Hill L L
Pediatric Nephrology, Baylor College of Medicine, Houston, Texas.
Pediatr Clin North Am. 1990 Apr;37(2):241-56. doi: 10.1016/s0031-3955(16)36865-1.
Because the internal environment of the body is largely a fluid medium, the preservation of the volume and composition of the body fluids is absolutely vital to circulatory status and the extraordinarily complicated functions of the human body. The fluid compartments do not exist as fixed spaces with identical compositions but rather are in constant interchange with each other and have strikingly different compositions. Methods of movement of solutes and water include diffusion along electrochemical gradients, by hydrostatic pressure, osmotic forces, bulk flow, primary and secondary active transport, capillary blood flow, and oncotic pressure. Complex feedback control mechanisms exist to ensure homeostasis or equilibrium and include participation by the kidneys, lungs, gastrointestinal tract, the circulatory system, the endocrine system, and the CNS. The maintenance of extracellular volume is centered around the control of balance of the sodium salts. Multiple afferent (or sensing) and efferent (or effector) mechanisms exist to accomplish this homeostasis. The most important determinants of the osmolality or tonicity of the body fluids is the excretion or retention of water by the kidney, thirst mechanisms, and the intake of water. The serum sodium concentration is the laboratory test most often used clinically to assess tonicity. The pH of the body fluids and the major acid-base buffer systems are also carefully regulated. The lungs are responsible for the elimination of the carbon dioxide produced by cellular metabolism, and the kidneys excrete hydrogen ions and regulate the concentration of bicarbonate in the body fluids. Urinary net acid excretion, the hydrogen ions excreted as titratable acid and ammonium ions minus any bicarbonate, equals the acid added to the ECF from the diet and metabolism plus any fecal losses of alkali.
由于人体的内环境主要是一种流体介质，因此保持体液的容量和成分对于循环状态以及人体极其复杂的功能绝对至关重要。体液腔室并非以具有相同成分的固定空间存在，而是相互之间不断进行交换，且成分差异显著。溶质和水的移动方式包括沿电化学梯度扩散、通过静水压力、渗透力、总体流动、原发性和继发性主动转运、毛细血管血流以及胶体渗透压。存在复杂的反馈控制机制以确保体内平衡或平衡状态，这包括肾脏、肺、胃肠道、循环系统、内分泌系统和中枢神经系统的参与。细胞外液量的维持以钠盐平衡的控制为核心。存在多种传入（或传感）和传出（或效应）机制来实现这种体内平衡。体液渗透压或张力的最重要决定因素是肾脏对水的排泄或潴留、口渴机制以及水的摄入。血清钠浓度是临床上最常用于评估张力的实验室检查。体液的pH值和主要酸碱缓冲系统也受到严格调节。肺负责清除细胞代谢产生的二氧化碳，肾脏排泄氢离子并调节体液中碳酸氢盐的浓度。尿净酸排泄量，即作为可滴定酸和铵离子排泄的氢离子减去任何碳酸氢盐，等于从饮食和代谢中添加到细胞外液的酸加上任何粪便中的碱损失。
