Digestive System ANATOMY AND PHYSIOLOGY

Function  The digestive system is the first organ system to develop in animals.  This system allows animals to take in nutrients that provide it with energy and materials necessary to grow and maintain their bodies.  The digestive system is at it’s core a tube that starts with the mouth and ends with the anus. This tube is called the alimentary canal, also known as the gastrointestinal tract (GI tract).  The GI tract travels through the pharynx, esophagus, stomach, small intestine, and large intestine.  Accessory organs such as the liver, pancreas, gallbladder, and salivary glands, secrete substances that help in the chemical breakdown and absorption of food.

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Six Steps of Digestion  1. Ingestion  2. propulsion  3. mechanical breakdown  4. chemical breakdown  5. absorption  6. defecation Animation

 Ingestion, the first step in the digestion process. The teeth and tongue mechanically break down the food. Increasing it’s surface area. Saliva begins the foods chemical breakdown.  Propulsion, swallowing and peristalsis – muscles in the esophagus contract to push food down. This delivers food to the stomach.  Mechanical breakdown, occurs in the stomach, the churning actions therein further reduce the food to smaller particles.  Chemical breakdown, in the stomach and small intestine requires enzymes secreted by the secondary digestive organs. The food then becomes chemically less complex, and easy to absorb.  Absorption, the small intestine and the large intestine absorbe the chemical nutrients and send it to the bloodstream so that these resources can be sent to the rest of the body.  Defecation, lastly, depleted of nutrients, the waste is removed from the body via the rectum and anus.

The layers of the Alimentary Canal  Mucosa: is the innermost layer, and composed of mucus covered epithelial cells. It lines the inner walls of the alimentary canal.  Submucosa: layer of irregular dense connective tissue, contains blood vessels, lymphatic vessels, and nerves. Lymphatic vessels and glands in the submucosa secrete substances that aid in the digestion and absorption  Muscularis Externa: composed of two layers of smooth muscle this layer propels food though the canal using peristalsis. Nerves run between the two muscle layers.  Serosa: A thin slippery layer that surrounds the alimentary canal and reduces friction between organs in the abdominal cavity.

Layers of the Alimentary Canal  The cavity, empty space inside the alimentary canal is called the lumen.

Organs of the Digestive System  Oral cavity :  Ingestion of food  Mechanical breakdown of food (chewing)  Chemical breakdown of food  Propulsion of food.

Organs of the Digestive System  Nasal Cavity:  Separated from the oral cavity via the palate. The uvula which hags down from the soft palate prevents food from entering the nasal cavity.

Organs of the Digestive System  Teeth and gums:  The gums are known as the gingiva, a soft tissue that covers the necks of the teeth and the upper jaw and mandible.  Children have 20 deciduous teeth  Adults have 32 permanent teeth.

Anatomy of the Tooth

Organs of the Digestive System  Salivary Glands:  Parotid: largest salivary glands, just in front of the ears.  Submandibular: near the lower law  Sublingual: under the tongue  Secrete saliva: Mostly water, contains mucus, antibodies, and several enzymes. The enzymes help break down starches into simpler sugars and begin the breakdown of fats.

Organs of the Digestive System  Pharynx:  The throat  3 Parts: nasopharynx, oropharynx, laryngopharynx  Epiglottis: moves down to prevent food passing into the trachea durring swallowing.  Propels food down into the esophagus  Swallowing x-ray Swallowing x-ray  Esophagus:  Posterior to the trachea and heart  Passes through a hole in the diaphragm  Uses a muscular contraction known as peristalsis to propel food

Major Organs of the Digestive System: Stomach  Matching Matching

 Stomach  4 major regions: cardia, fundus, body, and pyloric region  The stomach twists and contracts to churn food and break it down both physically and chemically before passing through the pyloric sphincter into the duodenum of the small intestine.  3 muscle layers  Longitudinal muscle layer (up and down) – top layer  Circular muscle layer (around the stomach) – middle layer  Oblique muscle layer (diagonally) – Inner layer  The rugae (folds) can flatten out increasing the stomach’s volume.  A full stomach can hold about 2 liters or more.  the stomach churning food (ramen) the stomach churning food (ramen)  Artificial stomach Artificial stomach  The mixture of gastric juice and food is known as chyme

Stomach lining  Stomach lining  Epithelia cells at the surface, covered in mucus ( mucus prevents our digestive juices from eating into our own tissues)  Gastric juice is secreted from gastric pits – controlled by the parasympathetic nervous system  The cells surround gastric pits that lead down into the gastric glands.  Mucus-secreting cells  Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor (helps the body absorb B 12 )  Chief cells secrete pepsinogen – activates protein-digesting enzymes  Enteroendocrine cells – produce gastrin (hormone) that stimulates the production of more gastric juice.

Small Intestine  Named for it’s diameter not its length.  Longest segment of the GI tract at 6-7 meters  Most chemical breakdown of food occurs in the small intestine  Parts of the small intestine  Duadenum – secretions from the liver, gallbladder, and pancreas enter the duadenum from the duadenal ampulla  Jejunum (8 feet)  Ileum (12-13 feet)  Chemical digestion, absorption, and propulsion via peristalsis occurs in all 3 regions.

Lining of the Small Intestine  Circular folds, villi, and microvilli all greatly increase the surface area of the lining of the small intestine.  Intestinal crypts are similar to gastric pits.

Chemical Breakdown in the Small Intestine.  When chyme enters the duodenum, secretions from the pancreas and bile from the gallbladder are secreted, to help break down the food.  Bile is important in the emulsification of fats.  The pancrease secretes many chemicals  Bicarbanate: to neutralize the acidic chyme  Pancreatic amylase: breaks down starches into disaccharides (2-sugar molecules)  Pancreatic lipase: chemically breaks down lipids and fats. The emulsification caused by the bile salts increases the surface area of the lipid, allowing more of the pancreatic enzyme to work on the lipids at one time.  Pancreatic proteases: Breaks down protein, stays inactive until it enters the small intestine. This protects the pancreas from being digested by it’s own enzymes.

Absorbtion  Epithelial cells in the villi freely absorb monosaccharaides and amino acids and transports them into the blood vessels in the villus. This nutrient rich blood is collected and sent to the liver.  Free fatty acids also enter the epithelial cells and get repackaged for transport.  B12 binds to intrinsic factor which will bind to receptors in the ileum. This will allow the celss to absorb the large molecule.  Vitamins A, D, E, and K are absorbed with fats.

Major Organs of Digestion: Large Intestine  Large diameter, shorter length.  Mostly propels waste and absorbs water.  Contains colonies of bacteria that live with us symbiotically. They help us with the synthesis of some B vitamins and vitamin K.  The cecum: where the ileum empties into the large intestine. The ileocecal valve opens in response to gastrin produced by the stomach.  The appendix protects the body from infectious organisms.