The Digestive System

Digestion Digestion the mechanical and chemical breakdown of foods into nutrients that cell membranes can absorb 2 Components of the digestive system: Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, large intestine, anal canal Accessory organs – secrete products into the canal; salivary glands, liver, pancreas, gallbladder

Alimentary Canal Around 9 meters (30 feet) long Muscular tube that passes through the ventral cavity

Alimentary Wall Structure Mucosa-innermost layer Submucosa- 2nd layer Muscular layer-3rd Circular fibers Longitudinal fibers Oblique fibers Serosa-Outside layer

Mouth Structure Surrounded by lips, cheeks, tongue, and palate Oral cavity Vestibule Cheeks Lips

Tongue Frenulum Secures tongue to floor of mouth Papillae Hyoid bone Lingual tonsils

Palate Hard palate Soft palate Uvula Palatine tonsils Pharyngeal tonsils (adenoids) Tonsils are lymphatic tissue used to protect you from infection

Primary and Secondary Teeth Primary teeth Deciduous teeth Erupt between 6 months and 2-4 years 20 teeth Secondary teeth Appear around 6 years 32 teeth All teeth have a coating of enamel (hardest substance in human body)

Salivary Glands Secrete saliva: Moistens food particles Helps bind food particles Amylase (enzyme) begins chemical digestion of carbohydrates (used first for energy) Dissolves food for tasting Helps cleanse mouth and teeth Mucus softens food & lubricates G.I tract

Regions of the Pharynx Nasopharynx Oropharynx Laryngopharynx Open to nasal cavity Passage for air during breathing Oropharynx Behind soft palate Passage for air and food Laryngopharynx Passage for food to the esophagus

Swallowing Reflex= deglutition Food is chewed (mastication)and mixed with saliva to form a mass called a bolus. Bolus is forced into the pharynx. Swallowing reflex is stimulated by sensory receptors around the pharyngeal opening. Soft palate rises and seals the nasal cavity off with the uvula to stop food from entering the nasal cavity. Hyoid bone and larynx are elevated, and the epiglottis of the larynx closes off the top of the trachea. Breathing is briefly inhibited.

Swallowing Reflex Tongue presses against the soft palate, sealing the oral cavity off from the pharynx. Longitudinal muscles in the pharyngeal wall contract, moving the pharynx up toward the bolus. Muscles in the lower pharynx relax, and the esophagus opens. Peristalsis moves the bolus through the esophagus.

Swallowing Reflex

Esophagus Straight, collapsible tube Approximately 25 cm long Passageway from pharynx to stomach Cardiac or esophageal sphincter (entry to stomach) Mucous glands for lubrication Disorders: Acid Reflux Ulcers Heart Burn

Stomach Receives food from the esophagus Mixes food with gastric juices Initiates protein digestion Performs limited absorption of water, salts, alcohol, and lipid-soluble drugs Moves food into the small intestine Bolus now turns into chyme (creamy paste) Food enters through esophageal sphincter and exits via pyloric sphincter Exploratory Surgery Stomach Disorders: Ulcers Tortion

Gastric Secretions Gastric pits Gastric glands Gastric juice HCl Goblet cells Chief cells – pepsinogen Parietal cells – HCl Gastric juice HCl Pepsin-digest protein Regulated by ACh, gastrin, and cholecystokinin Mucosal layer of stomach prevents stomach acid from eating through itself and releasing acid into the abdomen Lower pH (Acids) Gastric Secretions

Pancreas Secretes pancreatic juice from acinar cells Mixed gland Pancreatic duct Hepatopancreatic sphincter Helps digest carbohydrates, proteins, fats & nucleic acids

Pancreatic Secretions Pancreatic juice contains several digestive enzymes: Pancreatic amylase Pancreatic lipase Nuclease Trypsin, chymotrypsin, and carboxypeptidase Bicarbonate ions

What does the Pancreas do?

Liver Carbohydrate, lipid, and protein metabolism Storage of glycogen, iron, vitamins A, D, and B12 Blood filtering Detoxification Secretion of bile (emulsifies/breaks down fats)

How does the Liver work?

Gallbladder Pear-shaped sac on the inferior liver surface Connects to the cystic duct which feeds into the common hepatic duct Stores bile between meals Reabsorbs water to concentrate bile Releases bile into the small intestine Common bile duct

Small Intestine Extends from pyloric sphincter to the large intestine Receives secretions from the pancreas and liver Completes digestion of nutrients in chyme and absorbs nutrients of digestion Mixing movements and peristalsis – chyme moves through in 3-10 hours Transports digestive residue to the large intestine

Regions of the Small Intestine Duodenum Portion closest to stomach 25 cm long Most fixed portion of the small intestine Jejunum-middle portion Ileum Final portion of small intestines prior to large intestines

Tiny projections on the inner wall off the small intestine Densest in the duodenum Increase surface area for absorption Microvilli Secrete peptidases, sucrase, maltase, lactase, intestinal lipase Capillaries absorb simple sugars, amino acids, electrolytes (coenzymes that help control metabolism), and water Intestinal Villi

Large Intestine Ileocecal valve 1.5 meters long Proximal end functions primarily in water and electrolyte absorption Distal end functions primarily to store feces Little to no digestive function

Regions of Large Intestine Cecum Vermiform appendix Colon Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anal canal

To Gas or not to Gas?

Anal Canal Structure Anal columns Anus Hemorrhoids Internal anal sphincter External anal sphincter Hemorrhoids

Defecation Reflex Can be initiated by person (deep breath and abdominal contraction) Forces feces into rectum Reflex involves relaxation of the internal anal sphincter and peristaltic waves through the descending colon Can be prevented by contraction of the external anal sphincter

Feces Made of materials not digested or absorbed 75% water Electrolytes Mucus Bacteria 75% water Color from bile pigments altered by bacterial action Odor from compounds produced by bacteria