Describe the various methods of transport across the epithelial layer of the renal tubule.
The basic mechanisms of transport across the tubular epithelial cell membrane include active and passive transport mechanisms. Water and urea are passively absorbed along concentration gradients. Sodium, potassium, chloride, calcium, and phosphate ions, as well as urate, glucose, and amino acids, are reabsorbed using primary or secondary active transport mechanisms to move across the tubular membrane. Some substances, such as hydrogen, potassium, and urate ions, are secreted into the tubular fluids.