Module 2 The Endocrine System

1.2.1 page 4

Try This

TR 1.

Now it is your turn to try your hand at drawing a feedback loop using all of the conventions and symbols described above. Given the following information on the hormone ADH (antidiuretic hormone) and its regulation of water volume in the body, summarize by drawing a negative feedback loop. After checking it your diagram, file it in your course folder for future reference when studying.

• a person is losing water by sweating profusely while playing 3-on-3 basketball on a sunny hot summer day

• this basketball player forgot to bring a water bottle and has no water to drink

• this situation results in a decrease in the plasma (watery part) of the blood so that the solute (dissolved substances) concentration of the blood increases. As a result, both the blood volume and the blood pressure decrease

• sensors (or receptors) in the hypothalamus called osmoreceptors sense the increased solute concentration, decreased blood volume, and decreased blood pressure

• the hypothalamus sends messages to the posterior pituitary gland to increase the release of the hormone ADH

• an increased level of ADH causes the kidneys (kidney tubules) to retain (reabsorb) more water and release less in the urine

• reabsorbed water in the blood increases blood volume, increases blood pressure, and decreases solute concentration

• information on the above three factors is fed back to the sensors in the hypothalamus

Positive Feedback:

Sometimes the deviation from the set point or normal value is not corrected. Instead it leads to a further deviation. The result is a “runaway” situation in which a change triggers more change in the same direction. This is called positive feedback. At first sight, positive feedback would appear to be damaging and even destructive. It may cause illness or the onset of a disorder. Returning to the glucose example, if the pancreas were not able to decrease blood glucose after a meal because it could not produce insulin, the glucose level would remain high. You would feel hungry and perhaps eat a candy bar. As the glucose was absorbed from the stomach and intestines, even higher blood glucose levels would result. You might know this as the disorder diabetes (mellitus). Emily has this disorder and you will study it in considerable detail in Lesson 5 of this module. Using all of the conventions and symbols that you learned for drawing negative feedback loops, a positive feedback loop for glucose regulation might look like the following:

However in some cases positive feedback is a good thing. In Unit B you will study the process of natural childbirth which is dependent on positive feedback involving the hormone oxytocin.

Try This

TR 2.

Using the information below, practice drawing a positive feedback loop to show how water regulation functions when the hormone ADH cannot be secreted. Use the symbols and conventions that you learned when drawing a negative feedback loop. After comparing your work with the key, file your work in your course folder.

• a person is losing water by sweating profusely while playing 3-on-3 basketball on a sunny hot summer day

• this basketball player forgot to bring a water bottle and has no water to drink

• this situation results in a decrease in the plasma (watery part) of the blood so that the solute (dissolved substances) concentration of the blood increases and both the blood volume and the blood pressure decrease

• sensors (or receptors) in the hypothalamus called osmoreceptors sense the increased solute concentration, decreased blood volume, and decreased blood pressure

• the hypothalamus sends messages to the posterior pituitary gland to increase the release of the hormone ADH

• a tiny tumor in the posterior pituitary prevents the release of ADH

• the kidney cannot reabsorb more water so it is released as an increased volume of urine

• this information is fed back to the sensors in the hypothalamus