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Physiology 210 > Gastrointestinal Tract Physiology > Flashcards

Gastrointestinal Tract Physiology Flashcards

1
Q

4 Main Functions of the GIT

A

Digestion
Absorption
Excretion
Host Defense

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2
Q

What is Digestion?

A

Chemical alteration of food into absorbable molecules

Affected by GI motility, pH changes, biological detergents, enzymes

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3
Q

What is Absorption?

A

Movement of digested food from the intestine into the blood or lymphatic system

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4
Q

What is Excretion?

A

Non-absorbable components of food, bacteria, intestinal cells, and hydrophobic molecules exit the body

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5
Q

What is Host Defense?

A

The GIT is continuous with the exterior of the body
-if the GIT is injured we can get sick because bacteria can enter the body
Highly developed immune system

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6
Q

Mouth

A

Chopper

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7
Q

Stomach

A

Blender, acid sterilizer, reservoir

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8
Q

Duodenum

A

Reaction vessel

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9
Q

Jejunum and Ileum

A

Catalytic and absorptive surfaces

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10
Q

Large Intestine

A

Residue combuster, desiccator, pelleter

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11
Q

Pancreas

A

Enzyme supplier

Neutralizer

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12
Q

Liver

A

Detergent supplier

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13
Q

Structure of the GIT

A

Long muscular tube stretching from mouth to anus

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14
Q

Layers of the GIT

A

Mucosa
Submucosa
Muscularis externa
Serosa

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15
Q

Layers of the Mucosa

A

Epithelium
Lamina propria
Muscularis mucosa

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16
Q

Epithelial Layer

A

Basolateral and apical arrangement
Different transport proteins at the apical surface compared to the basolateral
-tight junctions confine transport proteins to specific membrane regions
Provides selective uptake of nutrients, electrolytes, and water
-prevents the passage of harmful substances
Epithelial cells are born in crypts and daughter cells migrate up towards the villous

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17
Q

Nutrient Transport Across the Epithelium

A

Paracellular pathway

Transcellular pathway

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18
Q

Paracellular Pathway

A

Limited by tight junction seal

Water and small ions can diffuse through tight junctions

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19
Q

Transcellular Pathway

A

Two-step process which requires a transport protein on the apical and basolateral surface of the cell

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20
Q

Lamina Propria

A 
Connective tissue
Small blood vessels
Nerve fibres
Lymphatic vessels
Immune and inflammatory cells
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21
Q

Muscularis Mucosa

A

A thin layer of smooth muscle

  • not involved in contraction of the GIT
  • might be important in the villi movement
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22
Q

Submucosa

A

Plexus (intricate network) of nerve cell bodies
-relay information to and way from the mucosa
Also composed of connective tissue, blood, and lymphatic vessels

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23
Q

Muscularis Externa

A

A thin layer of circular muscle
-fibers orientated to cause narrowing of the lumen
Myenteric nerve plexus
-regulates muscle function
Thinner outer layer of longitudinal muscle
-fibers oriented to shorten tube

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24
Q

Serosa

A

Thin layer of connective tissue

Forms connection between the intestines and the abdominal wall

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25
Q

Blood supply to the GIT

A

Blood perfuses the intestine and then flows to the liver via the portal vein

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26
Q

What is Portal Circulation?

A

The portal vein drains blood from the digestive tract and empties directly into the liver
Portal circulation = the circulation of nutrient-rich blood between the gut and the liver

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27
Q

What is the Purpose of Portal Circulation?

A

Allows the liver to:

  • remove harmful substances
  • process nutrients
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28
Q

Why is Portal Circulation Unusual?

A

The liver receives blood from both venous and arterial circulation
The venous supply is “in series” while most circulation to organs is “in parallel”

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29
Q

Regulation of GI Processes - Reflexes initiated by:

A

Distension of wall by volume of luminal contents
Osmolarity of contents
pH of contents
Concentrations of specific digestive contents

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30
Q

Regulation of GI Processes - Propagated by:

A

Mechanoreceptors
Osmoreceptors
Chemoreceptors

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31
Q

Enteric Nervous System (Intrinsic Neural Regulation)

A 
Controls the activity of the secretomotor neurons 
-motility and secretory functions 
Contained completely within the walls of the GIT
Dense and complex network of neurons 
Can function independently of the CNS
Two nerve networks
-myenteric plexus
-submucosal plexus
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32
Q

Myenteric Plexus

A

Influences smooth muscle

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33
Q

Submucosal Plexus

A

Influences secretion

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34
Q

Extrinsic Neural Regulation

A 
Regulation is through the ANS
Influences the motility and secretion of the GIT
-hunger
-sight/smell of food
-emotional state
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35
Q

Parasympathetic Response

A

Stimulates flow of saliva
Stimulates peristalsis and secretion
Stimulates release of bile

