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Digestive Physiology Flashcards

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1
Q

What are the digestive activities in the GI tract?

A
  • Ingestion
  • Secretion
  • Digestion
  • Motility
  • Absorption
  • Defecation
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2
Q

What happens during ingestion?

A 
•	Mechanical breakdown (from large to smaller particles)
o	Chewing (mouth)
o	Churning (stomach)
o	Segmentation (small intestine)
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3
Q

What happens during secretion?

A
  • Gastric mucosa and accessory organs will secrete

* Aids in the digestion and absorption

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4
Q

What happens during digestion?

A

• Chemical breakdown

o From polymer to monomer for absorption

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5
Q

What happens during motility?

A

• Propulsion (swallowing, peristalsis, mix and propel)

o Swallowing (oropharynx)

o Peristalsis (esophagus, stomach, small intestine, large intestine)

  • Starts with voluntary to involuntary actions of peristalsis
  • Wave of contractions and relaxation

o For better mixing and propel

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6
Q

What happens during absorption?

A
  • Absorption by mucosa lining, aided by microvilli

* Once absorbed -> transported to the lymph vessels and blood vessels

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7
Q

What happen during defecation?

A

• Elimination for undigested food via anus

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8
Q

Describe the events in the site of ingestion of oral cavity

A

• Mastication (mechanical breakdown of food, chewing)
o Teeth
o Muscles of mastication (masseter, tempolaris etc)

• Taste
o On tongue and sends signals to gastro centre in the brain

• Salivary glands
o Forms bolus by mixing food and saliva
o Enters esophagus and initiates swallowing

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9
Q

What are the properties and functions of saliva?

A

• Protection
o By producing lysozyme -> contains IgA -> prevents decay of tooth by killing bacteria

• Lubrication
o By mucoid secretion of saliva

• Formation of bolus
• Digestion of carbohydrates
o By salivary amylase

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10
Q

What are the regulation of saliva?

A

• ~1l of saliva produced a day
o aids in swallowing process by voluntary/reflexes

• controlled by Autonomic Nervous System (ANS)
o Sympathetic
- From T1-T3 help produce viscous saliva

o Parasympathetic
- From cranial nerve (7&9) help initiate watery salivary secretion

• Cerebral cortex and brain do take part in controlling
o Salivation decreases when person is asleep, rest, fatigue, scared

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11
Q

What is the regulation of the GIT?

A
  • Receptors
  • Emotional (cephalic reflexes)
  • Enteric Nervous System (ENS)
  • GI peptides
  • Hormone producing cells of stomach and small intestine
  • ANS Sympathetic
  • ANS Parasympathetic
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12
Q

Where are receptors for the regulation of the GIT?

A

GI tract wall

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13
Q

Where the role of cephalic reflexes for the regulation of the GIT?

A

Smell of food

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14
Q

Where the role of Enteric Nervous System (ENS) for the regulation of the GIT?

A

o Self-contained (intrinsic)

o Stimulates the inside/outside of tract

o Involves CNS/ANS centres

  • Centres: responds to different stimuli
  • Stimulis: stretching of organ for food in lumen, osmolarity, pH of content
  • Stimulated by chemical and mechanical receptors -> activates/inactivates -> gland that secrete digestive juice/hormones, or stimulate muscles to mix the content along the tract
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15
Q

Where the role of GI peptides for the regulation of the GIT?

A

o Regulatory role as hormones or paracrines

o Eg: gastrin, CCK, secretin

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16
Q

Where the role of hormone producing cells for the regulation of the GIT?

A

• Stomach and small intestine have hormone producing cells -> stimulated -> release product into interstitial fluid in extracellular space

o Blood and interstitial fluid -> distribute hormones -> to target cells in the same digestive organs -> induce for secretion or contraction

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17
Q

Where the role of ANS of parasympathetic for the regulation of the GIT?

A

enhance secretion of GI juices & digestion

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18
Q

Where the role of ANS of sympathetic for the regulation of the GIT?

A

decreases/inhibits hormones

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19
Q

What are the motility patterns in GI tract?

A
  • Peristalsis

* Segmentation

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20
Q

What happens during peristalsis?

A
  • Food comes from mouth
  • Adjacent segments of alimentary tract organs -> alternately contract and relax -> moves the food along the tract distally
  • One directional movements allows movement of food from one end to another
  • Takes place in esophagus and large intestine
  • Reverse peristalsis: vomiting
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21
Q

What happens during segmentation?

A

• Nonadjacent segments of alimentary tract organs -> alternately contract and relax -> moving food forward then backward -> for food mixing and slow food propulsion will occur

• Not in one direction movement, but zigzag movement
o For proper mixing and max absorption of chyme

• Takes place in small intestine

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22
Q

What are the phases of swallowing?

