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Yr 2 Gastrointestinal > Wk 3 - Physiology: Digestion and Absorption > Flashcards

Wk 3 - Physiology: Digestion and Absorption Flashcards

1
Q

In the fed state two types of motility occur, _____ and _____.

A

In the fed state two types of motility occur, segmentation (or mixing) and peristalsis.

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2
Q

The chyme itself moves at about 1 cm a second so it takes about _____ hours to traverse the small intestine.

A

The chyme itself moves at about 1 cm a second so it takes about 3 to 5 hours to traverse the small intestine

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3
Q

In the fasted state, the _____ sweeps the tract clean of debris and repeats every 90 minutes.

A

In the fasted state, the migrating motor/myoelectric complex sweeps the tract clean of debris and repeats every 90 minutes.

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4
Q

When one migrating myoelectric complex reaches the ____ at the end of the ileum another migrating myoelectric complex is just about starting at the ____.

A

When one migrating myoelectric complex reaches the ileocaecal valve at the end of the ileum another migrating myoelectric complex is just about starting at the stomach.

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5
Q

Where does the migrating myoelectric complex start?

A

Starts at the stomach

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6
Q

Another migrating myoelectric complex is initiated once it reaches…

A

The ileocaecal valve in the ileum

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7
Q

Migrating myoelectric complexes are stimulated by the hormone…

A

Motilin

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8
Q

Which cells secrete acid in the GI tract?

A

Parietal cells

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9
Q

Which cells secrete pepsinogen?

A

Chief cells

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10
Q

Pepsinogen is activated to ____ by the action of acids in the stomach.

A

Pepsinogen is activated to pepsin by the action of acids in the stomach.

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11
Q

List the different characteristics that protect the stomach from autodigestion.

A
  1. mucus lining
  2. secretion of bicarbonate into mucus
  3. high epithelial blood flow to wash acid away
  4. high epithelial cell turnover rate – re people with chemotherapy are in danger of getting ulcers bc chemo kills rapidly developing cells
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12
Q

In the stomach, gastrin secretion is stimulated by…

A

Food in stomach (detected via chemoreceptors)

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13
Q

Gastrin is released from the…

A

Pyloric antrum (G-cells there)

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14
Q

What is somatostatin?

A

Somatostatin produces predominantly neuroendocrine inhibitory effects across multiple systems. It is known to inhibit GI, endocrine, exocrine, pancreatic, and pituitary secretions, as well as modify neurotransmission and memory formation in the CNS.

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15
Q

How is somatostatin secretion stimulated in the gut?

A

By detection of excess acid + food in the duodenum stimulates CCK –> D-cells release somatostatin

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16
Q

When secreted, what does somatostatin do?

A

Inhibits gastrin –> decrease acid secretion

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17
Q

What are Brunner’s glands?

A

Brunner’s glands are located in the submucosa of the duodenum. They secrete an alkaline fluid containing mucin, which protects the mucosa from the acidic stomach contents entering the duodenum.

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18
Q

Brunner’s glands (in duodenum) secrete:

1.

2.

3.

4.

A
  1. mucus
  2. pepsinogen II
  3. EGF (possible role in enterocyte turnover)
  4. bicarbonate
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19
Q

How is protein digested?

A
  • Stomach - pepsin + acid denature protein (only 15% of dietary protein degraded at this stage)
  • Small intestines -
    • The brush borders of the duodenal and jejeunal epithelial cells contain a number of peptidases (main protein-digesting enzymes​), which further degrade proteins and produce peptide fragments
    • The fragments are reduced to single amino acids for absorption
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20
Q

What are the main protein-digesting enzymes? and what do they do?

A

Pancreatic proteases trypsin, chymotrypsin, carboxypeptidases A and B and elastase are the main protein-digesting enzymes. The action of these enzymes is to degrade ingested proteins and produce a number of peptide fragments.

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21
Q

Describe how peptide fragments and amino acids (from ingested and degraded proteins) are absorbed.

A
  • Peptide and amino acid uptake in duodenum and small intestine
  • Amino acids taken up into enterocyte/epithelial cell by Na-linked mechanism (active transport) + unlinked mechanism (faciliatted diffusion)
  • Peptides absorbed (TRH) –> some Na-linked and some unlinked
  • Amino acids inside cell more than gut lumen
  • Pass out into blood via diffusion, facilitated transport (not Na linked) and Na-linked transport
  • Co-transport: energy for some amino acid uptake and peptide transport (not shown in detail

in image) is provided by the inward sodium gradient generated by the sodium potassium ATPase

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22
Q

List the main types of dietary carbohydrates.

A
  1. Monosacchrides – main method of intake
    1. Glucose
    2. Fructose
  2. Disaccharides
    1. Sucrose
    2. Lactose
    3. Maltose
  3. Polysaccharides (complex carbs)
    1. Starch (amylose + amylopectin)
    2. Glycogen (‘animal starch’)
    3. Cellulose – add fibre and bulk to food to help gut motility
23
Q

How are carbohydrates digested?

