Digestion and Absorption

Digestion and Absorption Study Notes

Chapter 40

This chapter focuses on the processes of digestion and absorption in the body, detailing the mechanisms and biochemical changes that allow nutrients to be utilized by the body.

Overview of Digestive Function

Primary Function of the Digestive System

  • Essential Role: The primary function of the digestive system is to bring essential nutrients into the internal environment for availability to each cell in the body.

Mechanisms of the Digestive System

  1. Ingestion - Introduction of food into the body.

  2. Digestion - Process of breaking down food mechanically and chemically to extract the necessary nutrients.

Mechanisms of Digestion

Understanding the processes involved in digestion reveals how food moves through the gastrointestinal (GI) tract and the transformations it undergoes:

  1. Motility of the GI Wall - Involves the movement of food through the digestive system.

  2. Secretion - Enzymes and other substances are secreted to aid in digestion.

  3. Absorption - The process through which nutrients are absorbed into the bloodstream.

  4. Elimination - Removal of indigestible substances from the body.

  5. Regulation - Coordination of all digestive processes.

Mechanical Digestion

Definition

  • The process that changes ingested food from large particles into minute particles, facilitating chemical digestion.

Functions

  1. Churning: Mixes contents of the GI lumen with digestive juices, enhancing contact with the intestinal mucosa to facilitate absorption.

  2. Propulsion: Moves food along the alimentary tract.

  3. Elimination: Expels digestive waste from the body.

Mastication

  • The mechanical breakdown of food through chewing that facilitates the process of digestion.

Deglutition (Swallowing)

The process of swallowing consists of three stages:

1. Oral Stage

  • Location: From the mouth to the oropharynx.

  • Control: Voluntarily controlled by the tongue.

2. Pharyngeal Stage

  • Location: From the oropharynx to the esophagus.

  • Control: Involuntary movement. Combines contractions and gravity to move the food bolus into the esophagus.

3. Esophageal Stage

  • Location: From the esophagus to the stomach.

  • Control: Involuntary movement.

Peristalsis and Segmentation

Definitions

  • Peristalsis: Wavelike ripples of muscle contractions that propel food through the GI tract.

  • Segmentation: A mixing movement involving forward-and-backward motion within a single segment of the GI tract to mix contents thoroughly.

Combined Action

  • Peristalsis and segmentation can occur together and work alternately to enhance digestion.

Regulation of Motility

Gastric Motility

  • The activity in the stomach involves:

    • Churning the food (propulsion and retropulsion).

    • Mixing with gastric juices to form chyme.

    • Ejection of chyme into the duodenum approximately every 20 seconds.

    • Full gastric emptying takes between 2 to 6 hours and is controlled by hormonal and nervous mechanisms.

Intestinal Motility

  • Mixes chyme with digestive juices from the pancreas, liver, and intestinal mucosa.

  • As chyme approaches the end of the jejunum, the rate of peristalsis increases, pushing it into the large intestine.

  • The passage of chyme through the small intestine takes about 5 hours following its departure from the stomach.

Chemical Digestion

Definition

  • The process by which food is broken down into smaller molecules using hydrolysis, enabled by digestive enzymes.

Digestive Enzymes:

  • Definition: Extracellular organic (protein) catalysts that operate in the lumen of the digestive tract.

  • Properties:

    • Highly specific in action.

    • Function optimally at a specific pH, highlighting the need for varying conditions in different digestive environments.

Carbohydrate Digestion

Process

  • Carbohydrates are saccharide compounds, primarily polysaccharides.

  • Enzymatic Action:

    • Polysaccharides undergo hydrolysis by amylases to form disaccharides.

    • Final digestion steps are facilitated by enzymes such as sucrase, lactase, and maltase found on the cell membrane of epithelial cells in the intestinal villi.

Summary of Enzymatic Breakdown:

  1. Salivary Amylase - Begins hydrolysis in the mouth.

  2. Pancreatic Amylase - Continues process in the small intestine.

  3. Brush-border enzymes - Catalyze final digestion into monosaccharides.

Protein Digestion

Composition

  • Proteins consist of twisted chains of amino acids.

Digestive Process

  • Proteins are hydrolyzed into smaller peptides and amino acids by proteases:

    • Main Proteases:

    • Pepsin: Active in gastric juice.

    • Trypsin: Found in pancreatic juice.

    • Peptidases: Located in the intestinal brush border for further breakdown.

Summary of Enzymatic Breakdown:

  1. Proteolytic enzymes (proteases): Degrade proteins to peptides.

  2. Intestinal peptidases: Convert peptides to amino acids.

Fat Digestion

Overview

  • Fats require emulsification by bile in the small intestine before digestion can occur.

Main Digestive Enzyme

  • Pancreatic Lipase: The primary enzyme responsible for fat digestion.

Residues of Digestion

  • Not all food compounds are digested; some remnants are excreted as feces.

Secretion of Digestive Juices

Saliva

  • Source: Secreted by salivary glands.

  • **Functions: **

    • Lubricates food (via mucus) and facilitates mixing with water.

    • Contains Amylase: starts digestion of starches.

Gastric Juice

  • Source: Secreted by gastric glands.

  • Components:

    • Pepsin: Begins protein digestion.

    • Intrinsic Factor: Secreted by parietal cells, essential for vitamin B₁₂ absorption.

    • Mucus and water: Assist in lubrication and mixing of chyme.

Pancreatic Juice

  • Source: Secreted by acinar and duct cells of the pancreas.

  • Components:

    • Proteases (like trypsin and chymotrypsin): Digest proteins and polypeptides.

    • Lipases: Digest emulsified fats.

    • Nucleases: Break down nucleic acids (DNA and RNA).

    • Amylase: Continues digestion of starches.

Bile

  • Source: Secreted by the liver and stored in the gallbladder.

  • Functions:

    • Sodium bicarbonate raises pH for optimal enzyme function.

    • Contains waste products (cholesterol, detoxification by-products, bilirubin) to be excreted.

Control of Digestive Secretions

Salivary Secretion

  • Controlled reflexively; stimuli include:

    • Chemical and mechanical stimuli from food in the mouth.

    • Olfactory and visual stimuli from seeing and smelling food.

Control of Gastric Secretion

  1. Cephalic Phase (Psychic Phase): Mental factors induce secretion, activating the vagus nerve that stimulates gastrin production (by G cells).

  2. Gastric Phase: Protein digestion products stimulate gastrin release in response to food entering the pyloric region.

  3. Intestinal Phase: Mechanisms that adjust gastric secretion as chyme moves into the intestine.

Control of Pancreatic Secretion

  • Triggered by hormones released from the intestinal mucosa:

    • Secretin: Stimulates production of pancreatic fluid that is low in enzymes but high in bicarbonate, helping to neutralize stomach acid.

Control of Bile Secretion and Intestinal Secretion

  • Bile is secreted continuously by the liver to aid in digestion.

Absorption

Overview

  • Nutrients such as monosaccharides and amino acids are absorbed into the blood, then transported to the liver via the portal venous system.

Elimination

Definition

  • The process of expelling feces from the digestive tract, referred to as defecation.

Mechanism

  • Elicited by reflexes triggered by rectal mucosal stimulation due to rectal distention.

Disorders

  1. Constipation: Slower than normal movement of contents in the colon/rectum.

  2. Diarrhea: Rapid intestinal motility leading to reduced water absorption and watery stools.