7 – Regulation of Tubular Reabsorption

Question 1

What are the physical forces involved in reabsorption?

Answer 1

-hydrostatic and colloid osmotic forces

Question 2

what is the equation for reabsorption?

Answer 2

Reabsorption=Kf x net reabsorptive force

Question 3

What are the hydrostatic and colloid osmotic forces that make up the net reabsorptive force?

Answer 3

-hydrostatic pressure within peritubular capillaries (Pc)
-hydrostatic pressure in renal interstitium (Pif)
-colloid osmotic pressure of peritubular capillaries (pi c)
-colloid osmotic pressure of renal interstitium (pi if)

Question 4

What are the forces that oppose reabsorption?

Answer 4

-hydrostatic pressure within peritubular capillaries (Pc)
-colloid osmotic pressure of renal interstitium (pi if)

Question 5

What are the forces that promote reabsorption?

Answer 5

-hydrostatic pressure in renal interstitium (Pif)
-colloid osmotic pressure of peritubular capillaries (pi c)

Question 6

Peritubular hydrostatic force (Pc) is associated with

Answer 6

-arterial pressure
-resistance of afferent and efferent arterioles

Question 7

What does resistance in afferent and efferent arterioles do to peritubular hydrostatic pressure?

Answer 7

-reduced

Question 8

Peritubular colloid osmotic pressure is affected by:

Answer 8

-systemic plasma colloid osmotic pressure
-filtration fraction

Question 9

What does increased filtration fraction do to peritubular colloid osmotic pressure?

Answer 9

-more plasma filtered=proteins remaining in plasma leaving the glomerulus are more concentrated
*increased

Question 10

Solutes and water enter interstitum by active and passive mechanisms:

Answer 10

-enter the capillaries or leak back into lumen of tubule through leaky tight junctions

Question 11

What effects the hydrostatic and colloid pressure in the renal interstitum?

Answer 11

*peritubular capillary forces
1. Increased peritubular hydrostatic pressure OR reduced peritubular osmotic pressure=
2. Reduced reabsorption of water and solutes into capillaries=
3. Increase hydrostatic pressure in interstitum=
4. Increase reabsorption but leakage of water and solutes into tubules also increases

Question 12

Under normal conditions the regulation of interstitial fluid physical forces leads to:

Answer 12

-net reabsorptive force is positive
>from interstitial fluid toward peritubular capillaries

Question 13

How are interstitial fluid physical forces regulated?

Answer 13

1. Peritubular capillaries: increase hydrostatic pressure or decrease colloid osmotic pressure
2. Net positive reabsorption force toward capillaries decreases
3. Hydrostatic pressure in interstitial fluid goes sup
4. Leakage of water and solutes into tubule increases

Question 14

Glomerulotubular balance mechanism is important for:

Answer 14

-maintaining proper balance between filtration and reabsorption
>NS, hormonal and local factors
*reabsorption of some substances can be regulated independently

Question 15

Glomerulotubular balance mechanism:

Answer 15

-ability to adjust tubular reabsorption in response to increased tubular flow
-if GFR increases=increase in reabsorption rate
>especially in Loop of Henle
*reabsorption in proximal tubules remains about 65% of filtrate
*can happen independently of hormones

Question 16

what is the mechanism of Glomerulotubular balance?

Answer 16

-not fully known
>brush borders sense flow rate and can trigger cellular responses to increase transporters at luminal membrane

Question 17

Glomerulotubular balance prevents:

Answer 17

-overloading of distal tubule when GFR increases

Question 18

The need for accurate regulation of tubular reabsorption is important for:

Answer 18

-reabsorption of different solutes at different rate
-adjustment of reabsorption in response to intake changes

Question 19

Aldosterone is released from:

Answer 19

-adrenal cortex

Question 20

What stimulates the release of aldosterone?

Answer 20

1. Increased extracellular K+ (hyperkalemia)
2. Increased Ang II in response to reduced Na and volume depletion

Question 21

What are the major sites of action for aldosterone?

Answer 21

-principal cells (last distal tubule and collecting tubules)
-cortical collecting tubules

Question 22

What are the effects of aldosterone?

Answer 22

-increases sodium reabsorption
-increases potassium secretion

Question 23

What is the mechanism of aldosterone?

Answer 23

-increase activity of Na/K pump at basolateral membrane
-enhances Na permeability of luminal membrane

Question 24

What happens if the function of the adrenal gland is reduced?

Answer 24

-loss of Na
-accumulation of K

Question 25

What happens if the function of the adrenal gland is increased?

Answer 25

-loss of K
-Na retention

Question 26

What are the effects of Ang II?

Answer 26

-increases Na and water reabsorption

Question 27

What stimulates the release of Ang II?

Answer 27

-low BP
-low extracellular fluid (hemorrhage, diarrhea, excessive sweating)

Question 28

Ang II constricts efferent arterioles, which leads to:

Answer 28

-reduced hydrostatic pressure in hydrostatic capillaries
*increases filtration fraction
>protein concentration increases in peritubular capillaries=increased colloid osmotic pressure increases in peritubular capillaries

Question 29

What are the 3 mechanisms of Ang II?

Answer 29

1. Stimulates aldosterone secretion
2. Constricts efferent arterioles
3. Direct stimulation of Na reabsorption in all segments of tubules

Question 30

Ang II directly stimulates Na reabsorption in all segments of tubules by:

Answer 30

-stimulating Na/K pumps
-increases Na-H counter-transport at luminal membrane
-increases Na-HCO3- co-transport at basal membrane

Question 31

What are the effects of ADH?

Answer 31

-increase water reabsorption
*important under dehydration conditions
-increases water permeability of distal tubule, collecting tubule and collecting duct

Question 32

What is the mechanism of ADH (aka arginine vasopressin (AVP)?

Answer 32

1. Binds to V2 receptor and activates cAMP formation
2. PKA activation in cytoplasm
3. Movement of aquaporin-2 protein (AQP) to the cell apical membrane
4. AQP-2 proteins cluster together and fuse with cell membrane by exocytosis and form water channel at the membrane

Question 33

Atrial natriuretic peptide is released from:

Answer 33

-specific cells of cardiac atria

Question 34

Atrial natriuretic peptide is released in response to:

Answer 34

-stretching when blood/plasma volume is INCREASED

Question 35

What is the mechanism of atrial natriuretic peptide?

Answer 35

1. Reduces Na and water reabsorption mainly in collecting ducts
2. Inhibits renin secretion and Ang II formation

Question 36

Atrial natriuretic peptide inhibits renin secretion and Ang II formation which results in:

Answer 36

-less Na and water reabsorption
-more urine output

Question 37

Parathyroid hormone (PTH):

Answer 37

-increases Ca reabsorption

Question 38

Sympathetic nervous system and regulation of tubular reabsorption:

Answer 38

-activates alpha-adrenergic receptors on renal epithelial cells
>causes increased Na and water reabsorption

Question 39

What is the mechanism of how the SNS regulates tubular reabsorption?

Answer 39

1. Lower levels: increase Na and water reabsorption in proximal tubules and thick ascending segment of loop of Henle
2. Under severe conditions: reduces renal flow and GFR
3. Stimulates renin secretion and Ang II formation