Intestinal Absorption and Secretion of Fluids and Electrolytes Flashcards
What are the 2 key absorptive and secretory structures in the small intestine?
Absorptive - villi
Secretory - cryps of lieberkuhn
What are each of the cells in this image and what are their functions?
- Absorptive cell - at top of villi, absorb nutrients, electrolytes, fluid
- Goblet cell - secrete mucous
- Enteric endocrine cell - secreting cells, secrete CCK and secretin among other things
- Stem / progenitor cell - replenish absorptive cells
- Paneth cells - immune function (alpha defensin and lysozyme)
What is the turnover rate for absorptive cells on villi?
Slough off in 3-5 days
Under what conditions (not an exhaustive list, just some to know) is the turnover of shedding absorptive cells and producing new ones from stem cells diminished?
- Right after eating
- During lactation
- After intestinal resection
The absorptive cells that line the gut on the villi undergo rapid turnover. What does this make them susceptible to?
Vulnerable to radiation and chemotherapy
Brunner’s glands in the duodenum secrete what?
Alkaline mucus
- Where are brunner’s glands located?
- What is their function?
- What benefit does this provide for the GI tract?
- Pylorus –> sphincter of Oddi (place where pancreatic enzymes enter duodenum) –> ONLY IN DUODENUM
- Mucus secretion with high HCO3- content
- Protects duodenal wall and neutralizes acid in chyme from stomach
- What signals stimulate secretions from brunner’s glands?
- What signals inhibit secretions from brunner’s glands?
Where are crypts of lieberkuhn located?
Throughout small intestine
Within a crypt of Lieberkuhn, what are the 2 kinds of secretory cells that can be found and what do they secrete?
- Crypt enterocyte –> Fluid and electrolytes (Na+, Cl-, HCO3-)
- Goblet cell –> Mucous and HCO3-
- Throughout the small intestine, what is the purpose of secrete fluid into the GI lumen?
- How is fluid secretion accomplished?
- The secreted fluid picks up nutrients making it easier for the nutrients to be absorbed as they are absorbed with the fluid by absorptive cell
- The cells don’t actively secrete fluid. Instead they actively secrete Cl- and HCO3- and Na+ (both active and passive). Secretion of these ions into GI lumen causes osmotic movement of water into GI lumen as well –> fluid secretion.
What is the composition of fluid that is secreted by crypt enterocytes?
Isotonic, slightly alkaline for acid neutralization
Where does most absorption in the GI tract occur?
Duodenum and Jejunum
With regards to the secretory enterocytes in the crypts of Lieberkuhn, secretion of […] and […] is active while secretion of […] is both active and passive.
Cl-
HCO3-
Na+
In the colon, […] are absent while […] are present.
Villi
Crypts of Lieberkuhn
The colon lacks villi. How does it accomplish absorption and what does it absorb?
Colon still has surface enterocytes that absorb fluid and electrolytes (mostly fluid)
What is the function of crypt enterocytes in the colon?
Secretion of fluid and electrolytes (Na+, Cl-, HCO3-)
Are goblet cells found in the colon?
Yes –> secrete mucous and HCO3-
What is the purpose of having a large # of goblet cells in the colon to secrete mucus?
Helps with feces formation and mucosal protection of the colon wall
Surface area in both the small and large intestines are amplified in order to facilitate absorption. By what mechanisms is this accomplished in the small intestine and large intestine?
Where does most of the body’s fluid aborption occur?
Small Intestine
Different sections of the GI tract have different permeability to water. What determines how permeable a section of the GI tract is?
Degree of resistance posed by epithelial tight junctions
In column on right, state how permeable each section is to water.
Describe the general mechanism by which fluid is absorbed and secreted throughout the GI tract.
Fluid follows solutes due to creation of osmotic driving force for water to be either reabsorbed or secreted.
Fluid absorption in a particular region of GI tract is mostly due to net […] absorption
Na+, glucose, amino acids, and Cl-
Fluid secretion in a particular section of GI tract is usually due to net […] secretion
Cl-
What is the NET result of absorption and secretion in the small intestine and large intestine?
