The Digestive System

Digestive System Built around an alimentary canal (one-way tube) Includes Mount, pharynx, esophagus Stomach Small intestine Large intestine Salivary glands, pancreas, liver

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Mouth Begins mechanical digestion with chewing Teeth crush food to make a fine paste for swallowing Begins chemical digestion with saliva Digestive enzymes begin breakdown of carbohydrates into smaller molecules

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Teeth Anchored in jaw bones Protected by mineralized enamel Used to cut, tear, and crush food into smaller fragments

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Saliva Secreted by salivary glands in mouth to moisten food and make it easier to chew Controlled by nervous system Begins chemical digestion Enzyme amylase breaks down starches and releases sugars Lysozyme fights infection by digesting bacteria that may enter mouth with food

Esophagus Food (called a bolus) from mouth is swallowed (forced from mouth into esophagus by combined action of tongue and throat muscles) Bolus moved through esophagus by contractions (peristalsis) of smooth muscles to the stomach Cardiac sphincter at the bottom of esophagus prevents food in stomach from moving back into esophagus

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Stomach Muscular sac that continues mechanical and chemical digestion Three layers of smooth muscle thoroughly churn and mix food to help with digestion

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Chemical digestion in stomach Glands in the stomach lining release lubricants (to protect the stomach wall), hydrochloric acid (HCl), and pepsin (enzyme) Pepsin is activated by HCl to begin protein digestion to break protein into smaller pieces or amino acids Other enzymes are denatured by HCl and stop carbohydrate digestion

Mechanical digestion in the stomach Stomach muscles contract to churn and mix stomach contents This makes chyme After 1-2 hours the pyloric sphincter opens so chyme can pass into the small intestine

Small intestine Chyme enters the first part of the small intestine called the duodenum Digestive enzymes and fluids from the pancreas and liver also enter at the duodenum Most chemical digestion and absorption of nutrients occurs in the small intestine

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Effects of digestive enzymes Salivary amylase (mouth) Pepsin (stomach) Amylase (sm. Int. from pancreas) Trypsin (sm. Int. from pancreas) Lipase (sm. Int. from pancreas) Breaks down starch and disaccharides Breaks down proteins into large peptides Continues breakdown of starch Continues breakdown of protein Breaks down fat

Effects of digestive enzymes Maltase, sucrase, lactase (sm. Int.) Peptidase (sm. Int.) Breaks down remaining disaccharides into single sugars Breaks down dipeptides into amino acids

Accessory structures of digestion Pancreas (behind the stomach) has 3 roles Releases hormones to regulate blood sugar levels Releases enzymes that break down carbohydrates, proteins, lipids, nucleic acids Produces sodium bicarbonate that neutralizes stomach acids so the enzymes can function (without neutralization, enzymes denature and don’t function properly)

Accessory structures cont. Liver-(above and to the left of the stomach) produces bile loaded with lipids and salts Bile dissolves fats in fatty foods Bile is stored in the gallbladder

Absorption in the small intestine Last 2 parts of the small intestine are the jejunum and ileum, together are 6 meters long When chyme enters these it is a mixture of medium and small nutrient molecules (most chemical digestion is competed in the duodenum) Lining of the small intestine is covered with villi (fingerlike projections) each of which is covered with microvilli, both are needed to increase the surface area for absorption

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Absorption in the small int. cont. Cells of small intestine absorb nutrients Products of carbohydrate and protein digestion are absorbed into capillaries in villi Undigested fat and fatty acids are absorbed by lymph vessels When food leaves small intestine and enters the large intestine it is nutrient free and contains only water, cellulose, and undigestible substances

The large intestine Also called the colon Primary function is to remove water from undigestible material Water is absorbed quickly through the wall of the colon Colonies of bacteria produce useable compounds such as vitamin K Concentrated waste that remains passes through the rectum to be excreted