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RS 14 K+ Transport, General Considerations, Para-Aminohippuric Acid (PAH)…

- - - - i. The critical step is the active transport of K+
        from the intestitial fluid (blood) across the basolateral membrane into the DCT.
        This active transport step is associated with active Na+ efflux via the Na+ - K+ ATPase system.
      - ii. The elevated intracellular K+ so achieved favours the net K+ diffusion at the apical membrane down a concentration gradient into the tubular lumen.
        The transcellular transport of K+ is passive
        (DCT cells are freely permeable to K+).
- - - - The rate of K+ secretion is proportionate to the rate of flow of the tubular fluid through the distal portion of the nephron, because with rapid flow
        (as occurs with high sodium intake) there is less opportunity for the tubular K+ concentration to rise to a value that stops further secretion.
        
        Mineralocorticoids e.g. aldosterone, increases K+ secretion by:
        
        i. increasing the activity of basolateral Na+- K+ ATPase pump, which increases K+ entry into the DCT and the [K+] gradient from cell to lumen,
        
        ii. increasing the permeability of the luminal membrane to K+, further increasing K+ diffusion (secretion).
- - - - A counter current system is a system in which the inflow runs parallel to,
        counter to and in close proximity to the outflow for some distance.
        
        The operation of such a system in increasing the heating at the apex of a
        loop of pipe raises the temperature of water by 5°C, but the heated water flowing away from the heater warms the inflow.
        
        A gradient of temperature is thus set up a long the pipe, so that at the
        bend the temperature is raised not from 15 to 20°C but from 45 to 50°C.
- - - - The concentrating mechanism depends upon the existance of a
        gradient of increasing osmolality along the medullary pyramids. This
        gradient is:
        
        i. Produces by operation of the loop of Henle and collecting ducts as
        Counter Current Multipliers, and
        
        ii. Maintained by Vasa Recta as Counter Current Exchangers.
        (i) and (ii) together called as Counter Current Multiplier Exchange
        System or Counter Current System.
        
        Counter Current Multipliers
        
        In the PCT as about 70% of filtered solutes and water get reabsorbed, therefore, osmolar concentration of PCT fluid is equal to osmolar concentration of plasma i.e. 300 mosm/kg water.
        
        The Loop of Henle
        
        A. DLH - it is the ‘concentrating’ segment of the nephron. DLH is:
        
        highly permea0ble to water due to presence of Aquaporin-1, and
        
        relatively impermeable to solute.
        Therefore,
        i. solute-free water moves into the interstitium,
        ii. solute concentration in DLH increases; predominantly NaCl.
        
        B. ALH – The thick and thin segments of ALH constitute the ‘Diluting’ segment of the nephron.
        i. The thin segment is
        
        impermeable to water, and
        
        permeable to NaCl and urea.
        ii. The thick segment is
        
        impermeable to water and solute, but
        
        actively transports NaCl out of the lumen into interstitium by
        Na+ ─ 2Cl- ─ K+ symporter.
        
        DCT and collecting duct
        
        3 more items...
        
        Counter Current Exchangers
        
        The osmotic gradient in the medullary pyramids would not last long if
        the NaCl and urea in the interstitium were removed by circulation. The
        concentrations of Na+ and urea the medullary interstitium are kept high by the slow blood flow in the Vasa recta.
        These solutes remain in the pyramids to maintain hyperosmolality of the medullary interstitium
        primarily because the vasa recta operates as counter current exchanger.
        The arrangement of the descending and ascending limbs of the vasa
        recta in dose proximity to each other functions to maintain the
        hyperosmolality of the medullary interstitium.
- - - - Secretion of PAH occurs from the peritubular capillary blood into tubular
        fluid via carriers in the proximal tubule by a transport maximum Tm
        limited process.
- - - - Therefore, urea adds to the effects of NaCl in creating a
        hyperosmolal state in the med11/lanJ interstitium.
    - - At low urine flow rates, only 20-30% of the filtered urea is excreted
        and at high urine flow rates approx. 50-70% the filtered urea is
        excreted.
- - - - The medullary osmotic gradient decreases in water diuresis and increases when ADH is administered.
        
        The osmolality of the urine produced is usually equal to the osmolality
        of the tissue at the tip of the renal papilla.
        
        The production of concentrated urine depends on the relatively low
        renal medullary blood flow. Increase in medullary blood flow produces
        dilute urine.
        
        The amount of water reabsorbed in the collecting duct depends on:
        
        Osmolality of the medullary interstitium: This determines the magnitude of the force available for translocating water.
        
        Circulating level of ADH: The higher the plasma ADH level, the greater would be the concentration of urine formed and less its volume.