Digestive System

The digestive system is referred to as the – Digestive system (or tract) – Alimentary system – GI system (or tract) – The digestive system is basically a long, muscular tube that begins at the mouth and ends at the anus Functions of the digestive system – Intake and digestion of food and water – Absorption of nutrients – Elimination of solid wastes

Structures of Digestive System Mouth or oral cavity Pharynx Esophagus Stomach Small intestines Large intestines Accessory organs of digestion

Structures of the Mouth Mouth or oral cavity – Contains the Lips Cheeks Palates (hard and soft) Salivary glands Tongue Teeth Boundaries of the mouth are the maxilla and mandible (jaw) – Prognathia means having an elongated mandible (overshot) – Brachygnathia means having a shortened mandible (undershot)

The Throat The pharynx is the cavity in the caudal oral cavity that joins the respiratory and gastrointestinal systems – Also known as the throat

The Esophagus The esophagus is a collapsible, muscular tube that leads from the oral cavity to the stomach – Also known as the gullet – Enters the stomach through an opening that is surrounded by a sphincter Sphincter is a ring-like muscle that constricts an opening

The Abdomen The remaining digestive organs are found in the abdomen – Also known as the peritoneal or abdominal cavity – Located between the diaphragm and pelvis

Abdominal Structures The peritoneum is the membrane lining that covers the abdominal and pelvic cavity and some of the organs in this area – The layer that lines the abdominal and pelvic cavities is called the paxetal peritoneum – The layer that covers the abdominal organs is called the visceral peritoneum

The Stomach The stomach is a saclike organ that aides in digestion of food – Combining form is gastr/o – Animals can be classified as monogastric or ruminant Monogastric animals have one true, glandular stomach (one that produces secretions) Ruminants have one true, glandular stomach plus three forestomachs

Stomach Parts Parts of the stomach include – cardia (entrance near esophagus) – fundus (cranial, rounded part) – body (main part) – antrum (caudal part) – pylorus (narrow passage between the stomach and duodenum) – pyloric sphincter (muscle ring that controls flow of material from the stomach to the small intestine) – rugae (folds in the mucosa)

Small Intestine Small intestine extends from the pylorus to the large intestine It is held in place by the mesentery Enter/o means small intestine Three segments of the small intestine are – duodenum: proximal part duoden/i or duoden/o – jejunum: middle part jenun/o – ileum: distal part ile/o

Large Intestine Large intestine extends from the ileum to the anus Four segments of the large intestine are – cecum: pouch that takes food from the ileum – colon: varies among species – rectum: caudal portion – anus: caudal opening

Accessory Organs Liver: located caudal to the diaphragm Gallbladder Pancreas Salivary glands

Digestion Digestion is the process of breaking down foods into nutrients that the body can use Metabolism is the processes involved in the body’s use of nutrients – Meta- means change or beyond – Anabolism is building up of body cells – Catabolism is breakdown of body cells

Absorption Absorption is the process of taking digested nutrients into the circulatory system – also called assimilation Absorption occurs in the small intestine – Villi are tiny hairlike projections that help increase the surface area of the small intestine allowing more nutrients to be absorbed Vill/i means tuft of hair – The valleys that result from the projections of the small intestine are called crypts

Path of Digestion Food is grasped and collected into the oral cavity  This is called prehension Mastication (chewing) breaks food into smaller pieces Deglutition moves chewed food into the pharynx and on into the esophagus  The epiglottis closes off the entrance to the trachea Food moves down the esophagus by gravity and peristalsis  Peristalsis is a series of wavelike contractions of smooth muscle  -stalsis means contraction

Peristalsis vs. Segmentation Food moves through the small intestines by peristalsis and segmentation Peristalsis is a series of wavelike contractions that move ingesta caudally toward the anus Segmentation involves the side-to- side mixing of ingesta

Physiology of Digestion Introduction – Normal metabolism requires animal cells to acquire three major classes of nutrients in their simplest forms Carbohydrates – monosaccharides Proteins – amino acids Lipids – fatty acids – Delivered via blood

