Module 2

1. Module 2

1.6. Page 4

Lesson 1

Module 2—The Endocrine System

Self-Check

SC 1. Now it is your turn to try your hand at drawing a feedback loop using all of the correct conventions and symbols described earlier. Given the following information about the hormone ADH (antidiuretic hormone) and its regulation of water volume in the body, summarize by drawing a negative feedback loop. After checking your answer, store your feedback loop in your course folder. Use it as an example when reviewing how to draw feedback loops. This is an important skill in Biology 30. Drawing diagrams, feedback loops, and flow charts is often part of the open-response essay question on the Diploma Exam. In Module 2 you will have an opportunity to read about, study, and draw many feedback loops.

Harjinder is losing water through perspiration while playing three-on-three basketball on a hot summer day.

Harjinder forgot to bring a water bottle to the basketball court and has no water to drink.
This situation results in a decrease in the plasma (watery part) of the blood, so 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 about the preceding three factors is fed back to the sensors in the hypothalamus.

Check your work.

Self-Check Answers

SC 1. Your negative feedback loop should resemble the one shown below. It can be drawn horizontally; vertically, like the example shown on page 441 in your textbook; or in a combination of horizontal and vertical arrangements, like the diagram below. If you experienced difficulty drawing the negative feedback loop, or if you do not understand the concept, consult with your teacher.

A negative feedback loop illustrates how low water volume in the cells and extracellular fluid including the blood is raised.

Positive Feedback

The photograph shows a young man in bed. He is bundled up in a blanket and is wearing a scarf around his neck. His face is flushed, and he has his hand to his forehead as if checking for a fever. When homeostasis cannot be achieved, a person becomes ill.

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.

positive feedback: a mechanism where deviation from the optimum state causes further deviation from the optimum state; usually leads to instability and is tolerated by the body only for a short time

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 called diabetes mellitus, Type 1. You will study diabetes in considerable detail in Lesson 5 of this module.

Using the same conventions and symbols that you learned for drawing negative feedback loops, a positive feedback loop for glucose regulation might look like the following:

This diagram shows a positive feedback loop that results in a reinforcement of a high blood glucose level.

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.

Self-Check

SC 2. Using the information below, practise 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 suggested answer, save your work in your course folder.

Randall is losing water through perspiration while playing three-on-three basketball on a hot summer day.
Randall forgot to bring a water bottle to the basketball court and has no water to drink.
This situation results in a decrease in the plasma (watery part) of the blood, so 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 tumour 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.

Check your work.

Self-Check Answers

SC 2. In this example, positive feedback reinforces the loss of water in Randall’s body, not only from perspiring and drinking less, but also from excreting increased amounts of urine. Your positive feedback loop should look similar to the one below.

This diagram shows a positive feedback loop. Positive feedback results in even more water loss in the urine in addition to the loss in sweating and the lack of water intake.