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Theme 1: The Alimentary System > Salt and water transport and its control > Flashcards

Salt and water transport and its control Flashcards

1
Q

Overview of fluid movement in the GI tract

A

Net fluid entering: 8.5L/day

Net fluid absorbed by bowel: 8.4L/day

Net fluid loss via stool: 100ml/day

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2
Q

Large intestine absorption

A

Reabsorbs 2L/day

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3
Q

Jejunum reabsorbs

A

Na+

K+

Cl-

H2O

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4
Q

Ileum secretes

A

HCO3-

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5
Q

Ileium absorbs

A

H2O

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6
Q

Colon reabsorbs

A

Na+

Cl-

H2O

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7
Q

Colon secretes

A

K+

HCO3-

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8
Q

Absorption of water

A

Water transported through intestinal membrane by diffusion

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9
Q

Chyme hypoosmotic

A

Water is absorbed through intestinal mucosa into blood of the villi

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10
Q

Chyme hyperosmotic

A

Water transferred by osmosis to make chyme isoosmotic with the plasma

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11
Q

Epithelial lining

A

Simple columnar epithelium

Heterogenous population of cells

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12
Q

Four major types of epithelial cells making up intestinal mucosa

A

Enterocytes

Endocrine cells

Goblet cells

Paneths cell

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13
Q

Functions of intestinal epithelium

A

Barrier: enterocytes

Secretion of digestive enzymes: enterocytes

Nutrient absorption: enterocytes

Water and electrolyte exchange: enterocytes

Mucus secretion: goblet cells

Sensory and endocrine function: enteroendocrine cells

Innate immune function: paneth cells

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14
Q

Structural properties of enterocytes

A

Epithelial cells polarised

  • apical sides face lumen and has microfolds
  • basal side rests on basal membrane and communicates with blood stream and lymphatic lacteals
  • lateral side is in contact with neighbouring cells
  • apical and basolateral membranes separated by tight junctions
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15
Q

Tight junctions

A

Restrict passive flow of solutes after secretion and absorption

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16
Q

Types of epithelial transport

A

Paracellular pathway

Transcellular pathway

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17
Q

Transcellular transport

A

Move molecules and water through cells

Drives ion flux and established concentration gradients which dictates passive transport of water and solutes

May work against electrochemical gradient

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18
Q

Paracellular transport

A

Movement of solutes and water through tight junctions

Dictated primarily by electrochemical gradient

19
Q

Channels

A

Fluid filled pores built of protein aggregates

Gated: have a part that opens and closes and regulates ion entrance

Ion specific

Transport based on electrochemical gradient

20
Q

Carriers

A

Proteins that facilitate the movement of specific solutes across the membrane through conformational changes

Energy independent transport based on concentration gradient of energy dependent trasnport

Single molecule, co-transport or exchange

21
Q

Pumps

A

Transport proteins that move ions and other solutes across the membrane against the electrochemical gradient

Use energy by hydrolysis of ATP (ATPases)

Exchange

22
Q

Passive transport

A

Movement of solutes down its electrochemical gradient

Movement through openings of ion channels, carriers that facilitate diffusion or permeability of tight junctions

23
Q

Solvent drag

A

Water leaks from the lumen through paracellular space to reach osmotic equilibrium on the basolateral side

Water flow pulls additional solutes from the luminal to the basolateral space

Takes place in upper small intestine where tight junctions are the leakiest

24
Q

Active transport

A

Energy dependent

Transepithelial transport of NA+ using combination of Na+/K+ ATPase on the basolateral membrane

Depletes Na+ inside the cell

Na+ channel or nutrient/ Na+ cotransporter on apical side brings more Na+ in based on the concentration gradient created by the ATPase

25
Q

Concept 1: Na+/K+/ATPase

A

Critically important transport found on the basolateral aspect of the enterocyte

Drives sodium out of the cell

Creases a Na+ electrochemical gradient between enterocyte and lumen

26
Q

Concept 2: Na+ coupled transport

A

Na+ gradient created by Na+/K+ ATPase allows Na+ coupled transport from lumen into cell

