Active Reabsorption and transport maximum

Question 1

Reabsorption segments

Answer 1

-proximal tubule
-loop of henle (descending and ascending limbs)
-distal tubule
-connecting tubule
-collecting tubule
-collecting duct

Question 2

How much glucose needs to be absorbed daily?

Answer 2

-180g of glucose/day that needs to be absorbed

Question 3

% absorption of water, Na, glucose/amino acids

Answer 3

99%-100%

Question 4

% absorption of urea

Answer 4

50%

Question 5

% absorption of waste

Answer 5

0%

Question 6

Reabsorption pathways

Answer 6

1.through tubular epithelium into the interstitial fluid (can be both active and passive)
>Transcellular path
> Paracellular path
2. through peritubular capillary membrane into the blood (bulk flow derived by hydrostatic and colloid osmotic forces)

Question 7

Reabsorption of water

Answer 7

-passive transport

Question 8

Active transport in the kidneys

Answer 8

1.Na/K ATPase
2.Hydrogen ATPase
3.Hydrogen-potassium ATPase
4. Calcium ATPase

Question 9

Na/K pump

Answer 9

-pumps at basolateral membrane
-transport 3 Na out, 2 K in
-create 2 main forces for Na diffusion from the lumen into epithelial cells
>concentration gradient
>negative charge

Question 10

Where does Na reabsorption occur?

Answer 10

-happens in most parts of the tubules

Question 11

Na reabsorption at luminal (apical) membrane

Answer 11

-Na diffuses into the cells because of electrochemical gradient (created when Na pumped out, and K increased resulting in greater gradient.

Question 12

Na reabsorption at basolateral membrane

Answer 12

-active transport against electrochemical gradient

Question 13

Movement of Na to the peritubular capillaries

Answer 13

-occurs via passive ultrafiltration (hydrostatic and colloid osmotic pressure)
>Na also brings water with it

Question 14

Na facilitated diffusion

Answer 14

-also occur at luminal membrane and it is important for secondary active transport of glucose and amino acids
>Glucose and amino acids are transferred to interstitial fluid because of concentration gradient
»Glucose secondary active transport is very efficient in proximal tubules (SGLT2 90%, SGLT1 10%). At basolateral membrane, facilitated diffusion through GLUT 2 and GLUT 1 occurs to get glucose into interstitial fluid.

Question 15

Secondary active secretion of H+ & sodium reabsorption

Answer 15

-Active secretion of H+ into proximal tubules via a counter transport mechanism through Na/H exchanger (NHE)

Question 16

Pinocytosis

Answer 16

-active reabsorption of proteins (occurs in proximal tubules)
-proteins are trapped in invaginations of cell membrane and a vesicle forms inside the cytoplasm. Then protein is digested into amino acids (lysosomes) and are reabsorbed

Question 17

Fanconi’s syndrome

Answer 17

-occurs when the reabsorption process in renal tubules (especially proximal tubules) is compromised due to damage
>electrolytes and nutrients (Na, K, glucose, AA, HCO3-) are excreted and lost in urine

Question 18

Fanconi’s syndrome causes

Answer 18

-genetic reasons (primary)
-toxins, drug reaction (secondary)

Question 19

Fanconi’s syndrome symptoms

Answer 19

-excessive thirst
-excessive urination
-weight loss
-weakness
- lack of appetite

Question 20

How to manage Fanconi’s syndrome?

Answer 20

-supply nutrients and electrolytes supporting acid-base balance
**response varies depending on degree of damage

Question 21

Transport maximum

Answer 21

-indicates the limit of reabsorption or secretion of a substance
-related to the saturation of the transport proteins and enzymatic activities

Question 22

Transport max for glucose

Answer 22

375mg/min
-Means that if more than 375 mg of glucose is filtered every minute, then all the excess glucose appears in urine

Question 23

Transport max threshold level

Answer 23

-threshold level excretion is below transport max
**glucose threshold =250mg/min
-means that some traces of glucose appears in the urine. Due to variation in transport maximum of nephrons. Therefore transport maximum is assuming that all nephrons have reached their maximum reabsorption capacity