Human Digestive System Digestion is the process by

Human Digestive System ® Digestion is the process by which organisms break large molecules into smaller ones ® For example, breaking down carbs, fats, proteins ® Digestion must take place so that molecules will be small enough to pass through cell membranes

Mouth • Food enters here • Mechanical breakdown occurs • Teeth bite and grind food • Tongue moves and shapes food

• The digestive juice called saliva is added • Wets the food • Sticks food together into a bolus • Contains a digestive enzyme called salivary amylase • Salivary amylase breaks down starch (a polysaccharide) into maltose (a disaccharide) • Food is pushed into the throat • Food is kept from entering the larynx and trachea by a flap of tissue called the epiglottis

Esophagus • No mechanical or chemical breakdown occurs here • Food is pushed to the stomach by the muscular walls of the food tube ~ peristalsis • Food enters the stomach when a ring of muscle – the cardiac sphincter lets it through • Reverse peristalsis – cardiac sphincter opens and stomach contents are forced out – vomiting

Esophagus • Heartburn – when the cardiac sphincter relaxes causing stomach juices containing hydrochloric acid to come into contact with the esophagus

Stomach • a thick-walled muscular sac • Mechanical breakdown is completed here • Solid food is reduced to a thin soupy liquid called chyme (2 -6 h) • Liquids pass through in 20 min. or less • Stomach has pyloric glands • Secrete mucus to protect the stomach from being “digested”

Stomach • Stomach has gastric glands • Secrete gastric juices • Gastric juices contain hydrochloric acid and the digestive enzyme called pepsin • Pepsin can only break down proteins in an acidic environment • When acid is present it changes the proteins into shorter chains of amino acids called polypeptides • When stomach is empty only small amount of juice is present

Stomach • All stimula te gastric juices When food is eaten gastric juice increases 1. Thought, sight, smell, taste of food 2. Food touching the stomach lining 3. Food mass stretching stomach walls • Food exits through the pyloric sphincter into the small intestine

Stomach • Ulcers can occur when the mucous layer breaks down and part of the stomach is “digested”

Small intestine • About 6. 5 m long, 2. 5 cm in diameter • Coiled tube • Three sections: a. duodenum (25 cm) b. jejunum (several metres) c. ileum (largest section) • Chemical digestion is completed here • Absorption of nutrients is completed here

Small intestine • When food is present the small intestine is in constant motion (peristalsis) • Four main effects: 1. Squeezes chyme through 2. Mixes chyme with digestive enzymes 3. Breaks down food 4. Speeds up absorption • Small intestine fluids are alkaline (p. H > 7)

Small intestine • Three juices mix with the chyme 1. Bile • Made by liver • Stored by gallbladder • Emulsifies fats (breaks them into smaller drops to make it easier to break them down later) • Contains no enzymes • Neutralizes the acids from the stomach

2. Pancreatic juice • when acidic chyme enters the small intestine it stimulates hormones which signal the pancreas to release juice and enzymes into the upper part of the small intestine • Contains several enzymes: • Amylase ~ continues to breakdown starch to maltose) • Trypsin (protease) ~ breaks down the polypeptides called peptones and proteoses, into peptides • Lipase ~ breaks down fats and oils into fatty acids and glycerol

3. Intestinal juice • contains the enzymes • Peptidase ~ breaks down peptides into amino acids • Maltase ~ finishes carbohydrate digestion by breaking down maltose into glucose • Now all nutrients have been broken down: • carbohydrates • fats • proteins glucose (monosaccharides) fatty acids and glycerol amino acids

Small intestine (cont’d) • Absorption of these nutrients takes place through finger-like projections called villi that line the intestinal walls • Each villus has a network of capillaries and a lacteal • Through diffusion and active transport the nutrients are absorbed • Monosaccharides, amino acids, vitamins, minerals are all absorbed into the capillaries • Fatty acids and glycerol are absorbed into the lacteal and into the lymphatic system

Large intestine • No digestion here • About 1. 5 m long and 6 cm diameter • Small pouch called appendix situated where the small intestine joins the large intestine • Has no function but can cause problems

Large intestine (3 main functions): 1. Re-absorption of water (about ¾ of it) • Too much water absorbed = constipation • Too little absorbed = diarrhea 2. Absorption of vitamin K and some B vitamins that are produced by bacteria that live in the large intestine 3. Elimination of undigested and indigestible materials like plant cell walls, bacteria, bile, mucus, worn-out cells

Large intestine • As the material travels through it becomes feces or stool • Feces stored in the rectum (last part of colon) before being eliminated through the anus

Label diagram of respiratory system in resource booklet!!!