Control of Reabsorption and Secretion in the Nephron

Question 1

What substances are reabsorbed and what are excreted in the tubule?

Answer 1

• highly selective
• glucose and amino acid reabsorption almost done so excretion 0
• Na+, Cl-, and bicarbonate highly resorbed (at varied rates)
• waste products like urea, creatinine, uric acid and ammonia poorly absorbed and excreted in large amounts

Question 2

Describe active transport in the tubules

Answer 2

• transport of sodium with the sodium-potassium pump from interior of cell across basolateral membrane
• creates low intracellular sodium concentration and negative intracellular electrical potential
• causes sodium diffusion from tubular lumen into epithelial cells through brush border

Question 3

What is secondary active transport?

Answer 3

• when 2 or more substances interact with a specific membrane protein and are transported together across the membrane
• as one substance diffuses down electrochemical gradient, energy released is used to drive another substance against its electrochemical gradient
• therefore does not need ATP

Question 4

What are examples of secondary active transport in the nephron tubules?

Answer 4

• SGLT: sodium glucose co-transporter
• Na/amino acid transporter
• NHE: Na/H exchanger

Question 5

What are the transport characteristics of the loop of Henle?

Answer 5

• 20% filtered water, 25% filtered sodium, chloride and potassium reabsorbed
• thin descending segment permeable to water diffusion
• ascending limb impermeable to water
• thick ascending limb has active transporters and absorbs sodium, chloride and potassium
• other ions absorbed too
• co-transporters

Question 6

What are the transport characteristics of the early and late distal tubule?

Answer 6

• 5% filtered sodium absorbed here
• early part is impermeable to water
• pumps/absorbs sodium, chloride, and potassium
• urine becomes more dilute
• late section has principal cells (absorb H20 and Na+) and intercalated cells (absorb K+, secrete H+/HCO3-)

Question 7

What are the transport characteristics of the medullary collecting ducts?

Answer 7

• absorbs less than 10% filtered water and Na+
• final site of urine processing (determines urine output and composition)
• vital for producing dilute or concentrated urine
• impermeable to water unless ADH present
• permeable to urea and has urea transporters
• can secrete H+ (acid-base balance)

Question 8

What are the hormones that regulate tubular reabsorption in the nephron and their effects?

Answer 8

• aldosterone: increases NaCl and H20 reabsorption and increases K+ secretion
• angiotensin II: increases NaCl and H20 reabsorption and increases H+ secretion
• ADH: increases H20 reabsorption
• ANP: decreases NaCl reabsorption
• parathyroid hormone: decreases phosphate reabsorption and increases Ca2+ reabsorption

Question 9

Describe the activity of aldosterone

Answer 9

• regulates sodium reabsorption and potassium excretion
• site of action: principal cells of cortical collecting duct
• stimulates Na/K-ATPase pump on basolateral side of cortical collecting tubule membrane
• increases Na+ permeability of luminal side of principal cell membrane
• stimulates apical H+-ATPase in alpha-intercalated cells resulting in proton secretion

Question 10

Describe the activity of angiotensin II

Answer 10

• sodium-retaining hormone
• acts during haemorrhage and loss of salt and water to increase body fluid and solute levels
• stimulates aldosterone release from adrenal cortex
• constricts afferent and efferent arterioles in kidney to stimulate sodium reabsorption in PCT, loops of Henle, DCT and collecting ducts
• stimulates Na+ pump on basolateral side of tubular epithelia and Na/H exchange on luminal membrane esp in proximal tubule

Question 11

Describe the activity of ANP

Answer 11

• secreted by cardiac atria when they are distended by increased blood volume
• directly inhibit reabsorption of Na+ and water by renal tubules especially in collecting ducts
• increases solute and water loss to decrease blood volume

Question 12

Describe the activity of the sympathetic NS in reabsorption

Answer 12

• decreases Na+ and water excretion by constricting renal afferent arterioles decreases glomerular filtration rate
• decreases flow to vasa recta and increases medullary interstitial osmolarity

Question 13

Describe the action of the parathyroid hormone

Answer 13

increases tubular reabsorption of Ca2+ in distal convoluted tubule