Glands: hormones and functions

Endocrine System Regulation

The endocrine system and the nervous system together control other body systems. However, the endocrine system also controls itself.

Feedback

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.

Circulatory system

Introduction

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.

The blood

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.