Chapter 3: Digestion, Absorption & Transportation (DAT)

Overview of the Digestive System

• Functions of the Digestive System

  • Digestion: Mechanical and chemical processes that transform food into nutrients ready for absorption.

  • Absorption: Uptake of nutrients into intestinal cells for transport into the bloodstream or lymphatic system.

  • Transportation: Movement of nutrients to body cells for metabolic usage.

Learning Objectives (LO)

1. Describe the muscular and chemical actions in each major organ of the GI tract.

2. Identify stimulus, site of secretion, target organs, and effects of 4 key hormones in digestion.

3. Describe anatomical features in the small intestine aiding nutrient absorption.

4. Describe absorption processes in the small intestine.

5. Differentiate between nutrients traveling through the lymph system vs. directly into the bloodstream.

6. Discuss the consequences of flattened villi in the small intestine.

7. Analyze potential causes of chest pain post-meal (e.g., gastroesophageal reflux).

The Gastrointestinal (GI) Tract:

long, flexible, muscular tube from mouth to anus

Major Organs

1. Mouth: Initial digestion begins with mechanical chewing and salivary enzymes.

   —> muscles in mouth increase SA of food

2. Stomach: Chemical digestion of proteins with acidic gastric juices.

   —> pH = 1.7-1.5

3. Small Intestine: Main site for digestion and absorption; divided into duodenum, jejunum, and ileum.

4. Large Intestine: Absorbs water and electrolytes, produces waste.

Accessory Organs

1. Liver: Produces bile aiding in fat emulsification.

2. Pancreas: Secretes enzymes and bicarbonate for digestion.

3. Gall Bladder: Stores and releases bile into the small intestine.

Muscle Action in GI Tract

• 2 layers of muscle along GI tract (stomach has 3)

• longitudinal outer layer, circular inner layer

• diagonal innermost layer for stomach only

1. Chewing: breaks apart food, increases SA

2. Peristalsis: Longitudinal, wave-like muscular contractions that propel food along GI tract

3. Stomach churning: strongest muscles of GI tract found in stomach, 3 layers

   Mixing, churning stomach content, breaking large pieces into small, increases surface area

4. Segmentation: cinching of muscles breaks up food in local region of small intestine, increases surface area

5. Sphincter control: valves that control movement of food forward along GI tract, and prevent backflow

   Found at junctions between major organs of GI tract

   

Chemical Action in GI Tract

• chemical action occurs under the direction of hormones and is performed by enzymes

  Hormone:

• substance produced at one site in body in response to a specific stimulus

• travels in blood to different site

• elicits a response there

  Enzyme:

• a protein catalyst that accelerates rates of reactions without being changed or consumed in the process

• synthesis, degradation, rearrangement and exchange reactions

• lock and key fit with substrate to produce end product

• ‘ase’ suffix, although not for all enzymes

• active and inactive (pro-enzyme) forms

Hormonal Control of Digestion

• Gastrin: Secreted by stomach; stimulates gastric juices.

• Secretin: Released by the small intestine; stimulates pancreas.

• Cholecystokinin (CCK): Stimulates gall bladder and pancreatic juices.

Gastric Inhibitory Peptide (GIP): Slows gastric motility.

*first one stomach, last 3 in small intestine

Basic Reactions for Nutrient Handling

1. hydrolysis: requires water, catabolic, breakdown

   —> Digestion

2. Condensation: releases water, anabolic, synthetic (eg. protein synthesis)

   —> synthesis reactions

3. Reduction/oxidation: gain (reduction) or loss (oxidation) of electron

   —> absorption, transport

Absorption Processes

Nutrient Types and Absorption

1. Carbohydrates (CHO):

   • Absorbed as glucose (GLU), galactose (GAL), and fructose (FRU).

   • GLU and GAL via active transport; FRU via facilitated diffusion.

2. Proteins (PRO):

   • Absorbed as amino acids.

   • Active transport for single amino acids and small peptides.

3. Lipids:

   • Larger products diffuse in as micelles; absorbed via lymph as chylomicrons.

   • Smaller products enter bloodstream directly.

Anatomical Features of the Small Intestine

• Villi: Finger-like projections increasing surface area for absorption.

• Microvilli: Micro projections on intestinal cells aiding in nutrient capture.

• Folds: Increase surface area, enhancing absorption efficiency.

Transport Systems for Nutrients

1. Bloodstream:

   • Water-soluble nutrients (e.g., amino acids, monosaccharides) enter the portal vein leading to the liver.

   • Liver processes and distributes nutrients throughout the body.

2. Lymphatic System:

   • Fat-soluble nutrients (e.g., triglycerides) packaged into chylomicrons for transport into lymph before entering the bloodstream.

Key Differences in Nutrient Transport

• Bloodstream: Direct absorption in soluble form (hydrophilic compounds).

• Lymph: Fat-soluble nutrients packaged into lipoproteins for transport.

Consequences of Flattened Villi

• Leads to malabsorption conditions, impacting nutrient intake and health.

• May cause gastrointestinal symptoms and nutrition-related deficiencies.

Summary of Digestion and Absorption

• Proper functioning of the digestive system is crucial for nutrient assimilation and overall health.

• Disorders in the digestive processes can lead to significant health complications.

