Digestive System Chapter 14

The Digestive System – Overview Is a long, hollow pathway called the alimentary canal. Begins at mouth and ends with the anus Consists of primary and periphery organs Food passes through primary organs Periphery organs only play a role, food does not enter these organs.

The Digestive Tract Periphery Organs Fig. 14.1

4 Stages of Digestion Ingestion Digestion taking in of food; breaking down of food into smaller pieces Mechanical digestion Physical change only Breaks food into smaller pieces to increase surface area Chemical digestion Enzymatic breakdown to small organic molecules Enzymes – protein that chemically breaks down food Produced by the body's glands and organs. Results in simple molecules that easily pass into the blood.

4 Stages of Digestion cont’d Absorption transporting of molecules into the blood; Egestion removing of solid, undigested materials from the body.

Digestive Tract – Mouth Overall functions of mouth Begins process of digestion Mechanical digestion Chewing food Mixing with saliva Bolus formation Chemical digestion Begin digestion of starches to disaccharides

Adult mouth Fig. 14.2

Digestive Tract – Mouth cont’d Tongue Taste buds- chemical sensation of taste Rough surface- aids in mechanical digestion Composed of skeletal muscle Roof of mouth Hard palate- composed of bone ridges for mechanical digestion Soft palate- composed of muscle Uvula Closes off nares during swallowing

Digestive Tract – Mouth cont’d Tonsils Lymphoid tissue Help protect against infection 3 Types Palatine- on both sides of pharynx Pharyngeal (Adenoids)- in nasopharynx Lingual - at base of tongue Tonsillitis- inflammation of tonsils

Digestive Tract – Mouth cont’d Salivary glands 3 pairs Collective secretions are called saliva Lubricates food so it can pass through canal Dissolves food particles Allows us to taste food! Contains amylase enzyme for starch digestion

Digestive Tract – Mouth cont’d Teeth Mechanical digestion 20 Deciduous teeth Baby teeth, first 2 years 32 adult teeth Tooth structure Crown Outer enamel covering Dentin Pulp Root pulp Fig. 14.2

Digestive Tract – Mouth cont’d Teeth cont’d 4 Types, each with a purpose Incisors Chisel shaped for cutting an slicing Canines Forms a single point used to hold and tear food. Premolars & Molars Broad flat teeth Crushing and grinding

Digestive Tract – Mouth cont’d Teeth cont’d Dental caries Cavities in teeth Prevented by flouride Brushing and flossing can help prevent development

Digestive Tract - Pharynx Connects nasal and oral cavities Part of both respiratory and digestive systems Muscular structure, actively pushes food down Swallowing reflex Uvula closes off nares Trachea moves upward under epiglottis Epiglottis closes airway off Bolus of food moves down into esophagus Esophagus

Digestive Tract – Esophagus Passes from pharynx to stomach Peristalsis Rhythmic wave of contraction throughout tract Propels bolus down esophagus to stomach Sole purpose is conduction of food

Digestive Tract – Esophagus cont’d Sphinctors Muscles that encircle tubes Act like valves Contraction-closes tube Relaxation- opens tube Esophageal sphinctor At entrance to stomach Not well developed Relaxes with peristaltic wave Bolus pushed through into stomach

Digestive Tract – Esophagus cont’d Heartburn Gastroesophageal reflux Acidic contents enter esophagus Causes irritation Vomiting Abdominal muscles contract Diaphragm contracts Positive pressure pushes stomach contents upward through esophagus

Walls of Digestive Tract Mucosa Epithelium supported by connective tissue Lines lumen Glandular epithelial cells produce enzymes Goblet cells produce mucous Submucosa Loose connective tissue Contains blood vessels Lymph nodes- Peyer’s patches

Walls of Digestive Tract cont’d Muscularis- 2 layers of smooth muscle Longitudinal –outer, runs along length of gut Circular-inner, encircles tube

Walls of Digestive Tract cont’d Serosa Squamous epithelium Supported by connective tissue Secretes serous fluid Lacking in esophagus

Wall of digestive tract Fig. 14.4

The Stomach Thick-walled J-shaped organ Lies on left side of abdomen Overall function of the stomach Mechanical digestion Mixing of food with gastric juice Forms semi-liquid called chyme Chemical digestion Initiation of protein digestion Storage of food

The Stomach cont’d Rugae: folds in wall 3 muscle layers Aid in mechanical digestion-friction Allows expansion 3 muscle layers Longitudinal Oblique Circular

The stomach cont’d. Gastric glands Mucous cells Produce gastric juice Chief cells Pepsinogen Inactive form of proteolytic enzyme Parietal cells HCl Activates pepsinogen to pepsin Decreases bacterial growth Mucous cells Produce thick protective mucus layer

The Stomach cont’d. Fig. 14.5

The Stomach cont’d. Ulcers Open sore in stomach wall Helicobacter pyloris Infection decreases mucus production

The Small Intestine Duodenum First 25 cm Principal site of digestion of nutrients Receives bile from the liver Emulsification of fats Receives pancreatic juice from pancreas Many enzymes for digestion of nutrients Bicarbonate to neutralize pH

