GI electrolytes Flashcards
How much fluid is presented to the digestive system daily and from where?
8.5 L total; 1.5 L from saliva, 2L from stomach, 1/2 L in bile, pancreas 1.5 L, 1 L from intestine, 2 L by intake
Where is fluid reabsorbed and how much is absorbed in each site?
Small intestine reabsorbs 6.5 L and colon absorbs 1.9 L
What is the net movement of ions in the small intestine?
Net absorption: Na, Cl, H20
Net secretion: HCO3-
What is the net movement of ions in the large intestine?
Net absorption: Na, Cl, H2O
Net secretion: HCO3, K
What is the difference between a paracellular pathway and a transcellular pathway and where is the former more likely to appear?
Paracellular travel around tight junctions, more common in proximal gut; transcellular often require active transport somewhere
What is the driving force of sodium absorption in the intestines?
Na/K ATPase establishes gradients
What is the most common method of sodium uptake in feeding conditions and where does it occur in the gut?
Na/Glucose or Na/Amino acid cotransporters; in the villi of the jejunum
Which mechanism is triggered in an alkaline environment and where does it occur in the gut?
Na/H exchanger; in the proximal gut
Which mechanism predominates in fasting conditions and what cellular messengers promote or inhibit it? Where does it predominantly occur?
Parallel Na/H and Cl/HCO3 mechanism (electroneutral); decreased by cAMP, cGMP, and low Ca; promoted by Ca and angiotensin; happens in the ileum and proximal colon
Which mechanism occurs in the colon and how is it modulated?
Aldosterone allows uptake of sodium via electrogenic sodium channels (ENaC). It opens existing channels, transports vesicles to the surface, and produces new Na channels
What are the three mechanisms by which chloride is absorbed?
1) voltage dependent [Cl uptake linked with Na], can be paracellular or transcellular
2) Cl/HCO3 exchanger in ileum and colon
3) Na/H//Cl/HCO3 parallel exchanger
How can chloride be excreted?
Secretagogue promotes a channel to reach the apical surface of a cell in the crypts of the intestines; Cl will flow into the lumen and Na and H20 follow
How is potassium absorbed in the small intestine?
Usually passively via solvent drag; normally does not involve active transport to get into the body
How would the colon absorb potassium?
In low-potassium circumstances, the colon can send H/K ATPases to the apical surface to draw K in
How does the colon normally excrete K?
Na absorption creates an electrical gradient that promotes K secretion; can be induced by cAMP or aldosterone
What are the main secretagogues, where do they act, and what consequence do they all have in common.
Acetylcholine, laxatives [increase Ca], prostaglandins [Ca] and histamines from immune cells, angiotensin and aldosterone (humoral) [cAMP], bacterial toxins [cAMP, c GMP, or Ca]. All activate Cl- secretion
What are the main absorptagogues?
Aldosterone, Cortisol, Somatostatin—all promote Na absorption
What are the main types of starch and what are their chemical bonds?
Amylose–alpha 1-4 glycosidic bonds
Amylopectin–alpha 1-4 with alpha 1-6 branchpoints
Glycogen–animal starches with extensive branching
How do amylases break starches into smaller components? Where can they not work?
They cleave internal glycosidic 1-4 bonds; but cannot access terminal bonds or those near 1-6 bonds.
What are the products of amylase cleavages?
Maltose, maltotriose, alpha-limit dextrins (glucose never results!)
Which glycolytic enzymes are on the surface of enterocytes and what cleavages do they promote?
1) maltase–residual alpha 1-4 bonds
2) sucrase–cleaves sucrose
3) isomaltase–residual alpha1-6 bonds
4) lactase–cleaves lactose
How are the three main monosaccharides absorbed?
Glucose–facilitated diffusion via GLUT2 or Na-coupled
Fructose–GLUT5
Galactose–GLUT2
From where does the body get much of its protein load?
Recycled from endogenous proteins in saliva, secretions, and epithelial cells
What are the main proteases and in how much of protein digestion they function?
Pepsin–10-15% of total digestion
Endopeptidases (trypsin, chymotrypsin, elastase)–cleave short peptides, 70% of pancreatic digestion
Exopeptidases (carboxypeptidases)–cleave AA one at a time, 30% of pancreatic digestion
What enteric enzymes are involved in protein digestion and absorption?
Brush border peptidases, endopeptidases and exopeptidases; most peptides are absorbed in series of 2-4 AA via PepT1 transporter with Na; individual AA have special channels
How does fat emulsification begin?
Chewing and gastric churning start process, addition of bile prevents recombination
What enzymes begin fat digestion and how do they work?
Lingual and gastric lipases start functioning at low pHs and conduct 10-30% of digestion; they often cleave the first fatty acid off a TAG
What are the roles of pancreatic lipases?
They will remove fatty acids from the 1 and 3 positions of a TAG; colipases anchor lipase to micelles to help absorption
How are short and medium chain fatty acids absorbed?
Since they are smaller, they are more readily soluble and will be absorbed directly by the mucosa
How do enterocytes package fats for the rest of the body?
They put triacylglycerides into chylomicrons with proteins, phospholipids, and lecithin and enter lacteals.
How is folate absorbed?
1) Proximal enterocytes link glutamate residues which are cleaved down to one
2) folate-glutamate exchanged for a hydroxyl group
3) Hepatocytes and (minorly) enterocytes conjugate it into THF
How is Vitamin B12 absorbed?
1) Pepsin releases it from animal products
2) Initially binds to salivary haptocorrin
3) Parietal cells make intrinsic factor to bind to cobalamin
4) Enterocytes in the terminal ileum take up the complex via endocytosis
What class of antacids block B12 absorption?
Anti-histamine agents like ranitidine; proton pump inhibitors do not
How does calcium absorption occur?
Vitamin D promotes opening of Ca channels in the duodenum; calbindin takes up Ca ions to prevent action and to maintain a strong gradient in favor of entry; Ca shunted out of the cell via Ca-ATPase or Ca/Na antiporter
