Salt and water transport Flashcards
Net fluid entering bowel
~8.5 L/day
Net fluid reabsorbed by bowel
~8.4 L/day
Net fluid loss via stool
100 mL/day
Electrolytes from ingested food
Na+
K+
Cl-
HCO3-
Reabsorption by jejunum
Na+
K+
Cl-
H2O
Secretion by ileum
HCO3-
H2O
Reabsorption by colon
Na+
Cl-
H2O
Secretion by colon
K+
HCO3-
Electrolyte absorption summary
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
Absorption of water
Transported through intestinal membrane entirely by diffusion
Diffusion follows laws os osmosis
Chyme hypoosmotic
Chyme hyperosmotic
Chyme hypoosmotic
Water is absorbed through intestinal mucosa into the blood of the villi
Chyme hyperosmotic
Water transferred by osmosis to make chyme isoosmotic with the plasma
Epithelial lining
Simple columnar epithelium
Heterogeneous population of cells
Cells making up intestinal mucosa
Enterocytes
Endocrine cells
Goblet cells
Paneths cells
Functions of intestinal epithelium
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
Structural properties of enterocytes
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
Tight junctions
Intracellular tight junctions restrict passive flow of solutes after secretion or absorption
Types of epithelial transport
Paracellular pathway
Transcellular pathway
Transcellular transport
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)
Paracellular transport
Movement of solutes and water through tight junctions (as opposed to trancellular transport)
Dictated primarily by electrochemical gradient
Transepithelial transport determined by type of transport proteins
Channels
Carriers
Pumps
Channels
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
Carriers
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
Pumps
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
Types of transepithelial transport
Passive transport
Solvent drag
Active transport
Solvent drag
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
Concept 1: Na, K ATPase
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
Concept 1: Na coupled transport
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)
Oral rehydration solution
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
Concept 3: NaCl co-transport is mediated by two transport proteins
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
Concept 4” chloride secretion occurs in conjunction with basolateral Na, K, Cl transport
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
Concept 5: water follows NaCl
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
Water transport: absorption
Through tight junctions
Through ion channels along with active solute absorption by hydrostatic pressure, secondary to solute transport
Through apical Na+/ glucose co-transporter
Secretion: removal of water
In the small intestine Cl- secretion drags Na+ and water across the tight junctions
Transport in the jejunum
The highest absorption of Na+, coupled with nutrient absorption
Solvent drag is an important mechanism
Transport in the ileum
Similar to jejunum
The highest absorption of NaCl
Transport in the colon
Apical side: Na+ channels
Apical side: K+ channels
Aldosterone increases synthesis of Na+ channels so increases K+ secretion so hypokalemia
Nature of the absorbing epithelium
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