The Small Intestine cont’d. Jejunum and ileum Principal site of absorption of nutrients Lining has villi Increases surface area Outer layer of columnar epithelium Cells have microvilli- more surface area Villi contain lymph lacteals Absorb fatty acids and glycerol Villi contain blood capillaries Absorb sugars and amino acids

Anatomy of the small intestine Fig. 14.6

Regulation of Digestive Secretions Neurological control Pressure and distension Presence of particular types of food Hormonal control Gastrin Produced by stomach Stimulates gastric secretion Gastric inhibitory peptide (GIP) Produced by duodenum Inhibits gastric secretion

Regulation of Digestive Secretions cont’d Secretin and cholecystokinin (CCK) Produced by duodenum Acid stimulates secretin release Digested protein and fat stimulate CCK Effects of both hormones Increased pancreatic secretion Increased bile secretion

Regulation of Digestive Secretions cont’d Hormonal control of digestive gland secretions Fig. 14.7

The Large Intestine Cecum Colon Blind end of the large intestine, at juncture of s.i. Appendix Projection of cecum May play a role in fighting infections Rupture may cause peritonitis Colon Ascending, transverse, and descending portions Absorption of water, salts Terminates at the rectum

The Large Intestine cont’d. Fig. 14.8

The Large Intestine cont’d. Defecation reflex Feces forced into rectum by peristalsis Stretching of walls initiates reflex Rectal muscles contract Anal sphinctors relax Defecation occurs

The Large Intestine cont’d. Fecal composition 75% water Indigestible materials Intestinal flora (bacteria) 99% facultative anaerobes Produce vitamin K Color from breakdown of bilirubin and oxidized iron

Defecation reflex cont’d Fig. 14.9

Accessory organs The pancreas Endocrine function Exocrine function Produces Insulin and glucagon Regulate blood glucose Exocrine function Produces sodium bicarbonate: neutralizes chyme pancreatic amylase: chemical starch digestion Trypsin: chemical protein digestion Lipase: chemical fat digestion

Accessory organs cont’d. The liver Largest gland in the body Lobules Structural and functional units Triads between lobules bile duct branch of hepatic artery branch of hepatic portal vein Functions Removes and stores iron and vit. A, D, E, K, and B12 Makes plasma proteins Regulates cholesterol

Accessory organs cont’d The Liver cont’d … Functions cont’d Absorbs or chemically modifies toxic substances Alcohol and drugs metabolized into less harmful compounds. Other toxins, pesticides, and carcinogens are also detoxified. Less harmful compounds filtered from the blood in the kidneys. If the liver is chronically exposed to toxins, the cells become damaged and die. (cirrhosis)

Accessory organs cont’d The Liver cont’d … Functions cont’d Bile production Stored in gall bladder Composition Bilirubin – from breakdown of hemoglobin, greenish color Bile salts - derived from cholesterol, emulsifies fats in duodenum

Accessory organs cont’d The Liver The gall bladder, a separate organ on lower surface of liver Stores bile Secreted through common bile duct to duodenum Bile salts contain cholesterol can precipitate out of solution and form crystals Crystals can become gall stones, If large enough, they can block common bile duct and cause obstructive jaundice

Accessory organs cont’d The liver cont’d…Functions cont’d Glucose regulation Our body needs constant, stable levels of glucose Brain cells store little glucose, and cannot use other sources 0.1% blood glucose Intestines cannot regulate the rate of nutrient absorption Different foods digest at different rates Simple carbohydrates: digest quickly, causes high blood glucose levels after we eat Complex carbohydrates, proteins: digest more slowly, glucose absorbed into blood gradually.

Accessory organs cont’d The liver cont’d…Functions cont’d… Glucose Regulation cont’d High glucose levels in blood triggers the pancreas to release insulin Insulin in blood signals liver to convert excess glucose into glycogen. Low glucose levels trigger the pancreas to release glucagon Glucagon signals liver to convert glycogen back into useable glucose When glycogen is depleted, the liver converts lipids (fats) and amino acids (proteins) to glucose Requires deamination, break down, of amino acids then liver combines ammonia with carbon dioxide to form urea

Accessory organs cont’d The liver cont’d …Functions cont’d … Glucose Regulation cont’d Diabetes Low insulin levels, so no trigger to liver to convert glucose to glycogen. Insulin injections help to maintain balanced sugar levels. Low Carb Diets Insufficient ‘fast-burn’ foods consumed, body never has excess glucose to store as glycogen Puts body into starvation state, begins to digest its own fats and proteins. The liver cont’d…Functions cont’d… Glucose Regulation cont’d

Accessory Organs cont’d The liver cont’d…Functions cont’d… Glucose Regulation cont’d Both result in insufficient glycogen supplies Causes body to use fats as fuel, this produces small molecules called ketones Ketone build-up results in ketoacidosis Side effects include: weakness, nausea, headache, confusion, dehydration and acetone-like smelling breath. 

Hepatic portal system Fig. 14.11