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36
Q

Sympathetic Response

A

Stimulates flow of saliva

Inhibits peristalsis and secretion

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37
Q

Short Reflexes

A

Intrinsic

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38
Q

Long Reflexes

A

Extrinsic

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39
Q

Endocrine Chemical Messenger

A

Chemical messenger passes from cell which produced it into the blood and is carried by the blood to its target

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40
Q

Neurocrine Chemical Messenger

A

Chemical messenger is released from a nerve cell, travels across a synapse and acts on a post-synaptic target cell

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41
Q

Paracrine Chemical Messenger

A

Chemical messenger diffuses through the interstitial fluid to nearby cells

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42
Q

Autocrine Chemical Messenger

A

Chemical messenger acts on the cell that produced it

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43
Q

Hormonal Control of GI Activity

A

One surface of each endocrine cell is exposed to the GI lumen

  • chemical substances in lumen stimulate cell to release hormones across opposite surface of the cell into blood vessels in the lamina propria
  • hormones travel though blood to target cells
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44
Q

Best Understood GI Hormones

A 
Secretin
Cholecystokinin (CCK)
Gastrin
Glucose-dependent insulinotropic peptide (GIP)
All peptides
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45
Q

Generalized Facts about GI Hormones

A

Each participates in a feedback control system that regulates some aspect of the GI lumen
Most GI hormones affect more than one type of target cell

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46
Q

Types of Intestinal Motility

A

Peristalsis

Segmentation

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47
Q

Peristalsis

A

Circular muscle contracts on the oral side of a bolus of food (longitudinal layer relaxes)
Circular muscle contracted moves towards the anus, propelling the contents of the lumen in that direction
-as the ring moves, the circular muscle on the other side of the distended area relaxes (longitudinal muscle contracts) which facilitates smooth passage of the bolus

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48
Q

Segmentation

A

Contraction and relaxation of intestinal segments with little net movement of contents towards the large intestine
Mostly occurs in the small intestine
Allows mixing of contents with digestive enzymes
Slow transit time for absorption

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49
Q

Basic Electrical Rhythm

A

GIT had pacemaker cells throughout smooth muscle cells
-constantly undergoing spontaneous depolarization-repolarization cycles (slow waves)
In the absence of neural/hormonal input, spontaneous slow waves do not result in significant contraction

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50
Q

Phases of GI Control

A

Cephalic
Gastric
Intestinal

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51
Q

Cephalic Phase

A

Receptors in the had stimulated by:
-sight, smell, taste
-emotional state
Parasympathetic fibres activate neurons in the GI nerve plexuses

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52
Q

Gastric Phase

A

Receptors in the stomach stimulated by:
-distension, acidity, amino acids, peptides
Short and long neural reflexes mediate the response

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53
Q

Intestinal

A

Receptors in the intestine stimulated by:
-distension, acidity, osmolarity, digestive products
Mediated by short and long neural reflexes and by hormones secretin, CCK, and GIP

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54
Q

Parts of the brain involved in food intake

A

Hypothalamus

Ventromedial region

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55
Q

The hypothalamus and food intake

A

Feeding center in the lateral region
Activation increases hunger
-animals with damage to this area become anorectic and lose weight

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56
Q

The Ventromedial Region

A

Satiety centre
Activation makes you feel full
-animals with damage to this area overeat and become obese

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57
Q

Factors that Influence Food Intake

A

Orexigenic factors = increase intake

Anorexigenic factors = decrease intake

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58
Q

Orexigenic Factors

A

Neuropeptide Y
-neurotransmitter in the hypothalamus that stimulates hunger
Ghrelin
-synthesized and released from endocrine cells in the stomach during fasting
-stimulates the release of NPY and others in the hypothalamus feeding center

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59
Q

Anorexigenic Factors

A

Leptin (adipose)
Insulin (pancreas)
Peptide YY (intestines)
Melanocortin (hypothalamus)

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60
Q

4 Ways that Water Intake is Regulated

A

Increased plasma osmolarity
Decreased plasma volume
Dry mouth and throat stimulates thirst
Prevention of over-hydration

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61
Q

Water Intake - Increased Plasma Osmolarity

A

Osmoreceptors in thirst centre within the hypothalamus

When salt concentration increases, vasopressin is released and conserves water at the kidney

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62
Q

Water Intake - Decreased Plasma Volume

A

When we lose plasma due to vomiting or diarrhea stimulation of baroreceptors in the cardiovascular system
Baroreceptors in the kidney afferent arteries lead to activation of the renin-angiotensin system
-increases thirst

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63
Q

Water Intake - Prevention of over-hydration

A

A person stops drinking well before water is absorbed by the GIT
Probably mediated by stimulus from mouth, throat, and GIT

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64
Q

Main Salivary Glands

A

Parotid - watery (serous) secretion
Submandibular - serous/mucous secretion
Sublingual - mucous secretion

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65
Q

How much saliva does an adult produce per day?

