22nd June 2023

Filtration

Most substances contained within the plasma, with the exception of proteins and protein-bound substances, are readily filtered through the glomerulus to Bowman’s capsule. Therefore, the concentration of the filtrate in Bowman’s capsule is similar to that of the circulating plasma.
As the filtrate flows through the renal tubule, the concentration of the filtrate is modified by the reabsorption of solutes and water from the filtrate and by the secretion of other solutes into the tubular fluid from the peritubular capillaries.

End products of metabolism such as urea, creatinine and uric acid are readily filtered at the glomerulus but not reabsorbed, therefore are excreted in large quantities in the urine. Other substances requiring excretion, such as drugs, are also poorly reabsorbed but are secreted into the tubular fluid from the peritubular capillaries, resulting in excretion in the urine.

Electrolytes such as sodium, chloride, and bicarbonate ions are readily filtered and reabsorbed, resulting in mostly low levels of excretion in the urine, however, this is variable depending on the needs of the body. Amino acids and glucose are readily filtered but almost completely reabsorbed and therefore do not appear in the urine under normal circumstances.