Cardiovascular fitness ensures good circulation. The blood system is very sensitive to malnutrition. If microorganisms or unwanted substances penetrate the physical barrier, the blood and lymph system presents the second line of defense. Malnutrition lowers the bodies defense against invasive disease. The Hormonal and Nervous System 78 Blood also carries chemical messengers, called hormones , to tissue sites around the body.
Hormones act as messengers to initiate a response to a change in conditions.
Hormones regulate hunger and affect appetite. They perform numerous functions in the body including regulation of digestive juices, menstruation in women, the body's reaction to stress, and many others. Hormones are secreted and released by glands . Each gland in the body monitors certain conditions, and releases one or more hormones to regulate it. For example, the pancreas releases insulin when it detects high levels of blood sugar.