Lecture 16: Tubular reabsorption and secretion-Exam 3

Question 1

Ultrafiltration or bulk flow

Answer 1

Due to hydrostatic and colloid forces

water transported via specific channels: aquaporins

Question 2

Types of aquaporins

Answer 2

Aquaporin-1: widespread including renal tubules
Aquaporin-2: found in apical membranes of collecting tubules and controlled by ADH
Aquaporin-3: found in basolateral membranes of collecting tubules

Question 3

Characteristics of ATPases

Answer 3

Establish ionic gradient across nephron cell membranes
Gradient drives reabsorption and/or secretion of solutes
Solutes transported via secondary transport by symporters or antiporters.

Question 4

Characteristics of channels

Answer 4

ENAC channels: found in apical cell membranes, closed by drug such as amiloride and opened by a number of hormones
CFTR(Cl-) channel and K+ channels: found in apical cell membranes of some segments of nephrons
Uniporters: found in cell membranes and driven by concentration gradientb of substance concerned

Question 5

Types of transport

Answer 5

facilitated diffusion (glucose transport) occur through channels and uniporters
Active transport directly coupled with energy source
Secondary active transport indirectly coupled with energy source, example like glucose reabsorption y renal tubule

Question 6

Primary active transporters

Answer 6

Na+K+ ATPase
H+ ATPase
H+K+ATPAse
Calcium ATPase

Question 7

Sodium glucose transporters

Answer 7

Located on the brush borders of proximal tubule cells
SGLT2 reabsorbs 90% of glucose in early proximal tubule
SGLT1 reabsorbs 10% of glucose i late proximal tubule

Question 8

Substances that are actively secreted into the renal tubules

Answer 8

Creatinine

Para-Aminohippuric acid

Question 9

Transport maximum

Answer 9

Limit to the rate at which a solute can be transported

Due to saturation of specific transport system

Question 10

Threshold for glucose reabsorption

Answer 10

Transport maximum=375 mg/min

Filtered load of glucose=GFR x Plasma glucose=125mg/min

Question 11

Glucose reabsorption, excretion and transport maximum

Answer 11

Threshold for glucose excretion=200 mg/min even though transport maximum=375 mg/min.
Transport maximum representative of kidney as a whole and each nephron has limit on reabsorption

Question 12

Reasons for lack of transport maximum

Answer 12

Rate of diffusion determined by electrochemical gradient
Permeability of membrane to the substance
Time that fluid containing substance remains within the tubule

Question 13

Solvent drag

Answer 13

Movement of solutes(sometimes with existing gradient) due to movement of water by osmosis
Water reabsorption coupled mainly to sodium reabsorption

Question 14

Coupling reabsorption of water, chloride and urea with sodium reabsorption

Answer 14

Sodium reabsorption–> water reabsorption
Sodium reabsorption–>increase in lumen negative potential
Water reabsorption –> increase chloride ions concentration in lumen and increase urea concentration in lumen
Increase in lumen negative potential and increase concentration of chloride ions—> passive reabsorption of chloride ions
Increase urea concentration in lumen–>passive reabsorption of urea

Question 15

Characteristics of proximal tubule

Answer 15

Highly metabolic with large numbers of mitochondria
Extensive brush borders on luminal surfaces
Extensive intercellular and basal channels on interstitial surfaces

Question 16

Processes in proximal tubule

Answer 16

Reabsorb 65% of filtered sodium, potassium, chloride and bicarbonate
Reabsorb all filtered glucose and amino acids
Secrete organic acids, bases and hydrogen ions into tubular lumen

Question 17

Sodium reabsorption in proximal tubule

Answer 17

In first half ,via cotransport along glucose, AA and other solutes
In second half, mainly with chloride ions

Question 18

Sodium transport in proximal tubule

Answer 18

Most Na+ entry via antiporter with H+
Na+ pumped out of cell by ATPase pump
Electrical gradient in cytoplasm=-70mV and in lumen=-3 mV
Concentration gradient in cytoplasm=30 mosm and in lumen=140 mosm

