Intro to Digestion, Absorption, Transport & Storage

Introduction to Digestion, Absorption, Transport, and Storage

• Digestion is the process of breaking down complex food particles into smaller absorbable particles.
• Digestion involves both mechanical (e.g., chewing, churning) and chemical (e.g., enzymes) processes.
• Absorption involves the movement of digested nutrients across the intestinal wall into the bloodstream or lymphatic system.
• Elimination is the removal of undigested and unabsorbed materials from the body.
• Transport moves absorbed nutrients through the blood or lymphatic system to various parts of the body.
• Storage is the process of storing excess nutrients in specialized body regions.

Learning Goals

• Understand the difference between appetite and hunger/satiety.
• Identify the path food takes during digestion and describe muscular actions.
• Describe the actions and origins of digestive secretions.
• Describe the anatomy of the GI tract and intestinal cell features for nutrient absorption.
• Explain nutrient delivery from GI tract to body cells by vascular and lymphatic systems.
• Describe mechanisms that regulate GI function.

What Drives Us to Eat?

• Hunger is the physiological need to eat.
• It is non-specific (any food) and occurs due to biological changes, signaling the need to maintain energy levels.
• Satiety is the feeling of fullness. It occurs after a meal and prevents eating until the next meal.

Appetite

• Appetite is a desire to eat, mainly psychological.
• It's a learned response often influenced by the sight, smell, or thought of appealing foods.
• It can result from hunger but may have other causes such as emotional or environmental factors.

Hunger and Satiety

• Hunger and satiety are regulated primarily by the hypothalamus in the brain.
• The brain receives signals/hormones from the gut and blood based on nutrient and food levels.
• These signals inform the brain when it's time to eat or not to eat.

Foods and Satiety

• Different foods have varying satiety values: proteins have high satiety, meaning you feel full longer after eating them; carbohydrates and fats have lower values, leading to feeling hungry sooner.
• Bulky foods (high in fiber) provide a sense of satiety; solid foods are generally more filling than semi-solid or liquid foods.

Anatomy of the Gastrointestinal Tract

• Most absorption occurs within the small intestine.
• The large intestine includes the colon, rectum, and anus.

Digestion

• The process by which complex food particles are broken down to smaller absorbable particles.
• Digestion can be mechanical (physical breakdown of food), or chemical (enzymatic reactions that break down larger food molecules).

Digestion Begins in the Mouth

• Chewing is the main mechanical digestion step in the mouth.
• Saliva contains enzymes, like salivary amylase, which begin the process of chemical digestion.

Saliva

• Saliva is secreted by salivary glands and contains water, bicarbonate (pH regulation), mucus (lubrication), antibodies (protection), and enzymes (like amylase) to digest starch.

Stomach Digestion

• The gastroesophageal sphincter prevents food from returning to the esophagus.
• The stomach performs mechanical digestion (muscular churning) and chemical digestion (gastric juices breaking down food).

Gastric Juices

• Gastric juices contain water, hydrochloric acid (HCl), and pepsin to help digest proteins.

Chyme

• Chyme is a semi-solid mixture of partially digested food.
• It is expelled from the stomach into the duodenum (first section of the small intestine).

Digestion of Peptides and Fats in the Small Intestine

• Digestion of peptides and fats continues with pancreatic juices containing enzymes (e.g., pepsin for proteins and lipase for fats).
• Bile, produced by the liver and stored in the gallbladder, is released to emulsify fats for the enzymes to work on.

Liver and Bile

• The liver produces bile.
• The gallbladder stores and releases bile to aid in the digestion of fats.
• Bile acts as an emulsifier, dispersing fats in the water to allow enzymes to work properly.

Pancreas and Pancreatic Juices

• Pancreatic juice is produced by the pancreas and released into the small intestine through the pancreatic duct.
• The pancreatic juice contains enzymes that break down all macronutrients (carbohydrates, proteins, and fats). It also contains sodium bicarbonate to neutralize the stomach acid.

