The nervous system and circulatory system are vital for life and they rely on each other. The nervous system helps regulate heart rate and blood pressure while the circulatory system provides oxygen and nutrient-rich blood to the nervous system, and carries away waste. When one of these systems becomes compromised the other is directly affected. Neurotransmitter chemicals found in the nervous system are among the most powerful molecules in the body and play a crucial role in the function of other systems. According to the principles of herbology, the plants used to treat the nervous system work with receptors in the hormonal, immunological and other control systems within the body. These plants are thought to influence these systems through the digestive system. A reference to this can be found in the book,  Principles and Practice of Phytotherapy, where it is stated that “The ability of plant constituents to engage cell receptors is also a particular feature of activity within the digestive system, which in clinical reality is probably the most accessible interface to the chemical control of the nervous system for herbal remedies.”