Nutrient Categories and Digestion

Nutrient Categories and Their Roles

• Carbohydrates are the body's primary energy source and are found in grains, fruits, and veggies.
• Proteins provide amino acids for growth, repair, and enzyme production; they are found in meat, dairy, legumes, and nuts.
• Lipids serve as long-term energy storage, insulation, and are an essential component of cell membranes and can be sourced from oils, butter, and fatty meats.
• Vitamins act as coenzymes vital for metabolic processes such as water-soluble ones (Vitamin C and B-complex), which are not stored and must be consumed daily.
• Fat-soluble Vitamins A, D, E, and K are stored in body fat and the liver.
• Minerals are inorganic elements that play roles in bone formation, nerve function, and enzymatic reactions, such as calcium, potassium, iron, and zinc.
• Water is essential for maintaining body temperature, transporting nutrients, and eliminating waste.

Human Digestive System

• The oral cavity uses chewing for mechanical digestion and salivary amylase for chemical digestion of carbohydrates.
• The pharynx and esophagus transport food to the stomach by swallowing.
• Peristalsis begins in the pharynx and esophagus.
• The stomach churns and mixes food into chyme through mechanical digestion.
• Pepsin begins protein digestion, and hydrochloric acid (HCl) denatures proteins and kills bacteria through chemical digestion.
• The duodenum is a major site for chemical digestion, where enzymes from the pancreas and bile from the liver/gallbladder enter.
• The jejunum and ileum are primarily responsible for nutrient absorption.
• The large intestine absorbs water and electrolytes, houses beneficial bacteria, and prepares waste for secretion.
• The Rectum & Anus provide temporary storage and elimination of feces/waste.

Histology of The GI Tract

• Mucosa: innermost layer for absorption and secretion; contains epithelial cells, goblet cells (mucus), and sometimes glands.
• Submucosa: supports and transports the mucosa through blood vessels, nerves, and lymphatics.
• Muscularis externa: responsible for motility and peristalsis; contains two layers of smooth muscle (circular & longitudinal).
• Serosa: the outermost protective layer with outer connective tissue.
• Stomach distinguishing features include gastric pits, rugae (folds), and pepsinogen secretion.
• Small intestine distinguishing features include villi and microvilli increase surface area.
• Large intestine distinguishing features include a lack of villi, but many goblet cells.

Bird Digestive System

• The crop is an enlarged part of the esophagus that stores and moistens food.
• Proventriculus is a glandular stomach that secretes digestive enzymes.
• The gizzard is a muscular organ that grinds food, often with ingested stones or grit.
• The Small Intestine is adapted for nutrient absorption.
• The cloaca is the standard exit for digestive and urinary tracts.

Peristalsis and Reverse Peristalsis

• Peristalsis: a wave-like series of involuntary muscle contractions in the smooth muscle layers pushes food (bolus/chyme) through the GI tract.
• Peristalsis is coordinated by the enteric nervous system.
• Peristalsis transports food regardless of body position.
• Reverse Peristalsis: moves food in the opposite direction, from the stomach or esophagus back towards the mouth.
• Reverse peristalsis is common in vomiting humans.

Chambered Stomach and Ruminants

• Ruminants like cows, sheep, and goats have a four-chambered stomach adapted for breaking down cellulose through microbial fermentation.
• Reticulum: A honeycomb structure that traps foreign materials and works with the rumen for regurgitating cud.
• Rumen: The largest chamber, acts as a fermentation vat filled with symbiotic bacteria and protozoa that break down cellulose.
• Omasum: Absorbs water, electrolytes, and some fatty acids, the folded inner surface increases the surface area.
• Abomasum: The "true stomach" secretes enzymes and HCl, similar to monogastric stomachs.
• These adaptations allow ruminants to extract nutrients from tough plant matter that other animals cannot effectively digest.

Increased Surface Area in The Digestive Tract

• Digestive efficacy relies on maximizing surface area for chemical digestion and nutrient absorption, achieved through smaller food particles and folded organ linings.
• Gastric Rugae: folds in the stomach lining that allow expansion and increase surface contact with food and gastric juices.
• Villi and Microvilli (intestinal brush border) are located in the small intestine.
• Villi are finger-like projections that contain blood vessels and lymphatic vessels (lacteals) for absorption.
• Microvilli are microscopic extensions on the epithelial cells of villi that drastically increase surface area and contain enzymes for final-stage digestion.

Duodenum: Chemical Digestion Hub

• The duodenum is named for the two ducts that open into it and is the first section of the small intestine.
• Bile Duct, from the liver/gallbladder, delivers bile, emulsifying fats.
• Pancreatic Duct, from the pancreas, delivers pancreatic enzymes such as amylase (breaks down starch), lipase (digests fats), and trypsin and chymotrypsin (digest proteins).
• Duodenal epithelial cells also secrete carboxypeptidase (cleaves amino acids), disaccharides (compete with carb digestion), and peptidases (complete protein digestion).

Final Absorption of Nutrients

• Absorption mainly occurs in the jejunum and ileum of the small intestine.
• Proteins are absorbed as amino acids in the small intestine via active transport into the blood
• Carbs are absorbed as monosaccharides (glucose) in the small intestine via facilitated diffusion or active transport.
• Fats are absorbed as fatty acids and monoglycerides in the small intestine (via lacteals)

Cecum and Appendix

• Cecum: A blind-ended pouch at the junction of the small and large intestine.
• The cecum in many herbivorous mammals houses microbes that help digest cellulose from plant fibers.
• Appendix: In humans, the appendix is a small, vestigial cecum.
• The human appendix may play a role in gut immunity and bacterial storage for microbiome recovery after illness.