Digestion: Alimentary Canal and Pharynx

Overview of Digestion

• The digestive system contains two primary organ groups: the alimentary canal and accessory digestive organs.
• The alimentary canal facilitates nutrition, featuring the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory digestive organs include teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

Pharynx and Oral Cavity

• The oral cavity begins the digestive system and is framed by the lips and cheeks.
• The hard palate is superior to the oral cavity and consists of the maxillary and palatine bones.
• The soft palate is posterior to the hard palate, composed of muscular tissue for movement during swallowing.
• The uvula is a conical formation hanging from the soft palate.

Areas of the Pharynx

• The oropharynx extends from the soft palate to the epiglottis and contains palatine and lingual tonsils.
• Tonsils present in the oropharynx eliminate pathogens entering and contain lymphocytes.
• The nasopharynx is positioned superior and posterior to the soft palate.
• The nasopharynx contains pharyngeal and tubal tonsils.
• The laryngopharynx is inferior to the epiglottis and posterior to the larynx.
• The laryngopharynx division links to the esophagus and larynx.

Mouth and Associated Organs

• Food enters the GI tract through the mouth, initiating chewing, tongue manipulation, and saliva moistening.
• The tongue contains skeletal muscle which helps combine food with saliva into a bolus.
• The lingual frenulum secures the tongue to the mouth floor, limiting its posterior movement.

Salivary Glands

• The digastric muscle is responsible for opening the jaw.
• The masseter muscle is responsible for closing the jaw and inserts onto the mandible.
• The parotid gland, located superficially to the masseter and anterior to the ear, makes salivary amylase (ptyalin).
• Salivary amylase (ptyalin), a digestive enzyme.
• The parotid gland is the largest of the salivary glands.
• The submandibular gland (submaxillary) is anterior to the parotid gland.
• The submandibular duct empties this gland and runs on the lateral side of the digastric muscle.
• This gland transports saliva towards the lower jaw angle.
• The sublingual gland is found on the submaxillary duct.

Teeth

• Teeth resemble bone but consist of hydroxyapatite Ca5(PO4)3(OH), bone collagen, and cells.
• Normal oral cavity pH is roughly 7.2.
• Acids in the mouth are from stomach acid during vomiting, foods, or bacteria waste.

Tooth Anatomy

• Enamel is the hardest substance found in the body.
• The pulp cavity includes arteries, veins, and nerves.
• The alveolus is composed of alveolar bone.
• The root is composed of dentin.
• Gingiva is the gum.
• The periodontal membrane consists of periosteum surrounding the tooth.
• Cementum secures the tooth within the alveolus.

Types of Teeth

• Incisors are chisel-shaped for nipping food.
• Canines are cone-shaped for tearing food
• Premolars are for tearing food
• Molars grind food
• Permanent Dentition is composed of 32 teeth
• Deciduous Dentition is composed of 20 teeth.

How Digestion Occurs

• Digestion occurs through ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
• Peristalsis involves alternating muscular contraction and relaxation in digestive organs.
• Peristalsis helps squeeze food through the organs.
• Mechanical digestion is the physical preparation of food for digestion and segmentation which involves in mixing food with digestive juices in the intestines.
• Chemical digestion is where carbohydrates, fats, and proteins are broken down by enzymes.
• Absorption transports digested food components into the blood from the digestive canal.
• Defecation is the removal or elimination of waste.

Swallowing (Reflex)

• When the mouth has closed, the soft palate shifts upward, sealing off nasal passages.
• The sphincter valve closes the eustachian tubes
• The glottis is closed, stopping respiration and then bends to seal the entrance to the larynx.
• The esophagus then opens as pressure from the food bolus increases, allowing the epiglottis to open.
• Food then goes into the esophagus.

Chewing (Mastication)

• Chewing supports digestion by blending food with salivary digestive enzymes, boosting food surface area and facilitating food movement.

Saliva

• Saliva includes ptyalin or salivary amylase.
• Ptyalin or salivary amylase are enzymes breaking down starches and works only in specific pH levels.
• The pH of the mouth is roughly 7.2, being slightly alkaline above the pH scale midpoint 7.

Food Entering the Stomach

• When food enters the stomach, the pH lowers to 3, deactivating ptyalin.
• Once swallowed, smooth muscle in the esophagus uses peristalsis to move the bolus.
• The bolus advances into the stomach via the gastroesophageal sphincter, which stays closed through muscular contraction.