A

1500 mL

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66
Q

Composition of Saliva

A 
Water
-hypotonic, slightly alkaline
Electrolytes
-rich in potassium and bicarbonate
-poor in sodium and chloride
Digestive enzymes
-amylase, lipase
Glycoproteins
-mucin 
Other components
-anti-microbial factors
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67
Q

Functions of Saliva

A 
Moistens and lubricates food 
Initiates digestion 
Dissolves a small amount of food 
-allows diffusion to taste buds
Antibacterial actions
Aids in speech
Buffering action
-bicarbonate helps neutralize acid
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68
Q

Components of the salivary gland

A

Acinar cells
Ductal cells
Myoepithelial cells
Made up of many microscopic ducts that branch out from grossly visible ducts

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69
Q

Acinar Cells

A

Secrete the initial saliva

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70
Q

Ductal Cells

A

Create the alkaline and hypotonic nature of saliva

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71
Q

Myoepithelial Cells

A

Characteristics of both smooth muscle and epithelial cells

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72
Q

Formation of Saliva

A

Acinar cells secrete the initial saliva
-proteins are released by exocytosis
-chloride, bicarbonate, and potassium are actively secreted
-sodium and water follow paracellularly via leaky tight junctions
-initial secretion is isotonic
-myoepithelial cells contract and expel formed saliva from acinus into the duct
Ductal cells modify the initial saliva to a hypotonic, alkaline state
-net loss of sodium and chloride
-addition of potassium and bicarbonate
-duct cells are tightly joined and impermeable to water

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73
Q

Regulation of Salivary Gland Function

A

Both the parasympathetic and sympathetic systems stimulate salivary secretion
No hormonal regulation
Major influence is the parasympathetic system
-increases blood flow to glands which results in increased secretion
-also important for increase protein secretion from acinar cells and stimulate myoepithelial cells

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74
Q

Parasympathetic Salivary Gland Function

A 
Stimulated by:
-smell and taste
-pressure receptors in the mouth
-nausea (protective)
Inhibited by:
-fatigue, sleep, fear, dehydration, some drugs
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75
Q

Sympathetic Salivary Gland Function

A

Modestly increases saliva flow
Increased protein secretion from acinar cells
Stimulates myoepithelial cells

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76
Q

What is the role of saliva in digestion?

A

Amylase

  • starch digestion is initiated
  • inhibited by the acidic stomach pH

Lingual Lipase
-acid-stable and therefore active in the stomach

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77
Q

When would saliva play a bigger role in digestion?

A 
When there are pathological conditions (pancreatic insufficiency)
For neonates (immature digestive system)
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78
Q

What are some conditions that cause xerostomia?

A

Congenital conditions
Autoimmune conditions
Side effect of drugs
Radiation treatment

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79
Q

Consequences of Xerostomia

A 
Dry mouth
Decreased oral pH
-tooth decay
-esophageal erosions
Difficulty in lubricating and swallowing food 
-poor nutrition
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80
Q

Treatment of Xerostomia

A

Frequent sips of water and fluoride

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81
Q

How is swallowing initiated?

A

This reflex is initiated by pressure receptors in the walls of the pharynx
-stimulated by food/liquid entering the pharynx
Receptors send signals to the swallowing centre in the brainstem which in turn signals muscles in the:
-pharynx
-esophagus
-respiratory muscles

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82
Q

Larynx

A

The air passage between the pharynx and trachea

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83
Q

Glottis

A

The area around vocal cords - where air travels through

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84
Q

Epiglottis

A

A tissue flap that covers the trachea during swallowing

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85
Q

The Steps of Swallowing

A
  1. Tongue pushes food bolus to the back of the pharynx
  2. Soft palate elevates to prevent food from entering the nasal passages
  3. Epiglottis covers the glottis to prevent food or liquid from entering the trachea
  4. Food descends into the esophagus
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86
Q

What is the function of the esophagus?

A

Transfers food from mouth to stomach

Food passes very rapidly

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87
Q

Structure of the Esophagus

A

Skeletal muscle surrounds the upper third, smooth muscle surrounds the lower two-thirds

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88
Q

What type of epithelium does the esophagus have?

A

Stratified squamous epithelium (20-30 cells thick) because the esophagus is exposed to rough and abrasive food contents

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89
Q

Upper Esophageal Sphincter

A

Ring of skeletal muscle just below the pharynx

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90
Q

Lower Esophageal Sphincter

A

Ring of smooth muscle at the stomach

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91
Q

When are the only times these sphincters are open?

A

Swallowing
Vomiting
Burping

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92
Q

The Esophageal Phase of Swallowing

A
  1. Relaxation of the upper esophageal sphincter
  2. Peristaltic waves move food bolus down the esophagus
  3. Lower sphincter opens and allows food to pass into the stomach
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93
Q

What is the main driving force of swallowing?

A

Peristalsis

Gravity assists but is not necessary

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94
Q

What is heartburn?

A

When stomach acid comes back up into the esophagus

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95
Q

What happens when small amounts of acid are in the esophagus?

