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Biomedicine I > 7. Digestive System > Flashcards

7. Digestive System Flashcards

1
Q
A
  • Mouth
  • Oropharynx
  • Oesophagus
  • Stomach
  • Small Intestine
  • Large Intestine

Accessory organs:

  • Salivary Glands
  • Pancreas
  • Liver
  • Gall bladder and biliary tract
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2
Q

Gastrointenstinal Tract (GIT) Layers

A

The entire GIT contains the same basic 4-layer arrangement of tissues:

  1. Mucosa
  2. Submucosa
  3. Muscularis
  4. Serosa (peritoneum)
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3
Q

Mucosa

A
  1. Epithelium
  2. Lamina propria
  3. Muscularis mucosa
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4
Q

Mucosa: Epithelium

A
  • The mouth, pharynx, oesophagus & anus contains epithelium for protection.
  • Stomach & intestines: columnar epithelium with:
    • Microvilli–↑ surface area for absorption.
    • Goblet cells –secretion of mucous to lubricate food and protects against digestive juice erosion.
    • Enteroendocrine cells -specialised endocrine cells that secrete hormones(e.g. CCK) into blood.
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5
Q

Mucosa: Lamina Propria

A
  • The lamina propria consists of connective tissue containing many blood and lymphatic vessels that allows the absorption of nutrients.
  • The lamina propria contains MALT (mucosa associated lymphoid tissue). These are collections of immune cells (e.g. lymphocytes, macrophages etc.).
  • MALT is found in many places in the body. In the GIT, we call this GALT (gut associated lymphoid tissue).
  • MALT is especially prevalent in the tonsils, oesophagus, small intestine, appendix & large intestine.
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6
Q

GALT: Gut Associated Lymphoid Tissue

A

GALT CONTAINS 70% OF BODY’S IMMUNE CELLS

  • GALT is part of the first line of defense against ingested pathogens.
  • Composes around 25% of the intestine mucosal mass.

(70% OF THE IMMUNE SYSTEM IS IN THE GIT)

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7
Q

Muscosa: Muscularis Mucosa

A
  • The muscularis mucosa is a very thin layer of smooth muscle.
  • This layer of muscle creates the villi (small folds) which increases the surface area for absorption & digestion.
  • Movement of this ensures that all absorptive cells are fully exposed to the GIT contents.
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8
Q

Submucosa

A
  • The submucosa is a connective tissue layer that lies between the mucosa & muscularis.
  • Contains blood & lymph vessels which receive absorbed food molecules.
  • Contains network of neurons called the submucosal plexus (‘brain of the gut’).
  • May contain glands and lymphatic tissue e.g. ‘Peyer’s patches’ in ileum.
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9
Q

Muscularis Externa

A
  • The mouth, pharynx, upper oesophagus & anal sphincter contains skeletal muscle to allow voluntary swallowing & defecation.
  • The rest of the GIT is mostly smooth muscle, involuntary (autonomic). Contains two layers:
  1. Inner circular muscle
  2. Outer longitudinal.
  • Involuntary smooth muscle contractions aids:
    • Mixing of food with digestive juices.
    • Propelling food (chyme/bolus) along the digestive tract (peristalsis).
  • Between the two muscle layers are neurons –myenteric plexus.
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10
Q

Peritoneum

A

The peritoneum is the largest serous membrane in the body.

  • The peritoneum weaves between digestive organs and is supplied with many blood and lymph vessels. It provides a physical barrier to local spread of infection.

Consists of 2 layers:

  1. Parietal–covers the wall of the abdomen & pelvic cavity.
  2. Visceral–covers the organs.
  • The peritoneal cavity is the space between the 2 layers and contains a lubricating serous fluid.
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11
Q

Peritoneal Pathologies

A

Ascites:

  • The acumulation of fluid in the peritoneal cavity
  • Causes include liver cirrhosis, GIT malignancies, heart failure, pancreatitis

Peritonitis

  • Acute inflammation of the peritoneum
    • Causes of these can be bacterial infection, rupurted appendix, friction, surgical wounds
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12
Q

Greater Omentum

A

The largest fold of the peritoneum.

