ultrafiltrate

Ultrafiltrate is the portion of a solution that passes through a semipermeable membrane during ultrafiltration. Ultrafiltration uses membranes with pore sizes that allow solvent and small solutes to pass while retaining larger molecules such as proteins, polysaccharides, and cells. The composition of the ultrafiltrate depends on the membrane’s molecular weight cutoff (MWCO) and operating conditions; typical ultrafiltrates contain water, salts, sugars, amino acids, and other small metabolites, while larger macromolecules remain in the retentate.

In physiology, ultrafiltration is used to describe the glomerular ultrafiltrate produced in the kidney. Plasma water and small solutes are forced through the glomerular filtration barrier to Bowman's capsule, forming a filtrate that is largely free of proteins and cells and is osmotically similar to plasma. This glomerular ultrafiltrate then flows through the nephron, where water and solutes are reabsorbed or secreted as part of urine formation.

In laboratory and industrial settings, ultrafiltration serves to concentrate or desalt protein solutions and to separate macromolecules from smaller solutes. The ultrafiltrate is the liquid that passes the membrane, and its composition is governed by the membrane’s MWCO and the process conditions. Membranes typically range from about 1 to 1000 kDa MWCO, enabling selective removal of small molecules.

In medical practice, ultrafiltration is also used in dialysis to remove excess fluid from a patient’s blood; the resulting effluent, or dialysate, contains water and small-solute waste that is cleared from the body.