The Process of Thirst: A Systems Theory Overview
When people think of thirst, we often think of it in a very mechanistic fashion. I.e., when our body needs fluids, something sends this message to our brain and we become thirsty, thus causing us to want to drink something.
This, however, is very simplistic. In reality, a great many factors interact to produce the state which we call thirst. The chart above demonsrates this, showing how cellular dehydration and low blood volume (hypovolemia) both contribute to the thirst effect, in an interactive and complex system.
The pituitary gland, for example, can cause the kidneys to decrease urine production, which helps alleviate both dehydration and hypovolemia. However, this can be caused by either dehydration, hypovolemia, or some combination of the two. It furthermore can cause two other effects to take place. It can directly cause thirst by activating hypothalamus receptors. It can also indirectly cause it through the reabsorption of salt and water in the kidneys.
This is what's known as a complex interaction. The various systems in the body are interacting in a fashion which can not be viewed in simple mechanistic terms. It needs to be seen through a systems approach to be understood fully.
Strangely enough, you also have interactive effects which seem as though they'd fight against one another. Low blood volume (and low blood pressure) causes the blood vessels to contract, creating higher blood pressure, and that effect directly brings thirst about. So low blood pressure creates high blood pressure, creating a thirst effect.