A

Stimulates peristalsis
Increases salivary secretion
Results in neutralization and clearance

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96
Q

When does heartburn occur?

A

Due to an inefficient sphincter
After a big meal
During pregnancy

97
Q

Where is the stomach located?

A

Between the esophagus and the small intestine

98
Q

Functions of the Stomach

A

Storage of food
Mechanical breakdown of food
Chemical breakdown of food
-secretes pepsinogen and HCl
Controls the rate at which food enters the small intestine
Secretes intrinsic factor for absorption of vitamin B12
Very little absorption occurs across the stomach

99
Q

what does pepsinogen digest?

A

Protein

100
Q

What does HCl in the stomach do?

A

Dissolves food
Partially digests macromolecules in food
Sterilizes food

101
Q

The Fundus and Body of the Stomach

A 
A thin layer of smooth muscle
Secretes:
-mucus
-pepsinogen
-HCl
102
Q

The antrum of the Stomach

A 
The thicker smooth muscle layer
Secretes:
-mucus
-pepsinogen
-gastrin
103
Q

Exocrine

A

Chemical messengers secreted into ducts then on to an epithelial surface without passing into the blood

104
Q

Major Exocrine Secretions of the Stomach

A

Mucus = protective coating over the stomach epithelium to avoid self-digestion

HCl = hydrolysis of proteins

Pepsinogen = digestion of proteins

105
Q

Minor Secretions of the Stomach

A

Intrinsic Factor = for B12 absorption

Gastrin (endocrine) = stimulates HCl production and stomach motility

Histamine (paracrine) = stimulates HCl production

Somatostatin (paracrine) = inhibits HCl production

106
Q

Where is Chief Cells Found?

A

Gastric glands in all regions

107
Q

What do chief cells secrete?

A

Secrete pepsinogen

  • an inactive precursor to pepsin
  • pepsinogen cleaved by acid to pepsin
  • pepsin accelerates protein digestion
108
Q

Where are enteroendocrine cells found?

A

Gastric glands in the antrum

109
Q

What do enteroendocrine cells secrete?

A

Secretes gastrin (hormone)

  • stimulates HCl production by parietal cell
  • stimulates GI motility
110
Q

Where are ECL cells found?

A

Gastric glands in all regions but more so in the antrum

111
Q

What do ECL cells secrete?

A

Secrete histamine

-stimulate HCl release

112
Q

Where are D-cells found?

A

Gastric glands in all regions but more so in the antrum

113
Q

What do D-cells secrete?

A

Secrete somatostatin

-negative regulator of HCl secretion

114
Q

Where are parietal cells found?

A

Found in gastric glands contained in the fundus/body regions

115
Q

What do parietal cells secrete?

A

Secretes HCl and intrinsic factor

116
Q

What are Canaliculi?

A

They increase the surface area of the parietal cells and maximize secretion into the stomach lumen
Actively secreting cell has better defined canaliculus

117
Q

What pH does the stomach secrete?

A

2 L of 0.1 M HCl per day

118
Q

Steps of Acidification of the Stomach Lumen?

A
  1. H+/K+ ATPase
    - active transport
  2. Carbonic anhydrase
    - forms bicarbonate
    - bicarbonate dissociates into H+ and HCO3-
  3. Cl-/HCO3 exchanger
    - excess HO- is effluxed from the cell as HCO3- isn exchange for Cl-
    - critical step for the maintenance of neutral cellular pH
  4. K+ channels
    - K+ recycled back into stomach lumen
    - diffusion through channel
    - loss of positive charge
  5. Cl- channels
    - Cl- leaks back into stomach lumen
    - diffusion through channel
119
Q

Which 4 chemical messengers regulate the insertion of the H+/K+ ATPase into the plasma membrane of the parietal cell

A 
Gastrin
Acetylcholine
Histamine
Somatostatin 
-inhibits the release of HCl, gastrin, and histamine
120
Q

Phases of Gastric Secretion

A

Cephalic Phase
Gastric Phase
Intestinal Phase

121
Q

Cephalic Phase

A

Anticipatory, excitatory, mainly via the vagus

122
Q

Gastric Phase

A

Major phase, excitatory, mainly via gastrin

123
Q

Intestinal Phase

A

Mainly inhibitory, due to the presence of acid, fat, digestion products and hypertonic solutions in duodenum

124
Q

Regulation of Gastric Secretion

A

Acetylcholine, gastrin, and histamine all directly increase acid secretion by the parietal cell
Somatostatin inhibits acid secretion by the parietal cell
Acetylcholine also stimulates the release of gastrin from G cells and histamine from ECL cells and inhibits somatostatin release from D cells

125
Q

What happens when acid secretion is at a high rate?

