Urea diffuses from liver cells into the blood plasma.
In the glomerulus, most of the urea is forced out of the blood into the glomerular filtrate.
About half of the urea in the filtrate is reabsorbed as it diffuses passively through the cells of the proximal convoluted tubule and into the blood.
The concentration of urea in the fluid in the nephron increases as it passes along the proximal convoluted tubule, because the reabsorption of water and other solutes back into the bloodcauses the concentration of urea in the filtrate to increase.
