Lecture 7- Tubular transport

Question 1

Ways substances can be transported across the membrane

Answer 1

Diffusion
Osmosis
Carrier mediated transport (facilitated diffusion)
Active transport (Na/K ATPase and co-transportation with glucose and amino acids)

Question 2

Describe sodium overall in the kidney

Answer 2

Reabsorbed in the proximal tubule (65%), thin and thick limbs of loop of henle (25%), and collecting duct (4-5%)

NOT secreted by nephron

Reabsorption is active and powered by Na/K ATPase

Question 3

Describe water in overall kidney

Answer 3

Reabsorption is solute coupled

Reabsorbed in proximal tubule (64%), thin descending limb of loop of henle (10%), and collecting ducts (1-24%)

Question 4

Describe potassium overall in the kidney

Answer 4

Most is reabsorbed in proximal tubule (55%)

Some is co-transorted with Na+ in ascending limb of loop of henle (30%)

Actively reabsorbed in distal tubule (10%)

Secretion occurs in distal tubule and collecting duct

Question 5

What are some important features of the proximal tubule

Answer 5

Reabsorption is isosmotic
Na+, glucose, and other amino acids reabsorption is active
Cl, K, and water reabsorption is passive
H+ is secreted through Na/H anti-porter
2/3 of the filtrate is reabsorbed

Question 6

Describe the first half of the proximal tubule

Answer 6

Na is moved from tubular fluid to peritubular fluid with a co-transporter (glucose, amino acids, phosphate, chloride, or lactate)

Na is moved into peritubular fluid and H is moved into tubular fluid through a Na/H anti-porter
**the H ions come from carbonic anhydrase reaction

Na is moved into blood from peritubular fluid through Na/K ATPase

The cotransporter (glucose, amino acid, phosphate, chloride, lactate) is moved to blood from peritubular fluid through facilitated diffusion

Bicarbonate ions (from carbonic anhydrase) are moved into blood from peritubular fluid through facilitated diffusion

Question 7

Describe the second half of the proximal tubule

Answer 7

Na and Cl are moved into peritubular fluid through paracellular diffusion

Na and Cl is also moved into peritubular fluid through a complex co-transporter involving two anti-porters and weak acids

Na is moved into the blood through Na/K ATPase

Cl is moved into blood with K through facilitated diffusion

Question 8

Describe the secretion mechanisms in the proximal tubule for organic anions

Answer 8

Organic anions:
Anti porter that transports two different organic acids (for example, transport penicillin, contrast agent, lasix into tubule lumen while transporting organic acids like chloride, urate,and hydroxyl into the peritubular fluid

There is active transport using ATP and an organic acid into the tubule lumen

There is a complex tertiary active transport moving organic anions from the blood into the peritubular fluid

Question 9

Describe secretion mechanisms in the proximal lumen for organic cations

Answer 9

Organic cations get into the tubule lumen from the pertitubular lumen

Organic cations get to peritubular fluid from blood through co-facilitated diffusion

Question 10

Describe activity in the descending limb of the loop of henle

Answer 10

Cells have minimal metabolic activity- little active transport

High permeability to water, moderate permeability to urea, sodium, and other ions

Question 11

Describe the activity in the thin ascending loop of Henle

Answer 11

Little active transport
Little/no permeability to water
High permeability to sodium

Question 12

Thick ascending limb of loop of henle

Answer 12

Impermeable to water- primary sit of urine dilution

Net reabsorption of sodium, chloride, and potassium via Na/K/2Cl co-transporter

Establishes a high ion concentration gradient between lumen and peritubular fluid

Establishes a high osmolar concentration gradient between lumen and peritubular fluid

Question 13

Describe the activity of the early distal tubule

Answer 13

Can establish high ionic concentration gradient and high osmolar concentration gradient between urine and plasma

Impermeable to water

Sodium is reabsorbed through Na/Cl symporter

Question 14

Describe the activity of the late distal tubule and collecting duct

Answer 14

Permeability to water is variable and controlled by ADH

Principal cells secrete potassium- can vary from no net K+ secretion to net K+ secretion, reabsorb sodium, and are the site of ADH

H+ is actively secreted by intercalated cells