mixing of the food by the tongue  churning of food in the stomach

Mechanical Digestion Food Breakdown  regulated by hormones and neural factors  Sight and smell of foods can initiate the secretion of gastric juices Gastrin  hormone that causes the stomach to produce more: 1.) Pepsinogens (enzymes) 2.) Mucus 3.) Hydrochloric Acid

Chemical Digestion Chemical Digestion  the sequence of steps that occur in which large food molecules are broken down to their building blocks by enzymes Break down: Carbs, Lipids, Proteins

Chemical Digestion Carbohydrates  sucrose, lactose, maltose and starch ex.) Polysaccharides broken down into Monosaccharides Cellulose containing foodstuffs  cannot be digested by the human stomach (FIBER) *These provide bulk and help move foodstuffs along Proteins  broken down into their building blocks: Amino Acids Lipids  yield two types of building blocks when digested: Fatty Acids and Glycerols

Absorption Absorption  transport of digested end products (from Carbs / Proteins / Fats) to the blood

Defacation Elimination of solid wastes Mass Movements  Movement of the colon that will propel solid wastes along Goblet Cells  produces mucus that will lubricate the colon Disruptions to homeostasis: Constipation  slow movement of wastes Diarrhea  rapid waste movement with fluid loss

Nervous Control Autonomic Nervous System controls function Sensory receptors respond to: 1.) Stretch of an organ caused by food 2.) pH of the foodstuffs 3.) presence of certain breakdown products Receptors then: 1.) activate / inhibit glands that secrete essential digestive juices 2.) activate / inhibit the smooth muscles that mix and propel food along

Stomach Action As food enters the stomach: 1.) Walls are stretched 2.) Gastric Juices are produced 3.) Stomach smooth muscle layers (3) begin to churn and break apart food 4.) Chyme is produced (which will move along to the small intestine) Stomach Enzymes: 1.) Pepsin (acts to break down proteins) 2.) Rennin (acts to break down milk proteins)

Digestive Enzymes Brush Border Enzymes  break double sugars into simple sugars for digestion Pancreatic Juice: 1.) completes the digestion of starch 2.) carries of about half of the protein digestion 3.) responsible for fat digestion 4.) digest nucleic acids Insulin and Glucagon  Controls blood sugar

Digestive Hormones Hormones  Secretin and Cholecystokinin influence the release of pancreatic juice and bile Secretin  causes the liver to increase the output of bile Cholecystokinin  causes the gallbladder to contract and release stored bile