A

Parasympathetic input reduces
Negative feedback occurs for gastrin production (acid inhibits release)
Somatostatin release increases

126
Q

Stomach Size

A

50 mL

When we eat a meal, there is smooth muscle relaxation and the stomach can increase to 1.5 L without increased pressure

127
Q

Food arriving in the stomach causes:

A

Peristaltic waves

  • weaker contractions in the body of the stomach
  • powerful contraction in the antrum
  • mixes luminal contents, closes pyloric sphincter
128
Q

Closure of the pyloric sphincter results in:

A

Small amount of stomach contents released to the duodenum
Most antral contents forced backward towards the body of the stomach
-mixing of contents with enzymes and acid

129
Q

Electrical Basis of Stomach Motility

A

The stomach has pacemaker cells in the smooth muscle layer
-spontaneous slow waves of depolarization and repolarization = do not cause significant contractions
Excitatory hormones and neurotransmitters further depolarize and determine the strength of the contraction

130
Q

Causes of vomiting

A 
GIT disturbances
Psychogenic 
Motion sickness
Inner ear infection
Alcohol
Pressure on CNS
131
Q

What happens when we vomit?

A

Nausea, salivation, breath held in mid-inspiration
Glottis closes off trachea
Lower esophageal sphincter and esophagus relaz
Diaphragm and abdominal muscles contract
Reverse peristalsis moves upper intestinal contents into stomach
Stomach contents move up through esophagus and out through the mouth

132
Q

Benefits of Vomiting

A

Removal of harmful substances prior to absorption

Nausea and feeling bad should prevent individual from consuming noxious substance again

133
Q

Negatives of Vomiting

A

Dehydration
Loss of salts
Metabolic alkalosis due to loss of H+
Acid erosion of tooth enamel

134
Q

What is a Peptic Ulcer

A

A damaged/eroded area of the GIT mucosa, usually in regions that are acidic

135
Q

What causes an Ulcer?

A

An imbalance between aggressive factors (acid) and protective factors (bicarbonate)
Bacterial infection is a major cause

136
Q

Treatment for Ulcers

A

Antibiotics
H+/K+ ATPase inhibitor
Histamine (H2) antagonist
Prostaglandin-type drugs

137
Q

Gastric Bypass Surgery

A

Used in treatment of morbid obesity

Stomach is smaller

138
Q

The Pancreas

A

an exocrine and endocrine gland

139
Q

Exocrine Pancreas

A

Produces secretions that go into the gut
Source for the majority of enzymes required for meal digestion
Enzymes are produced in excess
Critical for secreting bicarbonate into the duodenum for the neutralization of stomach acid = critical for enzyme function

140
Q

Endocrine Pancreas

A

Non-digestive

Produces hormones that regulate the entire body (insulin)

141
Q

Exocrine Pancreas Structure

A

Secretion of substances into ducts that drain onto an epithelial surface

142
Q

Endocrine Pancreas Structure

A

Ductless gland, secretion occurs across the epithelial basolateral surface for diffusion into the blood

143
Q

Where do the pancreatic duct and common bile duct join?

A

Just before entering the duodenum

144
Q

Pancreatic Ducts

A

Similar to salivary glands
Acinar cells produce and secrete digestive enzymes
Duct cells secrete water and bicarbonate

145
Q

Pancreatic Juices

A

Isotonic and alkaline
Contains electrolytes
-high bicarbonate, low chloride
-sodium and potassium the same as in plasma
Contains digestive enzymes
-secreted by acinar cells
proteolytic enzymes are stored and secreted in inactive forms, activated in the duodenum

146
Q

Ductular Cell Secretion of Bicarbonate

A
  1. Chloride channel opens
    -allows diffusion of chloride into duct lumen
  2. Cl- in lumen is exchanged for bicarbonate in the cell
  3. Water and sodium follow paracellularly in response to electrochemical gradient across the epithelium
  4. Neutral pH of cytosol is maintained by exchange of H+ for Na+
    -secondary active transport
    Resulting in watery alkaline secretion neutralizes gastric acid and washes digestive enzymes through
147
Q

Alkaline Tide

A

After a big meal:
Parietal cells in the stomach are producing lots of acid so large amounts of bicarbonate are pumped across the basolateral surface into the blood stream

148
Q

Acid Tide

A

After a big meal:
Duct cells in the pancreas are producing and secreting bicarbonate so large amounts of protons are pumped across the basolateral surface into the blood stream

149
Q

Alkaline and Acid Tide

A

Bicarbonate and protons from the pancreas eventually meet up in the portal vein
Two processes compensate for each other and maintain acid-base balance

150
Q

Digestive Function of the Pancreas

A

Source for the majority of enzymes required for meal digestion
Starvation would occur without the pancreas

151
Q

Proteases

A

Digests proteins in peptides and amino acids

152
Q

Amylolytic Enzymes

A

Digest starch into sugars

153
Q

Lipases

A

Digest triglycerides into free fatty acids and monoglycerides

154
Q

Nucleases

A

Digest nucleic acids into free nucleotides

155
Q

Acinar Cells and Enzymes

A

Acinar cells synthesize and package pro-enzymes into zymogen granules that are stored at the apical pole of the cell

156
Q

Why are enzymes in the pancreas stored as inactive forms?

