GI system: L30 - Absorption Flashcards
What are the factors influencing absorption? (4 of them)
- Motility. Food must be be moved at a rate that maximises digestion and absorption. Also ensures that material is being exposed to digestive enzymes as well as absorptive surfaces.
- Surface area. Rate of absorption is proportional to SA. Anatomical adaptations are villi+microvilli+plicae circulares.
- Ability of molecules to pass through available SA. Intestinal epithelium acts as a barrier to absorption to prevent infection of pathogens. Thus there are 2 paths either paracellular (between cells) and cellular (across cell membranes, through cytoplasm).
- Removal of substances from interstitial space.
Large blood flow to allow swift removal of substances to allow a concentration gradient to be maintained especially for the paracellular pathway.
Explain what is meant by the paracellular pathway.
A passive pathway that requires a concentration gradient. Nutrients will pass between cells but must be small enough to cross the non-selective tight junction.
Discuss what is meant by cellular pathway.
Solutes absorbed must pass through 2 membranes (apical and basal cell membranes). Transporters are required for anything non-lipid-soluble as the membranes act as barriers. Transport proteins allow creating a selective barrier (as they can transport specific molecules across) and also use active transport to move solutes across membrane against concentration gradient.
Considering both paracellular and cellular pathways, how do we maximise absorption across available surface area?
Reducing the size of food molecules by chemical digestion for paracellular pathway. Using specific transport proteins to allow absorption through cellular pathway.
What are the sites of absorption?
Little absorption takes place in the mouth, oesophagus and stomach as insufficient digestion has taken place in these sites. The small intestine is the major site of absorption, responsible for 90% of water and sodium absorption and absorption of all the other nutrients. The large intestine absorbs 9% of water and sodium and vitamins generated by bacteria inside the colon.
How much water goes into our small intestine per day? Explain the mechanism of absorbing water.
We drink about 1.5L/day but secretions cause the small intestine to receive approximately 9-10L of water a day, 9L of which must be reabsorbed. The mechanism of water absorption is osmosis, which involves passive movement of water (due to osmotic gradients formed by absorbed salts+nutrients) from the lumen into the blood via aquaporins (water channels).
Explain the mechanisms of absorbing sodium. How does the gradient maintain?
Because sodium is a small solute and there is often large concentrations gradients between lumen and interstitial fluid, can be absorbed passively through paracellular pathway. Sodium can also be absorbed via the cellular pathway. There are 3 mechanisms in which it can do this:
1. Sodium transport alone: facilitated diffusion via sodium channels.
2. Sodium transport coupled to glucose: facilitated transport via a symporter (SGLT1).
3. Sodium transport coupled to amino acids: facilitated diffusion via a symporter.
With all this sodium movement into the cell, the concentration gradient must be maintained via Na+/K+ ATPase pump, which pumps sodium out of the epithelial cell into interstitial fluid while bringing K+ into the cell using energy from hydrolysed ATP. This keeps Na+ concentration low in the epithelial cell which drives constant diffusion. Note that in all 3 circumstances above that sodium moves into cell via facilitated transport which is passive.
Explain the mechanism of absorbing products of carbohydrate digestion.
Monosaccharides can possibly be absorbed by paracellular pathway but the more important mechanism is via coupling with Na+. Glucose (or galactose) is moved across membrane with Na+ using transport proteins on the membrane, using the sodium gradient to move monosaccharides against their concentration gradient. This method is called secondary active transport (as there is no direct coupling of ATP). Once in the cell, glucose and galactose concentrations are higher than interstitial fluid, so can therefore passively drain into interstitial fluid via independent basal transport protein (GLUT2).
How closely related are the products of carbohydrate and amino acid digestion, absorption pathways?
Basically identical use of Na+ secondary active transport in conjunction with Na+/K+ ATPase pump, with the exception that the basal membrane transport protein is specific for amino acids.
Explain the mechanism of absorbing products of fat digestion.
Micelles holding monoglycerides and fatty acids come into contact with the surface of the epithelium due to motility, deliver their products through simple diffusion (micelle does not diffuse into the same cell as products). Within the cell, the products are resynthesised into tryglycerides. The triglycerides are then packaged into chylomicrons and exit the cell via exocytosis before entering the lacteals (lymph capillaries). Once micelles have delivered their products the remaining bile salts are reabsorbed via diffusion in the jejunum and via sodium dependent active transport process in the ileum (same as protein and carb absorption).
How are vitamins absorbed?
Fat soluble vitamins (A,D,E,K) are absorbed with fats. Water soluble vitamins (C) are absorbed through co-transport with Na+.
Vitamin B12 is absorbed differently: Intrinsic factor from the stomach binds to Vit B12 forming a complex that is absorbed by an active process at the ileum.
Explain elimination.
We do not absorb everything, residues of digestion are expelled via faeces.
