tubular transport

Question 1

1. state the magnitude and regulated range of NaCl and water handling by the kidneys.

Answer 1

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.

Question 2

Describe the basic mechanism of sodium transport in kidney

Answer 2

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.

Question 3

Describe basic mechanism of chloride transport in kideny

Answer 3

Chloride is reabsorbed through the tight junctions (paracellular) from the lumen to the extracellular fluid on the serosal side

Question 4

Describe basic mechanism of water transport in kideny

Answer 4

Accumulation of Na and Cl on serosal side of membrane creates and osmotic gradient. Water moves either paracellularly or transcellularly or both

Question 5

Describe basic mechanism of glucose and amino acid transport in kideny

Answer 5

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

Question 6

Proximal convoluted tubule function

Answer 6

Reabsorbs majority of water and NaCl, and all of the filtered glucose and amino acids, and most of the filtered bicarbonate

Question 7

Tonicity of proximal convoluted tubule

Answer 7

Fuid remains isotonic since equivalent proportions of NaCl and water are reabsorbed.

Question 8

Define transport maximum

Answer 8

Since there are a finite number of transporters in the tubule, there is a maximal absorption rate for substances such as glucose.

Question 9

Function of loop of Henle

Answer 9

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.

Question 10

Permeability of ascending and descending loop of Henle

Answer 10

The descending loop has high water permeability, but no NaCl permeability. The ascending loop has high NaCl permeability but no water permeability.

Question 11

Describe Na transport in thick ascending limb of loop of Henle

Answer 11

Na/K/2Cl transporter couples the energy of sodium gradient to drive uphill reabsorption of potassium and chloride.

Question 12

Function of distal tubule and collecting duct

Answer 12

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

Question 13

compare reponse rates to aldosterone and ADH

Answer 13

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

Question 14

3. state the relative proportion of water and NaCl reabsorbed in each tubular segment.

Answer 14

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.

Question 15

7. state the Starling equation for the flow of solution from the renal interstitium to the peritubular capillaries.

Answer 15

Fic = K’ (Pint + πcap– Pcap - πint)

Question 16

8. give values for each of the Starling forces and the net pressure driving the flow in (7).

Answer 16

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

Question 17

9. describe qualitatively the effects of increasing and decreasing tubular flow on water and sodium excretion.

Answer 17

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

Question 18

Describe how diuretics affect tubular flow

Answer 18

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

Question 19

define “glomerulotubular balance” and describe the roles these processes play in the regulation of NaCl and water reabsorption

Answer 19

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.

Question 20

define “tubuloglomerular feedback” and describe the roles these processes play in the regulation of NaCl and water reabsorption

Answer 20

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