Angiotensin is part of the renin-angiotensin-aldosterone system. It does not generally exist simply as angiotensin, but rather as angiotensinogen, before it is cleaved into angiotensin I by renin and then converted to angiotensin II by angiotensin converting enzyme which is located mostly in the lungs.

Angiotensinogen and angiotensin converting enzyme are normally present in large amounts in the system, but cannot be activated and used, respectively, until renin is present. It is a large molecule, produced in the liver, and only a small part of it is cleaved off to become angiotensin I.

Angiotensin II is present in large amounts when body salt content is low, and in small amounts when there is enough sodium in the system.

These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)