Secondary active transport couples uphill movement of glucose/ aa to downhill movement of Na

The process electrogenic

27
Q

Oral rehydration solution

A

Utilising mechanism of glucose coupled sodium absorption

Carrier specific Na+/glucose transporter (SGLT 1) preserved in most diarrheal diseases and forms basis for oral rehydration therapy

SGLT 1 binds two Na molecules to one glucose molecule transporting them into the cell

28
Q

Concept 3: NaCl cotransport mediated by two transport proteins

A

NaCl absorbed in conjugation with export of H and HCO3-

Relies on Na+/K+ ATPase to establish electrochemical gradient

Na+/H+ exchanger works in conjunction with HCO3-/Cl- exchanger

Allows NaCl absorption

29
Q

Concept 4: Cl- secretion occurs in conjunction with basolateral Na+/K+/2Cl- transport

A

Involves coupled import of Na+/K+/2Cl-

Intracellular Cl- increases so is secreted via apical Cl- channels (CFTR)

Na+/K+ ATPase drives Na+ gradient allowing further Cl- secretion through apical CFTR

30
Q

Concept 5: water follows NaCl

A

Transport of ions, mainly NaCl leads to direction of fluid flow across tight junctions

Water will travel through intercellular tight junctions in the setting NaCl absorption

31
Q

Secretion of water

A

In the small intestine Cl- secretion drags Na+ and water across the tight junction

(not in colon)

32
Q

Jejunum transport

A

The highest absorption of Na+; coupled with nutrient absorption

Solvent drag is an important mechanism

33
Q

Ileum transport

A

Similar to jejunum

The highest absorption of NaCl

34
Q

Colon transport

A

Apical side: Na+ channels

Apical side: K+ channels

Aldosterone increases synthesis of Na+ channels .. increased K+ secretion.. hypokalaemia

35
Q

Nature of absorbing epithelium

A

Intestinal mucosa highly folded to generate villi

  • duodenum: broad and ridge-like
  • jejunum: tall
  • ileum: shorter

Brush border increases surface area

36
Q

Cholera toxin on electrolyte and H2O reabsorption

A

CFTR Cl- channel opening regulated by cAMP

Cholera toxin stimulate excess production of cAMP in crypt cells

  • massive influx of Cl-
  • Na+ follow to form osmotically active NaCl
  • followed by h2O secretion- more than can be reabsorbed

SECRETORY DIARRHOEA

37
Q

Large intestine- water and electrolyte absorption

A

Driven by Na+/K+ ATPase

Na+ entry by

  • Na+ channels
  • Na+/H+ antiport

Diffusion under aldosterone control- increases Na+ channels

Cl-/HCO3- provide buffer for acid

Tight junctions ensure no ion backflow

Na+ Cl- create osmotic gradient for transcellular water movement

38
Q

Na+ transport

A

Absorption

  • through Na+/H+ exchangers
  • nutrient coupled
  • through electrochemical Na+ channels in distal colon

Secretion
- in small intestine (Cl- secretion drags Na+ and water across tight junction)

39
Q

K+ transport

A

Absorption

  • through K+/H+ exchangers
  • through K+ channels

Secretion
- in colon through K+ channels, increased in pathophysiological conditions

40
Q

Gastrointestinal infections

A

Diarrhoea caused by:

  • bacterial enterotoxin
  • inflammation
41
Q

Bacterial enterotoxins

A

Cause secretory diarrhoea by interacting with receptors and signal transduction pathways in enterocytes
- secretion exceeds absorption

42
Q

Bacterial enterotoxins

A

Cholera toxin

Heat liable E.coli toxin

Salmonella toxin

Campilobacter toxin

Heat stable E.coli toxin

Yersina toxin

43
Q

Cystic fibrosis

A

Autosomal recessive disease

Deletions in CFTR gene

Sticky mucus and high viscosity of lumenal contents

Often presents as intestinal obstruction and meconium ileus in newborns

44
Q

Lactose intolerance

A

Lactase deficiency

Lactose not digested so remains in the lumen

Osmotic diarrhoea

Theme 1: The Alimentary System (16 decks)

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