Movement of ions between the nephron and renal capillaries

What are the main functions of the PCT?

• Reabsorption of NaCl (as isotonic solution)

• Reabsorption of glucose and amino acids

• Regulation of body fluid pH

• Secretion of organic molecules (e.g. hormones, drugs)

Does the concentration of tubular contents change in the PCT?

No, the fluid remains isotonic (~300 mOsm)

Which solutes are reabsorbed by diffusion or osmosis in the PCT?

Cl⁻, K⁺, Ca²⁺, Mg²⁺, urea, H₂O

What is diffusion?

Movement of molecules from high to low concentration

What is osmosis?

Movement of water across a membrane due to a solute gradient

What is active transport?

Movement of molecules using ATP against a concentration gradient

Which transporter helps reabsorb Na⁺ in PCT?

Na⁺/K⁺ ATPase on basolateral membrane

What is the role of the Na⁺/H⁺ exchanger?

Brings Na⁺ into the cell while secreting H⁺ into the tubule
→ helps regulate intracellular pH

How is glucose reabsorbed in the PCT?

Via Na⁺-glucose symporters on the apical side

Where does glucose go after entering the cell?

Exits to the interstitial fluid by facilitated diffusion into capillaries

Is glucose reabsorption carrier-mediated?

Yes, requires specific transport proteins

What is Tmax in glucose transport?

Tubular transport maximum — the max rate at which glucose can be reabsorbed

What happens if plasma glucose exceeds Tmax?

Excess glucose appears in urine (glucosuria)

At what plasma glucose concentration does this typically occur?

≈ 11–16 mM or > 200–300 mg/dL

What defines diabetes mellitus by glucose levels?

• Fasting plasma glucose ≥ 7 mM

• Random glucose test > 11.1 mM (on two occasions)