What is the definition of "Kinin"?

The term "Kinin" refers specifically to a group of polypeptides that play a crucial role in various physiological processes. These substances are primarily involved in the regulation of blood pressure, inflammation, and pain sensation. Kinins are known to influence the contraction of smooth muscle, which can lead to various effects on the cardiovascular and respiratory systems. Additionally, they are associated with the pain response, as they can trigger pain receptors when released during inflammatory responses.

By understanding that kinins are primarily polypeptides that have significant effects on smooth muscle function and pain modulation, it becomes clear why this definition is the correct answer. The other choices do not accurately reflect the characteristics or functions of kinins, making the choice of the correct definition important for understanding their role in the body.