describe and explain the formation of urine in the nephron, limited to: the formation of glomerular filtrate by ultrafiltration in the Bowman’s capsule, selective reabsorption in the proximal convoluted tubule

Published by Patrick Mutisya · 14 days ago

Homeostasis in Mammals – Urine Formation

Homeostasis in Mammals: Formation of Urine in the Nephron

1. Glomerular Filtration – Ultrafiltration in Bowman's Capsule

The first step in urine formation is the production of glomerular filtrate by ultrafiltration. Blood enters the glomerulus under arterial pressure (≈ 120 mm Hg). The capillary walls act as a filtration barrier composed of:

  • Fenestrated endothelium – allows plasma water and solutes to pass.

  • Basement membrane – negatively charged, restricts large proteins.

  • Podocyte slit diaphragms – size‑selective pores.

The net filtration pressure (NFP) can be expressed as:

\$\text{NFP}=P{GC}-P{BS}-\pi_{GC}\$

where PGC is glomerular capillary hydrostatic pressure, PBS is Bowman's space hydrostatic pressure, and \piGC is the oncotic pressure of glomerular plasma.

Typical values give an NFP of about 10 mm Hg, driving a filtration rate of \overline{125} mL min⁻¹ (glomerular filtration rate, GFR).

Suggested diagram: Cross‑section of a renal corpuscle showing blood flow, filtration barrier, and Bowman's space.

2. Selective Reabsorption in the Proximal Convoluted Tubule (PCT)

Approximately 65 % of the filtrate volume and the majority of valuable solutes are reclaimed in the PCT. Reabsorption is driven by active transport, secondary active transport, and passive diffusion.

Key Transport Mechanisms

  1. Na⁺/K⁺‑ATPase (basolateral membrane): Maintains low intracellular Na⁺, providing the gradient for secondary transport.

  2. Sodium‑glucose cotransporter (SGLT2): Uses Na⁺ gradient to co‑transport glucose (and galactose) into the cell.

  3. Na⁺/H⁺ exchanger (NHE3): Exchanges intracellular H⁺ for luminal Na⁺, facilitating bicarbonate reclamation.

  4. Amino‑acid transporters: Various Na⁺‑dependent carriers reabsorb amino acids.

  5. Water reabsorption: Osmotic gradient created by solute reabsorption drives water movement through aquaporin‑1 channels.

Substances Reabsorbed in the PCT

SubstanceApprox. % ReabsorbedTransport Mechanism
Water≈ 65 %Passive osmosis via aquaporin‑1
Sodium (Na⁺)≈ 65 %Na⁺/K⁺‑ATPase (basolateral) + Na⁺‑dependent cotransporters
Glucose≈ 100 %SGLT2 (Na⁺‑glucose cotransporter)
Amino acids≈ 100 %Na⁺‑dependent amino‑acid transporters
Bicarbonate (HCO₃⁻)≈ 100 %Na⁺/H⁺ exchanger + carbonic anhydrase
Urea≈ 50 %Passive diffusion

Reabsorption in the PCT is highly efficient, ensuring that essential nutrients and the majority of water are returned to the bloodstream, while waste products remain in the tubular fluid for later excretion.

Suggested diagram: Schematic of the proximal convoluted tubule highlighting Na⁺/K⁺‑ATPase, SGLT2, NHE3, and aquaporin‑1 channels.

Summary

Urine formation begins with ultrafiltration in Bowman's capsule, where plasma is forced through a size‑ and charge‑selective barrier, creating a filtrate that mirrors plasma but lacks proteins and cells. The proximal convoluted tubule then reclaims \overline{65} % of this filtrate volume, using active and secondary active transport to selectively recover glucose, amino acids, bicarbonate, Na⁺, and water. These processes are crucial for maintaining fluid balance, electrolyte homeostasis, and overall metabolic stability in mammals.