Movement of ions between the nephron and renal capillaries

What is the Proximal Convoluted Tubule (PCT)?

Key Functions:

• Reabsorption of water, ions, glucose, amino acids

• Secretion of waste products (e.g., H⁺, drugs)

Characteristics:

• Tubular fluid remains isotonic (~300 mOsm)

• Numerous microvilli on apical surface → increased surface area for reabsorption

• High mitochondrial density → supports active transport (e.g. Na⁺/K⁺ pumps)

What are important functions of the PCT?

Key Functions:

• Reabsorbs bulk of filtered NaCl
  → as an isotonic NaCl solution

• Reabsorbs essential solutes
  → e.g. glucose and amino acids

• Regulates body fluid pH
  → via H⁺ secretion and HCO₃⁻ reabsorption (detailed later)

• Secretes organic molecules
  → e.g. hormones, negatively charged molecules, lipid-based compounds (that aren't filtered efficiently)

What is reabsorption?

the process by which filtered material (e.g. water, ions, glucose, amino acids) is moved from the lumen of the nephron back into the blood.

Key Points:

• Occurs after filtration in the glomerulus.

• Happens primarily in the proximal convoluted tubule (PCT).

• Helps the body retain essential substances and maintain fluid and electrolyte balance.

• Involves active and passive transport mechanisms.

What is secretion?

the process by which selected molecules are actively transported from the blood (basolateral side) into the nephron tubule (apical side) for excretion in urine.

🧪 Key Features:

• Occurs mainly in the proximal and distal convoluted tubules, and also the collecting duct.

• Helps eliminate substances not efficiently filtered at the glomerulus.

• Includes the removal of:

  • Drugs (e.g. penicillin)

  • Toxins

  • Hydrogen ions (H⁺) for pH regulation

  • Potassium ions (K⁺) for electrolyte balance

• Involves carrier proteins and active transport mechanisms.

What are exchangers/antiporters?

are membrane transport proteins that move two different ions or molecules in opposite directions across a membrane.

In the Proximal Convoluted Tubule (PCT):

• The Na⁺/H⁺ exchanger (NHE) is a key antiporter located on the apical (luminal) side of tubular epithelial cells.

How It Works:

• Na⁺ enters the cell from the filtrate (tubular lumen).

• H⁺ is pumped out into the filtrate at the same time.

• This is driven by the Na⁺ gradient, which is maintained by the Na⁺/K⁺ ATPase on the basolateral membrane:

  • This pump moves Na⁺ out of the cell (into the interstitial fluid) and K⁺ in, keeping intracellular Na⁺ low and K⁺ high.

Functions:

• Reabsorbs Na⁺ into the blood.

• Secretes H⁺ into the tubular lumen (helps with acid-base balance).

• Contributes to the regulation of intracellular pH.

What are symporters?

are membrane transport proteins that move two or more molecules or ions in the same direction across a membrane.

In the Proximal Convoluted Tubule (PCT):

• Apical Na⁺-glucose symporter (also called SGLT):

  • Moves Na⁺ and glucose together from the tubular lumen into the cell.

  • Utilizes the Na⁺ gradient (maintained by the Na⁺/K⁺ ATPase) to drive glucose uptake even against its concentration gradient.

• Other symporters also couple Na⁺ transport with amino acids, helping reabsorb essential nutrients from the filtrate.

Functions:

• Reabsorbs Na⁺, glucose, and amino acids.

• Prevents loss of vital nutrients in urine.

• Supports energy-efficient transport using existing ion gradients

What is glucose transport?

in the kidney refers to the reabsorption of glucose from the filtrate back into the blood, primarily in the proximal convoluted tubule (PCT).

Key Points:

• Carrier-mediated process:
  Glucose reabsorption requires specific transport proteins. It cannot cross the membrane by simple diffusion.

• Tmax (Transport Maximum):
  The amount of glucose that can be reabsorbed is limited by the number of available transporters. This limit is called Tmax.

• Normal conditions:
  Under normal plasma glucose levels, 100% of filtered glucose is reabsorbed—no glucose in urine.

• High glucose levels (e.g. in diabetes mellitus):
  When blood glucose is too high, the filtered load exceeds Tmax.
  → Transporters are saturated
  → Excess glucose appears in the urine (glucosuria)