Digestive System

Question 1

Neural and Endocrine Input

Answer 1

• Enteric Nervous System: Controls peristalsis, which is up-regulated by parasympathetic nervous system and down-regulated by sympathetic nervous system.
• All digestive system glands stimulated by parasympathetic inputs and inhibited by sympathetic inputs.
• ADH and aldosterone promote thirst, glucagon and ghrelin promote hunger, and leptin and CCK promote satiety.

Question 2

Anatomy

Answer 2

• Oral Cavity -> Pharynx -> Esophagus -> Stomach -> Small Intestine -> Large Intestine -> Rectum.
• Oral Cavity: Salivary Glands release Salivary Amylase and Lipase.
• Pharynx: Epiglottis covers laryngeal inlet to prevent aspiration/choking.
• Esophagus: Propels food to stomach via Peristalsis; upper part is under somatic control, while lower part is under autonomic control.
• Stomach: Mucosa of stomach contains Gastric Glands and Pyloric Glands. Gastric Glands composed of Mucous Cells (produces bicarbonate-rich mucus), Chief Cells (secrete Pepsinogen zymogen), Parietal Cells (secrete HCl and Intrinsic Factor for vitamin B12 absorption). Hydrogen ions from HCl cleaves pepsinogen to Pepsin. Pyloric Glands composed of G-cells (secrete Gastrin to increase HCl production by parietal cells).
• Small Intestine: Duodenum is site of most chemical digestion via Brush-Border Enzymes (disaccharidases and peptidases) and secretes Enteropeptidase (activates pancreatic enzymes), Secretin (increases pancreatic enzyme secretions into duodenum), and CCK (stimulates secretion of bile and pancreatic juices, which makes duodenum alkaline compared to stomach). Absorption occurs primarily in Jejunum and Ileum via Villi, which contain capillaries for absorption of water-soluble nutrients (sugars, proteins, vitamin B complex and C, short-chain fatty acids) and Lacteals for transport of Chylomicrons (packages of triglycerides, cholesterol, and fat-soluble vitamins A, D, E, K) through lymphatic system. Water also absorbed by small intestine.
• Accessory Organs: All endoderm derivatives.
• Pancreas contains endocrine Islet of Langerhans cells and exocrine Acinar Cells, which produce bicarbonate-rich alkaline secretions (to neutralize acidic chyme) containing Pancreatic Amylase for carbohydrates digestion; Trypsinogen, Chymotrypsinogen, Carboxypeptidases A and B for protein digestion; and Pancreatic Lipase for Fat digestion.
• Liver produces Bile, which contains cholesterol, Bilirubin pigments from hemoglobin breakdown, and amphipathic Bile Salts (derived from cholesterol) for emulsifying fats and cholesterol into micelles (making them water-soluble) for Pancreatic Lipase to act on. Liver receives all blood draining from the digestive tract through the Hepatic Portal Vein so that excess sugar can be taken up to create Glycogen and so that fats can be stored as Triacylglycerols. Liver produces Albumin and Clotting Factors.
• Gallbladder stores and concentrates bile.

• Large Intestine: Primarily involved in water absorption to concentrate feces. Divided into Cecum, Colon, Rectum. Bacteria of intestines produce Vitamin K for coagulation and Vitamin B7 for metabolism.