Proximal tubular function and renal acidification in the aged

Publisher Summary As a consequence of the increase in life expectancy in the general population, the number of elderly persons continues to rise. This has meant that more attention must be paid to the possible existence of specific characteristics in this sector of the population when its members become ill. The kidney of elderly persons presents a series of characteristic modifications consecutive to organic or functional disturbances of the structures that constitute the organ. One such structure is the renal tubule. This chapter discusses the physiology and pathology of this structure. The proximal tubule reabsorbs a large fraction (two-thirds) of the glomerular filtrate. Sodium, several other solutes, and water are reabsorbed at a high rate. Active sodium reabsorption and hydrogen ion secretion are the essential processes to which transport of chloride, several organic solutes, and water are coupled by a variety of mechanisms. Fluid transport is isosmotic so that the concentration gradients of solute across the wall are small. The disturbance in such functions can lead to renal acidification. The clinical data can lead the physician to suspect the existence of some disturbance in renal acidification, it is also necessary to evaluate the acid–base equilibrium in blood, the urinary pH, and net acid excretion (ναɛ). When a disturbance in tubular acidification exists, a hyperchloraemic metabolic acidosis in blood will become apparent. Under such circumstances, the urinary pH will show the following characteristics: It may be acid, with normal ναɛ values, as in the case of proximal bicarbonate wasting or type 2 renal tubular acidosis (RTA); it may be consistently higher than 6.2 with a lowered ναɛ, as in the case of type I or distal RTA, or it may be acid,with subnormal ναɛ values; this is characteristic of type 4 RTA. In order to confirm diagnosis, overload tests must be carried out.