A

• Involves 22 muscles

• 3 phases
o Buccal phase
o Pharyngeal phase
o Esophageal phase

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23
Q

Describe the phases of swallowing

A

• Buccal phase
o Upper esophageal sphincter contract

o Food in the mouth -> mechanically broken down -> mix with saliva to form bolus

• Pharyngeal phase
o Tongue presses against hard palate -> forcing bolus into the oropharynx to laryngopharynx

o Voluntary control (eg. poisoned food can be spat out)

o Do not talk and eat (confuses the epiglottis to open or close)

• Pharyngeal-esophageal phase
o Begins as uvula and larynx rise to prevent food (or regurgitation) from entering the respiratory passageways

o Tongue blocks off the mouth, epiglottis close the larynx

o UES relaxes  food enter the esophagus

• Esophageal phase
o Constrictor muscles of pharynx contract -> forcing food into esophagus inferiorly (peristaltic activity begins) -> contract and relax -> propel food

o UES contracts (closes) after food enters

o Peristalsis moves food from esophagus to stomach in 8 seconds

• Gastroesophageal sphincter surrounding the cardiac orifice opens -> food enters the stomach

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24
Q

What are the features of the oesophagus?

A
  • Muscular tube that brings down food from pharynx to stomach
  • Has no digestive/absorptive functions
  • A conduit

• Motility by peristalsis
o Progressive wave of muscles contraction and relaxation alternation of inner circular and outer longitudinal muscles -> propel bolus of food to the stomach