Function of the GIT Reduce consumed foodstuffs to simpler molecules Transfer to blood – For delivery to cells for metabolism Physical digestion – mechanical breakdown of food Chemical digestion – chemical breakdown of food Indigestible substances or objects can pass through the GIT w/o being altered or harming animal

In the Walls Smooth muscle – Provides force to move digesta through the GIT Gastrointestinal motility – Regulated by Autonomic nervous system (ANS) Gastrointestinal hormones Enteric nervous system Presence of food and distension of GIT segments act as stimuli to initiate activity of enteric nervous system

Regulation The 3 regulatory systems also regulate – Secretions from glands in walls of GIT – Intestinal accessory organs Liver Salivary glands Pancreas – All 3 mechanisms may regulate a given segment or accessory organ Relative importance of each varies among segment of GIT & accessory organ Salivary secretion – regulated almost primarily by ANS Initiation of bile secretion - gastrointestinal hormones

Pre-Gastric Physiology Prehension & chewing – Prehension Act of brining food into the mouth Teeth, lips, and tongue are used by domestic animals Lips – horse Tongue – cow and sheep Snout – pig

Teeth and Chewing Type of teeth, arrangement of jaw, and chewing habits vary among species and food – Herbivores Upper jaw is wider than lower Chewing is thorough Mastication can be controlled voluntarily Presence of food will stimulate reflex chewing

Saliva and Salivary Glands Saliva consists of – Water – Electrolytes – Mucus – Enzymes Water & mucus – Soften and lubricate the ingesta – Helps mastication and swallowing Lysozyme – Salivary enzyme – Antimicrobial Amylase – Starch-digesting enzyme – Present in saliva of omnivores (pigs) and horses (limited) – Absent in ruminants and carnivores

Salivary Stimulation Sight and smell of food Presence of food in oral cavity Conditioned reflexes – Events associated with food/feeding – Pavlov’s Dog

Deglutination(Swallowing) Divided into 3 stages – Passage of food/water through mouth Voluntary control Food/saliva forms bolus Moved to upper surface of tongue Tongue raised against hard palate, tip first – Pushes bolus towards pharynx Same time, soft palate is raised, closing caudal nares Base of tongue acts as plunger, forcing bolus into pharynx

Deglutination… – Passage through pharynx Bolus enters pharynx – Stimulates receptors in walls – Reflexively initiates second stage Respiration is reflexively inhibited Larynx reflexively closes and pulls up and forward Base of tongue folds epiglottis over the laryngeal opening as it moves back Pharynx shortens and peristaltic action of pharyngeal muscles forces bolus into esophagus

Deglutination… – Passage through esophagus into stomach Reflex peristalsis of the esophagus Initiated by presence of food in the esophagus Peristalsis carries solid and semi-solid food through the esophagus of the horse at cm/second Liquids travel 5 times faster via squirting action of the mouth and pharynx

Small Intestine Physiology Primary site of chemical digestion and absorption of nutrients Most enzymes responsible are secreted from pancreas Small Intestine epithelial cells also have some enzymes that help in final stages of chemical digestion

Small Intestine Motility 2 basic movement types – Segmentation – Peristalsis Mix the digesta with intestinal juice and digestive enzymes Increase contact between digesta and epithelial surface of SI – Provides more exposure to enzymes and absorption surfaces

Liver Function Primarily to produce bile – Greenish-yellow salt solution – Consists mostly of Bile salt Cholesterol Phospholipids Bile pigments – Assist in digestion and absorption of lipids – Passes directly from liver to duodenum via bile duct

Physiology of Cecum and Colon Omnivores & some herbivores – Site of limited fermentation – Microbial digestion

Cecum and Colon Extremely large and complex Primary sites of fermentation and microbial digestion of cellulose – Roughage passes quick through the stomach and SI – Passage through the cecum/colon can take days – Fermentation occurs similar to rumen Junction between small colon and great colon are small, so potential sites of impaction

Rectum and Defecation Distention of rectum stimulates need to defecate Defecation requires  Contraction of smooth muscles in walls of rectum  Results from a spinal reflex stimulated by distended rectum  Contraction of abdominal muscles helps also

Stomach Small Intestine Cecum Colon Small Colon