Hormones—The Body’s Marvelous Messengers
YOU are crossing a street. “Look out!” somebody yells. You turn your head and see that a truck has run a red light and is thundering toward you at high speed.
Your body instantly equips you for emergency action. Your brain flashes an urgent message to your adrenal glands, which respond by pouring adrenaline and noradrenaline into your bloodstream. These hormones shut down the blood supply to parts of the body whose services are not immediately needed for escape, and they rush it to reinforce your brain, heart, and muscles.
Adrenaline and noradrenaline force your heart to beat hard and fast. They widen the airways of your lungs; your breathing quickens. They boost your blood sugar to supply maximum energy. In a twinkling, hormones have helped to equip you to perform feats of strength and endurance far beyond your normal ability.
The truck roars nearer, the ground shakes. There’s not a second to spare! You make a mighty leap to the safety of the sidewalk. You gasp for breath, your heart pounds, your stomach churns, your hands tremble—but you are alive!
In such situations hormones help save lives. But they do far more than that. They help us grow and develop into healthy men and women. They make possible our sexuality and reproduction. If we are cold or hot or hungry or thirsty or bleeding or sick, they assist us. And they are on the job 24 hours a day!
But how is all this work organized in our bodies? To help us understand that, let us consider what hormones are and how they function.
Hormones are chemical substances that are manufactured by our endocrine glands. The word “endocrine,” which means “secreting directly into,” describes these glands well, since they secrete hormones directly into the bloodstream. As the heart pumps the blood through the body, the hormones speed to various destinations, where they accomplish their work.
In order for the hormones to perform their functions, good communication is needed among our many body parts. All of us have complex communications systems that transmit information to keep us alive and operating smoothly—the endocrine system and the nervous system.
To illustrate how the two work together, let us take a look at Venice, Italy, a city famous for its network of canals. In Venice, people use the telephone system to send messages to other parts of the city. Similarly, the body sends its messages by means of the nervous system, a high-speed communications network that uses electrochemical signals. Like a telephone call, a nerve transmission is delivered virtually instantaneously.
Of course, a message could also be delivered by gondola, a long boat that travels through the maze of city canals. In the body, chemical messengers (hormones) travel through the bloodstream or other body fluids.
If we liken the bloodstream to the Venetian canals, then the hormones are like fleets of gondolas carrying messages hither and thither from many sources to many destinations. These hormones travel to muscles, organs, or glands far from their point of origin. Once they reach their destination, they set in motion a series of complex chemical reactions to accomplish their purpose.
But how is all this activity directed and coordinated? To answer that question, let us look at the headquarters of the endocrine system and see what work is done there.
Pituitary—The Master Gland
The overseer of the endocrine system is the pituitary, a small, reddish-gray organ that is attached to the brain by a slender stalk and that lies in a bony pocket just behind and above the nose.
The pituitary is not impressive to look at. It is only the size of a pea, and it weighs a mere 0.02 ounce [0.6 gm]. But even though the pituitary is small, its responsibility is immense. It has been called the master gland, the conductor of the endocrine orchestra. It is like a business executive whose office is a flurry of activity, with messages coming from and going to many departments.
Some jobs the pituitary delegates to other endocrine glands. For example, the pituitary launches a hormonal message into the bloodstream ordering the thyroid gland to produce and release three other hormones. These control metabolism, body heat, and bone maintenance. The pituitary likewise commands the sex glands to produce the hormones that will bring about the physical changes of puberty. The master gland can also instruct the adrenals to manufacture hormones that maintain blood pressure and salt balance in the body.
At times, though, the pituitary cares for matters directly, sending out hormonal messages that influence the growth of our bones and muscles. Its hormones even control how tall we will be.
The pituitary further plays a big role in delivering babies. To assist a woman in labor, the pituitary sends out oxytocin, a hormone that stimulates contractions of the womb. When the baby’s head reaches the birth canal, the brain sends a message to the pituitary requesting an extra supply of oxytocin to help with the final phase of delivery. All along, hormones from the pituitary have been stimulating the production of milk in the mother’s breasts. When baby is born, mother is equipped to feed it.
