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GI Tract Physiology > Digestion and Absorption > Flashcards

Digestion and Absorption Flashcards

1
Q

What is the importance of the Villous Lining of the Small Intestine?

A
  1. Allows for absorption

2. Houses Brush Border Enzymes that perform the final digestion of Proteins and Carbs

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2
Q

Brush Border Enzymes are responsible for final digestion of what materials?

A
  1. Proteins

2. Carbohydrates

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3
Q

What is the important Cl- Channel on the Villi of the Small Intestine?

A

The CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) which is stimulated by Secretin to secrete Cl- into the gut which is followed by Na and H2O creating a good buffer

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4
Q

What are the majority of the Carbs we ingest?

A
  1. Starches

2. Dissacharides (Sucrose and Lactose)

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5
Q

What monosaccharides make up Sucrose?

A
  1. Glucose

2. Fructose

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6
Q

What monosaccharides make up Lactose?

A
  1. Glucose

2. Galactose

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7
Q

Where does Starch Digestion start?

A

IN THE MOUTH

Salivary Alpha-Amylase is activated by Cl- to break Starches down into Malto-Oligosaccharides (Maltose and Isomaltose)

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8
Q

Where can Starch be digested?

A
  1. Mouth (25-30%)

2. Small Intestine (70-75%)

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9
Q

How is Starch Digested in the Small Intestine?

A
  1. Pancreatic Alpha-Amylase enzymes in the Duodenum break down the Starches into Malto-Oligosaccharides (Maltose and Isomaltose)
  2. Brush Border enzymes located in the Glycocalyx of the Villus lining in the Small Intestine take the Oligossacharides (Maltose and Isomaltose) and Disaccharides (Sucrose and Lactose) and break them down into Monosaccharides to be absorbed
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10
Q

Name the Brush Border Enzymes of the Small Intestines and the sugars they produce:

A
  1. Isomaltase (Glucose)
  2. Maltase (Glucose)
  3. Lactase (Glucose and Galactose)
  4. Sucrase (Glucose and Fructose)
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11
Q

What can undigested Carbohydrates in the Body cause?

A
  1. Diarrhea (because sugars are isoosmotic and draw water into the GI tract)
  2. Cramping and Gas (provides food for bacteria)
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12
Q

How do Monosaccharides enter the Enterocytes of the Small Intestine?

A

INSULIN-INDEPENDENT TRANSPORTERS

  1. SGLT-1- Glucose and Galactose enter via Secondary Active Transport with Na
  2. GLUT-5- Primary Active Transporter that is a Fructose Carrier
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13
Q

Does Glucose increase or decrease water and electrolyte reabsorption of the Small Intestine? How?

A

Glucose INCREASES Water and Electrolyte reabsorption in the Small Intestine because:

  1. Enters Enterocytes with Na via SGLT-1 (Na draws water and solute)
  2. Water follows Glucose as it is ingested and also draws solute with it
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14
Q

Assuming no digestive issues, what is the most easily digested compound that we ingest?

A. Lipids/Fats
B. Proteins
C. Carbohydrates

A

C. Carbohydrates

AS LONG AS THE AMYLASES AND BRUSH BORDER ENZYMES ARE ALL FUNCTIONAL

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15
Q

What are Olestra and Sucralose?

A

Carbohydrates that are UNDIGESTIBLE and cause cramps, gas, bloating, and diarrhea if ingested in large quantities because they CANNOT be absorbed leading to Water staying in the GI tract AND they provide food for bacteria

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16
Q

Where does Protein Digestion start? By what enzymes?

A

Protein Digestion starts in the stomach:

  1. Pepsinogens are secreted into the stomach as inactive Zymogens
  2. Pepsinogens are activated by the low pH of the stomach into Pepsin
  3. Pepsin cleaves Proteins into Oligopeptides
17
Q

Where does Protein Digestion happen in the body? What enzymes are used? What percentage of the Proteins are hydrolyzed in each place?

A
  1. Stomach- Pepsins cleave 25-30% of the Proteins into Oligopeptides
  2. Small Intestine- Pancreatic Proteases cleave 70-75% of Proteins into Oligopeptides
  3. Cytoplasmic Peptidases inside of the Enterocytes break down the absorbed Di- and Tri-Peptides into Amino Acids

NOTE: Brush Border Proteases hydrolyze the Oligopeptides into Di- and Tri-Peptides and Amino Acids

18
Q

What stimulates the release of Pancreatic Enzymes into the Duodenum?

A

CCK

19
Q

What stimulates the release of Pancreatic and Intestinal Buffers?

A

Secretin (increases the pH of the Acidic Chyme)

20
Q

What are the Pancreatic Proteases released into the Duodenum due to CCK stimulation? How are they activated?

A

The Pancreatic Proteases are:

  1. Trypsinogen
  2. Chymotrypsinogen
  3. Pre-Colipase
  4. Proelasatase

Enterokinase (EK) on the Brush Border activates Trypsin and Trypsin activates the others and also self-activates itself

21
Q

What kinds of Proteins are absorbed in the Small Intestines and how?

A
  1. Oligopeptides (created by Gastric and Pancreatic Proteases) are broken down by Brush Border Proteases into Di- and Tri-Peptides and Amino Acids
  2. Di- and Tri-Peptides AND Amino Acids are absorbed into Enterocytes via various transporters
22
Q

What part of digested Proteins is able to leave Enterocytes and enter the bloodstream?

