Reabsorption of Chloride, Urea, and Other Solutes by Passive Diffusion
When sodium is reabsorbed through the tubular epithelial cell, negative ions such as chloride are transported along with sodium because of electrical potentials. That is, transport of positively charged sodium ions out of the lumen leaves the inside of the lumen negatively charged, compared with the interstitial fluid. This causes chloride ions to diffuse passively through the paracellular pathway. Additional reab-sorption of chloride ions occurs because of a chloride concentration gradient that develops when water is reabsorbed from the tubule by osmosis, thereby con-centrating the chloride ions in the tubular lumen (Figure 27–5). Thus, the active reabsorption of sodium is closely coupled to the passive reabsorption of chlo-ride by way of an electrical potential and a chloride concentration gradient.
Chloride ions can also be reabsorbed by secondary active transport. The most important of the secondary active transport processes for chloride reabsorption involves co-transport of chloride with sodium across the luminal membrane.
Urea is also passively reabsorbed from the tubule, but to a much lesser extent than chloride ions. As water is reabsorbed from the tubules (by osmosis coupled to sodium reabsorption), urea concentration in the tubular lumen increases (see Figure 27–5). This creates a concentration gradient favoring the reab-sorption of urea. However, urea does not permeate the tubule as readily as water. In some parts of the nephron, especially the inner medullary collecting duct, passive urea reabsorption is facilitated by specific urea transporters. Yet only about one half of the ureathat is filtered by the glomerular capillaries is reab-sorbed from the tubules. The remainder of the urea passes into the urine, allowing the kidneys to excrete large amounts of this waste product of metabolism.
Another waste product of metabolism, creatinine, is an even larger molecule than urea and is essentially impermeant to the tubular membrane. Therefore, almost none of the creatinine that is filtered is reab-sorbed, so that virtually all the creatinine filtered by the glomerulus is excreted in the urine.