🐅Animal Physiology Unit 10 – Digestive System: Nutrient Processing

Overview of the Digestive System

• Consists of the gastrointestinal tract (GI tract) and accessory organs (liver, pancreas, and gallbladder)
• Primary function involves breaking down food into smaller, absorbable components for the body to utilize
• Mechanical digestion involves physical breakdown of food through chewing and peristalsis (muscular contractions of the GI tract)
• Chemical digestion occurs through the action of digestive enzymes and acids secreted by various organs
• Absorption of nutrients takes place primarily in the small intestine, where they are transported into the bloodstream
• Undigested material is eliminated from the body through the process of defecation
• Proper functioning of the digestive system is crucial for maintaining overall health and well-being

Anatomy of the Digestive Tract

• The digestive tract begins at the mouth and ends at the anus, consisting of several specialized organs
• The mouth contains teeth for mechanical breakdown of food and salivary glands that secrete enzymes (amylase) to initiate chemical digestion
• The esophagus connects the mouth to the stomach, using peristaltic contractions to move food
• The stomach secretes hydrochloric acid (HCl) and pepsin to further break down proteins
  • Rugae, folds in the stomach lining, allow for expansion and contraction during digestion
• The small intestine, divided into the duodenum, jejunum, and ileum, is the primary site of nutrient absorption
  • Villi and microvilli increase the surface area for efficient absorption
• The large intestine, or colon, absorbs water and electrolytes from undigested material and houses gut bacteria
• The rectum stores feces until elimination through the anus

Digestive Enzymes and Their Functions

• Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process
• Salivary amylase, secreted by the salivary glands, begins the breakdown of carbohydrates (starch) into simpler sugars
• Pepsin, activated by stomach acid, breaks down proteins into smaller peptides
• Pancreatic enzymes, released into the small intestine, include:
  • Trypsin and chymotrypsin for protein digestion
  • Pancreatic amylase for carbohydrate digestion
  • Pancreatic lipase for fat digestion
• Brush border enzymes, located on the surface of intestinal cells, complete the final stages of nutrient breakdown
  • Examples include maltase, sucrase, and lactase, which break down specific sugars
• Bile, produced by the liver and stored in the gallbladder, emulsifies fats to facilitate their digestion by lipases

Nutrient Breakdown and Absorption

• Carbohydrates are broken down into monosaccharides (glucose, fructose, and galactose) for absorption
  • Glucose is the primary energy source for cells and is absorbed via the SGLT-1 transporter
• Proteins are digested into amino acids, which are absorbed by the small intestine and used for protein synthesis and other functions
• Fats are broken down into fatty acids and monoglycerides, which form micelles with bile salts for absorption
  • Reassembled into triglycerides in intestinal cells and transported as chylomicrons in the lymphatic system
• Vitamins and minerals are absorbed through various mechanisms, such as active transport and facilitated diffusion
• Water-soluble nutrients (vitamin C, B vitamins) are absorbed directly into the bloodstream
• Fat-soluble vitamins (A, D, E, K) require bile salts for absorption and are transported with lipids

Hormonal Regulation of Digestion

• Gastrointestinal hormones play a crucial role in regulating digestive processes and signaling hunger or satiety
• Ghrelin, secreted by the stomach, stimulates appetite and promotes food intake
• Cholecystokinin (CCK), released by the small intestine, stimulates the release of pancreatic enzymes and bile, and induces feelings of fullness
• Secretin, produced by the small intestine, stimulates the release of bicarbonate-rich pancreatic juice to neutralize stomach acid
• Gastrin, secreted by the stomach, promotes the release of gastric acid and aids in stomach motility
• Peptide YY (PYY) and glucagon-like peptide-1 (GLP-1), released by the small intestine, signal satiety and slow gastric emptying
• Insulin, produced by the pancreas, regulates blood glucose levels and promotes the storage of nutrients

Gut Microbiome and Its Role

• The gut microbiome consists of trillions of microorganisms (bacteria, fungi, and viruses) that reside in the digestive tract
• Beneficial bacteria aid in the fermentation of undigested fibers, producing short-chain fatty acids (SCFAs) that nourish intestinal cells
• Gut bacteria synthesize certain vitamins (vitamin K and some B vitamins) and enhance nutrient absorption
• The microbiome plays a crucial role in the development and regulation of the immune system
  • Helps maintain the integrity of the intestinal barrier and prevents pathogen overgrowth
• Dysbiosis, an imbalance in the gut microbiome, has been linked to various digestive disorders (inflammatory bowel disease) and metabolic conditions (obesity, diabetes)
• Probiotics and prebiotics can help maintain a healthy gut microbiome by promoting the growth of beneficial bacteria

Digestive Disorders and Diseases

• Gastroesophageal reflux disease (GERD) occurs when stomach acid frequently flows back into the esophagus, causing heartburn and damage to the esophageal lining
• Peptic ulcers are open sores that develop in the lining of the stomach or duodenum, often caused by Helicobacter pylori bacteria or excessive use of NSAIDs
• Inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, involves chronic inflammation of the digestive tract
  • Symptoms may include abdominal pain, diarrhea, weight loss, and malnutrition
• Celiac disease is an autoimmune disorder triggered by the ingestion of gluten, leading to damage of the small intestine's lining
• Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by abdominal pain, bloating, and altered bowel habits (diarrhea or constipation)
• Gallstones are hardened deposits of bile that can cause pain and obstruct the flow of bile from the gallbladder
• Pancreatitis is inflammation of the pancreas, which can disrupt the production and secretion of digestive enzymes

Comparative Digestive Systems in Animals

• Herbivores (cows, horses) have elongated digestive tracts with a large stomach or cecum for fermenting plant material
  • Ruminants (cows, sheep) have a four-chambered stomach that allows for the regurgitation and re-chewing of food (cud)
• Carnivores (cats, dogs) have shorter digestive tracts and a simple stomach adapted for digesting meat
  • Lack salivary amylase and have a reduced ability to digest plant material
• Omnivores (humans, pigs) have digestive systems adapted for processing both plant and animal matter
• Birds have a unique digestive system that includes a crop for food storage, a gizzard for mechanical grinding, and paired ceca for fermentation
• Insects exhibit a wide range of digestive adaptations, such as the presence of a crop, gizzard, and Malpighian tubules for excretion
• Fish have a relatively simple digestive system, with a short intestine and the presence of pyloric ceca in some species to increase surface area
• Ruminants and hindgut fermenters (horses) rely on microbial fermentation to extract nutrients from plant material, while monogastric animals (pigs, humans) primarily digest food enzymatically