All of the following statements about histamine are true except:
Stimulates gastric acid secretion Causes constriction of bronchial smooth muscles
Increases blood pressure
Causes dilatation of capillaries
The correct answer and explanation is:
The correct answer is: Increases blood pressure.
Histamine does not directly increase blood pressure. In fact, histamine is more likely to cause a decrease in blood pressure due to its effect on blood vessels.
Histamine is a biogenic amine that plays a crucial role in several physiological functions. It is primarily stored in mast cells and basophils, and its release is triggered in response to injury, infection, or allergic reactions. Here’s a breakdown of its effects:
- Stimulates gastric acid secretion: Histamine binds to H2 receptors in the stomach lining, leading to the production of gastric acid. This is one of its well-known effects, important in digestive processes.
- Causes constriction of bronchial smooth muscles: Histamine can trigger bronchoconstriction when it binds to H1 receptors in the lungs. This is especially significant in allergic responses, such as in asthma, where histamine release causes narrowing of the airways.
- Causes dilatation of capillaries: Histamine causes vasodilation, particularly in the small blood vessels (capillaries). This increases blood flow to the affected area, which can lead to the redness and swelling characteristic of inflammation. The dilation of capillaries is part of the body’s response to injury or infection.
However, histamine does not increase blood pressure. In fact, histamine-induced vasodilation can cause a decrease in blood pressure, which is why it is associated with symptoms like hypotension in severe allergic reactions, such as anaphylaxis.
In summary, histamine’s primary effects involve altering the tone of smooth muscles in the lungs, digestive system, and blood vessels. While it plays a key role in immune responses and allergic reactions, its direct effect is to lower blood pressure, not raise it.