Digestive System and Metabolism: Anatomy & Physiology II Exam 4 Review

Digestive System Overview

Pleurae of the Intestines

The intestines are covered by two main layers of serous membrane called pleurae: the parietal peritoneum (lining the abdominal wall) and the visceral peritoneum (covering the organs). These layers help reduce friction and protect the intestines.

• Parietal peritoneum: Lines the abdominal cavity.

• Visceral peritoneum: Covers the external surfaces of the intestines.

• Mesentery: Double layer of peritoneum that supports and stabilizes the intestines.

Organs of the GI Tract and Accessory Organs

The gastrointestinal (GI) tract consists of organs forming the continuous tube from mouth to anus, while accessory organs aid in digestion but are not part of the tube.

• GI Tract Organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.

• Accessory Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Anatomy & Physiology of the Duodenum, Jejunum, Ileum, and Colon

• Duodenum: First part of the small intestine; receives chyme from stomach and digestive enzymes from pancreas and bile from liver.

• Jejunum: Middle section; primary site for nutrient absorption.

• Ileum: Last section; absorbs vitamin B12, bile salts, and remaining nutrients.

• Colon (Large Intestine): Absorbs water and electrolytes; forms and stores feces.

Diseases of the GI Tract

• GERD (Gastroesophageal Reflux Disease): Acid reflux from stomach into esophagus.

• Peptic Ulcers: Erosions in the stomach or duodenal lining due to acid and pepsin.

• Diarrhea: Increased frequency and liquidity of feces, often due to infection or malabsorption.

• Constipation: Infrequent or difficult bowel movements.

• Gallstones: Solid deposits in the gallbladder, often cholesterol-based.

Digestive Secretions and Enzymes

Major Secretions and Their Functions

• Saliva: Contains amylase for carbohydrate digestion.

• Gastric Juice: Contains hydrochloric acid (HCl) and pepsin for protein digestion.

• Bile: Emulsifies fats for digestion and absorption.

• Pancreatic Juice: Contains enzymes for digestion of carbohydrates, proteins, and fats.

Hormonal Regulation of Digestion

• Gastrin: Stimulates gastric acid secretion.

• Secretin: Stimulates bicarbonate secretion from pancreas.

• Cholecystokinin (CCK): Stimulates release of bile and pancreatic enzymes.

• Vasoactive Intestinal Peptide (VIP): Relaxes smooth muscle and increases secretion.

Enzymes and Their Substrates

• Amylase: Digests carbohydrates (starch to maltose).

• Proteases (e.g., trypsin, pepsin): Digest proteins to peptides and amino acids.

• Lipase: Digests fats to fatty acids and glycerol.

Digestion and Absorption

Digestion of Carbohydrates, Proteins, and Lipids

• Carbohydrates: Broken down by amylase and brush border enzymes to monosaccharides.

• Proteins: Digested by pepsin (stomach) and pancreatic proteases to amino acids.

• Lipids: Emulsified by bile, digested by lipase to fatty acids and monoglycerides.

Absorption Mechanisms

• Monosaccharides: Absorbed via active transport and facilitated diffusion.

• Amino acids: Absorbed via active transport.

• Fatty acids: Absorbed by diffusion into enterocytes, then assembled into chylomicrons.

Functions of the Stomach and Small Intestine

• Stomach: Mechanical and chemical breakdown of food; initiates protein digestion.

• Small Intestine: Major site of digestion and absorption of nutrients.

Liver and Pancreas Functions

Liver Functions

• Metabolism: Processes nutrients, detoxifies substances, synthesizes plasma proteins.

• Bile Production: Essential for fat digestion.

• Storage: Stores glycogen, vitamins, and minerals.

Pancreas Functions

• Exocrine: Secretes digestive enzymes and bicarbonate.

• Endocrine: Secretes insulin and glucagon for blood glucose regulation.

Metabolism and Nutrition

Metabolic Pathways

• Glycolysis: Breakdown of glucose to pyruvate, producing ATP.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

Equation for Glycolysis:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Vitamins and Minerals

• Vitamins: Organic compounds required in small amounts for metabolism (e.g., vitamin C, B vitamins).

• Minerals: Inorganic elements essential for body functions (e.g., calcium, iron).

GI Motility and Reflexes

GI Reflexes

• Gastrocolic Reflex: Increases motility of the colon after eating.

• Defecation Reflex: Initiates elimination of feces from the rectum.

• Vomiting Reflex: Coordinated contraction of GI muscles to expel contents.

Control of GI Motility

• Neural Control: Enteric nervous system regulates peristalsis and segmentation.

• Hormonal Control: Hormones like motilin and gastrin influence motility.

Summary Table: Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Amylase	Salivary glands, pancreas	Starch	Maltose
Pepsin	Stomach	Proteins	Peptides
Trypsin	Pancreas	Proteins	Peptides
Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
Lactase	Small intestine	Lactose	Glucose, galactose

Additional info:

• Some topics (e.g., matching polymers to monomers) relate to biochemistry and nutrition, which are covered in A&P under metabolism and digestion.

• Questions about diseases and reflexes are typical for exam review in Anatomy & Physiology II, focusing on the digestive system and metabolism chapters.