Stomach

• The lesser curvature is anchored to the liver by the lesser omentum, limiting movement.
• The esophagus and duodenal ends are anchored allowing the stomach to sag on the left side once it gets filled.
• Stomach walls have three muscle layers.
• Muscles of the stomach contract asynchronously, mixing and churning food.
• Mixing in the stomach involves water, hydrochloric acid (from the stomach), and pepsin, forming chyme.
• The pyloric sphincter controls the release of chyme.
• Longitudinal folds within the stomach lumen known as rugae increasing surface area.

Small Intestine

• The small intestine has a duodenum, jejunum, and ileum.
• The duodenum is 10 in long.
• The jejunum is 8 ft long.
• The ileum is 11 ft long.
• The small intestine produces 7 enzymes and releases a total of 17 into the duodenum for digestion.
• The small intestine is the primary site for digestion.
• About 74% of nutrient absorption occurs in the small intestine.
• Arteries, capillaries, and veins are within the plicae circularis.
• Veins channel into the hepatic portal system, which drains into the liver and inferior vena cava.
• Lymphatic drainage passes from lacteals to the cisterna chyli then into the thoracic duct.
• Segmental peristalsis takes place in the small intestine, producing a sausage-like pattern.
• The ileocecal valve (sphincter) controls flow into the large intestine and stops backflow into the small intestine.

Liver Functions

• Liver functions include blood detoxification and carbohydrate and lipid metabolism.
• Carbohydrate metabolism – glycogenesis forms glycogen from excess glucose in the circulation.
• Carbohydrate metabolism – glycogenolysis breaks down glycogen during fasting.
• Carbohydrate metabolism – gluconeogenesis forms glucose in hepatocytes from sources that are not carbohydrate.
• Lipid metabolism – synthesizes large quantities of cholesterol and phospholipids.
• Lipid metabolism – oxidizing triglycerides to produce energy.
• Liver functions include protein synthesis and bile secretion.

Bile

• Bile components are bile salts, water, pigments, cholesterol, and lecithin (a phospholipid).
• Bile salts emulsify fats, turning them into soluble droplets for better digestion.
• Bile is stored and concentrated in the gallbladder.
• The gallbladder contracts when fat is identified in the duodenum and bile is released into this area.
• The common bile duct enters the first part of the duodenum and its opening, the ampulla of Vater.
• The sphincter of Oddi manages this starting and relaxes as the gallbladder contracts.

Pancreas

• Roughly 10 enzymes made by the pancreas aid in digestion.
• The pancreatic duct transports enzymes into the common bile duct, and it occasionally pours them straight into the duodenum.
• The pancreas also secretes bicarbonate to neutralize duodenal components.
• The islets of Langerhans produce insulin and glucagon.

The Large Intestine

• The large intestine measures 5 feet in length.
• The cecum extends caudally from the sphincter as a 2.5-inch closed sac.
• The appendix extends from the cecum.
• Currently there is no known purpose for the appendix. Research is suggesting it has immune function.
• Ascending colon is 5 inches long.
• The ascending color is retroperitoneal.
• The right colic flexure joins the transverse colon, which measures 15 inches.
• The left colic flexure interfaces with the descending colon, spanning 10 inches, and is retroperitoneal.
• Descending colon leads into the sigmoid colon where the name changes at the sigmoid flexure.
• Rectum spans 5 inches and leads to the anal canal, spanning7 inches.
• The final sphincter in the tract is the sphincter ani.
• The sigmoid and rectum are also retroperitoneal.
• A piece of the mesocolic ligament supports the transverse colon, positioning it in the umbilical region.
• The colon's muscle bands only partially surround the structure, contrasting with the rectum, which has entire muscle coverage.
• Muscles function similarly to a drawstring, contracting the colon into small pockets. The pouches are called haustra.
• Bacteria that inhabit the colon contribute to synthesis of vitamins B12 and K and destroy harmful bacteria.
• E. coli is an example of such bacteria.
• The sphincter ani acts as an involuntary smooth muscle.
• The defecation reflex is maintained by the sigmoid flexure and peristaltic activity.
• Peristalsis causes the sphincter ani to relax, but this is countered by the external sphincter that's a skeletal muscle.
• This allows the user to hold “hold it in” until you reach a bathroom.