Glands: hormones and functions
|Pituitary, anterior gland||Growth hormone (GH, somatotropin)||All cells||Maintains protein production, releases fats and glucose|
|Thyroid-stimulating Hormone (TSH)||Thyroid gland||Stimulates production and secretion of thyroxine|
|Adrenocorticotropic Hormone (ACTH)||Adrenal cortex||Stimulates production and secretion of corticoids|
|Follicle-stimulating hormone (FSH)||Gonads||Plays a role in female monthly cycle, the production of female sex hormones and men gametes|
|Luteinizing hormone (LH)||Gonads||Plays a role in female monthly cycle, stimulates production of sex hormones|
|Prolactin||Mammary glands||Stimulates growth of gland and production of milk|
|Hypothalamus||Releasing hormones||Pituitary||Stimulates release of GH, TSH, LH, FSH, ACTH, and Prolactin|
|Inhibiting hormones||Pituitary||Inhibits release of GH and Prolactin|
|Oxytocin||Uterus, mammary glands||Stimulates muscle contracting during childbirth, milk release|
|Antidiuretic hormone (ADH, Vasopressin)||Kidneys||Controls water reabsorption|
|Thyroid||Thyroxine||All body cells||Stimulates metabolic rate|
|Parathyroid||Parathyroid hormone (PTH)||Bone||Controls level of calcium ions and potassium ions|
|Adrenal cortex||Aldosterone||Kidneys||Controls reabsorption of sodium, stimulates excretion of potassium|
|Hydrocortisone (cortisol)||Liver, various cells||Inhibits glucose uptake, aids healing, reduces inflammation|
|Androgen||Male gonads||Stimulates development of male secondary sex characteristics|
|Adrenal medulla||Epinephrine, Norepinephrine||Various cells||Controls stress reactions:increases heart and breathing rates, raises blood pressure and glucose level, inhibits digestion|
|Pancreas (islets of Langerhans)||Insulin, Glucagon||Liver muscle, Liver||Stimulates glucose uptake Triggers breakdown of glycogen into glucose|
|Female gonads (ovaries)||Progesterone, estrogen||Female sex organs||Controls female secondary sex characteristic development, Female sexual functions|
|Male gonads (testes)||Testosterone||Male sex organs||Controls development of male gametes and male secondary sex characteristics|
Endocrine System Regulation
The endocrine system and the nervous system together control other body systems. However, the endocrine system also controls itself.
The endocrine system controls itself through a process called negative feedback. This process is similar to the way a thermostat regulates a household furnace. When the temperature falls below the thermostat setting, the furnace switches on and begins producing heat. When the temperature reaches the thermostat setting, the furnace switches off. Similarly, the level of a hormone in the blood turns its own production off and on.
Negative feedback controls the thyroxine level in the blood. The hypothalamus plays the role of the thermostat. The hypothalamus has cells that detect the presence of thyroxine in the blood. When the thyroxine level is low, the hypothalamus secretes a releasing hormone that stimulates the pituitary to secrete TSH. TSH causes the thyroid to secrete thyroxine. When the thyroxine level returns to normal, the hypothalamus stops secreting the releasing hormone. As a result of this feedback mechanism, the pituitary stops secreting TSH, and the thyroid slows down secretion of thyroxine.
How Hormones Act
There are two types of hormones: steroids, which are fatlike organic compounds, and protein hormones. Sex hormones and corticoids are steroids. All others are protein hormones.
Steroids and protein hormones produce their effect differently. A steroid passes through the target cell membrane. It combines with a receptor molecule and moves into the cell nucleus. There it helps determine the manufacture of specific proteins. Protein hormones affect their target cells through a two-step procedure called a “two-messenger” system. The first messenger, the hormone, combines with the receptor on the target cell membrane. This combination activates an enzyme on the membrane’s inside wall. The enzyme helps change ATP into cyclic adenosine monophosphate, or cyclic AMP. Cyclic AMP triggers enzymes that bring about changes initiated by the original hormone. Thus, cyclic AMP is called the second messenger.
Substances within a single cell can be transported by movement of cytoplasm. Diffusion and active transport carry food, chemicals, gases, and waste products into and out of the cells of simple organisms. Large multicellular animals, however, require a more elaborate system for the transport of the nutrients, oxygen, and wastes that circulate throughout the body. Pickup and delivery within the human body are handled by an effective transport system: a pump, carriers, and thousands of kilometers of tubes that run throughout the body. The system is called the circulatory system. The circulatory system not only carries nutrients, oxygen, and bodily wastes but it also provides the body with a natural defense mechanism against disease.
Blood is the chief carrier of the body's transport system. It carries nutrients and oxygen to body cells and transports carbon dioxide and other waste products away from the cells. Blood also combats disease and helps maintain body temperature.
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