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Endocrine System Physiology
O B J E C T I V E S 1. To define the following terms: metabolism, hormone replacement therapy, type 1 diabetes, type 2 diabetes, and glucose standard curve. 2. To explain the role of thyroxine in maintaining an animal’s metabolic rate. 3. To explain the effects of thyroid-stimulating hormone on an animal’s metabolic rate. 4. To understand how estrogen affects bone density. 5. To explain how hormone replacement therapy works. 6. To explain how fasting plasma glucose is used to diagnose diabetes. 7. To understand how levels of cortisol and ACTH can be used to diagnose endocrine diseases.


he endocrine system exerts many complex and interrelated effects on the body as a whole, as well as on specific tissues and organs. Studying the effects of hormones on the body is difficult to do in a wet lab because experiments often take days, weeks, or even months to complete and are expensive. In addition, live animals may need to be sacrificed, and technically difficult surgical procedures are sometimes necessary. This computer simulation allows you to study the effects of given hormones on the body by using “virtual” animals rather than live ones. You can carry out delicate surgical techniques with the click of a button and complete experiments in a fraction of the time that it would take in an actual wet lab environment.

Hormones and Metabolism
Metabolism is the broad term used for all biochemical reactions occurring in the body. Metabolism involves catabolism, a process by which complex materials are broken down into simpler substances, usually with the aid of enzymes found in the body cells. Metabolism also involves anabolism, in which the smaller materials are built up by enzymes into larger, more complex molecules. When larger molecules are made, energy is stored in the various bonds formed. When bonds are broken in catabolism, energy that was stored in the bonds is released for use by the cell. Some of the energy liberated may go into the formation of ATP, the energyrich material used by the body to run itself. However, not all of the energy liberated goes into this pathway; some is given off as body heat. Humans are homeothermic animals, meaning they have a fixed body temperature. Maintaining this temperature is important to maintaining the metabolic pathways found in the body. The most important hormone in maintaining metabolism and body heat is thyroxine, the hormone of the thyroid gland, which is found in the neck. The thyroid gland secretes thyroxine, but the production of thyroxine is really controlled by the pituitary gland, which secretes thyroid-stimulating hormone (TSH). TSH is carried by the blood to the thyroid gland (its target tissue) and causes the thyroid to produce more thyroxine. So in an indirect way, an animal’s metabolic rate is the result of pituitary hormones. In the following experiments, you will investigate the effects of thyroxine and TSH on an animal’s metabolic rate (see Figure 4.1b). To begin, select Exercise 4: Endocrine System Physiology from the drop-down menu and click GO. Before you perform the activities, watch the BMR Measurement video to see an



Exercise 4


Hypothalamus TRH


Pituitary gland (hypophysis)

Thyroid gland

T3, T4 (b)

F I G U R E 4 . 1 Metabolism and the thyroid gland. (a) Opening screen of the Metabolism experiment. (b) The regulation of thyroid secretion. indicates stimulation of release, indicates inhibition of release, T3 triiodothyronine, T4 thyroxine, TRH = thyrotropin-releasing hormone, TSH thyroid-stimulating hormone.

Endocrine System Physiology


experiment in which basal metabolic rate is measured. Then click Metabolism. The opening screen will appear in a few seconds (see Figure 4.1a). Select Balloons On/Off from the Help menu for help identifying the equipment on-screen (you will see labels appear as you roll the mouse over each piece...
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