  • Drapes over transverse colon and small intestine like an apron.
  • It’s a double sheet that folds back on itself (hence 4 layers).
  • Stores fat: contains adipose tissue which can greatly expand with weight gain –the ‘beer belly’.
  • Has many lymph nodes containing macrophages & plasma cells (which produce antibodies) to combat infections of the GIT.
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13
Q

Lesser Omentum

A

The Lesser Omentumis a peritoneal fold that suspends the stomach and duodenum from the liver.

  • Pathway for blood vessels entering the liver.
  • The Lesser Omentum contains the hepatic portal vein, common hepatic artery, common bile duct and lymph nodes.
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14
Q

Enteric Nervous System

A
  • The enteric nervous system is the ‘brain’ of the gut and extends from the oesophagus to the anus.
  • Contains 100 million neurons (more than spinal cord).
  • Functions independently but regulated by the autonomic nervous system:

Parasympthetic (rest and digest)

  • Increases muscular activity (peristalsis -> myenteric plus)
  • Increases grandular secretion (submuscosal plexus)

Sympathetic (fight or flight)

  • Deceases muscular activity (peristalsis -> myenteric plus)
    • Decreases grandular secretion (submuscosal plexus)
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15
Q

Myenteric Plexus

A
  • This contains a network of sympathetic and parasympathetic nerve fibres between the circular and longitudinal muscles of the muscularis.
  • Controls strength and frequency of muscle contraction: gut motility.
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16
Q

Submucosal Plexus

A

• Network of sympathetic & parasympathetic nerve fibres within the submucosa –controls digestive secretions and detects sensory information

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17
Q

Enteric Nervous System: Types of Neurons

A
  1. Motor neurons (outgoing/action signal) in the myenteric plexus controls peristalsis & in the submucosal plexus control secretions.
  2. Sensory neurons (incoming signal) receive information about the mucosal environment: chemoreceptors & stretch receptors.
  3. Interneurons connect the two plexuses.
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18
Q

GIT Blood Supply

A
  • The GIT is supplied with arterial blood from branches of the abdominal aorta, which include the mesenteric arteries.
  • Nutrient rich blood in the intestines is returned by veins via the liver (portal system) or directly from iliac veins:
  • The liver filters the blood and processes nutrients which then enter systemic circulation.
  • The portal vein drains the lower oesophagus, stomach, pancreas, small & large intestine, upper rectum & spleen.
  • The iliac veins drain the lower part of the rectum & anal canal.
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19
Q

Oral Cavity

A
  • Inner surfaces are lined with mucous membranes.
  • Hard palate -bony partition between oral & nasal cavities. Allows simultaneous chewing and breathing.
  • Soft palate –an arch of muscle.
  • Uvula–swings up and blocks the nasopharynx preventing food from entering the nasal cavity.
  • Tongue–voluntary skeletal muscle structure attached to the hyoid bone and mandible. The superior surface covered with ‘papillae’, which contain taste buds.
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20
Q

Oral Cavity: Functions

A
  • Mastication
  • Speech
  • Taste
  • Swallowing–bolus (mixed digested food & digestive juices) is pushed by the tongue into the pharynx closing the nasopharynx –‘pharynx reflex action’.
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21
Q

Teeth

A
  • Adults have 32 teeth (20 temporary).
  • Teeth have a crown, neck & root.
  • ‘Dentin’ makes up the bulk of a tooth internally. Teeth are covered externally by enamel.
  • Sits within the gum (gingiva) and periodontal membrane (a ligament that fixes to bone/connects teeth).
  • Functions in Mastication (what chewing does not accomplish mechanically must be completed by the digestive tract chemically).
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22
Q

Saliva

A
  • Saliva is produced by the parotid glands, submandibular glands and sublingual glands via a reflex controlled by the autonomic nervous system.
  • Parasympathetic NS stimulates continuous salivation(1-1.5 L/day) which provides lubrication.
  • Saliva is swallowed & lubricates the oesophagus before eventually being reabsorbed.
  • During dehydration, salivation is stopped, which contributes to the sensation of thirst.
  • Sympathetic nervous stimulation (stress response) reduces salivation causing dryness of the mouth.
  • Touch, taste, smell, sight, sound can stimulate digestive salivation.
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23
Q

Saliva Composition

A
  • Water(99.5%).
  • Mineral Salts (Na, K, Ca, Cl, Bicarbonate, P).
  • Enzymes:salivary amylase (parotid), lysozymes (found in many body secretions, breaks down bacterial cell walls).
  • Mucous.
  • Immunoglobulins(IgA).
  • Blood clotting factors.
  • pH 6.35-6.85 (mildly acidic).
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24
Q