The Master Gland’s Master
While the pituitary is the overseer of other glands, it has its own overseer—the hypothalamus. This is a cluster of nerve cells no larger than the tip of your thumb. It is located at the base of the brain and is connected to the pituitary. Its job is not only to supervise the work of the endocrine system but also to coordinate the work of the autonomic nervous system.
Part of its work is to test the makeup and temperature of the blood. More blood gushes through the hypothalamus than any other part of the brain. Into this blood flow, the hypothalamus pokes wrinkled fingerlike sensors, much as a bather uses his finger to test the temperature of the water in his tub. If the blood is too cool, the hypothalamus sends instructions (via the pituitary and the thyroid) for more thyroxine, a hormone that boosts metabolism to produce heat to warm the blood.
Since the hypothalamus does its work automatically, we are usually unaware of its labors. Yet, it has a day-to-day effect on our lives. Are you hungry? Your hypothalamus has detected too little glucose in your blood, so it is telling you to eat. Are you thirsty? Your hypothalamus has decided that the salt level in your blood is a little too high. “Drink some water,” it tells you.
The hypothalamus also monitors levels of calcium in the blood. Without calcium our brain, muscles, and nerves would not work properly. When the level of blood calcium is too low, the hypothalamus withdraws calcium from the bones, much as a person withdraws money from a bank. How is the calcium withdrawal made? The hypothalamus sends a hormonal message to the pituitary. The pituitary launches its own command to the parathyroids, located in the neck. The parathyroids, in turn, secrete parathormone, which goes to the bones to request calcium for the bloodstream. Once the hypothalamus sees that the calcium level is correct, it cancels orders for further withdrawals.
But what if the hypothalamus learns that there is too much calcium in the blood? Once again messengers are sent to the ‘bone bank,’ but instead of making a withdrawal, they make a deposit. This is the procedure: The hypothalamus sends a message to its chief executive, the pituitary. The pituitary now issues a command to the thyroid. The thyroid, in turn, sends out the hormone calcitonin, which acts to transfer excess calcium from the blood to the bones.
What a masterpiece of organization! The hypothalamus controls the pituitary, the pituitary directs the glands, and the glands regulate the body. And all of this is done by more than 30 different kinds of hormones that flow silently through our body to care for our most basic physical needs. Yet, despite the complexity of all of this, the endocrine system operates with stunning efficiency.
The Bible states that “God has set the members in the body, each one of them, just as he pleased.” How fitting the psalmist’s expression to God: “I shall laud you because in a fear-inspiring way I am wonderfully made. Your works are wonderful, as my soul is very well aware”!—1 Corinthians 12:18; Psalm 139:14.
[Box/Diagrams on page 18]
ENDOCRINE GLANDS SECRETE HORMONES INTO THE BLOODSTREAM
THE HORMONES TRAVEL TO OTHER PARTS OF THE BODY TO ACCOMPLISH VARIOUS TASKS
This small gland, situated within the base of the brain, secretes melatonin, which is thought to affect wakefulness and various biorhythms in the body. The exact function of melatonin is not yet known.
Gonads, or Sex Glands
The two ovaries (in females) are found in the pelvic girdle, on each side of the womb. The hormones estrogen and progesterone are produced here. These control the menstrual cycle and affect the development of adult female physical characteristics.
The testes (in males), located in the scrotum, produce hormones that control the development of the body features of an adult male at puberty and stimulate sperm production.
This pea-sized organ is attached to the brain by a slender stalk and lies just above and behind the nose. The pituitary is supervisor of the other glands, sending out chemical messages to the thyroid, adrenal, and sex glands, as well as to other glands having endocrine functions. It is the main controller of height and influences the growth of bones and muscles. It also stimulates the production of milk by the breasts of a nursing mother.
Thyroid and Parathyroid Glands
These glands are situated in the neck. The parathyroid secretes hormones that regulate calcium levels to maintain healthy bones. The thyroid produces other hormones that control the speed at which oxygen and food are used to produce energy.
The Adrenal Glands
Located just above each kidney, the two adrenal glands produce adrenaline and noradrenaline, which equip the body for fight or flight in emergencies. Additional hormones manufactured here affect the metabolism of carbohydrates and proteins, regulate water handled by the kidneys, and activate the body’s food stores when there is little to eat.
Located underneath the stomach, this gland turns out glucagon and insulin, which regulate the level of sugar in the blood.
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