10. Transport of Na+ and Cl- Flashcards
How do kidneys maintain the body’s extracellular fluid (ECF)?
Kidneys help maintain body’s ECF volume by regulating the [Na] and [Cl] in the urine
Na is most important contributor to osmolality (electrolyte water balance) of the ECF
Where Na goes, H2O follows
Movement of solutes and water is driven by transepithelial transport across the different sections of the nephron
What are the mechanisms of epithelial transport?
Transport is governed by law of thermodynamics
Gibbs free energy
Equations on slide 2
Δμx=RTln[x]i/[x]o + zx FVm
Nernst equation
Vm= -(RT/zxF)ln[x]i/[x]o
Slides 2-4 Nov 21
How is epithelial transport done in the cells?
Polarized cells
Right junctions
Six pack model
Slide 5-6 Nov 21
Na has uphill movement in cell (not natural)
What direction is a transport moving into or out of the cell based on its sign?
Negative moves into cell
Positive moves out
Slide 7 Nov 21
How do membrane bound carriers aid epithelial transport?
Membrane bound carriers can couple the flows of 2 or more solutes with the final direction of flow determined by the cumulative thermodynamic gradient (adding up)
Slides 8-10 Nov 21
What is transcellular Na reabsorption?
Transcellular transport
Na and Cl sequentially traverse the apical and basolateral membranes before entering the blood (electrochemical gradients, ion channels, and transporters determine transport rates)
2 steps:
- Passive entry of Na into the cell across apical membrane (low intracellular Na and negative cell voltage allows this), proximal tubule, TAL, DCT have Na cotransporters and exchangers that move Na across
- Active extrusion of Na out of the cell across the basolateral membrane, mediated by Na-K pump (keeps [Na]i low and [K]i high)
Slide 14 Nov 21
Slide 22-23 Nov 21
What is paracellular Na reabsorption?
Paracellular transport
Na and Cl move entirely by an extracellular route through tight junctions between cells
Transepithelial electrochemical driving forces and permeability properties of the right junctions govern ion movements (transport rates)
Occurs in proximal tubule region
Slides 11-14 Nov 21
Slide 22-23 Nov 21
What types of transporters/channels are used in each area used in transcellular transport?
Proximal tubule, loop of Henle, a d distal tubule use a variety of Na coupled co transporters
Collecting ducts use epithelial Na channels (ENaC’s)
How does water move across cells?
Water moves from low solute concentration (high water potential) to high solute concentration (low water potential)
Osmotic gradient
Water cannot be actively pumped (can’t use ATP to move water)
Changes is osmolarity cause a transmembrane osmotic gradient, therefore water moves across the membrane affecting cell volume
Slide 15-16 Nov 21
What is osmolarity?
What is osmolality?
The measure of solute concentration (number of osmolarity per litre)
Osmolality is the number of osmoles per kg of solvent
Na is most important contributor to osmolality (electrolyte-water balance)
Where Na goes H2O follows
Slide 16 Nov 21
What is the osmolarity of the solution transported into the lateral intercellular space in epithelial transport?
What is the osmolarity of the solution transported into restricted basal spaces in epithelial transport?
The solution transported into the lateral intercellular space is slightly hyperosmotic, solution emerging into the interstitial space is nearly isosmotic
The solution transported into restricted basal spaces is slightly hyperosmotic, drawing water into these spaces. The solution emerging into the interstitial space is nearly isosmotic
Slide 17 Nov 21
What do even small variations in the fractional reabsorptive rate (kidneys reabsorb 99.6% of filtered Na) lead to?
Changes in total body Na+ that ultimately leads to altered ECF volume that leads to changes in blood pressure and body weight
What is the breakdown of reabsorption percentages through the nephron?
Reabsorption greatest at proximal tubule and along length on nephron (67%)
25% reabsorbed throughout loop of Henle
5% absorbed along the distal convoluted tubule, connecting tubule, initial collecting duct, and cortical collecting tubule
Nearly all remaining Na reabsorbed in the medullary collecting ducts leaving 0.4% of the filtered load excreted in urine
Slide 21 Nov 21
How is Na transported through loop of Henle?
Thin limbs Na transport is almost entirely passive & paracellular
Thick ascending limb has 2 major transcellular pathways:
1. Na:K:2Cl co transporter (NKCC2) moves 1 Na, 1K, 2Cl across apical membrane
2. Na/H exchanger is anti
porter that exploits gradient caused by Na/K pump across basolateral membrane
Paracellular diffusion accounts for 50% of Na reabsorbed
Slide 24 Nov 21
How is Na reabsorbed in the distal convoluted tubule?
Na reabsorption is almost entirely transcellular
Apically is uses the Na/Cl cotransporter
Basolaterally it uses the Na-K pump
No paracellular transport
Slide 25 Nov 21
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