Saliva Functions

A
  • Digestion-chemical breakdown of polysaccharides.
  • Lubricating & dissolving food.
  • Cleansing of oral cavity and teeth.
  • Defence-non-specific (IgA & lysozymes).
  • Taste.
  • Buffer–for acidic foods.
  • Waste removal–urea / uric acid from the body.
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25
Q

Oesophagus

A
  • A 25cm long muscular tube, attached to the larynx. Posterior to trachea, passes through the diaphragm (T10).
  • Lined with squamous epithelium for protection, lubricated with mucous.
  • The superior/middle oesophagus contains skeletal muscle & the lower contains smooth.
  • Food travels to the stomach via muscular contractions (‘peristalsis’).
  • Epiglottis–a flap of elastic cartilage which prevents food entering the trachea.
  • The lower oesophageal sphincter acts as a seal on the stomach to prevent reflux of stomach contents into the oesophagus.
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26
Q

Stomach

A
  • The stomach is a J-shaped organ with 4 main regions: cardia, fundus, body, pyloric with lesser and greater curvatures.
  • Same layers as the rest of the GIT, but with 3 layers of muscle (rather than 2) churning & mixing food with gastric juice.
  • 2 sphincters: cardiac and pyloric.
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27
Q

Stomach: Neural feedback

A
  • Food distends the stomach stimulating stretch receptors in its walls.
  • Chemoreceptors monitor pH changes.
  • Activates submucosal plexus causing waves of peristalsis and gastric juice flow.
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28
Q

Stomach: Composition

A

• The stomach secretes 2-3L of highly acidic (pH 2-3) gastric juice and mucous a day.
• The mucous produced by goblet cells in the stomach provide a ‘barrier’ against the acidity present within the stomach.
• The stomach contains simple columnar epithelial cells. These have a fast turnover (replacing the
lining every 3 days)
.

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29
Q

Stomach: Exocrine Cells

A
  1. Parietal cells
  2. Chief cells
  3. Goblet cells
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30
Q

Parietal cells

A

Intrinsic Factor: necessary for vitamin B12 absorption

HCl:

  • Secretes H+and Cl-separately which combine in the stomach.
  • HCl activates pepsin, acts as an anti-microbial agent and assists in activating bile and pancreas flow.
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31
Q

Chief cells

A

Pepsinogen: Protein digestion HCl converts pepsinogen to the active enzyme pepsin (digests protein).

Gastric Lipase: lipid digestion

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32
Q

Goblet cells

A

Mucous: protects against acid

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33
Q

Stomach: Functions

A
  • Mixing chamber –churns up food.
  • Holding reservoir –storage.
  • Defence–non-specific defence from gastric acidity.
  • Absorption (limited) –water, alcohol, drugs i.e. aspirin.
  • Digestion–mechanical but also chemical digestion of proteins & lipids.
  • Iron–made more soluble with stomach acid.
  • Satiation–tells you to stop eating.
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34
Q

Stomach: Hormones

A
  • Ghrelin–stimulates hunger.
  • Gastrin(produced by G cells) –responds to stomach distension. Stimulates gastric juice secretion & gastric motility.
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35
Q

Pancreas

A

The pancreas is an accessory digestive organof the digestive system that have both exocrine and endocrine functions.

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36
Q

Pancreas: Exocrine Function

A

Pancreatic juice is a clear liquid that is excreted into the small intestines (1.2-1.5L/day).

  • Sodium bicarbonate and water.
  • Protease enzymes: Trypsin, chymotrypsin, ribonuclease, deoxyribonuclease.
  • Lipase.
  • Pancreatic amylase.
37
Q

Pancreas: Endocrine Function

A

Hormones secreted (into the blood).

  • Insulin & Glucagon.
  • Somatostatin (growth-hormone-inhibiting-hormone).
38
Q

Pancreatic Enzymes

A

• Pancreatic enzymes are imperative for digestion.
• Pancreatic enzymes are secreted by the
pancreas into the lumen of the duodenum.
Proteases are secreted in their inactive form.

