tailieunhanh - Access: Acid-Base, Fluids, and Electrolytes - part 5

Net Nae bài tiết axit là tổng số tiền của H + bài tiết bởi thận, số lượng, chúng ta có thể tính toán Nae là số tiền của H (cả đệm và miễn phí) bài tiết trong nước tiểu trừ đi số tiền của HCO3-thất bại + được khai hoang và đã bị mất trong nước tiểu. Một số tác giả cũng trừ mất nước tiểu của các anion khác có tiềm năng | METABOLIC ACIDOSIS 185 TABLE 6-7 Net Acid Excretion NAE is the total amount of H excreted by the kidneys quantitatively we can calculate NAE to be the amount of H both buffered and free excreted in urine minus the amount of HCO3- that failed to be reclaimed and was lost in urine. Some authors also subtract the urinary loss of other anions that have the potential of being converted in the body to bicarbonate largely citrate Because H secretion into the tubule lumen results in a 1 1 HCO3- addition to the ECF NAE equals the amount of new HCO3- generated NAE is accomplished through two processes that are historically separated on the basis of a colorimetric indicator phenolphthalein that detects pH changes effectively between pH 5 and 8 that acid which can be detected by titrating sufficient alkali into urine to achieve color changes with this indicator is called titratable acid and is mostly phosphate in the monobasic H2PO4- form Nontitratable acid excretion occurs primarily in the form of NH4 this is not detected by phenolphthalein since the pK approximately for ammonium is too high The majority of NAE is in the form of protons bound to buffers either phosphate or ammonium this makes it possible to elaborate a much less acid urine but still achieve adequate NAE Even though most clinicians equate NAE with an acidic urine it is important to recognize that a low urine pH does not necessarily mean increased NAE for example at a urine pH of the free H concentration is only mmol in a 70-kg person on an average Western diet one can see that free protons would make up only a small fraction of the approximately 70 mmol of net acid excreted daily continued 186 METABOLIC ACIDOSIS TABLE 6-7 Continued There are several pathologic conditions discussed later in which the urine pH is relatively acid but NAE is insufficient In subjects that consume a typical Western diet adequate NAE occurs through the functions of both the proximal tubule to synthesize NH4 which .