Digestion, Absorption and Transport

Objectives for Chapter 3 List the organs and accessory organs of the digestive tract. Discuss the main roles of each of the organs of the digestive tract. Identify how other body systems assist the digestive tract. Name a digestive disorder that can occur for each of the following: mouth, esophagus, stomach, and intestines.

What Is Digestion? Digestion: a multi-step process of breaking down foods into absorbable components using mechanical and chemical means in the gastrointestinal (GI) tract Gastrointestinal tract consists of: –Mouth –Esophagus –Stomach –Small and large intestines –Other organs

What is Digestion? Anatomy of the GI –mouth  esophagus  stomach  small intestine  large intestine GI tract is about 23 feet long with extensive surface area for nutrient absorption. –Cells lining GI tract are replaced every 3–5 days.

What Is Digestion? Main roles of the GI tract are to: –Break down food into smallest components –Absorb nutrients –Prevent microorganism or other harmful compounds in food from entering tissues of the body GI tract is highly efficient: 92-97% of nutrients from food are digested and absorbed.

Digestion Is Mechanical and Chemical Mechanical digestion: chewing, grinding food to aid swallowing –Peristalsis: the forward, rhythmic muscular contraction that moves food through GI tract Chemical digestion: digestive juices and enzymes break down food into absorbable nutrients

Digestion The mouth –Saliva released: contains water, electrolytes, mucus and a few enzymes Softens, lubricates, dissolves food particles –Bolus (food mass) moves into pharynx, is swallowed, and enters the esophagus. –Epiglottis closes off trachea during swallowing to prevent food from lodging in the windpipe. –involves mastication (chewing),

The epiglottis prevents food from entering the trachea when you swallow.

Digestion The esophagus –Esophagus leads food into the stomach –Bolus is pushed down esophagus by peristalsis. Lower Esophageal Sphincter (LES): at bottom of esophagus relaxes and allows food into stomach LES then closes to prevent backflow of hydrochloric acid (HCl) from stomach –“Reflux” of stomach acid causes “heartburn” (irritation of esophagus lining).

Peristalsis: Muscles around organs of GI tract constrict in wavelike manner to move food along.

Digestion The stomach –Stomach stores, mixes and prepares food for digestion –grinds the bolus to a semi-liquid mass called chyme. Stomach produces powerful digestive secretions: –HCl: activates enzyme pepsin, enhances absorption of minerals, breaks down connective tissue of meat Makes the stomach acidic with a pH of 1.5 – 1.7 Mucus protects stomach lining from damage –Digestive enzymes, intrinsic factor (for vitamin B 12 absorption), stomach hormone gastrin

Digestion The Stomach (cont’d) The pyloric sphincter regulates the flow of partially digested food into the small intestine. Allows about 1 tsp of chyme to enter the small intestine every 30 seconds Gastrin: stimulates digestive activities and secretion of HCl, increases gastric motility and emptying Liquids, carbohydrates, low-fiber and low-calorie foods exit stomach faster High fiber, fat and protein foods exit slower, keep you feeling full longer

Digestion The small intestine –Most digestion and absorption occur in the small intestine –receives digestive juices from the gallbladder and the pancreas. –long, narrow, coiled, has three segments: Duodenum (10 inches) Jejunum (8 feet) Ileum ((12 feet) –Interior surface area tremendously increased by villi, microvilli, circular folds

Surface Area in the Small Intestine

Digestion –Peristalsis moves chyme through intestine –Segmentation mixes chyme with chemical secretions –Pendular movement enhances nutrient absorption Chyme moves at rate of 1 cm/minute Total contact time 3–10 hours, depending on amount and type of food

Digestion The large intestine –Large intestine absorbs water and some nutrients –Ileocecal sphincter: prevents backflow of fecal matter into ileum Most of nutrients in chyme have been absorbed when it reaches large intestine Large intestine has three sections: cecum, colon, rectum –About 5 feet long, 2.5 inches in diameter –Absorbs water and electrolytes –No digestive enzymes; chemical digestion done by bacteria

Digestion & Absorption Intestinal matter passes through colon in hours depending on age, health, diet, fiber intake –Bacteria in colon produce vitamin K and biotin and break down fiber and undigested carbohydrates Stool stored in rectum

Secretions of Digestion Enzymes – protein molecules which facilitates a chemical reaction Saliva – made by salivary glands Gastric Juice – made by the stomach, very acidic, HCL Pancreatic Juice/intestinal Enzymes – Contains bicarbonate to neutralize acidic gastric juices as it enters the small intestine Bile – secreted by the liver, stored in the gall bladder, emulsify fat. Only acts on fat

Accessory Organs Liver: largest gland in body Produces bile needed for fat digestion Metabolism of carbohydrates, fats and protein Stores nutrients: vitamins A, D, B 12, E, copper, iron, glycogen (glucose storage form) Detoxifies alcohol Gallbladder: concentrates and stores bile –Released into GI tract when fat is ingested

Accessory Organs Pancreas: –Produces hormones: insulin and glucagon, regulate blood glucose –Sodium bicarbonate neutralizes acidic chyme, protects enzymes from inactivation by acid –Digestive enzymes: Amylase: digests carbohydrate Lipase: digests fats Trypsin, chymotrypsin, and carboxypeptidase: digest protein

Other Body Systems Nervous system stimulates your appetite. –Hormone ghrelin signals brain to eat when stomach is empty. Circulatory system transports nutrients, oxygen, waste products through your blood. Lymphatic system distributes fat and fat- soluble nutrients through your lymph. Excretory system eliminates wastes from circulatory system via the urine.

The Circulatory and Lymphatic Systems

The Excretory System

Common Digestive Disorders Disorders of the mouth and esophagus : Gingivitis and periodontal disease – gum swelling, bleeding and oral pain Swallowing problems: –Choking –Food becomes lodged in the trachea. –The larynx cannot make sounds. –The Heimlich maneuver may need to be used.

Common Digestive Disorders Esophageal problems –Heartburn (acid reflux) may be caused by weak LES (lower esophageal sphincter) Certain foods, smoking, drinking alcohol, being overweight or obese, tight-fitting clothes, reclining after eating

Common Digestive Disorders Stomach Disorders: –Gastroenteritis – cause by bacteria or virus “stomach flu” –Peptic ulcers - a sore in the lining of the stomach (gastric ulcer) Gallbladder Disease: –Gallstones – cause by high cholesterol bile, may lead to obstruction of the bile duct

Common Digestive Disorders Disorders of the Small Intestine: –Nutrient malabsorption disorders Celiac disease: inability to digest gluten protein –Duodenal ulcers - a sore in the lining of the duodenum of the small intestine (duodenal ulcer). –Intestinal enzyme deficiencies –Short bowel syndrome

Common Digestive Disorders Disorders of the Large Intestine: –Constipation may be due to insufficient fiber and water intake, inactivity, stress, illness. –Diarrhea is treated by fluid and electrolyte replacement. –Irritable bowel syndrome (IBS): alternating patterns of diarrhea, constipation, abdominal pain –Colon cancer often begins with polyps. Curable when detected early and treated

Videos Digestion 1 Digestion 2 – Enzymes