Angiotensin I is a peptide hormone that plays a crucial role in the body's regulation of blood pressure and fluid balance. It is produced from angiotensinogen, a protein synthesized in the liver, through the action of the enzyme renin. Angiotensin I itself is relatively inactive but serves as a precursor to a more potent hormone. When angiotensin I is converted into angiotensin II by the enzyme angiotensin-converting enzyme (ACE), it exerts significant effects on the cardiovascular system. Angiotensin II causes blood vessels to constrict, increasing blood pressure, and stimulates the release of aldosterone, which promotes sodium and water retention in the kidneys.