5.1.2 The Principles of Homeostasis

Question 1

What are the principles of homeostasis?

Answer 1

The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance (i.e. to keep certain physiological factors, such as internal temperature or blood glucose concentration, within certain limits)
Negative feedback control loops involve:
A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
An effector (muscles and glands) – to carry out a response
The outcome of a negative feedback loop:
The factor/stimulus is continuously monitored
If there is an increase in the factor, the body responds to make the factor decrease
If there is a decrease in the factor, the body responds to make the factor increase

Question 2

How is negative feedback controlled?

Answer 2

Negative feedback loops help maintain a normal range or balance within an organism
They reduce the initial effect of the stimulus
Receptors detect any deviations from the normal range (stimuli) which results in a corrective mechanism to return the factor back to its normal range
In a negative feedback loop there are usually two corrective mechanisms:
One for when the factor becomes too low
One for when the factor becomes too high
The corrective mechanisms may involve the nervous system or the endocrine system
The magnitude of the correction required to bring a factor back within its normal range is monitored and regulated by negative feedback
As the factor gets closer to its normal value the level of correction reduces

Question 3

What is positive feedback?

Answer 3

Positive feedback
In positive feedback loops, the original stimulus produces a response that causes the factor to deviate even more from the normal range
They enhance the effect of the original stimulus
Positive feedback loop in bone repair
The repair of broken bones is carried out via a positive feedback loop involving special cells called osteoblasts and osteoclasts
The osteoblasts secrete a hormone called osteocalcin
Osteocalcin is a protein
They secrete the osteocalcin in an inactive form
The osteoclasts secrete acid which lowers the pH and the acidic conditions cause the inactive form of the protein osteocalcin to change into the active form of osteocalcin
The low pH alters the hydrogen and ionic bonds in the protein which changes the tertiary structure
The active form of osteocalcin binds to a receptor on beta (β) cells in the pancreas which stimulates them to release insulin
Osteoblast cells possess insulin receptors which when stimulated causes them to release more inactive osteocalcin
The osteoblast cells enhance the effect of the original stimulus (insulin) - positive feedback