The human digestive system, Lecture notes of Anatomy

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The Digestive System Functions - Ingestion - taking in food - Digestion - breaking food down both physically and chemically - Absorption - movement of nutrients into the bloodstream - Defecation - rids the body of indigestible waste Organs of the digestive system - Two main groups of organs - Alimentary canal (gastrointestinal or GI tract) — continuous coiled hollow tube - These organs ingest, digest, absorb, defecate - Accessory digestive organs - Includes teeth, tongue, and other large digestive organs Organs of the Alimentary Canal - Mouth - Pharynx - Esophagus - Stomach - Small intestine - Large intestine - Anus Mouth (Oral Cavity) Anatomy - Lips (labia)—protect the anterior opening - Cheeks—form the lateral walls - Hard palate—forms the anterior roof - Soft palate—forms the posterior roof - Uvula—fleshy projection of the soft palate - Vestibule—space between lips externally and teeth and gums internally - Oral cavity proper—area contained by the teeth - Tongue—attached at hyoid bone and styloid processes of the skull, and by the lingual frenulum to the floor of the mouth - Tonsils - Palatine—located at posterior end of oral cavity - Lingual—located at the base of the tongue Mouth Physiology - Mastication (chewing) of food - Mixing masticated food with saliva - Initiation of swallowing by tongue - Allows for sense of taste Pharynx anatomy - Nasopharynx—not part of the digestive system - Oropharynx—posterior to oral cavity - Laryngopharynx—below the oropharynx and connected to the esophagus Pharynx Physiology - Serves as a passageway for air and food - Food is propelled to the esophagus by two muscle layers - Longitudinal inner layer - Circular outer layer - Food movement is by alternating contractions of the muscle layers (peristalsis) Esophagus Anatomy and physiology - Anatomy - About 10 inches long - Runs from pharynx to stomach through the diaphragm - Physiology - Conducts food by peristalsis (slow rhythmic squeezing) - Passageway for food only (respiratory system branches off after the pharynx) Layers of tissue in the Alimentary Canal Organs - Four layers from deep to superficial: - Mucosa - Innermost, moist membrane consisting of - Surface epithelium - Small amount of connective tissue (lamina propria) - Small smooth muscle layer - Lines the cavity (known as the lumen) - Submucosa - Just beneath the mucosa - Soft connective tissue with blood vessels, nerve endings, mucosa- associated lymphoid tissue, and lymphatics - Muscularis externa (smooth muscle) - Inner circular layer - Outer longitudinal layer - Serosa - outermost layer of the wall contains fluid-producing cells - Visceral peritoneum—outermost layer that is continuous with the innermost layer - Parietal peritoneum—innermost layer that lines the abdominopelvic cavity Alimentary Canal Nerve Plexuses - Two important nerve plexuses serve the alimentary canal - Both are part of the autonomic nervous system - Submucosal nerve plexus - Myenteric nerve plexus - Function is to regulate mobility and secretory activity of the GI tract organs - External anal sphincter—formed by skeletal muscle and under voluntary control - Internal involuntary sphincter—formed by smooth muscle - These sphincters are normally closed except during defecation - No villi present - Goblet cells produce alkaline mucus which lubricates the passage of feces - Muscularis externa layer is reduced to three bands of muscle called teniae coli - These bands cause the wall to pucker into haustra (pocketlike sacs) Accessory Organs - Teeth - Salivary glands - Pancreas - Liver - Gallbladder Teeth - Teeth - Function is to masticate (chew) food - Humans have two sets of teeth - Deciduous (baby or “milk”) teeth - A baby has 20 teeth by age two - First teeth to appear are the lower central incisors - Permanent teeth - Replace deciduous teeth between the ages of 6 and 12 - A full set is 32 teeth, but some people do not have wisdom teeth (third molars) - If they do emerge, the wisdom teeth appear between ages of 17 and 25 Classification of teeth - Incisors—cutting - Canines (eyeteeth)—tearing or piercing - Premolars (bicuspids)—grinding - Molars—grinding Regions of a tooth - Crown—exposed part - Enamel—hardest substance in the body - Dentin—found deep to the enamel and forms the bulk of the tooth - Pulp cavity—contains connective tissue, blood vessels, and nerve fibers - Root canal—where the pulp cavity extends into the root - Neck - Region in contact with the gum - Connects crown to root - Root - Cementum—covers outer surface and attaches the tooth to the periodontal membrane Salivary Glands - Three pairs of salivary glands empty secretions into the mouth - Parotid glands - Found anterior to the ears - Submandibular glands - Sublingual glands - Both submandibular and sublingual glands empty saliva into the floor of the mouth through small ducts Saliva - Mixture of mucus and serous fluids - Helps to form a food bolus - Contains salivary amylase to begin starch digestion - Dissolves chemicals so they can be tasted Pancreas - Found posterior to the parietal peritoneum - Its location is retroperitoneal - Extends across the abdomen from spleen to duodenum - Produces a wide spectrum of digestive enzymes that break down all categories of food - Enzymes are secreted into the duodenum - Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from stomach - Hormones produced by the pancreas - Insulin - Glucagon Liver - Largest gland in the body - Located on the right side of the body under the diaphragm - Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament - Connected to the gallbladder via the common hepatic duct Bile - Produced by cells in the liver - Bile leaves the liver through the common hepatic duct - Composition is - Bile salts - Bile pigments (mostly bilirubin from the breakdown of hemoglobin) - Cholesterol - Phospholipids - Electrolytes - Function—emulsify fats by physically breaking large fat globules into smaller ones Gallbladder - Sac found in hollow fossa of liver - When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder - When digestion of fatty food is occurring, bile is introduced into the duodenum from the gallbladder - Gallstones are crystallized cholesterol which can cause blockages Functions of the Digestive system - Ingestion—placing food into the mouth - Propulsion—moving foods from one region of the digestive system to another - Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract - Segmentation—moving materials back and forth to aid with mixing in the small intestine - Food breakdown as mechanical digestion - Examples: - Mixing food in the mouth by the tongue - Churning food in the stomach - Segmentation in the small intestine - Mechanical digestion prepares food for further degradation by enzymes - Food breakdown as chemical digestion - Enzymes break down food molecules into their building blocks - Each major food group uses different enzymes - Carbohydrates are broken to simple sugars - Proteins are broken to amino acids - Fats are broken to fatty acids and alcohols - Absorption - End products of digestion are absorbed in the blood or lymph - Food must enter mucosal cells and then into blood or lymph capillaries - Defecation - Elimination of indigestible substances from the GI tract in the form of feces Control of digestive activity - Mostly controlled by reflexes via the parasympathetic division - Chemical and mechanical receptors are located in organ walls that trigger reflexes - Stimuli include - Stretch of the organ - pH of the contents - Presence of breakdown products - Reflexes include - Activation or inhibition of glandular secretions - Smooth muscle activity Digestive activities of the mouth - Mechanical breakdown - Food is physically broken down by chewing - Chemical digestion - Food is mixed with saliva