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Theme 2: The Liver and Regulation of Metabolism > Salt and water transport > Flashcards

Salt and water transport Flashcards

1
Q

Net fluid entering bowel

A

~8.5 L/day

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2
Q

Net fluid reabsorbed by bowel

A

~8.4 L/day

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3
Q

Net fluid loss via stool

A

100 mL/day

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4
Q

Electrolytes from ingested food

A

Na+

K+

Cl-

HCO3-

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5
Q

Reabsorption by jejunum

A

Na+

K+

Cl-

H2O

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6
Q

Secretion by ileum

A

HCO3-

H2O

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7
Q

Reabsorption by colon

A

Na+

Cl-

H2O

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8
Q

Secretion by colon

A

K+

HCO3-

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9
Q

Electrolyte absorption summary

A

Most ions absorbed along length of small intestine

Na+ coupled with absorption of glucose and amino acids

Anions passively follow electrical potential established by Na+

K+ diffuses across intestinal mucosa in response to osmotic gradients

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10
Q

Absorption of water

A

Transported through intestinal membrane entirely by diffusion

Diffusion follows laws os osmosis

Chyme hypoosmotic
Chyme hyperosmotic

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11
Q

Chyme hypoosmotic

A

Water is absorbed through intestinal mucosa into the blood of the villi

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12
Q

Chyme hyperosmotic

A

Water transferred by osmosis to make chyme isoosmotic with the plasma

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13
Q

Epithelial lining

A

Simple columnar epithelium

Heterogeneous population of cells

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14
Q

Cells making up intestinal mucosa

A

Enterocytes

Endocrine cells

Goblet cells

Paneths cells

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15
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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16
Q

Structural properties of enterocytes

A

Epithelial cells are polarised

  • apical sides: faces lumen, has microfolds
  • basal side: rests on basal membrane, communicates with bloodstream and lymphatic lacteals
  • lateral side: in contact with neighbouring cells
17
Q

Tight junctions

A

Intracellular tight junctions restrict passive flow of solutes after secretion or absorption

18
Q

Types of epithelial transport

A

Paracellular pathway

Transcellular pathway

19
Q

Transcellular transport

A

Employs membrane transporters to move molecules and water through cells

Their activity drives ion flux and establishes concentration gradients, which dictates passive transport of water and solutes

May work against electrochemical gradient (requires ATP/ energy)

20
Q

Paracellular transport

A

Movement of solutes and water through tight junctions (as opposed to trancellular transport)

Dictated primarily by electrochemical gradient

21
Q

Transepithelial transport determined by type of transport proteins

A

Channels

Carriers

Pumps

22
Q

Channels

A

Fluid filled pores built of protein aggregates (hydrophobic outside, hydrophillic inside)

Gate: have a part that opens and closes and regulates entrance of ions

Ion specific

Transport based on electrochemical gradient

23
Q

Carriers

A

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

Energy independent transport based on concentration gradient or energy dependent transport

Transport of single molecule, co-transporter or exchange

24
Q

Pumps

A

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

Use energy by hydrolysis of ATP, so they are ATPases

Exchange

25
Q

Types of transepithelial transport

A

Passive transport

Solvent drag

Active transport

26
Q

Solvent drag

A

Water leaks from the lumen through the 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

27
Q

Concept 1: Na, K ATPase

A

Critically important transport found on the basolateral aspect of enterocyte

Actively drives sodium out of cell

Na, K ATPase creates Na electrochemical gradient between enterocyte and lumen

28
Q

Concept 1: 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 to downhill movement of Na

Process is electrogenic (lumen becomes more negative and drives the parallel absorption on Cl)

29
Q

Oral rehydration solution

A

Promote fluid absorption by coupling sodium with glucose in solution

SGLT-1 binds two Na molecules to 1 glucose molecule and transports them into cell

30
Q

Concept 3: NaCl co-transport is mediated by two transport proteins

A

NaCL absorbed in conjunction with export of hydrogen and bicarboante

Relies on Na, K ATPase to establish the electrochemical gradient

Na/H exchanger works in conjunction with HCO3/CL exchanger allowing NaCL absoprtion

31
Q

Concept 4” chloride secretion occurs in conjunction with basolateral Na, K, Cl transport

A

Primarily at level of crypt, involves coupled import of Na, K and chlorine

As intracellular Cl increases, Cl is secreted via apical chloride channels (CFTR)

CFTR expression higher in the ileum and colon

Na, K ATPase drives Na gradient, further allowing Cl secretion through apical CFTR channel

32
Q

Concept 5: water follows NaCl

A

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

Water travels through interceullular tight junctions in the setting of NaCl absorption

33
Q

Water transport: absorption

A

Through tight junctions

Through ion channels along with active solute absorption by hydrostatic pressure, secondary to solute transport

Through apical Na+/ glucose co-transporter

34
Q

Secretion: removal of water

A

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

35
Q

Transport in the jejunum

A

The highest absorption of Na+, coupled with nutrient absorption

Solvent drag is an important mechanism

36
Q

Transport in the ileum

A

Similar to jejunum

The highest absorption of NaCl

37
Q

Transport in the colon

A

Apical side: Na+ channels

Apical side: K+ channels

Aldosterone increases synthesis of Na+ channels so increases K+ secretion so hypokalemia

38
Q

Nature of the absorbing epithelium

A

Intestinal mucosa highly folded to generate villi

Duodenum- broad, ridge like, tall in jejunum and shorter in ileum

Villi greatly increases surface area of absorption

SA further increased by brush border

Absorption by epithelial cells on sides and tips of villus

Theme 2: The Liver and Regulation of Metabolism (14 decks)

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