A

Amino Acids are the only part of digested Proteins able to leave Enterocytes and enter the bloodstream:

CYTOPLASMIC PEPTIDASES break down the absorbed Di- and Tri-Peptides into Amino Acids

23
Q

What happens to Amino Acids in the Liver?

A
  1. Shuttled via Lipoproteins to places needed
  2. Extra AAs are Deaminated to NH3+ which combines with CO2 to make Urea
  3. Synthesize 90% of Plasma Proteins
24
Q

Where does Lipid Digestion start? By what enzymes?

A

The MOUTH via Lingual Lipase (helps with the flavor of food)

25
Q

Where does Lipid Digestion take place in the body? What enzymes are used? What percentage of the Lipids are hydrolyzed in each place?

A
  1. Mouth- Lingual Lingual Lipase
  2. Stomach- Gastric Lipase
    (25-30% Pre-Duodenally)
  3. Small Intestine- Pancreatic Lipase (70% of Lipids)

Lipid hydrolysis produces Monoglycerides, Free Fatty Acids, and Cholesterol

26
Q

What are the products of Lipid hydrolysis?

A
  1. Monoglycerides
  2. Free Fatty Acids
  3. Cholesterol
27
Q

Describe the Pathway of Lipid Digestion, Absorption, and Utilization:

A
  1. Lingual Lipase in the mouth begins to hydrolyze Lipids that are ingested (enhances flavor)
  2. Gastric Lipase in the stomach also hydrolyzes ingested Lipids (25-30% of Lipids are digested by the time they reach the Duodenum)
  3. In the Duodenum, CCK causes the release of Bile which surrounds and emulsifies Lipids to increase surface area
  4. CCK also stimulates the Pancreas to release Proteases as inactive Zymogens
  5. Trypsinogen is activated by Enterokinase (EK) into Trypsin which activates Pre-Colipase into Colipase
  6. Co-Lipase binds to the Bile protective layer and “opens the door” to give Pancreatic Lipase access to the Lipids
  7. Pancreatic Lipase hydrolyzes the Lipids into Monoglycerides, Free Fatty Acids, and Cholesterol creating a “Micelle”
  8. The Micelle “Taxis” the digested Lipids (Monoglycerides, Free Fatty Acids, and Cholesterol) across the Unstirred Water Layer where they can diffuse into Enterocytes
  9. Inside of the Enterocyte, Lipids enter the Smooth Endoplasmic Reticulum (SER) where they are re-esterified to form Triglycerides, Phospholipids, and Cholesterol
  10. The re-esterified Lipids are surrounded by a Lipoprotein Coat called a CHYLOMICRON
  11. The Chylomicron exocytoses the cell and enters the Lymph Lacteals (too big for capillaries)
  12. Lipids travel through Lymphatics and get dumped into the Thoracic Duct
  13. Lipids now enter the blood and head to the Liver via the Hepatic Artery (Systemic Circulation)
  14. Liver metabolizes Lipids into Lipoproteins, steroids, etc.
28
Q

Where is Bile absorbed and how?

A
  1. Primary Bile is absorbed in the Terminal Ileum via Secondary Active Transport with Na (SODIUM DEPENDENT)
  2. Secondary Bile (if it crosses the Unstirred Water Layer) diffuses into the Enterocyte
29
Q

In the Jejunum and Ileum, Na reabsorption stimulates:

A

Water and Cl- Reabsorption

30
Q

How does Iron travel in the body?

A

THERE IS NO FREE IRON IN THE BODY:

  1. Transferrin (binds Fe in blood)
  2. Ferritin (stores Fe in cells)
31
Q

How is Iron lost in the body?

A
  1. Enterocytes sloughing off every few days (Ferritin stores lost)
  2. Blood Loss
32
Q

How does Vitamin D regulate Calcium in the body?

A
  1. Increases Ca in the blood when needed
  2. Increases Ca reabsorption by increasing number of Channels and number of Calbindin inside of cells to bind Ca
  3. Increases Ca ATPase used to pump Ca into the blood
33
Q

Where can Vitamin B12 be absorbed?

A

ONLY IN THE TERMINAL ILEUM

34
Q

How is Vitamin B12 absorbed?

A
  1. B12 binds to Transcobalamin-1 (TC-1) in the mouth to protect it from Pepsins of the Stomach
  2. Intrinsic Factor (produced by Parietal Cells) binds to B12-TC-1 forming a complex
  3. In the Duodenum, Trypsin cleaves TC-1 leaving B12-IF
  4. B12-IF dimerizes with another B12-IF to protect B12 from Pancreatic Enzymes
  5. B12-IF Dimer binds to receptors in the Terminal Ileum and enter the Cytosol
  6. In the Cytosol, IF dissociates and B12 binds to TC-II
  7. TC-II exocytoses the Enterocytes and enters to blood to take B12 to the Liver for storage or Bone Marrow for use in RBC production
35
Q

What are the two important functions of the Terminal Ileum?

A
  1. Vitamin B12 absorption

2. Bile reabsorption (recycling)

36
Q

What causes Diarrhea?

A

INCREASED MOTILITY via:

  1. Viral or Bacterial Infection
  2. Irritation to the Intestines
  3. Osmotic Agents

GI Tract Physiology (7 decks)

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