Explain Passive transport or diffusion - Mechanism of Urine Formation

Passive transport or diffusion: Diffusion means the random movement of particles in a fluid. The movement of particles from a high concentration area to a low concentration area takes place till the concentration is uniform. Although the particles move at random in all directions, a greater number of them move from a region of higher concentration to that of a lower concentration. When water is reabsorbed, the concentration of urea in the tubular fluid rises.

This, in turn, causes urea to diffuse from the tubular fluid to the peritubular fluid (fluid surrounding the tubule). Chloride, phosphate and bicarbonate ions are attracted from the tubular fluid towards the peritubular fluid. A few substances are secreted by diffusion in the same manner. For example, ammonium ions are synthesized inside the epithelial cells and diffuse into the tubular lumen. They help to control the degree of acidity of tubular fluid.