Advanced Biology THE DIGESTIVE SYSTEM

Overview of Digestive System  Alimentary canal (gastrointestinal – GI – Tract) is the continuous, muscular digestive tube that winds through the body  Digest = dissolve  Absorbs digested fragments through its lining into the blood

Digestive Process  1. Ingestion – taking in food through mouth  2. Propulsion – swallowing or the process of moving food through the Alimentary canal  Peristalsis – alternate waves of contraction and relaxation of muscles in the organ walls  3. Mechanical digestion – physically prepares food for chemical digestion by enzymes  Segmentation – rhythmic local constrictions of the intestine

Digestive Process  4. Chemical Digestion – a series of catabolic steps in which complex food molecules are broken down to their monomers  5. Absorption – the passage of digested end products from the lumen of the GI tract to the blood  6. Defecation – elimination of indigestible substances from the body

Alimentary Canal  Mouth  Ingestion of food  Propulsion  Mechanical digestion – chewing (mastication)  Chemical digestion - starches  Pharynx  Propulsion

Alimentary Canal  Esophagus  Propulsion  Stomach  Mechanical digestion and propulsion  Chemical digestion (proteins begun by pepsin); gastric juices  Absorption – fat-soluble substances (aspirin, alcohol, some drugs)

Stomach

Alimentary Canal  Small intestine  Mechanical digestion and propulsion  Continually mixes contents with gastric juices  Chemical digestion – digestive enzymes from pancreas  Border enzymes attached to villi – digestion of all foods  Absorption – breakdown of carbohydrate, protein, fat, nucleic acid, vitamins, electrolytes, and water

Small Intestine  Four layers of small intestine:  The mucosa  Submucosa  Muscularis externa  serosa

Small Intestine

Alimentary Canal  Large intestine (leads to the terminal opening – anus) “ the water-wringer ”  Chemical digestion – remaining food residues are digested by bacteria (produce vitamin K and some B vitamins)  Absorption – absorbs almost all remaining water, electrolytes (especially NaCl) and vitamins produced by bacteria  Propulsion – propels feces toward rectum  Defecation

Alimentary Canal  Human cadaver GI tract is about 30ft (9m) long  Shorter in living humans because of muscle contraction  Food is outside of the body because the canal is open to the environment

Accessory Digestive Organs  Teeth – grinding, tearing, etc.  Tongue – taste, swallowing, movement, etc.  Gallbladder – store bile (digestive enzyme) from liver  Digestive glands:  Salivary glands – produce saliva (digests starches)  Liver – produce bile  Pancreas – produce and secrete other digestive enzymes

Functional Concepts  1. Digestive activity is provoked by a range of mechanical and chemical stimuli  2. Controls of digestive activity are both extrinsic (CNS) and intrinsic (“In- house”)