Small Intestine

• The small intestine is the primary site of digestion and absorption.
• It includes three sections: duodenum, jejunum, and ileum.

Peristalsis

• Peristalsis is rhythmic, involuntary muscle contractions that move food through the GI tract.
• These contractions occur in waves, pushing the food forward.

Segmentation

• Segmentation mixes the chyme with digestive enzymes and brings nutrients into contact with the small intestine's lining.
• This mixing and pushing helps increase absorption efficiency.

Absorption

• Absorption is the process of moving digested nutrients from the small intestine into the bloodstream and lymphatic system.
• Small amounts of absorption occur in the stomach (e.g., water, alcohol).
• Most nutrient absorption occurs in the small intestine.

Absorption Structures

• The lining of the small intestine has specialized structures such as villi and microvilli to maximize absorption surface area.
• Villi are finger-like projections, and microvilli are even smaller hair-like structures.

Absorption Types

• Simple/passive diffusion: nutrients pass freely across the cell membrane along a concentration gradient.
• Facilitated diffusion: nutrients use a carrier protein to cross the membrane along a concentration gradient (no energy required).
• Active transport: nutrients move against a concentration gradient using a carrier protein and energy.
• Endocytosis: a small amount of intestinal content is engulfed by the membrane (e.g., proteins).

Absorption/Elimination

• Undigested food moves to the large intestine through the ileocecal valve.
• Materials are stored 12-24 hours before elimination.
• Water and some nutrients are absorbed.
• Bacteria produce beneficial vitamins (like vitamin K and B12).

Gut Flora

• The gut is colonized by a variety of bacteria (flora), mostly in the large intestine.
• Imbalance in gut flora can cause problems such as diarrhea or constipation.

Probiotics

• Probiotics are live microorganisms that can improve the host's intestinal microbial balance when consumed.
• They are found in fermented dairy products and need to be consumed in sufficient amounts.
• Probiotics offer health benefits such as supporting the digestive system.

Prebiotics

• Prebiotics are non-digestible substances that promote growth of beneficial bacteria in the large intestine.
• They are found in various fruits and vegetables.

Synbiotics

• Synbiotics combine probiotics and prebiotics for potential synergistic benefits for the digestive system.

Transport

• The vascular system transports water-soluble nutrients in the blood.
• The lymphatic system transports fat-soluble nutrients in the lymph.

Hepatic Portal System

• The hepatic portal system is a specialized vein that delivers absorbed water-soluble nutrients first to the liver for processing before entering the general circulation.
• The liver acts as a filter and processes these nutrients before distributing them.

Regulation

• Homeostasis is the tendency to maintain a stable internal environment in response to external changes.
• Negative feedback loops are a key regulatory mechanism in maintaining homeostasis.

Regulation Components

• CNS (central nervous system), including the brain and spinal cord.
• PNS (peripheral nervous system), including enteric nerves in the GI tract.
• Hormones, which act as chemical messengers to regulate processes remotely.

Gastrin

• Gastrin is a hormone released by G cells in the stomach.
• Gastrin stimulates parietal cells to produce hydrochloric acid (HCl).
• The release of gastrin is triggered by the presence of peptides and proteins in the stomach.

Secretin

• Secretin is a hormone released by the duodenum when acidic chyme arrives.
• Secretin inhibits gastric motility and gastrin.
• Secretin stimulates bicarbonate release from the pancreas to help neutralize the chyme.

Cholecystokinin (CCK)

• CCK is released by the duodenum in response to fat in the chyme.
• CCK stimulates the pancreas to release digestive enzymes and stimulates the gallbladder to release bile.

Gastric Inhibitory Peptide (GIP)

• GIP is produced by the duodenum in response to glucose.
• GIP inhibits gastric activity (motility and acid secretion).
• GIP also stimulates insulin release from the pancreas.

Test your knowledge about the human digestive system, specifically focusing on the small intestine. This quiz covers important functions, structures, and processes involved in digestion and nutrient absorption. Challenge yourself and see how well you understand the critical roles of various components in this essential system.