A

The enzymes can digest the pancreas

They are activated in the duodenum

157
Q

Enterokinase

A

Enzyme embedded in the luminal membrane of the duodenum and cleaves trypsinogen to trypsin

158
Q

Trypsin

A

Inactive trypsinogen is activated by enterokinase

Endopeptidase that results in mixture of peptides and amino acids

159
Q

Chymotrypsin

A

Inactive chymotrypsinogen is activated by trypsin

Endopeptidase that results in a mixture of peptides and amino acids

160
Q

Elastase

A

Inactive pro-elastase is activated by trypsin

Endopeptidase that results in a mixture of peptides and amino acids

161
Q

Carboxypeptidase A and B

A

Inactive pro-carboxy peptidase A and B is activated by trypsin
Exopeptidase that results in a mixture of peptides and amino acids

162
Q

Amylase

A

Cleaves starches to sugars

163
Q

Lipase

A

Hydrolyzes triglycerides into free fatty acids

164
Q

Phospholipase A2

A

Inactive prephospholipase A2 is activated by trypsin

Hydrolyzes phospholipids into free fatty acids

165
Q

Cholesterolesterase

A

Hydrolyzes cholesterol-esters into free fatty acids and cholesterol

166
Q

CCK and Secretion of Pancreatic Juice

A

Fatty acids and amino acids in the small intestine trigger CCK secretion from cells in the small intestine into the blood
Circulating CCK stimulates
-the pancreas to increase digestive enzyme secretion
-gall bladder contraction
-relaxation of the sphincter of Oddi
Fat and amino acids are absorbed and stimulation of CCK release is stopped due to their removal

167
Q

Secretin Regulation of Pancreatic HCO3-

A

Acid enters the duodenum
Reduced pH triggers secretion from cells in the small intestine into the blood
Circulating secretion stimulates
-pancreas duct cells to increase HCO3- secretion
-liver duct cells to increase HCO3- secretion
Stomach acid is neutralized and stimulation of secretin release stopped

168
Q

Secretin and CCK Influence on the Stomach

A

Secretin and CCK inhibit gastrin secretion

Results in reduced stomach motility and reduced acid secretion

169
Q

Phases of Pancreatic Secretion

A

Cephalic phase = minor phase but sight, taste, smell will stimulate pancreatic secretion vis parasympathetic nerves

Gastric phase = minor phase but distension of the stomach will stimulate pancreatic secretion via the parasympathetic nerves

Intestinal phase = major phase of regulation. acid from the stomach in the duodenum results in secretin release. digested fat and protein in duodenum results in CCK release

170
Q

Cystic Fibrosis

A

Still produce digestive enzymes but HCO3- and water secretion is low and enzymes do not get flushed from the ducts
Retained proteolytic enzymes can result in pancreatic autodigestion
Need supplements of digestive enzymes

171
Q

Hepatic and Lobule Structure

A

Hexagonal structure with a central vein running through the centre and portal triads at each corner

172
Q

What is the portal triad composed of?

A

Hepatic artery
Portal vein
Bile duct

173
Q

Microanatomy of the liver

A

Bile components produced by the hepatocytes are put into the canalicular networks
Bile components flow towards the bile ducts
Blood flow occurs on the other surface of the hepatocyte

174
Q

Major Functions of the Liver

A

Exocrine gland = formation and secretion of bile
Metabolism and storage of nutrients = liver matches supply demand
Deactivation and detoxification = drugs, hormones, waste products, toxins
Production of circulating proteins = blood coagulation factors, lipoproteins

175
Q

Constituents of Bile

A 
Bile acids = synthesizes within the hepatocyte from cholesterol and amphipathic 
Cholesterol = slightly amphipathic 
Salts and water
Phospholipids
Bile pigments = bilirubin 
Trace metals
176
Q

Role of Bile in Fat Digestion

A

Pancreatic lipase is water-soluble enzymes and can only work on the surface of liquid droplets
Large lipid droplets need to be made smaller for efficient access by lipase in a process called emulsification

177
Q

Emulsification

A

Mechanical disruption to make lipid droplets smaller (GI motility)
Emulsifying agent to prevent droplets from re-aggregating = bile acids

178
Q

Bile Acid and Micelles

A

Bile acids also form mixed micelles with phospholipids and products of lipase digestion
Micelle is a soluble cluster of amphipathic molecules with nonpolar groups in the middle and polar groups on the outer layer

179
Q

Micelle Function

A

Fatty acids are really insoluble in water
Micelles keep fatty acids in small soluble aggregates
Equilibrium between the micelle and free fatty acids (free forms diffuse across the SI epithelium
Micelles are like holding station for small nonsoluble lipids

180
Q

Formation of Bile

A

Hepatocytes = produce and secret bile acids
-also secrete phospholipids, cholesterol, and bile pigments
-all components secreted through primary active transport pathways
Bile ducts add HCO3- and water to bile
Gallbladder = stores and concentrates the bile between meals then expels it into the duodenum after a meal

181
Q

The Enterohepatic Circulation of Bile Acids

A

Bile acids are conserved
Recycling of bile acid occurs through the enterohepatic circulation
Allows the secretion rate to greatly exceed the synthesis rate

182
Q

Steps for Bile Acid Recycling

A
  1. Bile acids are released by the liver/gallbladder into the duodenum for fat digestion
  2. Bile acids are reabsorbed across the Ileum into the portal circulation
  3. Bile acids are transported back into hepatocytes
183
Q

How do Dietary Fibers Lower Cholesterol?

