Pradella M, Dorizzi R M, Rigolin F
Laboratorio Analisi Chimico-Cliniche, Ospedale di Legnago, Italy.
Crit Rev Clin Lab Sci. 1988;26(3):195-242. doi: 10.3109/10408368809105890.
The physical properties and chemical composition of urine are highly variable and are determined in large measure by the quantity and the type of food consumed. The specific gravity is the ratio of the density to that of water, and it is dependent on the number and weight of solute particles and on the temperature of the sample. The weight of solute particles is constituted mainly of urea (73%), chloride (5.4%), sodium (5.1%), potassium (2.4%), phosphate (2.0%), uric acid (1.7%), and sulfate (1.3%). Nevertheless, urine osmolality depends only on the number of solute particles. The renal production of maximally concentrated urine and formation of dilute urine may be reduced to two basic elements: (1) generation and maintenance of a renal medullary solute concentration hypertonic to plasma and (2) a mechanism for osmotic equilibration between the inner medulla and the collecting duct fluid. The interaction of the renal medullary countercurrent system, circulating levels of antidiuretic hormone, and thirst regulates water metabolism. Renin, aldosterone, prostaglandins, and kinins also play a role. Clinical estimation of the concentrating and diluting capacity can be performed by relatively simple provocative tests. However, urinary specific gravity after taking no fluids for 12 h overnight should be 1.025 or more, so that the second urine in the morning is a useful sample for screening purposes. Many preservation procedures affect specific gravity measurements. The concentration of solids (or water) in urine can be measured by weighing, hydrometer, refractometry, surface tension, osmolality, a reagent strip, or oscillations of a capillary tube. These measurements are interrelated, not identical. Urinary density measurement is useful to assess the disorders of water balance and to discriminate between prerenal azotemia and acute tubular necrosis. The water balance regulates the serum sodium concentration, therefore disorders are revealed by hypo- and hypernatremia. The disturbances are due to renal and nonrenal diseases, mainly liver, cardiovascular, intestinal, endocrine, and iatrogenic. Fluid management is an important topic of intensive care medicine. Moreover, the usefulness of specific gravity measurement of urine lies in interpreting other findings of urinalysis, both chemical and microscopical.
尿液的物理性质和化学成分变化很大，在很大程度上取决于所摄入食物的数量和种类。比重是尿液密度与水密度的比值，它取决于溶质颗粒的数量和重量以及样本的温度。溶质颗粒的重量主要由尿素（73%）、氯（5.4%）、钠（5.1%）、钾（2.4%）、磷酸盐（2.0%）、尿酸（1.7%）和硫酸盐（1.3%）构成。然而，尿渗透压仅取决于溶质颗粒的数量。肾脏产生最大浓缩尿和形成稀释尿可归结为两个基本要素：（1）产生并维持肾髓质溶质浓度高于血浆；（2）内髓质与集合管液之间的渗透平衡机制。肾髓质逆流系统、抗利尿激素的循环水平和口渴感之间的相互作用调节着水代谢。肾素、醛固酮、前列腺素和激肽也发挥着作用。浓缩和稀释能力的临床评估可通过相对简单的激发试验进行。然而，夜间禁水12小时后的尿比重应达到或超过1.025，因此晨尿是用于筛查目的的有用样本。许多保存程序会影响比重测量。尿液中固体（或水）的浓度可通过称重、比重计、折射计、表面张力、渗透压、试纸条或毛细管振荡来测量。这些测量方法相互关联，但并不相同。尿密度测量有助于评估水平衡紊乱，并区分肾前性氮质血症和急性肾小管坏死。水平衡调节血清钠浓度，因此低钠血症和高钠血症揭示了相关紊乱。这些紊乱是由肾脏和非肾脏疾病引起的，主要包括肝脏、心血管、肠道、内分泌和医源性疾病。液体管理是重症医学的一个重要课题。此外，尿液比重测量的用途还在于解释尿液分析的其他化学和显微镜检查结果。
