PSIO EXAM 3 Digestive System

3.6 Learning Objectives:

1. List and define the main functions and processes that occur
   along the GI tract.

2. Identify and describe the distinguishing characteristics of each
   of the layers within the GI tract. Describe the functions of the
   tissues and cells within each layer.

What are the four general processes in the GI tract?

Motility
• Movements of the tube. Mediated by
smooth muscle action. This is autorhythmic
2. Secretion
• Adding stuff to the tube
• Enzymes, water, other chemicals
3. Digestion
• Mechanical and chemical breakdown of
substances in tube
4. Absorption
• From tube into body

3 major mechanisms of digestion?

Local factors
- pH, physical distortion of the wall, chemicals (like nutrients)
Neural control
- Short and long reflexes
Hormonal control
- Enteroendocrine cells => at least 18 different hormones

Mass balance of the digestive system?

Everything that enters the digestive system has to come back out
• Food & drink – 2.0 L/day
• Secretions – 7.0 L/day
• Ions & Water
• Digestive molecules
• Acid, enzymes
• Mucus
• Saliva
• Bile
• Reabsorb 8.9 L/day & excrete 0.1
L/day as feces

Myenteric Plexus

• Part of ENS
• Controls smooth muscle layers

Visceral Peritonium?

• consists of areolar connective tissue covered with simple
squamous epithelium

• Covers most of the digestive tract
• Portions in the abdominopelvic cavity
• NOT the oral cavity, pharynx, esophagus, or rectum

• Secretes watery fluid to lubricate
organs

Where do short reflexes take place?

Integrated in the GI tract and ENS

Where do long reflexes take place?

Integrated in the CNS

What helps regulate digestive processes?

Neural, endocrine, and local signals

How is the digestive system regulated?

It’s regulated directly by changing secretions and motility, however, digestion and absorption are not regulated

3.7 Learning Objectives:

1. Describe the composition and functions of saliva.
2. Describe how saliva secretion is regulated.
3. Describe the structures involved and the processes leading up to and during
swallowing.
4. Explain the structural and functional characteristics of smooth muscle tissue in
the GI tract.
5. Describe the major regions and functions of the stomach.
6. Describe the cell types and secretions of cells in the walls of the stomach.
7. Describe the process and regulation of HCl secretion in the stomach, including
the location of the key transporters involved in the apical and basolateral
membranes.
8. Describe the response of the stomach secretory cells and muscular walls to the
ingestion of food.

What does bicarbonate do?

keeps saliva at near neutral pH and protects teeth and oral tissues from acids

What is a lysosome?

An antibacterial enzyme that breaks down cell walls and helps control oral bacteria

What stages of swallowing are voluntary and involuntary?

Oral is voluntary so when bolus goes through the soft palate, uvula, and epiglottis. Pharyngeal and Esophogeal are involuntary so when bolus goes through the esophogus and pharynx.

Swallowing in the esophageal stage?

upper esophogeal sphinctor relaxes, peristalis moves bolus down, (4-8 seconds for solids, 1 second for fluids), lower esophogeal sphinctor relaxes,

Smooth muscle in the gi tract

Autorhythmic behavior (modified by ANS)
• Electrical coupling (gap junctions)
• Activation of 1 SM cell is transferred to the neighboring
SM cell
• Mechanical coupling (desmosomes)
• SM cells are linked together
• Thick/thin filament arrangement
• No sarcomere/striations
• Slow, sustained contraction without fatigue (↓↓
energy cost)
• Maintains a “basal tone” which is a partial contraction even at rest

Mechanical digestion in the stomach?

1. Gentle mixing waves - bolus w/gastric
juice => chyme
2. More vigorous waves (body => pyloric
region)
3. Intense waves near pylorus => opens
pyloric sphincter (chyme => duodenum)

Cell types in the stomach?

Surface mucous cells and mucous neck cells secrete mucous. Partietal cells secrete hydrocloric acid and intrinsic factor. Cheif cells secrete pepsinogen and gastric lipase. G cells secrete the hormone gastrin.

What do parietal cells secrete?

1-3L of HCl acid per day
• Activates pepsin to break down proteins
• Denatures proteins → opening up peptide bonds to pepsin
• Kills pathogens
• Inactivates salivary amylase
Intrinsic factor
• Vitamin B12 absorption
• Key for RBC production

Chemical digestion in the stomach?

Protein digestion begins in the stomach
HCl denatures (unfolds) proteins
HCl converts pepsinogen into pepsin (protease)
Fat digestion continues*
gastric lipase => triglycerides in milk fat (most effective at pH 5-6)
HCl kills microbes in food
Mucous cells => mucus

Absoption of substances by stomach?

