tubular transport Flashcards
- state the magnitude and regulated range of NaCl and water handling by the kidneys.
Water: 190L are filtered, 0.5-25L are excreted, and 165-189.5L (up to 99.95%) are reabsorbed. NaCl: 1500g are filtered, 0.05-30g are excreted, and 1470-1500g (up to 99.997%) are reabsorbed.
Describe the basic mechanism of sodium transport in kidney
Na-K pump on basolateral side actively pumps Na out of the cell and into the extracellular fluid. This creates a driving force for Na entry into the cell from the lumen via sodium channels in the apical membrane.
Describe basic mechanism of chloride transport in kideny
Chloride is reabsorbed through the tight junctions (paracellular) from the lumen to the extracellular fluid on the serosal side
Describe basic mechanism of water transport in kideny
Accumulation of Na and Cl on serosal side of membrane creates and osmotic gradient. Water moves either paracellularly or transcellularly or both
Describe basic mechanism of glucose and amino acid transport in kideny
Secondary active transport in which Na movement down its concentration gradient into the cell also brings glucose (or aa) into the cell with it. This allows glucose or aa to be concentrated in the cell, so that it can then passively move out of the cell on the basolateral side
Proximal convoluted tubule function
Reabsorbs majority of water and NaCl, and all of the filtered glucose and amino acids, and most of the filtered bicarbonate
Tonicity of proximal convoluted tubule
Fuid remains isotonic since equivalent proportions of NaCl and water are reabsorbed.
Define transport maximum
Since there are a finite number of transporters in the tubule, there is a maximal absorption rate for substances such as glucose.
Function of loop of Henle
Creates a hypertonic interstitium and a hypotonic tubular fluid: NaCl reabsorption in the ascending loop creates a hypertonic interstitium and an osmotic gradient for water reabsorption from the descending loop, but less water is reabsorbed than NaCl, so the fluid becomes hypotonic.
Permeability of ascending and descending loop of Henle
The descending loop has high water permeability, but no NaCl permeability. The ascending loop has high NaCl permeability but no water permeability.
Describe Na transport in thick ascending limb of loop of Henle
Na/K/2Cl transporter couples the energy of sodium gradient to drive uphill reabsorption of potassium and chloride.
Function of distal tubule and collecting duct
fine tuning: Only a small amount of obligatory NaCl and water reabsorption occurs, most is fine tuning. Tight junctions prevent paracellular transport. Na channels (upregulated by aldosterone) allow Na to move from the lumen into the cell, then Na is pumped out the basolateral side. Water channels (upregulated by ADH) allow for water reabsorption
compare reponse rates to aldosterone and ADH
Aldosterone is slower (takes an hour or more) due to the time it takes for aldosterone to bind to receptor, then turn on genes that increase synthesis of transporter proteins. A fast version of this does exist. ADH is very rapid b/c vesicles containing pre-made aquaporins are fused with the apical membrane and there are already pre-existing aquaporins in the basolateral membrane
- state the relative proportion of water and NaCl reabsorbed in each tubular segment.
Proximal tubule: 65% NaCl, 65% H20. Descending loop of Henle: 0% NaCl, 15% H20. Ascending loop of Henle: 25% NaCl, 0% H20. Distal tubule and collecting duct: 8-10% NaCl, 6-20% H20.
- state the Starling equation for the flow of solution from the renal interstitium to the peritubular capillaries.
Fic = K’ (Pint + πcap– Pcap - πint)
- give values for each of the Starling forces and the net pressure driving the flow in (7).
Pint = 7 mm, πcap = 35 mm, Pcap = 11 mm, πint = 6 mm, Net flow presure= 25mm. The capillary oncotic pressure is is large because the plasma had a significant amount of its water extracted from it upstream when it was subjected to glomerular filtration
- describe qualitatively the effects of increasing and decreasing tubular flow on water and sodium excretion.
Increased tubular flow decreases the amount of time substances have in contact with transporters for reabsorption, so a greater proportion of tubular substances escape reabsorption. Reduced tubular flow rates increases water and sodium reabsorption
Describe how diuretics affect tubular flow
Diuretics increases urine output by decreasing water reabsorption. This results in a larger volume of water in the tubule, thus it increases tubular flow. As a consequence, all solutes flow past transporters faster, so there is an increase in urinary excretion of sodium, potassium, and chloride indirectly
define “glomerulotubular balance” and describe the roles these processes play in the regulation of NaCl and water reabsorption
Glomerulotubular balance refers to the ability of the obligatory reabsorption mechanisms in the proximal tubule to compensate for changes in filtered load. If GFR increases, proximal tubule reabsorption increases so that 65% of filtered load of water and NaCl is always reabsorbed. However, there will still be a surplus or deficit in the tubular filtrate after the proximal tubule.
define “tubuloglomerular feedback” and describe the roles these processes play in the regulation of NaCl and water reabsorption
This mechanism actually directly regulates the GFR of each nephron in response to changes in NaCl concentration at a specialized group of epithelial cells called the macula densa. These cells are in direct contact with afferent arteriole and can cause arteriole constriction or dilation. Increased tubular flow will cause an increase in NaCl (due to less reabsorption time), so macula densa cells detect the increased NaCl and signal afferent arteriole to contract
