The formation of urine is the result of the simultaneous occurrence of three processes, only one of which is explicable in terms of known forces. These processes are: (1) glomerular filtration, or the passage of a colloid-free filtrate of blood plasma through the glomerular capillary endothelium and its investing squamous epithelium owing to an excess of hydrostatic over colloid osmotic pressure; (2) tubular excretion, or the passage of solutes from the plasma in the peritubular capillaries through the cells of the proximal convolution of the tubule to its lumen; (3) tubular resorption, or the partial removal of solutes and water from the lumen of the distal half of the tubule by its cells (the corrective conservation of certain solids and of water wastefully excreted by the renal corpuscle).
The adequacy of these processes in the indispensable regulation of the composition of the plasma is dependent on the functional integrity of
EDWARDS JG. THE FORMATION OF URINE. Arch Intern Med (Chic). 1940;65(4):800–824. doi:10.1001/archinte.1940.00190100141006