Digestive System

Functions of Digestive System

Mechanical digestion

Chemical digestion

Secretion vs Absorption

Mechanical digestion

is the process of physically breaking down food into smaller pieces to increase surface area for enzymatic action. This includes actions such as chewing and churning in the stomach.

Chemical digestion

is the process in which food is broken down into simpler molecules by enzymatic action. This occurs primarily in the stomach and small intestine, where enzymes and acids further breakdown nutrients for absorption.

Secretion vs Absorption

refers to the distinction between the release of substances such as digestive enzymes and hormones into the digestive tract and the uptake of nutrients from the digestive tract into the bloodstream.

Alimentary Canal

is the continuous tube in the digestive system that extends from the mouth to the anus, comprising the digestive tract and associated organs. It plays a crucial role in the digestion and absorption of food.

Accessory organs

are organs that assist in digestion but are not part of the alimentary canal, including the liver, pancreas, and gallbladder. They produce and store enzymes and substances necessary for digestion.

Secrete into the alimentary canal

Alimentary canal structure

Mucosa

Submucosa

Muscularis externa

• 2 layers

Serosa

Sphincters

• circular bands of smooth muscle, controls entry (2 in stomach)

Autonomic Innervation

Why surface area is important in digestion

Surface area is crucial in digestion because it allows for more efficient absorption of nutrients by providing more sites for digestive enzymes to act and increase contact with nutrients.

Done to increase Surface Area

Ensures most food is absorbed

Highest in small intestines

Autonomic Nervous System in Digestion

A complex network that regulates involuntary processes of the digestive system, controlling functions such as peristalsis and secretion without conscious effort.

Sympathetic dominates when physically active and inhibits digestion

Parasympathetics dominate at rest and stimulates digestion

Mixing

The process of combining food with digestive juices in the stomach to enhance enzymatic activity and facilitate nutrient absorption.

Parasympathetics controls this

Happens in small intestine via segmentation

Segmentation of Small Intestine

A rhythmic contraction of the circular muscles in the small intestine that mixes food with digestive juices and enhances nutrient absorption.

Pinching happens via circular muscle

Contracting happens via longitudinal muscle

Circular muscle

A type of muscle in the digestive tract that encircles the lumen and is responsible for constricting it during processes like segmentation.

Longitudinal Muscle

A type of muscle in the digestive tract that runs along the length of the intestine and is responsible for shortening the intestine during peristalsis and segmental contractions.

Propelling via Peristalsis

A wave-like muscle contraction in the digestive tract that moves food through the intestines, enabling effective digestion and absorption.

Happens via Circular muscle

Similar to Propagation of Nerves

Salivary Glands

Glands in the mouth that produce saliva, aiding in the digestion of food and maintaining oral health.

Saliva composition

• Salivary Amylase

• Mucus

Control of Secretion

Salivary Amylase

An enzyme found in saliva that begins the digestion of carbohydrates by breaking down starch into simpler sugars.

Mucus of Salivary Glands

A viscous, slippery secretion produced by salivary glands that helps to lubricate food, facilitating easier swallowing and protecting the mucous membranes of the mouth.

Control of Secretion of Salivary Glands

The regulation of saliva production primarily controlled by the autonomic nervous system, influenced by the presence or anticipation of food.

What type of glands are the 3 salivary glands

exocrine glands that secrete saliva through ducts.

Gastric Secretions

The fluids produced in the stomach that aid in digestion, including hydrochloric acid, pepsinogen, and mucus.

Pepsin

Hydrochloric Acid (HCl)

Mucus

Gastric Lipase

Intrinsic Factor

Pepsin

A digestive enzyme that breaks down proteins into smaller peptides, secreted in its inactive form, pepsinogen, by the stomach lining.

Hydrochloric Acid

A strong acid produced by the stomach that aids in digestion by providing an acidic environment for enzymes and killing bacteria.

Mucus in Gastric Secretions

A viscous fluid produced by the stomach lining that protects the gastric mucosa from erosion by stomach acid and aids in the smooth passage of food.

Gastric Lipase

An enzyme secreted by the stomach that helps digest dietary fats by breaking them down into fatty acids and glycerol.

Intrinsic Factor

A glycoprotein produced by parietal cells in the stomach that is essential for the absorption of vitamin B12 in the intestine.

Chief Cells

Specialized cells in the stomach lining that produce gastric enzymes, primarily pepsinogen, which is essential for protein digestion.

Parietal Cells

Cells in the stomach lining that secrete hydrochloric acid and intrinsic factor, playing a key role in digestion and vitamin B12 absorption.

