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Alimentary System > Small Intestine > Flashcards

Small Intestine Flashcards

1
Q

Which 2 sphincters separate the SI from stomach and LI?

A
  1. Pyloric sphincter (proximal)

2. Ileocaecal valve (distal)

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2
Q

Which part of the SI is the largest?

A

Ileum largest (3.75m), Duodenum smallest (0.25m)

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3
Q

What are the invaginations in the SI called?

A

Crypts of Lieberkuhn

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4
Q

What are the different types of cells in the SI mucosa?

A
  1. Enterocytes
  2. Goblet cells
  3. Enteroendocrine cells
  4. Paneth cells
  5. Stem cells
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5
Q

Describe enterocytes (each one has 2000 microvilli)

A
  1. Most abundant in the gut - absorption.
  2. Columnar and have microvilli on apical surface (apical region has lots of RER)
  3. 1-6 day lifespan
  4. Contains glycocalyx - rich carbohydrate network which traps water, mucus and enzymes on cell surface for protection
  5. Layer of trapped mucus, water and enzymes = unstirred layer
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6
Q

Describe goblet cells (2nd commonest)

A
  1. Apical part contains mucin - distorts cell shape.
  2. Mucous = water and GPs, lubricant for gut passage.
  3. Number of goblet cells increases along length of intestine (as water is absorbed as you go along it)
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7
Q

Describe enteroendocrine cells

some enteroendocrine cells are closed and not exposed to gut lumen

A
  1. Hormone secreting epithelial cells (found commonly at bottom of crypts). Columnar-ish.
  2. Sensory apparatus = apical portion
  3. G = gastrin
    I = cholecystokinin
    S = secretin
    D= somatostatin
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8
Q

Describe Paneth cells

A
  1. Immunological cells
  2. Contain many acidophilic granules.
  3. Acidophilic granules contain: lysozyme, GPs, Zn (cofactor for lysosomes)
  4. Located near stem cells - priority is to protect progenitor cells
  5. Engulf bacteria and regulate intestinal flora
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9
Q

Describe stem cells

A

Stem cells continuously replace goblet cells and enterocytes (every 36 hrs)

Escalator birth, migration and death is energy intensive.

Allows lesions to be short lived, and gut borne toxins/drugs to eventually run their course

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10
Q

Distinguishing features of duodenum, jejunum and ileum?

A

Duodenum - mucous secreting submucosal gland. Secretes alkaline solution, neutralises acid chyme and protects SI proximal lining.

Jejunum - large submucosal folds (Place Circularis). Much larger than folds in other parts of SI and closer together - frill like.

Ileum - 100 Peyers patches (lymphoid tissue). Can initiate leukocyte and Ig responses. Patches contain M-cells, which don’t have microvilli.

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11
Q

What is the purpose of intestinal motility?

A
  1. Mix ingested food with digestive secretions
  2. Facilitate contact between contents and mucosal surface
  3. Propel contents along SI
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12
Q

What is the major effect of segmentation? (alternate contraction and relaxation)

A

Mixing and mechanical breakdown

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13
Q

What is the major effect of peristalsis? (sequential contraction)

A

Circular muscles contract, then longitudinal muscle.

Major effect is propulsion =.

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14
Q

What is the migrating motor complex? (less ordered in fed state)

A

Periodic contractions which prevent colonic flora from travelling backwards, and cleanse SI of residual food.

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15
Q

Where are big daddy enzymes?

A

Gut lumen

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16
Q

Where are dimeric cleaver enzymes - for absorption

A

Brush border of enterocytes

17
Q

Describe carbohydrate digestion

Break long polymers into simple monomers for absorption

A
  1. Salivary amylase
  2. Duodenum - pancreatic amylase and brush border enzymes
  3. Sucrase/Maltase/Lactase
  4. Maltose = 2xglucose
    Sucrose = glucose, fructose
  5. Glucose/galactose absorbed through apical surface by SGLT-1 (secondary AT).
  6. Fructose = facilitated diffusion via GLUT-5 channels.
  7. Glu/Gal/Fru diffuse down conc gradient in basolateral membranes using GLUT-2.
  8. Na gradient in the cell is ensured by Na/K ATPase as Na is pumped out.
18
Q

Describe protein digestion

A
  1. Pepsin in stomach breaks down long polypeptides into smaller peptides (pepsin doesn’t work in the duodenum)
  2. Digestion continues in duodenal lumen - pancreatic juice breaks polypeptides into tripeptides and dipeptides. Pancreatic juice contains trypsin, chymotrypsin and carboxypeptidase.
  3. Final stage = brush border, enzymes secreted by epithelial cells (di/tripeptidases). Enzymes are endo/di/aminopoly/carboxy peptidases.
19
Q

Describe protein absorption.

A
  1. Single amino acids absorbed into enterocytes by secondary active transport - (AA/Na transporter).
  2. AA/H+ supporter used for di/tripeptides. Cytoplasmic peptidase enzymes break them down into AA before moving across basolateral membrane via facilitated diffusion.
20
Q

Describe lipid digestion

A
  1. Lingual lipase hydrolyses triglycerides.
  2. Digestion in stomach, due to ingested lingual lipase and gastric lipase.
  3. Mechanical stomach churning slightly emulsifies fats, increasing SA.
  4. Acid chyme leaves stomach, but fats largely undigested. Bile enters duodenum - chemical emulsification - huge SA increase as small fat droplets.
  5. Pancreatic juice simultaneously secreted, and cleaves 2 fatty acid chains leaving a monoglyceride and free fatty acids (pancreatic lipase and colipase) = luminal digestion.
  6. At brush border, they are combined with bile salts forming micelles - soluble enough to cross aqueous unstirred layer.
21
Q

What does gastric lipase do?

A

Cleaves fatty acid chains from free triglycerides.

22
Q

What is luminal digestion?

A

Pancreatic lipase and colipase in duodenum.

23
Q

Describe Absorption of lipids

A
  1. In brush border, lipolytic products diffuse through apical membrane, bile salts remain in lumen.
  2. In the enterocyte, MAG becomes TAG. Either through monoglyceride acylation pathway, or phosphatidic pathway.
  3. TAGs packaged with proteins, phospholipids and cholesterols - form chylomicrons (lipoprotein).
  4. Chylomicrons exocytosed and enter lymphatic system via lacteal.
24
Q

What do Brunners glands do?

A

Found only in duodenum.

They are submucosal glands that neutralise acidic chyme entering the duodenum.

Alimentary System (15 decks)

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