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25
What is the gastric motility?
• Stomach stores food mix with gastric juice for digestion -> forms chyme -> empties it as chyme • Peristaltic contraction (3-5/min) o Mix the content and empties part of chyme into duodenum (each contraction) - Propulsion a. Peristaltic waves moves from fundus to pylorus b. Allows mixing of the bolus with gastric content -> forms chyme c. Pyloric valve and LES closed - Grinding a. Mixing action occur close to the pylorus - Retropulsion a. Pyloric end of stomach acts as pump -> delivers small amounts of chyme into duodenum, simultaneously forcing most of its contained material backward into the stomach b. Small amount of chyme will enter c. Most amount propelled in main stomach area & mixed again with stomach content
26
What is vomiting?
• Protective reflex • Complex reflex coordinated in the brain stem o Toxic gas of stomach/noxious -> stimulate excessive salivation -> deep breaths -> vomit occur when: - Intra-abdominal pressure or intragastric pressure overcomes LES pressure -> content of stomach thrown out via esophagus to mouth and out
27
What are the stimulis that contributes to vomiting?
``` o Stomach distension o Seas sickness/dizziness o Pain o Increased intracranial pressure o Others: noxious chemicals, toxins, drugs ```
28
What is the consequence of excessive vomiting?
causes dehydration, electrolyte loss and acid-base imbalance
29
What is the stomach motility?
Gastric Phase: - Stimulus: Increased gastric motility -> to push content into small intestine - Mechanism: 1. Long neural reflexes 2. Gastrin (gastroileal reflex) - Small intestine motility: Increased activity of ileum
30
What is the small intestine motility?
Intestinal Phase: - Stimulus: Chyme in small intestine causes distension of small intestine - Mechanism: Neural reflexes - Small intestine motility: Increases strength of segmenting contractions -> initiates proper mixing and breaking down
31
What are the functions of large intestine?
* Water absorption * Synthesis of vitamins * Storage and defecation
32
What is the motility of the large intestine?
• Haustral contraction o Slow segmental movement o Last for 11 min, occurs every 30 mins • Mass movement o Long and slow o Powerful o Occurs in 3-4 times a day -> gives the urge to defecate ``` • Gastrocolic reflex o Regulate mass movement o can be enhanced by fibre in the food (fibres in food -> enhances colon contraction of GI tract and soften stool)   ```
33
Describe the defecation process of the large intestine
1. Feces move into and distend the rectum --> stimulate stretch receptors on rectum wall --> transmit signals along afferent fibers --> to spinal cord neurons 2. Spinal reflex initiated --> parasympathetic motor (efferent) fibers --> stimulate contraction of rectum and sigmoid colon and relaxation of internal anal sphincter 3. If convenient to defecate --> inhibition of voluntary motor neurons --> allowing external anal sphincter to relax --> feces pass
34
How does the cerebral cortex control defecation?
* By conscious control * Humans will evaluate if it’s the time to defecate * Voluntary motor nerve to external anal sphincter relax --> relaxes skeletal muscles --> parasympathetic fibers relax the internal anal sphincter --> external anal sphincter relax
35
How factors for defecation to start?
* Good anal tone * Good pressure of rectal content to overcome the pressure of anal sphincter * Both aided by water and fibres
36
What breaks down starch to oligosaccharides and disaccharides?
- salivary amylase (in mouth) | - pancreatic amylase (in small intestine)
37
What is the path of absorption for carbohydrates?
- Glucose and galactose -> absorbed via cotransport with sodium ions - Fructose passes via facilitated diffusion - Monosaccharides leave the epithelial cells via facilitated diffusion -> enter capillary blood in villi -> transported to liver via hepatic portal vein
38
What breaks down disaccharides to monosacchartides?
Brush border enzymes (dextrinase, gluco-amylase, lactase, maltase, sucrase) (at the small intestine)
39
What breaks down protein to large polypeptides?
Pepsin (stomach glands) in presence of HCI (at the stomach)
40
What breaks down large polypeptides to small polypeptides/small peptides?
Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) (at the small intestine)
41
What breaks down small polypetides/small peptides to amino acids?
Brush border enzymes (aminopeptidase, carboxypeptidase, dipeptidase) (at small intestine)
42
What is the path of absorption for proteins?
- Amino acids: absorbed by cotransport with sodium ions - Dipeptides and tripeptides: absorbed and hydrolyzed (with hydrogen ions) to amino acids within cells - Amino acids: leaves the epithelial cells via facilitated diffusion -> enter capillary blood in villi -> transported to liver via hepatic portal vein
43
What breaks down non-emulsified fats to monoglycerides, fatty avids, glycerol and fatty acids?
- Emulsification by detergent action of bile salts ducted in from the liver (at small intestine) - Pancreatic lipases (produced by pancreas) (at small intestine)
44
What is the path of absorption for fat digestion?
- Fatty acids and monoglycerides: enter the intestinal cells via diffusion - Fatty acids and monoglycerides: recombined to form triglycerides -> combined again with other lipids and proteins within cell -> form chylomicrons -> then extruded by exocytosis - Chylomicrons: enter lacteals of villi -> transported to the systemic circulation -> via lymph in thoracic duct - Short fatty acids: absorbed and moved into villi by diffusion -> transported to liver via hepatic portal vein
45
Describe the process of fat-digestion/absorption
1. Bile salts in the duodenum -> emulsify large fat globules -> break into small fat droplets 2. Digestion of fat by pancreatic enzyme -> lipase -> yields free fatty acids and monoglycerides -> these associates with bile salts -> forms micelles -> micelles are ferries into the intestinal mucosa a. Micelles = fatty acids + monoglycerides + bile salts 3. After entering the intestinal mucosa -> fatty acids and monoglycerides leave micelles -> diffuse into epithelial cells -> then recombined and packaged with other fatty substances and proteins -> form chylomicrons a. Chylomicrons = fatty acids + glycerols + protein 4. Chylomicrons extruded from epithelial cells -> by exocytosis 5. Chylomicrons enter lacteals -> carried away from the intestine in lymph
