Alternative names for angiotensin
The different forms of angiotensin are denoted by Roman numerals, angiotensin I-IV. The hormones and the way they are activated are often referred to together as the renin-angiotensin system.
What is angiotensin?
The liver creates and releases a protein called angiotensinogen. This is then broken up by renin, an enzyme produced in the kidney, to form angiotensin I. This form of the hormone is not known to have any particular biological function in itself but, as it passes in the bloodstream through the lungs and kidneys, it is further metabolised to produce angiotensin II by the action of angiotensin-converting enzyme. Angiotensin II binds to receptors throughout the body and affects several parts of the body including:
Effects on the blood vessels (vascular), such as constricting the blood vessels and hence increasing blood pressure
Effects on the nerves (neurological), such as causing the sensation of thirst, desire for salt and encouraging release of anti-diuretic hormone from the pituitary gland and noradrenaline from sympathetic nerves
Effects on the adrenal glands, such as stimulating aldosterone production resulting in the body retaining sodium and losing potassium from the kidneys
Effects on the kidneys, such as increasing sodium retention and altering the way the kidney filters blood. This increases water reabsorption in the kidney to increase blood volume and blood pressure.
The overall effect of angiotensin II is to increase blood pressure, body water and sodium content.
How is angiotensin controlled?
An increase in renin production occurs if there is a decrease in blood pressure and sodium levels which is sensed by the kidneys. In addition, low blood pressure can stimulate the sympathetic nervous system which goes on to increase renin production. More renin means an increase in the conversion of angiotensinogen to angiotensin I. Angiotensin I has to be converted to the more active angiotensin II hormone by angiotensin-converting enzyme which limits angiotensin metabolism. The renin-angiotensin system is also activated by other hormones, including corticosteroids, oestrogen and thyroid hormones. On the other hand, natriuretic peptides (produced in the heart and central nervous system) can impede the renin-angiotensin system in order to increase sodium loss in the urine.
What happens if I have too much angiotensin?
Too much angiotensin II is a common problem resulting in excess fluid being retained by the body as well as raised blood pressure. This often occurs in heart failure where angiotensin is also thought to contribute to growth in the size of the heart. To combat these adverse effects, drugs such as angiotensin-converting enzyme inhibitors and angiotensin receptor blockers are used, although these can lead to excessive retention of potassium (hyperkalaemia).
What happens if I have too little angiotensin?
Control of plasma sodium and potassium concentrations, and the regulation of blood volume and pressure, are all hormonal mechanisms which are impaired by low angiotensin levels. Absence of angiotensin can be associated with retention of potassium, loss of sodium, decreased fluid retention (increased urine output) and low blood pressure.
Written: March 2011