L10: absorption of ions and water Flashcards

1
Q

Significance of water

A
  • 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
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2
Q

What are the three body fluid compartments?

A

Interstitial fluid and plasma which make up the extracellular fluid compartment, and intracellular fluid compartment

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3
Q

Role of the gut in absorption of water

A
  • 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
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4
Q

Water movement

A
  • moves by osmosis

- tightly coupled water and solute/sodium movement

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5
Q

What is water secreted and absorbed by?

A

The SI secretes water from the crypts which is then absorbed along with ingested water by the villous enterocytes, predominantly tips of the villi.

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6
Q

How is water absorbed by osmosis?

A
  • 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
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7
Q

How does rate of water transport vary along the small intestine?

A
  • 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
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8
Q

Importance of sodium in ion and nutrient absorption

A

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

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9
Q

What part of the SI are most sodium co-transporters located?

A

Jejunum

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10
Q

Importance of sodium in water absorption

A

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.

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11
Q

Water absorption linked to food absorption

A

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.

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12
Q

What is diarrhoea?

A

Loss of faecal fluid >500ml/day

Major cause of death/ill health due to dehydration in children

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13
Q

How is diarrhoea caused?

A

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
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14
Q

How does motility affect diarrhoea?

A

Increased motility

  • increased propulsive activity
  • water and electrolytes sent to gut faster than they can be absorbed
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15
Q

Osmotic diarrhoea

A
  • 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)
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16
Q

What is dumping syndrome?

A
  • rapid gastric emptying
  • no gradual entry of chyme with large volumes creating a hyperosmotic chyme in the intestine
  • too much water secreted
  • multiple symptoms soon after a meal, especially carb rich meals
  • common in 20-50% of patients following bariatric surgery
17
Q

Oral rehydration therapy (ORT)

A
  • death from infection related diarrhoea and subsequent dehydration can be prevented
  • sugar and salt pull water into your bloodstream and speed up rehydration
  • water containing: glucose, sodium, chloride
  • cheap, safe, effective
18
Q

Absorption of calcium

A
  • essential for bones and teeth
  • regulated according to need - related to blood levels of calcium
  • more needed in: growing children and lactating women
  • regulated by vitamin D and parathyroid hormone
  • absorbed along entire SI length
19
Q

How is calcium absorbed?

A
  • calcium is actively transported (primarily) on the basolateral side of enterocytes
  • calcium then enters the enterocytes on the lumenal side by secondary active transport
  • binds to calbindin to stop it precipitating, and delivers it to the basolateral channels
  • channels heavily influenced by vitamin D3.