Intestinal Transport

What are methods of water intake?

Drinking water, feed, and metabolic water gain

How does metabolic water gain occur?

Hydrolysis will consume water but anabolism will produce water

What are methods of water loss?

Urination (main), feces, evaporation, respiratory, milk

Diarrhea can be problem

The internal environment created by epithelial cells will be most similar to what?

Extracellular space

Why will GI tract will secrete apporxamly an Extracellular volume of fluid per day?

to make sure the lumen stays isosmolar because water intake is not enough

Water absorption mainly occurs in ______, secretion mainly occurs in ________.

Villus

Crypts

Secretory and absorption will change in response to what?

Weaning

Resection surgery

Chronic dehydration

Infection

GI tract wall stretch

Hormones

The small intestine will absorb water and electrolytes while secreting bicarb. The fluid absorbed is?

Isoosmotic

The large intestine will absorb water NaCl and secrete K+ and bicarb. The fluid absorbed is?

Hypoosmotic fluid

Cl- is secreted by the ___ and will be triggered by what active mechanism?

Crypts

NKCC pumps and the Na/K pump on the basolateral membrane

What factors can increase presence of Cl- channels? What is the normal secretion of Cl-?

Neurotransmitters

Bacterial toxins

Inflammation

Immune system

normally low basal secretion

How do prostaglandins increase Cl- secretion?

Promote VIP production from parietal neurons which will increase cAMP and activate PKA that will activate Cl- channels

Toxins that activate Cl- secretion can lead to what?

Increased water in lumen then diarrhea

What is the most important transporter for fluid absorption in the GI tract?

Na cotransporters (Na/glucose or Na/ AA)

What transporter in the large intestine will allow for it to absorb 3x the amount of water in the intestines? What hormone controls it?

ENaC

Aldosterone (promotes embedding into apical membrane/new proteins to be made + NaK pump)

How does most all K+ absorption take place?

Solvent drag (passive)

Most Cl- absorption takes place ____ via?

Passively via pracellular and transcellular due to electric gradients of Na+

electroneutral exchange w bicarb

Cl is absorbed via Na/Cl transporter on the ____ membrane and is secreted by Na/Cl transporter on the ____ membrane.

Apical

Basolateral

What are the two coupled transporters in the ileum and proximal colon that absorb Na and Cl? What helps catalyze the reaction?

Na/h exchanger coupled w/ the Cl/bicarb exchanger

carbonic anhydrase

At what osmolarity is the body kept at?

~ 300 mOsm/L

digestion takes place in this environment

What are the concentrations of Na+, K+, and Ca2+ kept at in the body?

Na - 130 mM extracellularly, 10 mM intracellularly

K - 4 mM extracellularly and 120 mM intracellularly

Ca2+ - low intracellularly

Why is Na+ balance so important?

sets up chemical/osmolar gradient important to cell functions for electric signals

needed for secondary active transport

Digestion _______ osmolarity of the lumen.

increases

water secreted to lumen moved by osmotic gradient intensified by nutrient digestion and chyme made isosmotic

Cl- transport primarily aids in _____ function of the GI.

secretory

How does Chyme remain isosmotic though GI?

water absorption/secretion

hyperosmotic chyme → water secretion → nutrient absorption → hypoosmotic chyme → water reabsorption

What transport is targeted for oral/enteral (PO/NG) rehydration? What does this promote absorption of?

Na/glucose and Na/amino acid, they are energized by NaK pump

Cl- is absorbed via paracellular transport

What transporter may have advantages over Na/glucose for rehydration therapy? Why?

Na/Gln (glutamine)

promotes mucosal repair, serves for gluconeogenesis, as fuel (makes a-ketoglutarate) and building block for protein synthesis

How can diarrhea cause hypokalemia? In times of hypokalemia where and how K+ be increased?

inhibits K+ absorption and is lost to feces

in distal colon have K-H ATPase to pump K+ into enterocytes

Where is Na, Cl, K , bicarb, and water primarily absorbed in the GI?

Na + water - proximally (duodenum/upper jejunum)

K, Cl, bicarb - distally (ileum + colon)