Why do Chief Cells secrete pepsinogen and not pepsin

Pepsinogen is an inactive precursor (zymogen) that is converted to pepsin in the acidic environment of the stomach. This prevents autodigestion of the gastric lining by the active enzyme.

Regulation of Gastric Secretion

The process by which the body controls the production and release of gastric juices, including enzymes and acid, primarily through hormonal and neural mechanisms.

Cephalic Phase

Gastric Phase

Intestinal Phase

Cephalic Phase

The initial phase of gastric secretion triggered by the sight, smell, or thought of food, which stimulates the brain to send signals to the stomach to prepare for digestion.

Parasympathetics

Gastric Phase

The phase of gastric secretion that occurs when food enters the stomach, leading to increased acid and enzyme production due to the stretching of the stomach wall and the release of gastrin.

Gastrin

Gastrin

A hormone produced by G cells in the stomach that stimulates gastric acid secretion and promotes gastric motility.

Intestinal Phase

The final phase of gastric secretion that occurs when food enters the small intestine, leading to a decrease in gastric acid production as hormones like secretin and cholecystokinin are released, regulating digestive processes.

Intestinal Gastrin

Cholecystokinin

Intestinal Somatostatin

Intestinal Gastrin (initially)

A hormone released in the small intestine that stimulates gastric acid secretion and influences digestive processes during the intestinal phase.

Cholecystokinin

A hormone released by the small intestine that stimulates the contraction of the gallbladder to release bile and promotes the secretion of digestive enzymes from the pancreas.

Intestinal Somatostatin

A hormone produced in the small intestine that inhibits gastric acid secretion and regulates various digestive functions by suppressing the release of other hormones.

Gastric Absorption

The process by which nutrients from digested food are taken up into the bloodstream from the stomach and small intestine.

Main Job is Mixing

Small Job in Absorption

Stomach is designed for low pH, esophagus is not so the sphincters help in this role and prevent acid reflux

Enterogastric Reflex

A physiological process in which the presence of food in the small intestine inhibits gastric motility and secretions to regulate digestion.

It helps coordinate digestion by signaling the stomach to slow down its activity when food enters the small intestine.

Stretch is picked up and rate of stomach emptying decreases and more controlled

Pancreas and Digestion

The pancreas is a gland that plays a crucial role in digestion by producing digestive enzymes and hormones, including insulin and glucagon. It secretes these substances into the small intestine to aid in breaking down carbohydrates, proteins, and fats.

Pancreatic Secretions

Pancreatic Amylase

Pancreatic Lipase

Proteolytic Enzymes

Nucleases

Bicarbonate ions

Pancreatic Amylase

An enzyme produced by the pancreas that helps break down carbohydrates into simple sugars, facilitating digestion in the small intestine.

Ensures all carbs end up as disaccharides

Pancreatic Lipase

An enzyme secreted by the pancreas that aids in the digestion of fats by breaking them down into fatty acids and glycerol in the small intestine.

Proteolytic Enzymes

Enzymes produced by the pancreas that break down proteins into smaller peptides and amino acids during digestion in the small intestine. These enzymes include trypsin, chymotrypsin, and carboxypeptidase, which are crucial for protein digestion.

Nulceases

Enzymes that catalyze the breakdown of nucleic acids into nucleotides, playing a key role in the digestion of DNA and RNA.

Bicarbonate Ions

Are secreted by the pancreas to neutralize stomach acid in the small intestine, creating a more alkaline environment for digestive enzymes to work effectively.

Raise pH to neutralize stomach acid in intestinal contents.

• Acts as a buffer so we can use multiple enzymes

Pancreas Anatomy and Digestion

The pancreas is a vital organ in the digestive system that produces digestive enzymes and bicarbonate, which are released into the small intestine to aid in digestion. It plays a crucial role in both nutrient absorption and blood sugar regulation.

Acini - Exocrine Glands

Separation of Proteins

Multiple different types of enzymes are needed in order to break the many different peptide bonds

Activation of Proteolytic Enzymes

refers to the process by which inactive enzyme precursors, or zymogens, are converted into active enzymes that can break down proteins in the digestive system.

Enterokinase activates trypsin

Trypsin activates all others

Origin of Enterokinase

Enterokinase is produced in the duodenum and plays a key role in activating trypsin from its inactive form, trypsinogen, initiating the proteolytic enzyme activation cascade.

