L10: absorption of ions and water Flashcards
Significance of water
- water is a major component of plasma, cells and the interstitial fluid
- if electrolyte levels or fluid volumes change, the concentration or osmolality of body compartments can change, water can shift between the three body fluid compartments and lead to various clinical symptoms like oedema
What are the three body fluid compartments?
Interstitial fluid and plasma which make up the extracellular fluid compartment, and intracellular fluid compartment
Role of the gut in absorption of water
- gut must absorb huge volumes of water
- it secretes 6-7 litres (saliva, pancreas, SI etc)
- almost all is reabsorbed - only 0.1-0.2L in faeces
Water movement
- moves by osmosis
- tightly coupled water and solute/sodium movement
What is water secreted and absorbed by?
The SI secretes water from the crypts which is then absorbed along with ingested water by the villous enterocytes, predominantly tips of the villi.
How is water absorbed by osmosis?
- paracellular (between cells)
- transcellular (water channels through cells)
- water moves in both directions
- follows the movement of ions/nutrients
- 95% absorbed by SI and 5% by colon
How does rate of water transport vary along the small intestine?
- greater absorption in jejunum than ileum (to keep gut moist at start)
- proximal villi are larger - so they can absorb food better
- proximal villi has leakier tight junctions - to encourage water to be absorbed
- differences in transporter, channels etc
- e.g. bile salts and vitamin B12 absorbed in terminal ileum
Importance of sodium in ion and nutrient absorption
Establishing a sodium electrochemical gradient across the enterocyte is the most important process for the subsequent absorption of water, ions and nutrients like glucose and amino acids.
Energy invested to absorb sodium, then everything else follows passively - tight coupling between water and solute absorption
What part of the SI are most sodium co-transporters located?
Jejunum
Importance of sodium in water absorption
Osmotic gradient created with more sodium accumulating on the basolateral side. Water follows by osmosis thus leaving less water in the intestinal lumen.
Increased potassium and chloride luminal osmolarity. Potassium and chloride diffuse passively down their electrochemical gradient via a paracellular route.
Water absorption linked to food absorption
When food arrived from the stomach, the undigested chyme molecules have a low osmolality. As they get digested they become smaller and the chyme becomes hyper-osmotic, so water is drawn into the intestinal lumen from the gut wall.
Nutrients get absorbed so the chyme becomes hypo-osmotic, so water follows the nutrients back into the bloodstream.
What is diarrhoea?
Loss of faecal fluid >500ml/day
Major cause of death/ill health due to dehydration in children
How is diarrhoea caused?
Inhibition of sodium transport:
- prevents osmotic gradient being set up
- inflammation, colon irritation by non-absorbed bile
Massive secretion of fluid:
- volume secreted too great for colon to reabsorb
- bacterial food poisoning - Vibrio cholerae, E.coli
How does motility affect diarrhoea?
Increased motility
- increased propulsive activity
- water and electrolytes sent to gut faster than they can be absorbed
Osmotic diarrhoea
- increased concentration of slowly absorbable substances in chyme such as magnesium, sulfates, phosphates
- osmotic gradient created that draws water into the lumen (same as hyperosmotic nutrients after they’ve been digested)