39
Q

Pacnreatic amylase

A

Breakdown of starches into sugars

40
Q

Pancreatic Lipase

A

Lipd/fat digestion

41
Q

Typsin

A

Protein digestion

42
Q

Chymotrypsin

A

Protein digestion

43
Q

Ribonuclease

A

Digest RNA

44
Q

Deoxyribonuclease

A

Digest DNA

45
Q

Gallbladder

A
  • The gallbladder is a pear-shaped sac in the liver, measuring 7-10 cm in length.
  • Bile ducts project from the gallbladder and liver, meeting at the common bile duct.
  • Bile ducts collect bile produced by hepatocytes which pools in the gallbladder.
  • Bile enters the small intestine via the common bile duct.
  • When the individual has ingested fat, the gall bladder contracts to eject bile down the common bile duct and into the duodenum.
46
Q

Bile

A
  • The gall bladder ejects ‘bile’, which is an agent that emulsifies fats.
  • Emulsification breaks the lipid into smaller molecules. This increases the surface area for lipid enzymes (lipase) to work.
  • Bile is composed of bile salts, cholesterol and bilirubin.
  • pH 7.6-8.6 (mildly basic).
  • 90-95% of bile are absorbed & transported back to the liver from the ileum: enterohepatic circulation
47
Q

Small Intestine

A
  • After food combines with stomach secretions, the resulting chyme is pushed through the pyloric sphincter into the small intestine.
  • Most digestion & absorption occurs in the small intestine.
  • A long structure (6.5 metres) with villi to maximise surface area.
  • Contains a variety of cells e.g. goblet (mucous), endocrine, absorptive.
48
Q

Duodenum

A

Emulsification & most digestionoccurs here (30cm).

49
Q

Jejunum

A

most absorptionoccurs here (2.5m).

50
Q

Ileum

A

vitamin B12is absorbed (3.5m).

51
Q

Villi

A
  • Finger-like projections with blood capillaries & lacteals (lymphatic capillaries)
  • Absorbed nutrients enter the blood & fatty acids enter the lymph.
52
Q

Microvilli

A

(brush border) are projections of absorptive cell membranes.

53
Q

Brush border enzymes

A
  • Brush border enzymes are imperative for absorption.
  • The enzymes are attached to the lining of the intestine (not free in the lumen)

• Includes

  • maltase, sucrase, lactase. -> sugars
  • Dipeptidase: break down proteins into amino acids
  • Nucleoasidases and phosphatases -> digest RNA and DNA

• Hence enzymatic digestion occurs on absorptive cell surface. Release when cells slough off.

54
Q

Cholecystokinin (CCK)

A

• Cholecystokinin (CCK) is a peptide hormone that is secreted by enteroendrocrine cells in the
duodenum (small intestine).
• CCK is released in response to high fat or protein chyme entering the duodenum

55
Q

CCK Functions

A
  • Stimulates the pancreas to secrete pancreatic enzymes (& hence digest fat/protein/carbohydrates).
  • Increases hepatic production of bile and stimulates contraction of the gall bladder (bile -> fat digestion).
  • Mediates satiety (feeling of fullness).
56
Q

Small Intestine: Functions

A
  • Movement / peristalsis of food.
  • Digestion.
  • Absorptionof nutrients & water.
  • Hunger / satiety.
  • Immunity-Peyer’s patches & bacterial microflora.
57
Q

Small Intestine: Absorption

A

• 90% of absorption occurs in small intestine.
• Carbohydrates and amino acids are transported into capillaries:
• Monosaccharides: active & passive transport
• Amino acids:active transport
• Fatty acids, glycerol, cholesterol and fat soluble vitamins (A,D,E,K) are:
1. Emulsified by bile.
2. Enterintestinal cells by simple diffusion.
3. Packaged into chylomicrons and absorbed into lacteals.
4. Travel through the lymphatic system and enter the blood in the subclavian vein.

• Vitamin B12is absorbed in the terminal ileum (needs intrinsic factor for absorption).

  • Vitamins, mineral salts & water enter blood capillaries and are sent to the liver (hepatic first pass).
  • Bile salts are reabsorbed & recycled from the ileum.
58
Q

Fluid Movement

A

• All water absorption in the GI tract occurs via osmosis from the lumen into absorptive cells and into blood capillaries.
• Osmosis is bi-directional – therefore absorption of water depends on electrolyte, monosaccharide
and amino acid levels to maintain
an osmotic balance / concentration gradient with the blood.

