Mouth, Digestion, and Salivary Glands

Mouth and Initial Digestion

• Initial digestion starts in the mouth, or buccal cavity.
• Several structures in the mouth contribute to the initial breakdown of food.

Teeth (Mastication)

• Teeth physically break down food into smaller pieces, increasing the surface area for enzymatic action.
• Types of teeth include incisors, canines, premolars, and molars, which are specialized for different functions in mastication
• Tooth structure (enamel, dentin, pulp) is crucial for function and susceptibility to decay.

Tongue

• The tongue manipulates food during chewing.
• The tongue forms the bolus (a mass of chewed food) and initiates swallowing.
• Taste buds on the tongue identify sweet, salty, sour, bitter, and umami tastes.
• Tongue musculature is important for speech articulation.

Salivary Glands

• Three pairs of salivary glands (parotid, sublingual, submandibular) secrete saliva.
• Salvia moistens food, facilitating swallowing, and speech.
• Saliva contains amylase, an enzyme that begins the breakdown of carbohydrates (starch) into simpler sugars (maltose).
• It also contains lysozyme, an antibacterial enzyme.

Palates

• The hard palate is the bony roof of the mouth that separates the oral cavity from the nasal cavity.
• It provides a firm surface for the tongue to work against during mastication.
• Cleft palate is a congenital condition affecting the hard palate.
• The soft palate is the muscular posterior portion of the palate, separating the oral cavity from the nasopharynx, with its fleshy extension, the uvula.
• The soft palate and uvula prevent food from entering the nasal cavity during swallowing.
• The soft palate also has a significant role in speech production.

Esophagus and Peristalsis

• After the creation of a bolus, it is swallowed and enters the pharynx (throat).
• The epiglottis closes over the larynx (voice box) to prevent food from entering the respiratory tract.
• The esophagus transports the bolus from the pharynx to the stomach via peristalsis.
• Peristalsis is a series of wave-like muscle contractions that move food through the alimentary canal.
• Peristalsis is an involuntary process controlled by the autonomic nervous system.
• Coordination of circular and longitudinal muscle layers is essential for efficient peristalsis.
• Dysfunction in peristalsis can lead to conditions like esophageal reflux.

Stomach and Gastric Juices

• The stomach is a J-shaped organ that receives food from the esophagus.
• Rugae are folds in the stomach lining that allow the stomach to expand as it fills with food.
• The cardiac sphincter is a circular muscle at the junction of the esophagus and stomach that prevents food from refluxing back into the esophagus.
• The pyloric sphincter is circular muscle at the junction of the stomach and small intestine.
• It regulates the passage of chyme (partially digested food) into the small intestine.

Gastric Juices

• Gastric juices are secreted by gastric glands in the stomach lining, which contain hydrochloric acid (HCl).
• Hydrochloric acid (HCl) kills bacteria, activates pepsin, and facilitates iron absorption.
• Pepsin is an enzyme that begins the digestion of proteins.
• Lipase is an enzyme that begins the digestion of fats (in smaller amounts than in the pancreas).
• Rennin (in infants) aids in the digestion of milk proteins (absent in adults).

Intestines and Absorption- Small

• The small intestine is the primary site of nutrient absorption.
• It's a long, narrow tube divided into three sections: Duodenum, Jejunum, and Ileum.
• The duodenum is the first section, where chyme mixes with bile and pancreatic juices.
• The jejunum is the middle section, where most nutrient absorption occurs.
• The ileum is the final section, where the absorption of vitamin B12 and bile salts takes place.

Villi

• Villi are finger-like projections lining the small intestine's inner surface.
• They greatly increase the surface area for absorption.
• Each villus contains blood capillaries and a lacteal (lymphatic vessel).
• Digested nutrients are absorbed through the villi.
• Capillaries absorb monosaccharides (simple sugars), amino acids, and water-soluble vitamins.
• Lacteals absorb fatty acids and glycerol (products of fat digestion).
• Mechanisms of nutrient transport across the intestinal epithelium are highly specific and energy-dependent.

Digestive Enzymes

• Maltase, sucrase, and lactase break down disaccharides (double sugars) into monosaccharides.
• Peptidase completes protein digestion.
• Lipase (steapsin) aids in fat digestion.
• Pancreatic amylase (amylopsin) acts on sugars.
• Trypsin and chymotrypsin act on proteins.

Large Intestine and Waste Elimination

• The large intestine is the final section of the alimentary canal.
• It has three primary functions: water absorption, waste storage, and vitamin synthesis.
• Waste storage occurs by storing feces (indigestible material) before elimination.
• Bacteria in the large intestine synthesize some vitamins (vitamin K and some B vitamins).
• Sections of the large intestine include the cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anus.
• The gut microbiome plays a crucial role in large intestinal function.

Accessory Organs- Liver

• Liver is the largest gland in the body.
• It has six primary functions: bile production, glycogen storage, nutrient storage, heparin production, blood protein production, cholesterol production, and detoxification.
• Bile emulsifies fats and makes them easier to digest while glycogen stores glycogen (a form of glucose).
• Nutrient Storage stores iron and some vitamins.
• The liver produces heparin (an anticoagulant) and blood proteins involved in clotting.
• The liver produces cholesterol and detoxifies harmful substances (alcohol, drugs, etc.).
• The liver's metabolic functions are incredibly diverse and essential for homeostasis.

Gallbladder

• Gallbladder stores and concentrates bile produced by the liver.
• It releases bile into the duodenum when needed and gallstones can form in the gallbladder, causing pain and inflammation.

Pancreas

• The pancreas produces pancreatic juices, containing enzymes for digestion (amylase, lipase, protease).
• It also produces insulin and glucagon, hormones that regulate blood glucose levels.
• Pancreatic insufficiency can lead to malabsorption of nutrients.

Key terms

• Mastication: The process of chewing and grinding food.
• Bolus: A mass of chewed food.
• Amylase: An enzyme that breaks down carbohydrates.
• Peristalsis: Wave-like muscle contractions that move food through the alimentary canal.
• Chyme: Partially digested food in the stomach.
• Hydrochloric Acid (HCl): Kills bacteria and activates pepsin.
• Pepsin: An enzyme that breaks down proteins.
• Villi: Finger-like projections in the small intestine that increase surface area for absorption.
• Lacteals: Lymphatic vessels in the villi that absorb fats.
• Bile: Emulsifies fats, aiding in their digestion.
• Pancreatic Juice: Contains enzymes for carbohydrate, protein, and fat digestion.
• Insulin: Hormone that regulates blood glucose levels.
• Glycogen: Storage form of glucose in the liver.
• Large Intestine Functions: Water absorption, waste storage, vitamin synthesis.
• Epiglottis: Prevents food from entering the respiratory tract during swallowing.
• Cardiac and Pyloric Sphincters: Control the movement of food through the digestive tract.
• Rugae: Folds in the stomach lining that allow expansion.
• Absorption in Small Intestine: Capillaries absorb monosaccharides, amino acids, and water-soluble vitamins; lacteals absorb fatty acids and glycerol.