Reabsorption Forces

Question 1

physical forces involved in reabsorption

Answer 1

-hydrostatic force
-colloid osmotic force

Question 2

Peritubular reabsorption rate

Answer 2

124ml/min

Question 3

Reabsorption eqn

Answer 3

Reabsorption=Kf x (net reabsorptive force)

Question 4

Net reabsorptive force

Answer 4

1.hydrostatic pressure within peritubular capillaries (Pc) – opposes reabsorption
2.Hydrostatic pressure in the renal interstitium (Pif)- promotes reasorption
3.Colloid osmotic pressure of peritubular capillaries (pi c)- promotes reabsorption
4. Colloid osmotic pressure of renal interstitium (pi if)- opposes reabsorption

Question 5

Reabsorption rate measurement

Answer 5

-can be measured, whereas Kf cannot
>use 124ml/min reabsorption rate and the net reabsorptive rate to determine Kf

Question 6

Peritubular hydrostatic force

Answer 6

-associated with arterial pressure and the resistance of afferent and efferent arterioles
>an increase in resistance of afferent and efferent arterioles will decrease peritubular hydrostatic pressure

Question 7

Peritubular colloid osmotic pressure

Answer 7

-affected by the systemic plasma colloid osmotic pressure and filtration fraction
>more proteins in the plasma means an increased colloid osmotic pressure within the capillaries
>increased filtration fraction means more plasma is filtered in the glomerulus, so proteins in the remaining plasma leaving the glomerulus are more concentrated and therefore there will be increase in colloid hydrostatic pressure within peritubular capillaries

Question 8

Glomerulotubular balance

Answer 8

-maintain balance between filtration and reabsorption
>regulated by nervous system, hormones, and local factors
>reabsorption of some substances can be regulated independently

Question 9

How does the glomerulotubular balance occur?

Answer 9

-Balance involves adjusting tubular reabsorption in response to increased tubular inflow therefore ensuring that distal tubules are not overloaded when GFR increases
>If GFR is increased, the reabsorption rate increases… keeps reabsorption in proximal tubules at ~65% of filtrate
>Other areas (especially loop of henle) of renal tubules also adjust their reabsorption

Question 10

Glomerulotubular balance mechanism

Answer 10

-not completely known
-idea is that brush borders sense the flow rate and this can trigger cellular responses to increase transporters at the luminal membrane

Question 11

Aldosterone

Answer 11

-released from adrenal cortex
And acts on principle cells or cortical collecting tubules to trigger increased Na reabsorption and K secretion

Question 12

What triggers release of aldosterone?

Answer 12

1.increased extracellular concentration of K
2. Increased angiotensin II in response to reduced Na and volume depletion

Question 13

Mechanism of aldosterone

Answer 13

-increases Na/K pump at basolateral side enhancing the Na permeability of luminal membrane

Question 14

Reduced function of adrenal gland

Answer 14

-results in loss of Na and accumulation of K

Question 15

Increased function of adrenal gland

Answer 15

-results in loss of K and Na retention

Question 16

Angiotensin II

Answer 16

-released when low blood pressure and low extracellular fluid volume (hemorrhage, diarrhea, excessive sweating)
-leads to increases Na and Water reabsorption

Question 17

3 mechanisms of Ang II

Answer 17

1.stimulates aldosterone secretion
2.constricts efferent arterioles which leads to reduced hydrostatic pressure in peritubular capillaries.
3. direct stimulation of Na reabsorption in all segments of tubules (stimulates Na/K pumps and increases Na-H counter transport at luminal membrane, increasing the Na-HCO3- cotransport at basal membrane)

Question 18

What does constriction of efferent arterioles by Ang II do?

Answer 18

-increases the filtration fraction and therefore the protein concentration increases in the peritubular capillaries and colloid osmotic pressure within the capillaries also increases

Question 19

Antidiuretic Hormone (ADH)

Answer 19

-increases water reabsorption by increasing the water permeability of the distal tubule, collecting tubule and collecting duct

Question 20

Mechanism of ADH

Answer 20

1.ADH binds to its V2 receptor
2.activates protein kinase A in the cytoplasm
3.results in movement of aquaporin-2 protein to the cell apical membrane
4.many aquaporin-2 proteins cluster together and fuse with the cell membrane by exocytosis and forms water channel at the membrane

Question 21

Atrial natriuretic peptide

Answer 21

-released by specific cells of cardiac atria in response to stretching when blood/plasma volume is increased

Question 22

Atrial Natriuretic mechanism

Answer 22

1.reduces Na and water reabsorption mainly in collecting ducts
2.Inhibits renin secretion and Ang II formation and this results in less Na and water reabsorption and more urine output

Question 23

Parathyroid hormone

Answer 23

-increases calcium reabsorption

Question 24

Sympathetic nervous system control of tubular reabsorption

Answer 24

Activates alpha-adrenergic receptors on renal epithelial cells and causes increased Na and Water reabsorption

Question 25

Mechanism of sympathetic nervous control of tubular reabsorption

Answer 25

1.Lower levels of sympathetic nervous system increases Na and water reabsorption in proximal tubules and the thick ascending segment of the loop of Henle

2. Under severe conditions, it reduces the renal flow and GFR
3. Stimulates renin secretion and angiotensin II formation