59
Q

Large Intestine

A
  • Terminal portion of the GIT: where the final stages of digestion, absorption, some vitamin synthesis (through bacterial activity) & stool formation occurs.
  • The ileocaecalvalveallows one way flow.
  • Mucous is produced but no enzymes.
  • Absorption of water, minerals, vitamins & some drugs
60
Q

Large Intestine Regions

A
  1. Caecum
  2. Colon
  3. Rectum
  4. Anal canal
61
Q

Caecum

A

appendixis attached -a twisted tube about 8cm in length. Part of our immune system: contains macrophages, lymphocytes, bacteria.

62
Q

Colon

A
  • ascending
  • transverse
  • descending
  • sigmoid colon
63
Q

Rectum

A
64
Q

Anal Canal

A

mucous membranes folds. Contains an internal and external anal sphincter.

65
Q

Large Intestine: Microbes

A
  • The large intestine hosts a rich community of microbes: 100,000,000,000,000(1x1014).
  • Mixture of anaerobic and aerobic bacteria, generally acid-producing.
  • Mostly symbiotic, but pathogenic species may flourish according to local pH, poor nutrition, digestive processes further upstream and antibiotic use.
  • The final stages of nutrient extraction occur in the colon through microbial fermentation.

•
Bacterial fermentation of remaining carbohydrates produces hydrogen, CO2and methane (flatulence).
• Fermentation of residual amino acids to various compounds (including hydrogen sulphide) which contribute to faecal odour.
• Some toxic products of bacterial fermentation are re-absorbed & transported to the liver where they are excreted in urine.
• Bilirubinis decomposed into simpler molecules, these pigments contribute to the colour of faeces.
• Produce some vitamins (B12& vitamin K) & fatty acids.
• Faeces are 30-50% bacteria.

66
Q

Defacation

A
  • Mass peristaltic movements push fecal matter into the rectum from the sigmoid colon.
  • The resulting distension of the rectal wall stimulates stretch receptors & the defecation reflex. This includes sensory impulses to the sacral part of the spinal cord.
  • Motor impulses travel down parasympathetic nerves back to the rectum and anus.
  • Rectal muscles (with abdominal muscles and diaphragm) contract, increasing abdominal pressure that opens the internal sphincter.
  • The external anal sphincter is voluntarily relaxed.
67
Q

Liver

A
  • The liver is the heaviest gland of the body & second largest organ (skin being the largest).
  • Uses 27% of our resting metabolism.
  • It filters 1.4 litres of blood every minute.
  • The hepatic artery brings oxygenated blood from the heart.
  • Kuppfer cells are phagocytic cells which remove foreign bodies from the blood (they are a type of white blood cell that specialise in engulfing microbes).
68
Q

Portal Vein

A
  • The portal vein brings in nutrient (& toxin) rich blood from the GIT.
  • “Hepatic first pass”: all blood from the GIT is transported to the liver (via the portal vein) to be filtered / metabolised before entering systemic circulation.
  • Blood from the portal vein & hepaticartery mix in the ‘sinusoids’ (columns between hepatocytes).
  • The filtrate enters the central vein which enters the hepatic vein & in to the systemic circulation.
69
Q

Liver Functions

A

The liver is estimated to have over 500 functions.

  1. Cleansing blood of microbes.
  2. Detoxification-metabolising drugs, toxins & alcohol.
  3. Bile production & secretion.
  4. Haemolysis (kuppfercells)
  5. Synthesis of plasma proteins -blood clotting & coagulation factors.
  6. Hormone homeostasis –deactivating all hormones.
  7. Metabolism of glucose (glycogen), fats (hepatocytes store triglycerides) & amino acids.
  8. Heat production –thermogenesis.
  9. Synthesis -vitamin A (from beta carotene), Co-Q10 & activation of vitamin D.
  10. Storage-vitamins (A,D,E,K, B12), iron, copper, glycogen.
70
Q

Liver Metabolism

A

Carbohydrate
• Excess glucose is converted to glycogen for storage.
• Glycogen to glucose as required.

Fat
• Metabolises fat from storage as required.
• Synthesises cholesterol & triglycerides.

Protein
•Converts essential amino acids into non-essential amino acids .
• Removes nitrogen groups from amino acids to form urea to be excreted.
• Breaks down nucleotides to form uric acid to be excreted.

