Small intestines

Question 1

Function and length of the duodenum

Answer 1

Digestion and gut regulation (0.25m)

> coiled mucous-secreting submucosal gland that secretes bicarbonate-rich alkaline solution

> Neutralise acidic chyme / optimum pH for other enzymes / protect lining of proximal SI

Question 2

Function and length of the jejunum

Answer 2

Absorption (2.5m)

> Large and close submucosal folds (plicae circularis) = seem like frills

Question 3

Function and length of the ileum

Answer 3

Absorption (3.75m)

> Up to 100 Peyer’s patches (specialised lymphoid tissue involved in the immune response to gut pathogens)

>Initiate leukocyte and immunoglobulin responses.

> contain m cells which don’t have microvilli

Question 4

The overall structure of the small intestines

Answer 4

> Finger-like projections (villi) = motile, rich blood supply, lymph drainage > Folded mucosa > Crypts of Lieberkühn (invaginations)

Question 5

How is the small intestines separated from the large intestines and stomach?

Answer 5

Pyloric sphincter = proximal end

ileocaecal valve = distal end

Question 6

Enterocytes

Answer 6

> most abundant (absorption)

> columnar epithelia with villi (brush-border) Apical region = RER basolateral membrane = nucleus and mitochondria

Life span => 1-6days

Question 7

Apical membrane of enterocytes

Answer 7

Rich network of carbohydrates (glycocalyx)

> help to trap layer of water mucus and enzymes for protection from luminal contents

>regulate digestion and absorption Known as the unstirred layer

Question 8

Goblet cells

Answer 8

Second most abundant Apical = mucin granules (distort the shape of the cell)

contains golgi, RER and nucleus

Mucous contains water + glycoproteins = lubricant to facilitate passage

Question 9

Enteroendocrine cells

Answer 9

> Hormone-secreting epithelial cells

> bottom of crypts and fairly columnar

> Sensory apparatus at apical membrane, hormones at basolateral membrane (secrete into the blood)

> can be closed (not exposed to the gut lumen)

Question 10

Examples of enteroendocrine cells

Answer 10

G cells = gastrin

I cells = Cholecystokinin

S cells = secretin

D cells = somatostatin

Question 11

Paneth cells

Answer 11

> Immunological cells containing acidophilic granules

> bottom of crypts

> Near stem cells (protect progenitor cells)

> Ingest bacteria and regulate intestinal flora

Question 12

What do the granules of paneth cells contain?

Answer 12

Lysozyme (antibacterial enzyme), Glycoprotein (to protect local cells from enzymes), Zinc (common lysozyme cofactor)

Question 13

Stem cells

Answer 13

> Continuously replace goblet cells and enterocytes

> Pluripotent cells that proliferate in the crypts and move upwards

> Become senescent at the top of the villi (slough off, broken down and reabsorbed)

Question 14

What do stem cells allow?

Answer 14

>Allows toxins to run their course and affected cells to continuously be replaced

> Lesions will be repaired and short-lived

Radiation therapy can impair cell function resulting in severe GI dysfunction

Question 16

3 functions of intestinal mobility

Answer 16

> Mix food with digestive secretions/enzymes

> Aid contact between contents and mucosal surface

> Propel contents along SI

Question 17

First stage of carbohydrate digestion

Answer 17

Mouth via salivary amylase and resumes in the duodenum via pancreatic amylase + brush border enzymes

Complex = long polymers

Simple = disaccharides and monosaccharides

Question 18

3 types of motility mechanisms

Answer 18

Segmentation (alternate contractions of non-adjacent segments = mixing and mechanical breakdown)

Peristalsis (squential contractions of adjacent segments of circular muscle and longitudinal muscle = propulsion)

Migrating motor complex ( periodic contractions from the stomach to the distal ileum during a fasted state and restarts after completion, lees ordered = fed = prevention of colonic flora from travelling upwards)

Question 19

Outline the action of pancreatic amylase

Answer 19

Polysaccharides to disaccharides (sucrose, maltose and lactose)

Maltose = glucose + glucose (via maltase)

Sucrose = glucose + fructose (via sucrase)

Lactose = glucose + galactose (via lactase)

Question 20

Describe the absorption of monosaccharides

Answer 20

Apical transporter

Glucose and galactose (SGLT-1) = secondary active transport/symporter, Fructose (GLUT-5) = facilitated diffusion

Basolateral transporter

All use GLUT-2

Question 21

Overall degradation of proteins

Answer 21

Pepsin breaks down long chains to smaller chains in the stomach (needs acidic env) = doesn’t work well in the duodenum

Pancreatic juice containing protease continues breakdown (trypsin, chymotrypsin, carboxypeptidase)

tri/dipeptidases breakdown proteins to 2 or 3 amino acids (secreted from the epithelium brush border)

Question 22

Outline the first stage of lipid digestion

Answer 22

The first stage = lingual lipase (hydrolyses triglycerides)

Ingested lingual lipase and gastric lipase (cleaves fatty acid chains from triglycerides)

Mechanical churning = emulsify fats = increase in surface area

Question 23

Describe what happens to lipids after leaving the stomach

Answer 23

Bile is added = chemical emulsification and increases S.A by forming fat droplets

Pancreatic juice = cleave 2 fatty acid chains from triglycerides = monoglyceride and free FA

Combine with bile salts to form micelles = soluble enough to cross the aqueous unstirred layer

Question 24

Outline the absorption of lipids

Answer 24

• Lipolytic products diffuse through the apical membrane and bile salts remain in the lumen (reabsorbed in the terminal ileum)
• Monoglycerides and FA is re-synthesised = triglycerides via monoglyceride acylation pathway and phosphatidic pathway
• Packaged with proteins, phospholipids and cholesterol = chylomicrons = exocytosis into the lymphatic system