A
  • Mouth - salivary glands secrete amylase
    • breaks the bonds between the monomeric sugar units of disaccharides, oligosaccharides, and starches
    • coverts starch and glycogen into smaller chains of glucose called dextrins and maltose
  • Stomach - ongoing mechanical digestion but no breakdown because amylase does not function in acidic environments
  • Small intestines - breakdown continues
    • pancreas releases pancreatic juice through a duct – pancreatic amylase
    • villi that line gut epithelia secrete enzymes – sucrase, maltase, lactase and glucamylase
      • sucrase breaks down sucrose to glucose and fructose
      • maltase breaks down maltose to individual glucose units
      • lactase breakdown lactose to glucose and galactose units (NOT INDUCEABLE)
      • glucamylase (present in small amounts) breakdown malto-oligosacchrides to glucose units
24
Q

Describe how glucose is taken up in the small intestine.

A
  • Fructose, glucose and galactose are absorbed into the enterocyte
    • active transport via co-transport using Na gradient made by Na/K ATPase
  • Fructose is absorbed by facilitated diffusion while glucose and galactose are actively transported across the cell membrane into the blood
  • The first organ to receive glucose, fructose, and galactose is the liver
25
Q

What happens in people who are ‘lactose intolerant’?

A

When people do not have enough of the enzyme lactase, lactose is not sufficiently broken down resulting in a condition called lactose intolerance. The undigested lactose moves to the large intestine where bacteria are able to digest it. The bacterial digestion of lactose produces gases leading to symptoms of diarrhoea, bloating, and abdominal cramps.

Lactose intolerance usually occurs in adults and is associated with race (less in those of European background)

26
Q

What are the main groups of dietary fat?

A

Dietary fat includes:

  • triglycerides
  • phopholipids
  • cholesterol and its esters
27
Q

Describe fat digestion.

A

Mouth:

  • The enzyme lingual lipase, along with a small amount of phospholipid as an emulsifier, initiates the process of digestion
  • As a result, the fats become tiny droplets and separate from the watery components

Stomach:

  • Gastric lipase starts to break down triacylglycerols into diglycerides and fatty acids.
  • Very little fat digestion occurs in the stomach

Small intestine:

  • Bile secreted from duct – Bile contains bile salts, lecithin, and substances derived from cholesterol so it acts as an emulsifier
  • Products of fat digestion and the bile salts form “micelles”
28
Q

The main enzyme involved in fat digestion is…

A

Lipase

29
Q

Describe the absorption process of fats.

A

Monoglycerides and the free fatty acids diffuse through the epithelial cell membrane. The bile acid micelles are left behind to pick up more fat from the chyme.

In the enterocyte:

  • Monoglycerides and fatty acids are reconstituted into triglycerides
  • Which form into globules

Bloodstream:

  • Exocytosed into the central lacteal lymph where they are called chylomicrons
  • The chylomicrons travel in the lymph to the thoracic ducts and hence the great veins
30
Q

List the fat-soluble vitamin needed in the human body.

A
  • Vitamin A - required for retinal pigments and for normal growth of epithelial cells
  • Vitamin D - involved in Ca++ metabolism
  • Vitamin E - antioxidant. Protective role in preventing the oxidation of unsaturated fats
  • Vitamin K - required for the synthesis of clotting factors II, VII, IX and X
31
Q

What function does vitamin K have in the human body?

A

Vitamin K - required for the synthesis of clotting factors II, VII, IX and X

32
Q

Net movement of water across cell membranes always occurs by…

A

Osmosis

33
Q

Which part of the small intestine is most water permeable?

A

The duodenum is the most water permeable part

34
Q

Absorption of water in the gut is highly dependent on ____ particularly ____.

A

Absorption of water in the gut is highly dependent on salts/solutes particularly sodium.

35
Q

Describe sodium absorption in the gut.

A
  • Sodium is absorbed from the intestinal lumen by several mechanisms, most prominently by cotransport with glucose and amino acids, and by Na+/H+ exchange, both of which move sodium from the lumen into the enterocyte.
  • Absorbed sodium is rapidly exported from the cell via sodium pumps - when a lot of sodium is entering the cell, a lot of sodium is pumped out of the cell, which establishes a high osmolarity in the small intercellular spaces between adjacent enterocytes.
36
Q

Diarrhoea will result in _____ (metabolic acidosis/metabolic alkalosis).

A

Diarrhoea will result in metabolic acidosis.

Pushing out bicarbonate into colon – results in generation of H proton (acid) that goes into blood stream.

37
Q

What is ‘gastric dumping’/’dumping syndrome?

A

Dumping syndrome is a medical condition in which your stomach empties its contents into the first part of your small intestine (the duodenum) faster than normal. Dumping syndrome is also known as rapid gastric emptying. People with dumping syndrome experience symptoms like nausea and abdominal cramping.

