Which physiological process is NOT regulated by hormones?

The regulation of physiological processes by hormones typically involves endocrine signals that are responsible for maintaining homeostasis and promoting growth, metabolism, and other critical bodily functions. Among the listed options, muscle contraction is primarily regulated by the nervous system through neural impulses, and respiratory rate is predominantly controlled by the autonomic nervous system, although it can be modulated by certain hormonal influences. However, hormonal regulation is less direct in the case of respiratory rate compared to metabolism, which involves hormones such as insulin and glucagon to manage energy balance, and growth, which involves growth hormone and various growth factors.

Respiratory rate is predominantly influenced by factors like carbon dioxide levels, oxygen levels, and pH of the blood rather than hormones. Therefore, while hormones can play a supportive role in modulating respiratory functions (such as during stress or exercise), they are not the primary regulators of the respiratory rate. This is why the process that is not primarily regulated by hormones is accurately identified as the respiratory rate.