36 Questions
What percentage of water is present in normal human stool?
65-85%
What is the primary function of the crypts in the colon epithelium?
To act as capillaries to suck water out of feces
What is the pH level of normal human stool?
5-6 acidic
What is the primary mechanism of water absorption in the colon?
Capillary suction pressure
What percentage of solid matter in stool is composed of bacteria?
40-50%
What is the main function of the goblet cells in the colon epithelium?
To produce mucus
What is the name of the process by which the gut maintains a standing gradient of absorption?
Standing gradient osmosis
What is the percentage of water absorbed by the GI tract?
99%
What is the mechanism of sodium transport in the proximal bowel?
Counter-transport in exchange for H+
What drives the transport of Cl- and HCO3- into the intercellular spaces?
Electrical potential created by Na+ transport
What is the result of the high concentration of ions in the intercellular spaces?
Hypertonic fluid in the intercellular spaces
What is the role of Na+K+ATPase in the lateral plasma membrane?
Active transport of Na+ into the lateral intercellular spaces
What is the mechanism of water transport from the gut lumen?
Osmotic flow through tight junctions
What is the direction of ion and water transport across the basement membrane?
From the intercellular spaces into the blood
What is the effect of the high hydrostatic pressure in the intercellular spaces?
Increased ion and water transport across the basement membrane
In which part of the intestine does sodium transport occur through co-transport with Cl-?
Ileum
What is the primary mechanism of water absorption in the small intestine?
Osmosis following sodium and solute absorption
What is the primary function of Na/K ATPase in the colon?
To maintain the sodium gradient
What is the characteristic of osmotic diarrhea?
It is characterized by a high osmotic gradient (>350 mosmoles)
Which of the following ions is NOT absorbed via secondary active transport?
Potassium
What is the consequence of slow movement of faeces in the colon?
It leads to the formation of very hard faeces
What is the primary mechanism of sodium absorption in the colon?
Restricted movement through ion channels
Which of the following is NOT a function of aquaporins?
Modulating ion channels
What is the underlying mechanism of congenital chloride diarrhea?
A lack of Cl-/HCO3- exchange in the ileum and colon
What is the primary mechanism by which cholera toxin causes diarrhea?
Stimulation of intracellular cAMP leading to increased Cl- secretion
What is the fate of undigested carbohydrates in the large intestine?
They are converted into SCFA by bacteria
What is the primary mechanism of chloride absorption in the ileum?
Co-transport with sodium
What is the characteristic of normal stool?
It is isotonic (10-300 mosmoles)
What is the colon classified as in terms of epithelial resistance?
High resistance epithelia
What is the primary function of the microbiome in the large intestine?
Fermentation of undigested carbohydrates
What is the primary mechanism of iron absorption?
Active transport through specific receptors
What is the primary mechanism of potassium absorption in the small intestine?
Passive diffusion through paracellular pathways
What is the primary cause of osmotic diarrhea?
Malabsorption of nonelectrolytes
What is the primary mechanism of bicarbonate absorption in the colon?
Exchange with chloride ions
What is the primary function of CFTR in the luminal membrane?
Cl- channel
What is the effect of hypermotility on intestinal absorption?
Decreased absorption of nonelectrolytes
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
This quiz covers the structure of the colon, including its epithelium and crypts, as well as its functions, such as water and electrolyte absorption, formation of stool, and vitamin absorption.
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