Colon Structure and Functions

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Questions and Answers

What percentage of water is present in normal human stool?

65-85% (correct)
50-60%
40-50%
70-80%

What is the primary function of the crypts in the colon epithelium?

To absorb vitamins
To act as capillaries to suck water out of feces (correct)
To produce mucus
To ease the passage of feces

What is the pH level of normal human stool?

7-8 alkaline
6-7 alkaline
4-5 acidic
5-6 acidic (correct)

What is the primary mechanism of water absorption in the colon?

Capillary suction pressure (B) Signup and view all the answers

What percentage of solid matter in stool is composed of bacteria?

40-50% (D) Signup and view all the answers

What is the main function of the goblet cells in the colon epithelium?

To produce mucus (B) Signup and view all the answers

What is the name of the process by which the gut maintains a standing gradient of absorption?

Standing gradient osmosis (A) Signup and view all the answers

What is the percentage of water absorbed by the GI tract?

99% (A) Signup and view all the answers

What is the mechanism of sodium transport in the proximal bowel?

Counter-transport in exchange for H+ (C) Signup and view all the answers

What drives the transport of Cl- and HCO3- into the intercellular spaces?

Electrical potential created by Na+ transport (A) Signup and view all the answers

What is the result of the high concentration of ions in the intercellular spaces?

Hypertonic fluid in the intercellular spaces (B) Signup and view all the answers

What is the role of Na+K+ATPase in the lateral plasma membrane?

Active transport of Na+ into the lateral intercellular spaces (C) Signup and view all the answers

What is the mechanism of water transport from the gut lumen?

Osmotic flow through tight junctions (B) Signup and view all the answers

What is the direction of ion and water transport across the basement membrane?

From the intercellular spaces into the blood (A) Signup and view all the answers

What is the effect of the high hydrostatic pressure in the intercellular spaces?

Increased ion and water transport across the basement membrane (A) Signup and view all the answers

In which part of the intestine does sodium transport occur through co-transport with Cl-?

Ileum (D) Signup and view all the answers

What is the primary mechanism of water absorption in the small intestine?

Osmosis following sodium and solute absorption (B) Signup and view all the answers

What is the primary function of Na/K ATPase in the colon?

To maintain the sodium gradient (C) Signup and view all the answers

What is the characteristic of osmotic diarrhea?

It is characterized by a high osmotic gradient (>350 mosmoles) (A) Signup and view all the answers

Which of the following ions is NOT absorbed via secondary active transport?

Potassium (B) Signup and view all the answers

What is the consequence of slow movement of faeces in the colon?

It leads to the formation of very hard faeces (C) Signup and view all the answers

What is the primary mechanism of sodium absorption in the colon?

Restricted movement through ion channels (D) Signup and view all the answers

Which of the following is NOT a function of aquaporins?

Modulating ion channels (B) Signup and view all the answers

What is the underlying mechanism of congenital chloride diarrhea?

A lack of Cl-/HCO3- exchange in the ileum and colon (A) Signup and view all the answers

What is the primary mechanism by which cholera toxin causes diarrhea?

Stimulation of intracellular cAMP leading to increased Cl- secretion (D) Signup and view all the answers

What is the fate of undigested carbohydrates in the large intestine?

They are converted into SCFA by bacteria (C) Signup and view all the answers

What is the primary mechanism of chloride absorption in the ileum?

Co-transport with sodium (C) Signup and view all the answers

What is the characteristic of normal stool?

It is isotonic (10-300 mosmoles) (A) Signup and view all the answers

What is the colon classified as in terms of epithelial resistance?

High resistance epithelia (D) Signup and view all the answers

What is the primary function of the microbiome in the large intestine?

Fermentation of undigested carbohydrates (D) Signup and view all the answers

What is the primary mechanism of iron absorption?

Active transport through specific receptors (C) Signup and view all the answers

What is the primary mechanism of potassium absorption in the small intestine?

Passive diffusion through paracellular pathways (D) Signup and view all the answers

What is the primary cause of osmotic diarrhea?

Malabsorption of nonelectrolytes (C) Signup and view all the answers

What is the primary mechanism of bicarbonate absorption in the colon?

Exchange with chloride ions (B) Signup and view all the answers

What is the primary function of CFTR in the luminal membrane?

Cl- channel (C) Signup and view all the answers

What is the effect of hypermotility on intestinal absorption?

Decreased absorption of nonelectrolytes (C) Signup and view all the answers

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Study Notes

Structure of the Colon

The colon has a flat epithelium with deep crypts
Crypts can act as capillaries to suck water out of feces
Many mucus-secreting goblet cells throughout the epithelium ease the passage of feces

Functions of the Colon

Water and electrolyte absorption
Formation of hard stool suitable for defecation
Fermentation of undigested carbohydrates
Absorption of some vitamins, e.g., Vitamin K

Water Absorption and Electrolytes

99% of water presented to the GI tract is absorbed
Water absorption follows Na+ and other solute absorption by osmosis
Water transport can be paracellular (via tight junctions) or transcellular (across cell membranes via aquaporins)

Standing Gradient Osmosis

Driven by Na+ transport from lumen into enterocyte
Becomes more efficient as you travel down the intestine
Counter-transport in exchange for H+ (small intestine)
Co-transport with amino acids, monosaccharides (small intestine)
Co-transport with Cl- (small intestine)
Restricted movement through ion channels (colon)

Absorption of Other Ions

Cl- co-transported with Na+ (ileum) and exchanged with HCO3- (colon) into enterocytes
K+ diffuses in via paracellular pathways in small intestine and leaks out between cells in colon
Passive transport

Colonic Disorders

Diarrhea: congenital diarrhea, bacterial infection of gut (e.g., cholera, C. difficile), osmotic diarrhea
Constipation: slow movement of feces allows too much water absorption, resulting in hard feces
Irritable bowel syndrome: combination of the above, causing abdominal discomfort, with unknown cause

Colon as a High-Resistance Epithelium

Normal stool: 10-300 mosmoles, isotonic
Osmotic diarrhea: >350 mosmoles

Congenital Diarrhea

Deficiency of normal ion transport system
Cl-/HCO3- exchange lacking in ileum/colon

Bacterial-Induced Diarrhea

Enhanced secretion of electrolytes and water, e.g., cholera
Intracellular cAMP controls Cl- channel in luminal membrane (CFTR)

Osmotic Diarrhea

Failure to absorb nonelectrolytes (various syndromes)
Use of laxatives or broad-spectrum antibiotics
Hypermotility of intestine - various causes, rapid flow of intestinal contents past the absorptive epithelium

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