38
Q

Gastric dumping may lead to…

A
  1. massive water fluxes
  2. hypovolaemia
  3. syncope (in extreme cases)
39
Q

Which part of the small intestine does most sodium, chloride, bicarbonate, sugar and amino acid absorption occur?

A

In jejunum

40
Q

What is usually absorbed in the terminal part of the ileum?

A
  • bile salts
  • vitamin B12
41
Q

What is usually secreted in the colon?

A

Bicarbonate and potassium from enterocytes into colon

42
Q

In the colon, bicarbonate is usually secreted in exchange for ____.

A

In the colon, bicarbonate is usually secreted in exchange for chloride.

43
Q

Inflammatory conditions (e.g. coeliac disease) in the GI tend to cause issues in ____ because the villi are responsible for it while the junctions remain functional and thus there are no issue in ____.

A

Inflammatory conditions (e.g. coeliac disease) in the GI tend to cause issues in absorption because the villi are responsible for it while the junctions remain functional and thus there are no issue in secretion.

44
Q

Describe the control of salt and water transport in the lower GI tract.

A
  • Factors which increase cellular cAMP favour secretion and decrease absorption
  • Stimulation of the sympathetic nervous system decreases secretion and increases absorption
  • Stimulation of the parasympathetic nervous system increases secretion and decreases absorption
  • Aldosterone and angiotensin II promote absorption (angiotensin II works partly by promoting Noradrenaline release from sympathetic nerve endings)
  • Histamine causes increased secretion and decreased absorption (role in inflammatory bowel disease)
45
Q

List the different factors/system that increase secretion and decrease absorption.

A
  1. Any factor that increases cAMP
  2. Stimulation of the PNS
  3. Histamine release
46
Q

Stimulation of the SNS will generally lead to ____ secretion and _____ absorption.

A

Stimulation of the SNS will generally lead to decreased secretion and increased absorption.

47
Q

In the GI tract, stimulation o the PNS will lead to ___ secretions and ____ absorption while the SNS will lead to ___ secretions and ____ absorption.

A

In the GI tract, stimulation o the PNS will lead to INCREASED secretions and DECREASED absorption while the SNS will lead to DECREASED secretions and INCREASED absorption.

48
Q

Explain the different causes of diarrhoea.

A
  1. Increased rate of transit
  2. Failure to absorb
  3. Increased secretion
  4. Increased secretion AND decreased absorption
49
Q

List some factors that would lead to increased transit (gut motility) that leads to diarrhoea.

A
  • Some laxatives increase gut motility
  • Bacteria produce hydroxylated fats which stimulate motility
  • Dumping due to an incompetent pyloric sphincter perhaps following surgery
  • Thyrotoxicosis (excess thyroid hormone synthesis and secretion by the thyroid gland)
  • Irritable bowel syndrome
50
Q

List some factors that would lead to failure of absorption in the gut that leads to diarrhoea.

A
  • Familial chloride diarrhoea (absence of ileum and colon chloride bicarbonate exchanger)
  • Some laxatives e.g. MgSO4 are not absorbed and act osmotically (c.f. glucose and osmotic diuresis in the kidney)
  • Failure to digest e.g. lack of lactase activity will mean that lactose exerts an osmotic action – lactose intolerance
  • Reduced and/or damaged mucosa or epithelium e.g. coeliac disease due to gluten sensitivity results in villi which are flattened and less functional
  • Bile salts in the colon increase motility and inhibit absorption (bile salts usually absorbed in terminal ileum)
51
Q

List some factors that would lead to increased secretion in the gut that leads to diarrhoea.

A
  • Zollinger Ellison syndrome (gastrinoma produces excess gastrin) – very rare
  • “Pancreatic cholera” (due to excess VIP secretion)
  • Disordered prostaglandin metabolism
52
Q

List some factors that would lead to increased secretion AND decreased absorption in the gut that leads to diarrhoea.

A
  • Cholera and other bacterial infections – leading to MORE than 11L of diarrhoea in a DAY!!!
53
Q

Describe what happens in cholera that leads to increased diarrhoea.

A
  • Ganglioside GM1 is recognised by cholera toxin
  • Cholera toxin enters the cell. Cholera toxin is an enzyme which catalyses the transfer of ADP ribose onto the αs subunit
  • As a result, the subunit can no longer hydrolyse the bound GTP
  • And therefore the adenylyl cyclase (cAMP) is permanently stimulated
  • cAMP increases secretion and decreases absorption
  • Massive amounts of fluid in the gut – diarrhoea may exceed 11 litres
    • Life-threatening in children and the elderly, adults may tolerate
54
Q

How is cholera usually managed?

A

Cholera requires rapid fluid replacement. This is best done with water containing a mixture of salt and sugar in water (oral rehydration therapy).

+ antibiotic.

Yr 2 Gastrointestinal (37 decks)

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