A

Bile acids are made out of cholesterol

High fibre foods bind with bile acids which prevent it’s reabsorption and the acids are lost in feces

184
Q

Regulation of Hepatobiliary Secretion During Intestinal Phase - Bile Salts

A

As more bile salts are absorbed from the ileum and return to the liver, more will be secreted back into the bile
Bile salt synthesis is reduced when the enterohepatic circulation is working well

185
Q

Regulation of Hepatobiliary Secretion During Intestinal Phase - Secretin

A

Secretin is produced and released by S-cells in the duodenum when acid stimulates the duodenum
Secretin increases HCO3- secretion by the bile duct cells and the pancreas

186
Q

Regulation of Hepatobiliary Secretion During Intestinal Phase - CCK

A

Produced by the I-cells in the duodenum and jejunum
CCK increases contraction of the gallbladder and releases the sphincter of Oddi
-bile is released into the duodenum

187
Q

Causes of Gallstones

A

Cholesterol stones
-when [cholesterol] gets too high then cholesterol will start to precipitate out

Pigment stones

  • less common
  • caused by excessive hemolysis which increases [pigment] which will precipitate with calcium ions
188
Q

Consequences of Gallstones

A

May cause obstruction/infection of the gallbladder, liver, pancreas
Pain, nausea, jaundice

189
Q

Treatment of Gallstones

A

Cholecystectomy
Removal of stones
Drugs to dissolve gallstones

190
Q

The 3 Sections of the Small Intestine

A

Duodenum
Jejunum
Ileum

191
Q

Major Functions of the Small Intestine

A

Digestion and absorption of protein, fat, carbohydrate, electrolytes, water, minerals, vitamins

192
Q

Function of the Duodenum

A

Mixing of pancreatic digestive enzymes and bile with food
Absorption of nutrients, iron, calcium
Release of endocrine hormones secretin and CCK

193
Q

Function of the Jejunum

A

Digestion and absorption

194
Q

Ileum

A

Digestion and absorption

  • bile acids
  • vitamin B12
195
Q

How is the Surface Area Increased in the SI?

A

In the lumen of the SI are folds of Kerckring (circular folds)
On these folds on villi with microvilli on the epithelium of the villi
Crypts are projections in the opposite direction

196
Q

Villus

A

Protrusion of the epithelium into the lumen of the GIT

197
Q

Crypts

A

Invaginations of the epithelium

198
Q

Absorptive Cell (Enterocyte)

A

Absorption

Brush border enzymes

199
Q

Goblet Cell

A

Secretion of mucus

200
Q

Enteroendocrine Cell

A

Release of hormones

201
Q

Paneth Cell

A

Secrete antibacterial proteins

202
Q

What is a Brush Border?

A

Microvilli of epithelial cells covering the villi of the small intestine, major absorptive surface of the small intestine

203
Q

What is a Brush Border Enzyme?

A

An enzyme anchored to the brush border with catalytic activity in the lumen
Important for breaking down carbs and peptides into sugars and amino acids prior to transport across the enterocyte

204
Q

Digestion of Carbohydrates in the SI

A

Maltose and alpha-limit dextrins are broken down into glucose by brush border enzymes

Sucrose and lactose are also broken down by brush border enzymes

205
Q

How are glucose and galactose absorbed?

A

Move from the intestinal lumen into the enterocyte through the Na+-dependent glucose transporter SGLT on the apical membrane
Glucose and galactose are transported across the basolateral surface through a facilitated glucose transporter, GLUT

206
Q

How is fructose absorbed?

A

Moves into the enterocyte across the apical membrane through a facilitated carrier, GLUT5
Fructose is transported across the basolateral surface of the enterocyte through a facilitated glucose transporter, GLUT2

207
Q

Digestion of Proteins in the SI

A

Proteins are further broken down into free amino acids by carboxypeptidase, aminopeptidase, other brush border enzymes

208
Q

How are amino acids absorbed?

A

Free amino acids are absorbed by secondary active transport coupled to Na+
Small peptides can also be absorbed by different secondary active transport proteins coupled to H+
Amino acids then undergo facilitated diffusion across the basolateral surface of the enterocyte

209
Q

Fat Digestion in the SI

A

Lipid droplets are emulsified by mechanical disruptions and pancreatic lipase
Products of lipase are incorporated into micelles
As micelles breakdown they release fatty acids and monoglycerides that can diffuse across the SI epithelium

210
Q

How are fatty acids absorbed?