Water (minimal)
Electrolytes (minimal)
Some drugs (aspirin)
Alcohol

3.8 Objectives:

1. List the major regions of the small intestine and the distinguishing features of the intestinal wall in the small intestine.
2. Describe the types of cells that comprise the small intestine mucosa
and their functions.
3. Describe the major intestinal hormones (cholecystokinin [CCK],
secretin, and glucose-dependent insulinotropic peptide [GIP], GLP-1,
serotonin) and their general functions.
4. Describe the digestive process for carbohydrates, lipids (including the
role of bile salts in emulsification) and proteins and how they’re
absorbed by the body.
5. Describe how water is absorbed along the GI tract and the
importance of this absorption.

Small intestine?

Primary Site for:
1. Chemical Digestion = Enzymes and Acids (Either Brush border or
pancreatic juice)
• Carbohydrates
• Proteins
• Fats
2. Absorption
• Monosaccharides
• Amino acids
• Fatty acids
• Vitamins/minerals
• Water and electrolytes

What do absorptive cells do?

absorb nutrients

What do globlet cells do?

secrete mucous

what do S cells do?

secrete secretin

What do CCK cells do?

Secrete cholecystokinin

What do K cells do?

secrete GIP

What do L cells do?

Secrete GLP-1

What do EC cells do?

secrete 5-HT (serotonin)

What do Paneth cells do?

secrete lysosome, phagocytosis

Brush border enzymes within the small intestine?

oligo- & di-saccharidases
oligo- & di-peptidases

Pancreatic Juice?

Pancreatic Juice (1.2-1.5 L/day)
Clear, colorless liquid (pH 7.1 to 8.2):
H2O, salts, sodium bicarbonate, enzymes

Pancreatic Amylase?

Breaks down carbohydrates.

Proteases?

Enzymes are trypsin, chymotrypsin, carboxypeptidase, and elastase.

Pancreatic Lipase?

Breaks down fats

Nucleases?

Degrade nucleic acids. Enzymes are ribonuclease and deoxyribonuclease.

Carbohydrate digestion?

Mouth – salivary amylase
Stomach – mechanical only
Small Intestine —
Pancreatic juice (pancreatic amylase)
Brush border enzymes (maltase,
sucrase, and lactase)
Disaccharides = > monosaccharides
(fructose, glucose & galactose)

Absorption of monosaccharides?

Glucose-sodium symporter, galactose-secondary active transport, fructose-transporter, they enter the enterocyte, then facilitated diffusion takes them to the blood.

Digestion of Proteins?

Mouth – mechanical only
Stomach – HCl (unfolds) and pepsin
Small intestine –
Pancreatic juice (pancreatic proteases)
Trypsin, chymotrypsin, carboxypeptidases, elastase
Brush border enzymes (aminopeptidase and dipeptidase)

Absorption of amino acids and di/tripeptidase?

amino acids, dipeptides, tripeptides → sodium symporter (secondary active transport) → Enterocyte → Facilitated diffusion → Blood

Digestion of Lipids?

Mouth – lingual lipase
Stomach – gastric lipase
Small intestine —
Pancreatic juice (pancreatic lipase)
triglycerides => fatty acids + monoglycerides
Bile (liver), emulsifies lipids => tiny micelles

Absorption of Lipids?

Bile → Break → Micelle → Enterocyte → Chylomicron → Lymph

Electrolytes?

Sources of electrolytes:
GI secretions, ingested foods, and liquids
Absorption (facilitated diffusion and active transport)
sodium, potassium, calcium (active transport)
chloride, iodide, nitrate (passive)
iron, magnesium, copper, zinc, phosphate ions (active transport)

Absorption of vitamins?

Fat-soluble vitamins (A, K, D3, E): absorbed w/in micelles
Water-soluble vitamins: absorbed by facilitated diffusion & cotransport
Vitamin B12: combines w/intrinsic factor (parietal cells), absorbed by receptor-mediated endocytosis (ileum)

Absorption of H2O?

Osmosis: small and large intestine
H2O
Through cells (transcellular)
Between cells (paracellular)
Interstitial fluid ~9.3 L/day H2O
enters the GI tract
Blood capillaries

3.9 Objectives:

1. List the major digestive secretions of the pancreas and describe their
general function.
2. Describe the blood flow to and from the liver – note which vessels carry
oxygen-rich blood, nutrient-rich blood, and mixed blood to and from the liver.
3. Describe the major cell types of the liver and their functions.
4. List and describe the major functions of the liver, detailing the metabolic
functions of the liver related to carbohydrate, lipid, and protein metabolism.
5. List the major regions and structural features of the large intestine.
6. Describe the functions of the large intestine, including the control of
defecation.
7. Describe the underlying concepts related to water movement that lead to
diarrhea and constipation.

Liver Functions?

