Angiotensin is a peptide hormone produced in the human body. It is an oligopeptide and is converted from Angiotensinogen. Angiotensin is a significant hormone that plays a vital role in the Renin-angiotensin-aldosterone system (RAAS). Angiotensin is produced in the liver of the human body. Renin converts angiotensinogen to angiotensin. Renin is also a hormone produced in the kidney. Different types of angiotensin hormones perform different functions. The types of angiotensin hormones are Angiotensin I, Angiotensin II, Angiotensin III, etc. All the types of angiotensin play different roles in the human body.

Angiotensin Classification

Angiotensin is classified into four types.

• Angiotensin I- It is also known as angiotensin as it is converted from Angiotensinogen in the presence of Renin. Renin cleaves the peptide bond present between the amino acid Leucine and Valine. This leads to the formation of the decapeptide Angiotensin I. Renin is also a hormone produced in the kidney. The biological activity of Angiotensin I is not known, but it acts as a precursor of Angiotensin II.
• Angiotensin II has formed from Angiotensin I as the enzyme Angiotensinogen Converting Enzyme(ACE) cleaves angiotensin I to form angiotensin II. ACE is found mainly in the lungs but also in small amounts of kidney epithelial cells, endothelial cells, and the brain. It is known to act on the central nervous system (CNS) to increase the production of Vasopressin. It also causes vasoconstriction by acting on the venous and arterial smooth muscles. It is also known to increase the amount of aldosterone.
• Angiotensin III- It is synthesised from angiotensin II due to the action of angiotensin cases which are found in the red blood cells. Angiotensin Ashes is an aminopeptidase A that removes Asp residue from angiotensin II to form angiotensin III. Angiotensin III shows 40% vasoconstriction activity and 100% Aldosterone forming activity compared to other types. It activates the AT2 receptor to cause natriuresis only when the AT1 receptor is blocked.
• Angiotensin IV- This is a hexapeptide containing six peptide bonds. Its activity is less compared to angiotensin III. Its main activity is observed in the central nervous system. The receptor for Angiotensin IV is the AT4 receptor and is an aminopeptidase regulated by insulin. It is also observed that the AT4 receptor interacts with the HGF system through the c-met receptor. An analogue of Angiotensin IV has been developed that has the ability to penetrate through the blood-brain barrier.

Renin-Angiotensin- Aldosterone system

It is also known as the RAAS system, which involves different hormones to regulate blood pressure and sodium ion concentration in the plasma. Here Renin acts on Angiotensinogen to convert it to angiotensin I. Further, Angiotensin-Converting Enzyme (ACE) cleaves Angiotensin I to form Angiotensin II. This system builds resistance vessels which increases blood pressure. It increases the flow of sodium ions in the renal tubules and increases the body’s ability to reabsorb more water. The release of Aldosterone causes further release of Vasopressin from the posterior pituitary; this leads to increased reabsorption of water in the kidney. 

Importance of angiotensin

• It upregulates the lipogenesis of adipose tissue and downregulates its lipolysis.
• They act as potent vasoconstrictors by narrowing arteries and veins and increasing blood pressure. This is done through the ATI receptor, which is a GPCR. The Gq protein here activates Phospholipase C, followed by an increase in intracellular calcium.
• Angiotensin II is known to increase thirst, decrease the response of baroreceptor reflex action, increase salt consumption, act on the posterior pituitary to increase ADH secretion, and act on the anterior pituitary to increase ACTH secretion. 
• Angiotensin II can also increase the release of norepinephrine.
• Angiotensin II can increase the release of Aldosterone by acting on the adrenal cortex of the adrenal gland. Aldosterone causes the loss of potassium ions and prevents the loss of sodium ions.
• During the luteal phase of the menstrual cycle, the aldosterone levels are increased due to an increase in the angiotensin II hormone. 

Conclusion

Angiotensin is an oligopeptide hormone produced in the human body. Angiotensin is produced in the liver of the human body. Renin converts angiotensinogen to angiotensin. Renin is a hormone produced in the kidney. Different types of angiotensin hormones perform different functions. The types of angiotensin hormones are Angiotensin I, Angiotensin II, Angiotensin III, etc. Angiotensin I is formed from Angiotensinogen with the help of the enzyme renin. Angiotensinogen is an alpha globulin formed in the liver. Angiotensinogen concentration can be increased by different hormones like corticosteroids, oestrogen, etc. Angiotensin I is cleaved to form Angiotensin II with the help of an enzyme Angiotensin-converting Enzyme. Angiotensin is a significant hormone that plays a vital role in the Renin-angiotensin-aldosterone system (RAAS). It plays a vital role in maintaining blood volume and blood pressure.