A

As soon as fatty acids are absorbed, they are converted into extracellular fat droplets called chylomicrons by the ER and Golgi
Chylomicrons enter the lymphatic system because lacteals are leakier and eventually enter systemic circulation via the thoracic duct
Lipoprotein lipase releases triglycerides from chylomicrons to be absorbed by tissues

211
Q

Absorption of Iron in the SI

A

Iron is actively transported into the enterocyte and incorporated into the protein ferritin (stores iron)
Iron that is not stored is released on the blood side and transported by transferrin

212
Q

Accumulation of Iron in Tissues

A

We do not expel iron

This can result in toxicity including skin pigmentation and heart failure

213
Q

Why Would Someone Accumulate Iron?

A

Genetic defects in absorption control pathways
Adult males/post-menopausal women excessively supplementing
Poisoning

214
Q

Iron-deficiency Anemia

A

Reduced number of red blood cells

215
Q

Why is the Control of Fluid in the Intestine Critical for GI Function?

A

Permits contact between food and digestive enzymes
Diffusion of digested nutrients to absorption site
Fluidity provides for transit without damage to the epithelium

216
Q

Generalizations about Water Absorption and Secretion

A

Water absorption at the villi, secretion from the crypts

Intestinal epithelium establishes an osmotic gradient and water follows through tight junctions

217
Q

Absorption of Water in the SI

A

Predominantly depends on Na+ gradients generated during secondary active nutrient uptake

218
Q

Secretion of Water in the SI

A

Predominantly depends on Cl- gradients generated by secondary active Na+/K+/2Cl- transporter

219
Q

What is Segmentation During Digestion?

A

Continuous division and subdivision of intestinal contents
-mechanical breakdown of food
-mixing of food with digestive enzymes
Frequency is set by basic electrical rhythm
Contraction force determined by neurohormonal input
Slow net migration towards the LI
-allows digestion and absorption of food

220
Q

What happens to Motility in the SI After Absorption?

A

Segmentation contractions stop

Replaced by a peristaltic activity called the migrating myoelectric complex

221
Q

What is the purpose of the MMC?

A

Pushes any undigested material from the small to the large intestine
Prevents bacteria from remaining in the small intestine

222
Q

Lactose Intolerance

A

Cannot completely digest lactose which causes
Results in decreased water absorption
Lactose goes to the LI and bacteria eats it
Causes diarrhea and gas

223
Q

Cholera

A

Occurs after eating or drinking something that is contaminated with the bacteria
Vomiting and diarrhea
The treatment is fluid replacement and intravenous fluids

224
Q

The Large Intestine

A

A tube approximately 6.5 cm in diameter and 1.5 m in length

225
Q

Ileocecal Valve

A

Sphincter between the cecum and ileum
Open when ileum contracts post meal
Closed when large intestine distended
Retains large intestine contents

226
Q

Function of the Cecum/Appendix

A

No apparent function

227
Q

Function of the Ascending/Transverse/Descending/Sigmoidal Colon

A

Reabsorption of water
Reservoir for the storage of wastes and indigestible materials prior to elimination by defecation
Absorption of products of bacterial metabolism

228
Q

Function of the Rectum

A

Reservior for feces

229
Q

Function of the Anus

A

Two sphincters that control defecation

230
Q

Lining in the Colon

A

Only contains crypts - no villi

Surface area is much lower than SI

231
Q

Large Intestine Cell Types

A 
Absorptive cells
-no brush border enzymes 
Goblet cells
-abundant
Very few paneth and endocrine cells 
Ecosystem of bacteria (10^12 bacteria/g of large intestine)
-produce vitamins
-produce gas
232
Q

Absorption of Water in the Large Intestine

A

Predominantly depends on Na+ gradients generated by Na+/K+ ATPase

233
Q

Secretion of Water in the Large Intestine

A

Predominantly depends on Cl- gradients generated by secondary active Na+/K+/2Cl- transporter

234
Q

The Purpose of Motility in the Large Intestine

A

Mixing the contents and retaining them for optimal salvage of fluid and bacterial products

235
Q

Mixing in the Large Intestine

A

Segmentation but slower

Allows colon retention for 18-24 hours

236
Q

Propulsion in the Large Intestine

A

Three to four times a day a wave of intense contraction known as a mass movement spreads rapidly over the large intestine, pushing contents towards the anus
Occurs after eating and prior to defecation

237
Q

Defection

A

Rectum contracts, internal anal sphincter relaxes and the outer anal sphincter contracts
Increased peristaltic activity in the sigmoid colon, increasing pressure results in reflex relaxation of the external anal sphincter
Feces voided

238
Q

What is feces made of?

A

Water, undigested food, bacteria, sloughed epithelial cells

Physiology 210 (12 decks)

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