Made my small intestine

Activates trypsinogen into trypsin

Regulation Pancreatic Secretions

Hormonal control

Neural Control

Hormonal Control

CCK Secretin

Neural Control of pancreatic secretions

is mediated by the autonomic nervous system, primarily through the vagus nerve, which stimulates pancreatic juice release in response to food intake.

CCK

A hormone secreted by the duodenum that stimulates the secretion of pancreatic enzymes and bile.

Secretin

A hormone produced by the S cells of the duodenum that stimulates the pancreas to release bicarbonate-rich fluids to neutralize stomach acid in the small intestine.

How secretin is controlled

Controlled by CCK and the presence of acidic chyme in the small intestine.

Emulsification

The process of breaking down fats into smaller droplets to increase the surface area for digestion, primarily facilitated by bile salts.

Makes it fat soluble

Separates Fat Droplets

Similar to Soaps

Allows Lipases to act better and digest easier

Cholecystokinin (CCK) Pathway

1. Chyme with fat enters duodenum

2. Cells of the intestinal mucosa secrete the hormone CCK into bloodstream

3. CCK stimulates muscular layer of gallbladder wall to contract

3 Regions of the small intestine

Duodenum

Jejunum

Ileum

Duodenum

The first region of the small intestine, where most chemical digestion occurs, particularly with the presence of bile and pancreatic enzymes.

Jejunum

The second region of the small intestine, located between the duodenum and ileum, responsible for nutrient absorption.

Ileum

The final region of the small intestine, responsible for the absorption of nutrients and bile salts, as well as the connection to the large intestine.

Cecum

The portion of the large intestine located at the junction of the small intestine and colon, playing a role in the absorption of fluids and salts. It is also the site where the appendix is attached.

Intestinal Villi Intestinal Gland

Small finger-like projections in the small intestine that increase surface area for absorption, containing cells that secrete digestive enzymes and absorb nutrients.

Enzymes of Small intestine the digest peptides

The group of enzymes that facilitate the breakdown of peptides into amino acids in the small intestine, primarily including peptidases and proteases.

Enzymes of small intestine the digest disaccharides

These enzymes break down disaccharides into simple sugars, primarily including sucrase, lactase, and maltase.

Absorbed via diffusion until gradient shifts to being equal

Enzymes of small intestine that digest triglycerides

These enzymes, primarily lipases, break down triglycerides into free fatty acids and glycerol for absorption in the small intestine.

Internal Lipase

Absorption in small intestine

Monosaccharides

Amino acids and small peptides

Monosaccharide absorption in small intestine

occurs primarily through the intestinal lining via active transport and facilitated diffusion, allowing simple sugars to enter the bloodstream for energy.

Facilitated diffusion until gradient evens

Active transport after gradient equals

Amino acids and small peptide absorption in small intestine

occurs mainly through active transport and facilitated diffusion, enabling these nutrients to enter the bloodstream for protein synthesis and energy.

Active transport

Additional enzymes needed

• Dipeptidases

• Tripeptidases

Fatty acid and glycerol absorption

occurs in the small intestine primarily through passive diffusion, allowing the fatty acids and glycerol to enter the bloodstream and be utilized for energy and cellular functions.

Turns fatty acids into chylomicrons

Chylomicrons

are lipoprotein particles that transport dietary lipids from the intestines to other locations in the body. They facilitate the absorption of fatty acids and glycerol after digestion.

Then gets inserted into lacteal of villus and through lymphatic system

Electrolyte Absorption

is the process by which essential ions are taken up from the intestinal lumen into the bloodstream, primarily in the small intestine. This includes sodium, potassium, calcium, and magnesium, which are vital for various physiological functions.

Some active transport

Some facilitated diffusion

Water absorption

Osmosis

Gradient established by monosaccharides, amino acids, and electrolytes

Movement of small intestine

Segmentation and Peristalsis

Gastroenteric Reflex

Gastroileal Reflex, ileocecal sphincter

Segmentation of Peristalsis

are the two primary types of muscular contractions in the small intestine that facilitate the mixing and propulsion of chyme through the digestive tract.

Gastroenteric Reflex

is a neural reflex that stimulates gastrointestinal motility and secretion in response to food entering the stomach, promoting digestive activity in the small intestine.

Gastroileal reflex

is a physiological response that increases peristalsis in the ileum and relaxes the ileocecal sphincter, allowing the passage of chyme from the small intestine to the large intestine.

Ileocecal Sphincter

is a valve that separates the ileum of the small intestine from the cecum of the large intestine, controlling the flow of chyme into the colon.