Releases bile salts
Detoxifies blood
Removes bilirubin
Stores fat-soluble vitamins (A, D3, E, K)
Stores iron, copper, and vitamin B12
Phagocytosis of old blood cells
Metabolism

Gluconeogenesis?

Convert some amino acids and glycerol from lipids into new molecules of glucose

Glycogenolysis

Breakdown of glycogen polymers into glucose if blood glucose drops

Glycogenesis

Convert excess blood glucose into glycogen polymers for storage

Lipid Metabolic Functions?

Synthesize cholesterol
Synthesize lipoproteins (HDL and LDL)
Stores some fat
Lipolysis and beta-oxidation

Protein Metabolism?

Deamination - removes -NH2 (amine group) from amino acids so we can use what is left as an energy source
Transamination - converts one amino acid into another
Synthesizes plasma proteins
Metabolic Functions

Large Intestine Functions?

Storage
Absorption (remaining H2O & electrolytes)
Bacterial contributions

Large Intestine bacterial contributions?

ferment undigested carbohydrates (CO2 & methane gas)
fiber digestion
vitamin production (B and K)

Gastroileal reflex?

stomach is full => Gastrin hormone is released and relaxes the ileocecal sphincter => the small intestine will empty into the large intestine

Gastrocolic reflex?

stomach fills => triggers a strong peristaltic wave to move through the transverse colon => contents are propelled to the rectum

Defecation reflex

substances are propelled to the rectum and cause local stretch => sends signals to the sacral spinal cord to initiate the defecation reflex to expel feces

Constipation?

↓ intestinal motility
↑ H 2O absorption
remedies: fiber, exercise, and water

Diarrhea?

↓ H2O absorption
secretory and osmotic

3.10 Objectives:

1. Describe how digestion is regulated by local physical and
chemical changes that occur as a bolus or chyme moves through
the different portions of the GI tract.
2. Describe the processes that occur during the cephalic, gastric,
and intestinal phases of digestion.
3. Describe how short reflex loops, long reflex loops, and hormones
from enteroendocrine cells regulate secretion, motility, and
digestion along the GI tract.

Regulation of Digestion?

1. Cephalic phase
2. Gastric phase
3. Intestinal phase

Cephalic Phase?

‐ anticipation
• Prepares the mouth and stomach for food
• Cerebral cortex –
sight, smell, taste & thoughts of food stimulate the parasympathetic
nervous system to stimulate...
→ Salivary glands to secrete saliva
→ Gastric glands to secrete gastric juice (Vagus)

Gastric Phase?

Neural influences over stomach activity
– stretch receptors & chemoreceptors (pH) signal bolus entry
– vigorous peristalsis and gastric gland secretions
– chyme periodically released into the duodenum
Endocrine influences over stomach activity
– distention and presence of food in stomach cause G cells to
secrete gastrin into the bloodstream
– gastrin also increases gastric gland secretions and motility,
and causes pyloric sphincter relaxation

Neural and hormonal regulation of gastric emptying?

Distention of the stomach & increased pH
Increase secretion of gastrin & increase parasympathetic activation of the vagus nerve
The vagus nerve & gastrin...
→ both stimulate the gastric gland (& increase gastric secretions)
→ both stimulate contraction of the lower esophageal sphincter and the
stomach muscles, as well as relaxation of the pyloric sphincter (& thus
increase churning + gastric emptying)

Control of gastric secretions?

Increased Gastrin & Vagus verve input (parasympathetic) → Increase activity of H+/K+ exchanger

Controlled Release of Chyme?

Neural influences: distension of the duodenum and chemical contents of the chyme activate
sympathetic nerves, which slow gastric activity
(enterogastric reflex)
• Endocrine influences: distension of duodenum and contents of chyme trigger hormonal release from enteroendocrine cells in the duodenum
– secretin decreases stomach secretions
– cholecystokinin (CCK) decreases stomach emptying

GI Reflexes?

Enterogastric reflex - regulates the amount of chyme released into the duodenum
• Gastroileal reflex - when stomach is full, gastrin hormone relaxes ileocecal sphincter so SI will empty into LI
• Gastrocolic reflex - when stomach fills, a strong peristaltic wave moves contents of transverse colon into rectum
• Defecation reflex – when rectum fills, input to sacral spinal cord return commands to expel feces

Enterogastric Reflex?

These sensory impulses:
• inhibit parasympathetic stimulation of the stomach
• stimulate sympathetic impulses to the stomach
• stimulate parasympathetic impulses to the SI & accessory organs
• Cause CCK & secretin release from the duodenum...
• (which recall also decrease gastric activity & emptying)

Secretin?

Acidity in SI causes secretin release, which causes increased sodium bicarbonate release by the pancreas

CCK

Fats and proteins in SI cause CCK release, which causes increased digestive enzyme release by the pancreas