The glandular and nervous systems work intimately together to maintain balance among all of the body systems. Together, these systems regulate voluntary and involuntary actions within the body such as growth, metabolism, digestion, elimination, menstruation and sleep. These systems also serve as the body’s means of communication between cells.
The glandular system communicates using chemical messengers called hormones, which stimulate reactions that may last from a few hours to several days.
Glandular system concerns include hormone imbalances, weak adrenal glands, thyroid imbalance, diabetes, hypoglycemia and hyperglycemia.
EXAMINING THE GLANDULAR SYSTEM
While the glandular system communicates using hormones, the nervous system uses electrical impulses and chemicals called neurotransmitters to relay fast, short-lived messages. Some neurotransmitters, e.g., noradrenaline, also act as hormones and may be secreted in more than one place throughout the body. Both neurotransmitters and hormones bind to receptor cells to initiate responses.
The nervous and glandular systems are linked by the hypothalamus—the control center for the body’s emotional and physical responses. The hypothalamus, part of the brain stem, has been found to control hunger, thirst, blood pressure, pain, pleasure, water balance, temperature, sexual desire, hostility and many other emotions and responses. However, it is not the only area of the brain associated with emotions. Many structures, including the hypothalamus, make up the limbic system, an area of the brain that processes feelings and emotions.
The relationship between the hypothalamus and the pituitary gland provides the link between the nervous and glandular systems. The pituitary is a small gland located at the base of the brain. Many refer to it as the master gland because it regulates the release of hormones from most other glands, much like the maestro of a symphony directs all the musicians to play together to produce harmonious music. Divided into two lobes, the anterior and posterior, the pituitary is a double gland. Some hormones are produced in the hypothalamus and stored in the posterior lobe of the pituitary. The hypothalamus also manufactures hormones that stimu late the release of other hormones from the anterior pituitary. These are called releasing, or tropic hormones.
Many glands have a “feedback loop” that prevents the over-secretion of certain hormones. For instance, if the anterior pituitary secretes thyroid-stimulating hormone (TSH), the thyroid subsequently releases thyroxine and tri-iodothyronine, two hormones that trigger an increase in metabolism. These hormones inhibit the release of TSH so the thyroid will not be stimulated by the anterior pituitary.
The pituitary also produces hormones that it secretes directly into the bloodstream. One of these, growth hormone, is responsible for growth, development, protein synthesis, the breakdown of fats, and increases in blood sugar levels. Other hormones produced by the pituitary and secreted directly into the bloodstream include prolactin, which stimulates milk secretion in females; follicle-stimulating hormone (FSH), which stimulates the growth of ovarian follicles in females and seminiferous tubules in males; and luteinizing hormone (LH), which initiates ovulation during the menstrual cycle.
The pineal gland is a small, pea-sized gland located in the center of the brain. It is associated with biological responses to light and regulation of the body’s circadian rhythms such as sleep. Exposure to light inhibits the secretion of melatonin, a pineal hormone, and darkness stimulates it. Secretion of this natural antioxidant peaks around midnight and troughs in the morning. Melatonin production decreases with age.
The thymus gland is located behind the upper part of the breastbone and produces the hormone thymosin. Thymosin plays a role in immune response.
Just above the thymus, in the neck, is the thyroid gland. The thyroid secretes two different hormones: thyroxine and tri-iodothyronine (mentioned previously). Together, these two hormones regulate metabolism, growth and development. Calcitonin, another thyroid hormone, regulates blood levels of calcium, preventing excessive amounts from being released into the blood.
The parathyroid gland lies adjacent to the thyroid. Parathyroid hormone (PTH) acts as an antagonist to calcitonin to increase calcium levels in the blood by stimulating its release from the bones. At the same time, PTH causes the kidneys to excrete phosphate, which is also released from the breakdown of bone. The kidneys, intestines and bones all play a role in maintaining proper blood levels of calcium and phosphate.
The adrenal glands are located on top of the kidneys. Each gland is divided into two parts, the cortex and the medulla. The adrenal cortex (outer layer) produces “cortical” steroid hormones from cholesterol. Among the many cortical steroid hormones produced by the adrenal cortex are the sex hormones, DHEA, cortisone and aldosterone. The sex hormones signal development of secondary characteristics such as facial hair and a deeper voice. DHEA (dehydroepiandros terone), an intermediate hormone, aids in the production of sex hormones. Cortisone and its derivatives (called the glucocorticoids) help maintain normal blood sugar levels by stimulating formation of carbohydrates from protein and fat through secretion of enzymes from the liver. These glucocorticoids also decrease inflammation, help repair damaged tissue and act as the body’s buffer for stress.
Another cortical steroid hormone is aldosterone, which regulates mineral balance within the body by stimulating the absorption of sodium, chloride and water, and decreasing absorption of potassium by the kidneys. A careful balance of these minerals is vital for proper function of the sodium—potassium pump, which transports molecules into cells, and for conduction of nerve impulses.
Many plants produce hormones similar to the cortical steroid hormones; these are often called phytosteroids, and in the body they act similarly to the way the cortical steroid hormones function.
The medulla (or inner layer) of the adrenal gland produces adrenaline and noradrenaline. These hormones stimulate the sympathetic nervous system in fight-or-flight responses. When the body senses stress—whether it be emotional, physical or environmental—elevated blood pressure, increased blood sugar, accelerated heart rate and constricted blood vessels can result as the body redistributes its energy in order to deal with the stress. Simultaneously, functions that are less crucial to survival (like digestion and elimination) temporarily stop.
The ovaries are the female sex organs that produce estrogen and progesterone. Estrogen maintains and helps develop other female sex organs, stimulates secondary sexual characteristics and stimulates growth of the uterine lining during the first two weeks of the menstrual cycle. Progesterone has been called the pregnancy hormone because it prepares the uterus for the fertilized egg during the last two weeks of menstruation and helps maintain a healthy fetus during pregnancy.
The male hormone testosterone is produced in the testes, the main male reproductive organs. Testosterone stimulates development of sperm cells in men, maintains and develops male reproductive organs, and is responsible for male secondary sexual characteristics.
The pancreas is a digestive organ that also functions as a gland. It secretes glucagon, which converts glycogen into glucose, and insulin, which acts as an antagonist to glucagon to prevent excessive levels of blood glucose. Both of these conversions involve enzymes produced by the liver.
The digestive tract also contains hormones. Gastrin, secreted in the stomach, triggers secretion of gastric juices and enzymes in the pancreas. Stomach acidity causes the release of secretin in the small intestines, which in turn causes the pancreas to secrete its enzymes, and stops the secretion of acid. Cholecystokinin (CCK) stimulates the gallbladder to secrete bile.
Proper nutrition is crucial to the function of this complex system. Most glands need increased amounts of particular minerals to function as enzymes in their reactions. For example, the pancreas needs chromium, the prostate requires zinc, and the thyroid uses iodine. Many problems with the glandular system are associated with poor nutrition and low levels of these minerals in the diet.
Did You Know?
• The pituitary gland is about the size of a kidney bean.
• Giants and midgets are the result of too much or too little growth hormone.
• Diabetes mellitus, the most common glandular system disorder, affects between 10 and 20 million people in the U.S.