71
Q

Liver Detoxification

A

• Hepatocytes convert toxins into nontoxic
metabolites which can then be excreted from the body.
• A healthy liver deals with thousands of toxins each day. These include airborne pollutants, those in food, drugs etc.
• Highly energy dependent (ATP) and nutrient dependent.
• Induced by toxicants, drugs, phytonutrients: enzymes made on the spot.

  1. Hydrophilic: Excreted in urine or bile.
  2. Lipophilic: Must be chemically altered into hydrophilic compounds to facilitate elimination
72
Q

Liver Detox: Phase I

A

Bio-activation

  • Involves CYP450 family of enzymes (a class of more than 50 enzymes). These enzymes are particularly important in metabolising toxins and medications.
  • The enzymes are mostly found in liver cells but are also found in the small intestine, lungs, placenta and kidney.
  • Converts water insoluble toxins into water soluble substances to be excreted by the kidneys
  • Converts toxins to more reactive substances which can be metabolisedin Phase II.
73
Q

Liver Detox: Phase II

A
  • Conjugation reactions –molecules are attached to the toxins to neutralise them making them stable (non-reactive) & water-soluble to be excreted.
  • Various enzymes are involved to induce many chemical reactions

‘Glutathione’ is a key anti-oxidant in phase II liver detoxifcation.

74
Q

Abdominal Quadrants

A
  1. Right & left hypochondrium, epigastric region
  2. Right & left flank, umbilical region
  3. Right & left groin, pubic region.
75
Q

Monosaccharides

A
  • Onesugar unit
  • Glucose(key source of energy).
  • Fructose (fruits).
  • Galactose.
76
Q

Disaccharides

A
  • Twosugar units
  • Maltose–digested into glucose + glucose.
  • Sucrose glucose + fructose.
  • Lactoseglucose + galactose.
77
Q

Polysaccharides

A
  • Manysugar units
  • Starch–digested into glucose (potatoes, wheat, rice etc.)
  • Glycogen –digested into glucose.
  • Cellulose–indigestible (it is indigestible fibre-in the cell walls of green plants).
78
Q

Dietary Lipids

A

Triglycerides

Phospholipids

Cholesterol

79
Q

Triglycerides

A
  • Predominant dietary lipid.
  • Composed of glycerol & 3 fatty acidchains.
  • Fatty acids are saturated or unsaturated.
80
Q

Phospholipids

A
  • Composed of 2 fatty acid tails & a phosphatehead.
  • Digested to free fatty acids and absorbed.
81
Q

Cholesterol

A
  • A steroid particularly in animal foods.
  • Vital for cell membrane integrity, vitamin D synthesis and sex hormone synthesis.
82
Q

Dietary Lipids: Functions

A

Energy, insulation, cell membranes, hormones
production, protection of organs.

83
Q

Saturated

A

a molecule containing the greatest number of hydrogen atoms without any double bonds

84
Q

Unsaturated

A

Unsaturated fatty acids have one or more double bonds between carbon atoms (C=C)

85
Q

Cis Fatty Acids

A

Hydrogrens are on same side

86
Q

Trans Fatty Acids

A

Hydrogens are on opposite sides

87
Q

Proteins

A
  • Proteins are long molecules of amino acids, making up 15% of the total body mass.
  • Amino acid chains create a 3D structure of each protein, which is essential for them to function.
  • Proteins must first be denatured before protease enzymes are able to efficiently cleave the amino acids apart. Body temperature regulation allows proteins to keep 3D shape.
  • Amino acids are absorbed in the small intestine.
88
Q

Proteins: Functions

A

Immunity (antibodies, immunoglobulins),

structures (muscles, collagen),

enzymes,

hormones,

neurotransmitters,

energy.

89
Q

Enzymes

A
  • Enzymes are biological catalysts which speed up chemical reactions up to thousands of times.
  • They are highly specific with what substrate they react with (lock & key).
  • Enzyme names are usually based on the substrate they react with, and end in –ase e.g. Lactase.However some enzymes involved in protein digestion end with –in e.g. pepsin.
  • Enzymes are not changed or consumed in a chemical reaction –they can perform the same reaction many times.
  • Proteins are easily denatured by pH & temperature changes.

Biomedicine I (18 decks)

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