46
What are some features of stomach?
- Secretes 2-3l of gastric juice - Acidic a. Contains HCL -> to denature dietary protein to prepare for enzymatic digestion b. Important for protein digestion c. Low pH helps inactivate proteins d. Pepsinogen to pepsin, a powerful protease - Pepsin inactivated by intestine by alkaline juice
47
What are the activities of the stomach?
- Digestion a. Mechanical: bolus -> chyme b. Chemical: breakdown of protein - Secretion a. Gastric juice and intrinsic factor - Motility a. Causes churning movements, aided by 3 muscular layers b. Causes peristalsis to propagate chyme into duodenum - Absorption a. 2 lipid soluble substances absorb into stomach - Alcohol and aspirin
48
What is the regulation of the gastric secretion?
- By neural or hormonal - 3 phases 1. Cephalic Phase 2. Gastric phase 3. Intestinal phase
49
Describe the regulation of the gastric secretion
- Cephalic phase – Vagus nerve (parasympathetic) a. Before entry of food in stomach b. Emotional/neural control c. Vagus nerve -> stimulates stomach -> gastric secretion d. Smell, taste, sight or thought e. Last a few minutes - Gastric phase – vagus and gastrin a. Distention, food, rising pH - Activates the neural receptors of vagus nerve b. Gastrin (g cell) -> stimulates HCl release - Increased pH stimulates gastrin secreting & enteroendocrine glands -> release enzyme acting on protein (pepsinogen) c. Last 3-4 hours - Intestinal phase – gastrin a. Aka intestinal gastrin release phase b. Partially digested food (chyme) and distention in duodenum wall
50
What is the mechanism of HCl secretion?
- HCl secreted by parietal cells - H+ and HCO3-: generated by dissociation of carbonic acid - Parietal cells stimulate -> H+-K+ ATPase -> pumps H+ into lumen against the concentration gradient of H+-K+ ATPase pump, K+ enters the cell - HCO3-Cl antiporter transports HCO3- into interstitial space -> exchange for chloride ions a. HCO3- leaves cell but causes bicarbonate alkaline tide - Alkaline tide -> important -> for the maintenance of blood pH - Cl- and K+ diffuse into lumen -> through membrane channels a. Cl- combine with H+ to form HCl in the lumen
51
What are small intestines?
- Major organ for digestion and absorption - Big surface area a. Presence of villi and microvilli b. Plicae in SI absorbs nutrients - Connected to liver and pancreas
52
What are the features of the small intestine?
- Has 1-2l of intestinal juice by the intestinal gland a. Produced by enterocytes - Secretes intestinal juice - Maximise the absorption by absorptive cells - pH of 7.4 – 7.8 - Neutralizes acidic chyme from the stomach and makes it isotonic - Contains brush border enzymes (converts disaccharide to monosaccharide)
53
What are livers?
- Important organ for nutrient metabolism and regulation - Made up of hepatocytes and rich in blood supply - Produces bile (1l/day) a. By liver cells b. Network of canaliculi -> connected to duct -> brings bile to gallbladder c. Stored in gallbladder d. Released upon CCK activation into duodenum
54
What are the functions of liver?
- Organ for nutrition - Control of metabolism regulation - Produces bile -> stored in gallbladder -> release by CCK -> digestion function
55
What does liver produce?
Bile
56
Where are bile stored?
Gallbladder
57
What does bile contain?
Bile salts, bile pigments, cholesterol, triglycerides, phospholipids, electrolytes
58
What are the actions of bile salts?
- Emulsification of fat: a. By trapping fat globule to form micelle b. Micelle ready for absorption by small intestine - are recycled a. enters the enterohepatic circulation -> after emulsification and absorption
59
What are the different bile pigments?
- Bilirubin a. RBC broken down in spleen -> forms heme and globin -> converted to bilirubin - Biliverdin - Stercobilin: brown colouration of feces - Urobilin: urine colour
60
What is the role of bilirubin?
- Bilirubin enter the splenic vein -> join the superior mesenteric vein -> enter portal vein -> enter the liver a. In normal situations: - Bilirubin recycled -> enter duodenum -> excreted as stercobilinogen -> turns poop brown - Bilirubin -> secreted in urine -> turns urine yellow -> presence of urobilin b. In diseased liver: - Bilirubin will not be excreted out -> retains in blood -> increase bilirubin in blood -> JAUNDICE (poop not brown but pale grey, increases fat content)
61
What are the roles of pancreas?
- Exocrine (secretion of pancreatic juice) | - Endocrine (secretion of insulin, glucagon)
62
What are pancreatic juice?
- pH 8 - HCO3- high level of content -> therefore more alkaline pH 8 - Inactivated proteases (self-protect -> as pancreas will not autodigest) - Active lipases, amylases, nucleases (act on nucleic acid DNA, RNA) - Most are proenzymes (inactive) a. Are activated in the duodenum
63
What are the exocrine role of pancreas?
- Formation of pancreatic juice - 2 anatomical features a. Gland (acinar) b. Tube (ductar)
64
What are the role of acinar in pancreas?
- Produces proteases -> inactive form | - Proteases enters the duodenum -> gets activated
65
What are the role of ducts in pancreas?
- the lining -> produces bicarbonate rich watery fluid -> neutralizes acidic chyme - plays a vital role in acid-base balance a. eg: diarrhea -> content of gastric juice -> lost -> bicarbonic decreases -> acidosis decreases
66
What is the regulation of bile and pancreatic secretion?
- Neural: parasympathetic (weak) - Hormone: cholecycystokinin (CCK) - Stimulate the pancreatic secretion (enzymes), gallbladder contraction and relaxes the sphincter
67
What are the functions of CCK?
- Gallbladder contraction a. Bile from gallbladder -> enters the bile duct - Ductal cells of pancreas to contract a. Enzyme rich juices at entry - Relaxes the sphincter of oddi a. Content of pancreatic and bile duct enters the duodenum
68
What are the roles of secretin?
- Causes ductal cells to produce more bicarbonate | - Causes liver to produce more bile and store in gallbladder = initiate and complete the fat digestion

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