Chapter 3: Digestion, Absorption & Transportation (DAT)

Overview of the Digestive System

The digestive system is a complex network responsible for breaking down food into essential nutrients. This process involves several organs and mechanisms working in harmony to ensure effective digestion, absorption, and transportation of nutrients throughout the body.

Functions of the Digestive System

• Digestion: Involves mechanical and chemical processes, beginning in the mouth and continuing in the stomach and intestines, transforming food into simpler forms that can be absorbed as nutrients.

• Absorption: This occurs mainly in the small intestine, where nutrients pass through the intestinal wall into the bloodstream or lymphatic system.

• Transportation: Nutrients are transported via the bloodstream to body cells, where they are utilized for energy, growth, and repair.

Learning Objectives (LO)

1. Describe key muscular and chemical mechanisms occurring in each major organ of the gastrointestinal (GI) tract.

2. Identify the stimulus, secretion site, target organs, and effects of four key hormones in the digestion process.

3. Explain the anatomical features of the small intestine that enhance nutrient absorption, such as villi and microvilli.

4. Elaborate on the various absorption processes occurring within the small intestine, including specific mechanisms for different nutrient types.

5. Differentiate between nutrients that are absorbed into the lymphatic system versus those that enter directly into the bloodstream.

6. Discuss the potential health consequences associated with flattened villi in the small intestine and their effects on nutrient absorption.

7. Analyze the causes and implications of symptoms like chest pain that can occur post-meal, focusing on conditions such as gastroesophageal reflux disease (GERD).

The Gastrointestinal (GI) Tract

Major Organs

• Mouth: The process of digestion starts here, where teeth mechanically break food down and salivary enzymes begin the chemical breakdown of carbohydrates.

• Stomach: Here, gastric juices, which contain hydrochloric acid and pepsin, chemically digest proteins and create a semi-liquid substance called chyme.

• Small Intestine: The primary site for both digestion and nutrient absorption, this organ is subdivided into three sections—duodenum, jejunum, and ileum—each playing a unique role in the digestive process.

• Large Intestine: Responsible for absorbing water and electrolytes, as well as compaction and elimination of waste. It also houses beneficial bacteria that play a role in fermentation and vitamin production.

Accessory Organs

• Liver: Produces bile, which is essential for emulsifying fats and aiding their digestion.

• Pancreas: Secretes digestive enzymes (e.g., amylase, lipase, proteases) and bicarbonate to neutralize stomach acids in the small intestine, thereby facilitating optimal enzyme function.

• Gall Bladder: Stores and concentrates bile before releasing it into the small intestine when fats are present.

Hormonal Control of Digestion

• Gastrin: A hormone secreted by the stomach that stimulates the production of gastric juices, essential for protein digestion and maintaining the acidic environment in the stomach.

• Secretin: Released by the small intestine in response to acidic chyme, it triggers the pancreas to release bicarbonate, neutralizing stomach acid and optimizing digestive enzyme efficacy.

• Cholecystokinin (CCK): This hormone promotes the release of bile from the gall bladder and stimulates pancreatic enzyme secretion, enhancing the digestion of fats and proteins.

• Gastric Inhibitory Peptide (GIP): Serves to inhibit gastric motility and secretion when fats and sugars enter the small intestine, thereby regulating the speed of digestion.

Absorption Processes

Nutrient Types and Absorption

1. Carbohydrates (CHO):

   • Absorbed as glucose (GLU), galactose (GAL), and fructose (FRU).

   • GLU and GAL are absorbed through active transport mechanisms, while FRU utilizes facilitated diffusion, allowing for efficient uptake without energy expenditure.

2. Proteins (PRO):

   • Broken down into amino acids during digestion.

   • Single amino acids and small peptides are absorbed via active transport.

3. Lipids:

   • Larger fat molecules are emulsified and form micelles, which facilitate their absorption.

   • Fat-soluble products are incorporated into chylomicrons and transported through the lymphatic system, whereas smaller fatty acids directly enter the bloodstream.

Anatomical Features of the Small Intestine

• Villi: These finger-like projections significantly increase the surface area for absorption, optimizing the nutrient uptake process.

• Microvilli: Located on the surface of villi, they form a brush border which contains enzymes that further aid in the digestion of nutrients.

• Folds (circular folds): Enhance the surface area, contributing to more efficient nutrient absorption in the intestine.

Transport Systems for Nutrients

• Bloodstream: Nutrients that are water-soluble, including amino acids and monosaccharides, enter the portal vein, which routes them to the liver for processing before distribution to the body.

• Lymphatic System: Fat-soluble nutrients are packaged into chylomicrons and enter the lymphatic system before being released into the bloodstream, where they are eventually delivered to tissues.

Key Differences in Nutrient Transport

• Bloodstream: Involves direct absorption of soluble (hydrophilic) nutrients.

• Lymph: Functions for fat-soluble nutrients, which require packaging into lipoproteins to facilitate transport to the circulatory system.

Consequences of Flattened Villi

Flattened villi can lead to significant malabsorption issues, fostering nutrient deficiencies and gastrointestinal symptoms such as bloating, diarrhea, and potential long-term health complications owing to insufficient nutrient intake, including iron-deficiency anemia or osteoporosis.

Summary of Digestion and Absorption

The seamless operation of the digestive system is vital for nutrient assimilation and maintaining overall health. Disorders in this system can severely impact an individual’s health and nutritional status, warranting prompt attention to gastrointestinal symptoms or dysfunctions.