肾素-血管紧张素系统（英语：renin-angiotensin system，简称为RAS）或肾素-血管紧张素-醛固酮系统（renin-angiotensin-aldosterone system, RAAS）是一个激素系统。当大量失血或血压下降时，这个系统会被启动，用以协助调节体内的长期血压与细胞外液量（体液平衡）。

当血压降低时，肾脏分泌肾素。肾素催化血管紧张素原水解产生血管紧张素I。血管紧张素I基本没有生物学活性，而是经血管紧张素转化酶（Angiotensin Converting Enzyme, ACE）剪切C-末端两个氨基酸残基而形成血管紧张素II。血管紧张素II具有高效的收缩血管作用，从而使血压升高；血管紧张素II也能刺激肾上腺皮质分泌醛固酮。醛固酮能促进肾脏对水和钠离子的重吸收，继而增加体液容量，升高血压。

The renin–angiotensin system (RAS) or the renin–angiotensin–aldosterone system (RAAS) is a hormone system that regulates blood pressure and fluid balance.

When renal blood flow is reduced, juxtaglomerular cells in the kidneys convert the prorenin already present in the blood into renin and secrete it directly into the circulation. Plasma renin then carries out the conversion of angiotensinogen, released by the liver, to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin-converting enzyme (ACE) found in the lungs. Angiotensin II is a potent vaso-active peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood, while at the same time causing the excretion of potassium (to maintain electrochemical balance). This increases the volume of extracellular fluid in the body, which also increases blood pressure.