Homeostasis, negative feedback, and thyroxin

Tutoring biology, the concept of negative feedback is important to explain.  Negative feedback is used by all organisms to maintain their living state.  You also use it while driving your car.

More or less, the body needs all its processes to proceed at a constant, continuous rate.  Change is the enemy for biological entities.  However, as you interact with your surroundings – eating, exercising, and so on – change is imposed on your internal environment.  Negative feedback means that when a change happens, your body responds to negate that change – i.e., to bring itself back to normal.  Once back to normal, your body stops its negative response.  It won’t respond again until another change brings it out of normal range.

Consider driving a car – specifically, steering.  Imagine the road is straight, but your car starts drifting to the right.  You correct by steering left, to bring the car back on course.  In so doing, you are applying negative feedback.  The car is going too far right, so you oppose that change of course by steering left.  Once the car is back on course, you stop correcting:  you let the wheel slide back to center.  You won’t steer again until the car begins to drift off course once more.  That process of opposite, corrective response is negative feedback.  You can also call it a feedback loop, since you base your reaction on what is already happening.

Biology has many examples of negative feedback, but we’ll look at the one involving the hormone thyroxin:

The hypothalamus – in the brain – monitors the body’s metabolism.  If the metabolism gets too low, the hypothalamus stimulates the anterior pituitary.  In response, the anterior pituitary releases a hormone that causes the thyroid to release thyroxin.  Thyroxin increases the body’s metabolism.

Once the metabolism reaches an acceptable level, the hypothalamus stops stimulating the anterior pituitary.  In turn, the anterior pituitary ceases to release the thyroid-stimulating-hormone.  Therefore, the thyroid stops releasing thyroxin.  The body’s metabolism remains constant – until another external change depresses it again.

Therefore, the connection among the hypothalamus, anterior pituitary, and thyroid gland constitutes a negative feedback system that maintains the body’s metabolism at a constant rate.  This negative feedback system promotes homeostasis.  Homeostasis means the maintenance of a constant internal environment.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.