Water is absorbed throughout the whole GI tract. What areas of the GI tract does most of the reabsorbing?
Na+ and Cl- are absorbed throughout the small and large intestines. What areas of do most of the absorbing?
Bicarb is absorbed or secreted along the length of the small intestine and colon.
- Where is it secreted?
- Where is it absorbed?
- Secreted - Duodenum, Ileum, colon
- Absorbed - jejunum
K+ is secreted and absorbed throughout small intestine and colon.
- Where is it secreted?
- Where is it absorbed?
- Secreted - Ileum, colon
- Absorbed - Duodenum, jejunum, colon
Where in the small intestine is:
- Ca2+ absorbed?
- Fe2+ absorbed?
- Duodenum, jejunum
- Duodenum
Describe how Ca2+ is absorbed in the small intestine.
- Occurs in duodenum and jejunum.
- These parts of SI have leaky tight junctions, so Ca2+ can diffuse paracellularly –> Passive
- Can also have active transport when VitD (calcitriol) is around –> upregulates insertion of Ca2+ channels into apical and basolateral membranes of intestinal epithelium –> Ca2+ enters and binds to calbindin to maintain Ca2+ gradient and keep intracellular available Ca2+ regulated –> calbindin delivers Ca2+ to basolateral membrane where Ca2+ is pumped into blood via a number of transporters –> Active / transcellular
Describe the absorption of Iron
- Occurs in duodenum
- Ingestion of iron from either heme sources (Fe2+) or non-heme sources (Fe3+)
- Heme enters via heme transporter, free Fe3+ reduced to Fe2+ by apical membrane cytochrome, then enters via DMT1
- Cells either
- Store the iron as ferritin (which is lost)
- Release iron into the body via transport through ferroportin on the basal epithelial membrane.
- Transported Fe2+ is oxidized to Fe3+(by Hephaestin); Fe3+ binds transferrin for travel in the blood
- Utilized or stored in liver
Describe intestinal absorption of Na+ and Cl- in the FED STATE
- Important to note - mostly duodenum and jejunum that are affected by fed/fasting
Describe intestinal absorption of Na+ and Cl- in the FASTING STATE
- Important to note: this diagram illustrates how the duodenum and jejunum handle absorption of Na+ and Cl- in fasting but it illustrates how ileum and colon handle absorption all the time, regardless of fed/fasting conditions
How is Na+ and Cl- absorbed in the distal colon?
Aldosterone regulates Na+ absorption in the […]
Distal colon
- Describe how Cl- is secreted in the intestine (small and large, is same mechanism)
- Why is chloride secreted in intestines?
- What is the relationship between presence of cholera toxin and Cl- secretion?
K+ is absorbed in the […] and secreted in the […]
Small/large intestine
Distal colon
Describe K+ absorption in small intestine.
- Describe K+ secretion in the distal colon.
- What transporter drives this secretion?
- What hormone is this affected by?
- How is this influenced when in hypovolemic?
Describe the effects of aldosterone in the distal colon.
Aldosterone stimulates the increased activity and # of Na+/K+ ATPase and ENac channels in the distal colon cells as well as the increased activity and # of the Na+/K+/2Cl- transporter. This results in net Na+ absorption and K+ secretion. It is important to note that these mechanisms are not coupled –> they occur by different mechanisms and I assume in different cells, unlike in the kidney where they were directly coupled. Thus when a person is having really bad diarrhea, they are losing a lot of volume –> triggers aldosterone secretion –> increases Na+ absorption and K+ secretion –> hypernatremic and hypokalemic.
What 4 things regulate absorption and secretion in GI tract?
ENS/ANS
Paracrine factors
Endocrine factors
Immune cells
How does the ENS/ANS regulate secretion/absorption?
What is the stimulus?
How do paracrine factors regulate secretion/absorption?
What is the stimulus?
How do endocrine factors regulate secretion/absorption?
What is the stimulus?
How do immune cells regulate secretion/absorption?
What is the stimulus?
What is the definition of diarrhea
Increased stool volume > 0.2L/day due to:
- Increased bowel movements
- Decreased stool consistency
What are the different types of diarrhea?
C - glucose
Na+, Cl-, K+
