Digestion and absorption Flashcards
What are the organs of the GI tract?
Mouth, salivary gland, stomach, pancreas, liver, gall bladder, small intestine, large intestine
What does the mouth do?
Mechanical digestion
What does the salivary gland do?
Secretes fluid and digestive enzymes
What does the stomach do?
Body of stomach secretes mucus, pepsinogen and HCl; antrum of stomach secretes mucus, pepsinogen and gastrin; stores large quantities of food until it can be accommodated in intestines; mixes food with gastic secretions to form a homogenous, acidic chyme; regulates rate of emptying chyme into duodenum; secretes intrinsic factor
What does HCl do within the stomach?
Activates pepsin from pepsinogen; kills many pathogens; denatures and breaks down food proteins so are more vulnerable to enzyme action; inactivates hormones of foreign origin; increases absorption of Fe and Ca
What does gastrin do?
Stimulates motility and HCl production
What is intrinsic factor necessary for?
Vitamin B12 absorption
What does the pancreas do?
Secretes NaHCO3 (to neutralise acidic content from stomach) and digestive enzymes
What does the liver do?
Secretes bile
What does the gall bladder do?
Temporarily stores bile; makes more concentrated
What do the small intestines do?
Absorb water, nutrients, and electrolytes
How does the small intestine have such a large SA?
Has folds of kreckring, villi, and microvilli; villi have folds in mucosal layer, microvilli, and brush border membrane of mucosal epithelial cells
What do the large intestines do?
Absorb small amount of water and electrolytes; contains large colony of bacteria which: metabolise fibre to SCFA, which can be absorbed by diffusion; and produce Vitamin K
How many litres of chyme enters the large intestines each day?
1.5l
What is peristalsis?
The contraction of adjacent segments of the GI tract in sequence
What are the enzymes involved in the digestion of carbs, and the reactions they catalyse?
Salivary amylase: starch –> dextrins and maltose. Pancreatic amylase: dextrins –> maltose. In small intestine:- lactase: lactose –> galactose + glucose; maltase: maltose –> glucose + glucose; sucrase: sucrose –> fructose + glucose
How are glucose and galactose absorbed?
Glucose and galactose through sodium-dependent glucose transporter 1 (SGLT1) into epithelial cell from intestinal lumen (process is facilitated diffusion with Na+); Na+ actively transported into blood; Na+ diffuses back into intestinal lumen; glucose and galactose through GLUT2 into blood
How is fructose absorbed?
Fructose through GLUT5 into epithelial cell from intestinal lumen; fructose through GLUT2 into blood
Why do sports drinks often have a mixture of fructose and glucose?
As they use different transporters, rate of absorption within the small intestine will be increased
What are the enzymes involved in the digestion of proteins, and the reactions they catalyse?
In stomach, pepsin: proteins –> large peptides. In small intestine:- pancreatic enzymes (trypsin, chymotrypsin, proelastase): large peptides –> small peptides; brush border enzymes (aminopeptidase, dipeptidases): small peptides –> amino acids; pancreatic enzyme (carboxypeptidase): small peptides –> amino acids
How is pepsin activated?
Chief cell in gastric pit releases pepsinogen; parietal cell in gastric pit releases HCl; HCl activates pepsinogen –> pepsin
How are pancreatic proteases activated in the duodenum?
Membrane bound enterokinase activates trypsinogen –> trypsin; trypsin activates chymotrypsinogen –> chymotrypsin; chymotrypsin activates precarboxypeptidase –> carboxypeptidase
How are proteins absorbed?
Within the small intestine. Amino acids are absorbed directly into epithelial cell with help of Na+; dipeptides absorbed into epithelial cell with help of H+; dipeptides –> amino acids by cytoplasmic peptidases; amino acids diffuse into capillary
How are amino acids utilised in protein turnover?
Dietary protein digested; broken down into amino acids; aa’s to liver; processed and stored; aa’s taken to muscle to build muscle/ be used used in signalling processes; aa’s transported to other places within body that need protein for different cellular turnover
What are the enzymes involved in the digestion of lipids, and the reactions they catalyse?
Saliva and stomach hold lipase: hydrolyse TGs (slow process b/c TGs are not water soluble, which is where lipase is found. Duodenum:- pancreatic lipase: hydrolyses TGs –> liberates LCFAs. Small intestine:- bile salts from gall bladder: emulsify lipid droplet –> emulsion droplets; bile salts and pancreatic lipase: emulsion droplets –> monoglycerides + FFAs
What is a micelle?
A ball with FFAs and MGs (hydrophobic core) surrounded by bile salts (hydrophilic side outwards)
How are lipids absorbed?
In small intestine. Micelles –> MGs + FFAs at edge of epithelial cell; MGs and FFAs diffuse into epithelial cell; water-based environment causes MGs and FFAs to be repackaged at ER –> TGs; TGs within lipoproteins –> chylomicrons; chylomicrons diffuse into lympthatic system; MCFAs diffuse into blood stream, coated with albumin
Which is absorbed more quickly, carbs or fats?
Carbs (within 30-60 minutes). Fats involve more complex processes
Where are fats taken to if the body is exercising?
To muscles; if are not exercising, stored in adipose tissue
What occurs if there are too many fats to store (spillover)?
Fats taken to liver, where they are recycles to be used later if needed
How are fat soluble vitamins absorbed?
Absorbed along with FFAs; diffuse into epithelial cell of small intestine; aid formation of chlyomicrons; diffuse into lympthatic system
Where are vitamins mostly absorbed?
Jejunum and ileum, mostly by diffusion
How are water soluble vitamins absorbed when at low concentrations?
Active transport. Vitamin B1 and and C involve Na+ dependent active processes. Vitamin B12 requires formation of complex with intrinsic factor; complex binds with specific receptor; complex undergoes endocytosis and is engulfed into epithelial cell
Where are coenzymes, such as B vitamines, cleaved from their enzymes?
In small intestine
What is the absorption of minerals like?
They aren’t very well absorbed: 35% Ca, 20-30% Mg, 14-41% Zn
How is Na absorbed?
By active transport; much more Na is secreted back into intestinal lumen than kept within body, so need to have high reabsorption
2-4 hours before an event, what is the recommended ingestion of fluids and foods?
5-7ml/kg of BW; salt snack and small meals may help; increase fluids if urine is dark/ no urine
During an event, what is the recommended ingestion of fluids and foods?
Sufficient fluid to limit body mass loss to <2% (0.4-0.8l/hr); limit excessive electrolyte imbalance; if >1hr, consider CHO addition; if >2hrs/ salty sweater, add Na
<12 hours after an event, what is the recommended ingestion of fluids and foods?
1.5l/kg of BM loss; Na
How do water and carbs impact each other?
Increased concentration of carbs decreases % of max delivery of H2O
How does Na impact water and carbs?
Increased Na increases absorption of water and CHO
How does Na increase water absorption?
Na is actively absorbed, increasing water potential gradient and pulling water into epithelial cell by osmosis
