**2. Reabsorption

What is reabsorption?

Movement of molecules out of the tubule and into peritubular blood

What are the 2 mechanisms of reabsorption and secretion?

• Active transport

• Passive transport

Active Transport for Reabsorption

• What is transported

• What is actively secreted

• Each actively transported substances have

What is transported: Sodium, glucose, amino acids, calcium, potassium, phosphate, urate ions

What is actively secreted out of tubule: Hydrogen ions

Each actively transported substances have: Transport maximum (Tm) rate and plasma threshold value

Passive Transport for Reabsorption

• What

• What is transported

What: Diffusion, osmosis

What is transported: Potassium secreted in distal tubule and collecting duct

Proximal Tubule Transport

• Cells of proximal tubule reabsorb what nutrients from filtrate

  • Involves

  • Uses

• Which nutrient has a limit

  • Called

  • If blood glucose too high

Cells of proximal tubule reabsorb what nutrients from filtrate: Glucose and amino acid

• Involves: Secondary active transport

• Uses: Sodium-glucose co-transporter

Which nutrient has a limit of reabsorption: Glucose

• Called: Transport maximum (max rate at which glucose can be reabsorbed by tubule cells —> blood)

• If blood glucose too high: Glomerular filtration > transport maximum —> renal threshold exceeded —> glucosuria (glucose in urine)

  • Glucose filtered exceeds the transport maximum of SGLT transporters in PCT

  • Meaning that the transporters are saturated

  • So excess glucose stays in nephron and flows into urine

Proximal Tubule Transport: Bicarbonate Ions

• What is the predominant base in plasma and ECF?

• How much bicarbonate normally reabsorbed in proximal tubule

• What enzyme converts bicarbonate for reabsorption

  • Where

  • How

• What happens to CO2 and H2O once inside proximal tubule

• How does bicarbonate leave the proximal tubule cell

• Which ion is reabsorbed together with bicarbonate

  • Maintains

• What are the 2 predominant osmolytes in initial filtrate

• Percentage reabsorbed can by increased by

  • By

  • When does this occur

What is the predominant base in plasma and ECF: Bicarbonate ions

How much bicarbonate normally reabsorbed in proximal tubule: 85-90%

What enzyme converts bicarbonate for reabsorption: Carbonic anhydrase

• Where: Tubular lumen

• How: With the help of H+ ions

What happens to CO2 and H2O once inside proximal tubule: Reconverted into bicarbonate and H+ by carbonic anhydrase

How does bicarbonate leave the proximal tubule cell: Membrane transporter

Which ion is reabsorbed together with bicarbonate: Sodium ions

• Maintains: Electrical neutrality

What are the 2 predominant osmolytes in initial filtrate: Na+ and Cl-

Percentage reabsorbed can by increased by: Angiotensin II and sympathetic nerves on tubule cells

• By: Vasoconstriction of renal blood vessels

  • Longer contact time between tubular fluid and tubule cells → allows more reabsorption of water and solutes before the fluid is excreted as urine.

• When does this occur: During dehydration or blood loss to retain sodium chloride and water

Bicarbonate Reabsorption/H+ Excretion: Acidosis #ff00e7

• What is generated

• What happens to H+

• What is reabsorbed in exchange for H+

• What ion movement occurs between cells and plasma

What is generated: Bicarbonate ions (HCO3-)

What happens to H+: Binds to buffers in urine and is excreted

What is reabsorbed in exchange for H+: 1 Na+ and 1 HCO3-

What ion movement occurs between cells and plasma: K+ moves out of cells —> extracellular fluid

Bicarbonate Reabsorption/H+ Excretion: Alkalosis #a500e8

• What happens to HCO3-

• What happens to H+

• What happens to K+

• What is the process compared to acidosis

What happens to HCO3-: It’s secreted into —> tubuluar lumen —> excreted in urine

What happens to H+: Reabsorbed to acidify blood

What happens to K+: Excess K+ excreted

What is the process compared to acidosis: Opposite process