Human Biology Digestive System
Digestion - Defined Prepares food for use by all body cells The physical and/or chemical breakdown of food Did you know: the average person eats more than 500kg of food per year!
Physical Digestion Physically breaking food stuff into smaller pieces WITHOUT chemically changing it into different molecules Increases the exposed SA for chemical digestion to occur Mechanical Changes the physical form of food Chew, tear, grind, mash, mix
Chemical Digestion The breakdown of large molecules (polymers) into smaller, soluble molecules (monomers) to allow for absorption Changes the chemical composition of food with the aid of digestive enzymes Lipids to fatty acids Proteins to individual amino acids Carbohydrates into simple sugars Note: Digestive enzymes are special proteins that help break up large molecules of food into very tiny molecules that can be absorbed and used by the cells in the form of nutrition.
Phases of Digestion: 1. Ingestion 2. Movement 3. Digestion 4. Absorption 5. Further Digestion
The Digestive Tract A long, muscular tube with many sections and areas Begins with the mouth and ends with the anus
The Digestive Tract Major Organs Mouth Pharynx Esophagus Stomach Small Intestine Large Intestine Anus
The Digestive Tract - Accessory Organs/Parts Organs that are not in the digestive tract but help in the process of digestion: Teeth Tongue Salivary Glands Liver Gall Bladder Pancreas
Mouth (Oral Cavity) Teeth Physical / mechanical digestion Incisors - cut Canines - hold, tear Premolars - crush, grind Molars - chew, grind
Carnivore Herbivore Omnivore (Human)
Mouth (Oral Cavity) Saliva Mostly water and mucus Acts as a lubricant Moistens food Contains enzymes (ptyalin or salivary amylase) that begins digestion of starch (to glucose) Salivary Glands produce and secrete saliva into the oral cavity Parotid (beneath the cheeks) Submaxillary (below the jaw bone) Sublingual (below the tongue) Tongue Mixes and rolls food into tiny mashed up bits (BOLUS) Pushes food toward pharynx
Swallowing A coordinated activity of the tongue, soft palate, pharynx, and esophagus
Swallowing 1. A mass of chewed, moistened food (bolus) is pushed into the pharynx by the tongue (voluntary) 2. In pharynx, the bolus triggers an involuntary swallowing reflex Prevents food from entering lungs and directs bolus to esophagus Soft palate seals off the nasal cavity 3. Larynx (Adam s Apple) rises so the Epiglottis (a flap of tissue) can close the opening of the trachea This response stops breathing 4. Cardiac (Gastroesophageal) sphincter relaxes 5. Esophageal muscles propel bolus by peristalsis (involuntary muscle contractions) into the stomach
Esophagus straight muscular tube 10 inches (25 cm) long connects mouth to the stomach Food takes about 4 to 8 seconds as it passes through to stomach Peristalsis propels food and liquid slowly down the esophagus into the stomach Cardiac Sphincter (ring-like valve) relaxes allows food into the stomach
Peristalsis
Stomach
Stomach J-shaped muscular sac Has inner folds (rugae) increases SA Churns and grinds bolus into smaller pieces Food mixes with gastric juices secreted by stomach wall
Stomach - Sphincters Cardiac (Gastroesophageal) Sphincter Limits backflow of gastric juices up the esophagus Failure = heartburn (acid reflux) Pyloric Sphincter Limits flow of gastric juices into the small intestine
Gastric Juices
Hydrochloric Acid (HCl) From Parietal cells (within gastric pits) Function Breaks up connective tissues to increase SA for pepsin enzyme activity Kills bacteria Activates pepsin by reacting with pepsinogen
Pepsinogen From Chief cells (within gastric pits) Function Pepsin Precursor Pepsinogen + HCl pepsin (protein) (enzyme)
Pepsin Function Enzyme that digests proteins into peptides Optimal activity is at low ph (pepsin) Protein + water a.a. strands (polypeptide fragments)
Mucus From Goblet cells (within gastric pits) Function Protects stomach lining Lubricant for food passage
Chyme Semi-fluid mass of partially digested material Exits the stomach (expelled into duodenum) ph of 2 (very acidic)
Small Intestine (in 3 parts ) 1. Duodenum Approx. 10in; connected to stomach ph: 6-6.5 Site of DIGESTION Produces and receives numerous digestive enzymes and hormones, pancreatic juices and BILE Pancreatic juice adds sodium bicarb. To neutralize acidic chyme For optimal enzyme activity
Small Intestine (in 3 parts ) 2. & 3. Jejunum and Ileum Jejunum - Approx. 8ft Ileum - Approx. 12ft ph: 7-8 Site of ABSORPTION Folds allow for an increased SA
Small Intestine - Villi
Villi
Small Intestine - Enzymes
Large Intestine (aka. colon, bowel) Larger in diameter than S.Intestine 5ft (1.5m) in length ph: 7-5.5 Includes Cecum, colon, rectum, anus
Large Intestine Function: Water reabsorption Store indigestible materials (ie. Cellulose) Site of 100 trillion bacteria!!! Including E. coli; generally a mutualistic symbiont that prevents colonization of harmful bacteria Synthesizes Vitamin K and various B vitamins Compacts undigested wastes for defecation Feces: 40% water
You should be able to. 1. Identify all digestive system structures in a diagram 2. List functions for each digestive structure 3. Describe the roll each structure plays in the digestive process 4. List the composition of gastric juice 5. Describe the components of intestinal juice and their functions 6. Describe the function of the villus and how nutrients are absorbed and transported
Next Up: Accessory Organs!