Question 19

Hydrogen and bicarbonate ions in proximal tubule

Answer 19

H+ increases in lumen due t antitransport with Na+
H+ combines with bicarbonate and forms carbonic acid
Carbonic anhydrase splits carbonic acid into CO2 and H20
CO2 and H20 form carbonic acid
Carbonic acid splits into H+ and bicarbonate ions
Bicarbonate ions diffuse into interstitial space
H+ removed from cell via antitransport or H+ATPase

Question 20

Characteristics of loop of Henle

Answer 20

Thin descending segment highly permeable to water and moderately permeable to most solutes including urea and sodium. It reabsorbs 20% of filtered water
Thick ascending segment impermeable to water and site of action of powerful loop diuretics (Furosemide, ethacrynic acid, bumetanide)

Question 21

Transport mechanisms in ascending loop of Henle

Answer 21

ATP pump maintains low intracellular sodium concentration and negative electrical potential in the cell (-70 mV) by reabsorption of K+ into cells against concentration gradient
Secondary active transport via cotransporters of 1 sodium, 2 chloride and 1 potassium ions into cells
Slight back leak of potassium ions creates positive charge of + 8mV in lumen which forces cations to diffuse through paracellular pathway

Question 22

Characteristics of early distal tubule

Answer 22

Forms macula densa
Reabsorbs most ions
Impermeable to water and urea
Possess Na+ Cl- cotransporter inhibited by thiazide diuretics

Question 23

Characteristics of late distal tubule and collecting duct

Answer 23

Highly convoluted
has characteristics similar to ascending segment
Reabsorbs sodium and water from tubular lumen by principal cells
Secretes K+ into tubular lumen
Primary site of K+ sparing diuretics(Spironolactone, eplerenone, amloride, triameterene)
Intercalated cells reabsorb K+ from tubular lumen and secrete H+ into tubular lumen by H+ATPase

Question 24

Characteristics of medullary collecting duct

Answer 24

Cuboidal epithelial cells with smooth surface and a few mitochondria
Permeability to water controlled by ADH
Permeable to urea by urea transporter
Capable of secreting H+ against a large concentration gradient

Question 25

Aldosterone

Answer 25

Produced by adrenal cortex in response to increased extracellular K+ and/or increased level of angiotensin II
Increases sodium reabsorption
Stimulates potassium secretion and Na+K+ pump on basolateral membrane of cortical tubule membrane
Acts on principal cells of cortical collecting ducts
Absence results in marked loss of sodium and accumulation of potassium and causes Addison’s diseases
Hypersecretion leads to Conn’s syndrome

Question 26

Angiotensin II

Answer 26

Increases sodium and water reabsorption
Returns blood pressure and extracellular volume towards normal
Stimulates aldosterone secretion
Constricts efferent arterioles

Question 27

ADH (Anti Diuretic Hormone)

Answer 27

Produced by posterior pituitary gland
Increases water reabsorption
Binds to V2 receptors in late distal tubules, collecting tubules and collecting ducts
Increases formation of cAMP by stimulating movement of aquaporin-2 proteins to luminal side of cell membranes (form clusters)

Question 28

ANP (Atrial Natriuretic Peptide)

Answer 28

Produced by cardiac atrial cells in response to distension

Inhibits reabsorption of sodium and water

Question 29

Parathyroid hormone

Answer 29

Produced by parathyroid glands

Increases calcium reabsorption

Question 30

Renal clearance

Answer 30

Volume of plasma that is completely cleared of the substance by the kidney per unit time
Cs x Ps = Us x V
Cs= clearance rate of substance= GFR for substance not reabsorbed r secreted in body
Ps=Plasma concentration of substance
Us= urine concentration of substance
V= Urine flow

Question 31

Passive routes

Answer 31

For a substance to be reabsorbed it must be transported:
-To the tubular epithelial cells –> renal interstitial fluid
-Through the peritubular capillaries–> renal blood flow
Water is transported from lumen to renal interstitial fluid via transcellular and paracellular routes by osmosis

Question 32

Plasma flow in the kidney

Answer 32

20% of 625 mL/min=125 mL