The range of human motivation is quite broad in controlling behaviors. Motivation can be defined as a condition that energizes and directs behavior in a particular manner. Different aspects of motivation can be attributed to instinctive behavior patterns, the need to reduce drives, or learned experiences.
Thirst is one of many biologically based motivational factors; among other such factors are those that involve food, air, sleep, temperature regulation, and pain avoidance. Biologically based motivational factors help humans and other organisms to maintain a balanced internal environment. This is the process of homeostasis. Deviations from the norm, such as hunger, excessive water loss, and pain, will cause an organism to seek out whatever is lacking.
Biologically based motivational factors, such as thirst, have been explained by the drive-reduction hypothesis proposed by Clark Hull in 1943. The lack of some factor, such as water or food, causes the body to feel unpleasant. This motivates one to reduce this feeling of unpleasantness, thus reducing the drive. Thirst is considered what is called a primary drive. Primary drives, which are related to biologically based needs such as hunger, thirst, and sleepiness, energize and motivate one to fulfill these biological needs, thus helping the body to maintain homeostasis. Secondary drives fulfill no biological need.
One may wonder what it is that makes one thirsty and how one knows when one has had enough to drink. Seventy-five percent of a human's weight is water. The maintenance of water balance is an ongoing process. In an average day, a person will lose approximately 2.5 liters of water; 60 percent of the water loss occurs through urination, 20 percent is lost through perspiration, and the remainder is lost through defecation and exhalation from the lungs. These 2.5 liters of water must be replaced.
What is the stimulus that motivates one to drink when one is thirsty? The simplest hypothesis, which was proposed by Walter Cannon in 1934, is the dry mouth hypothesis. According to Cannon, it is a dry mouth that causes one to drink, not the need for water. This hypothesis has not held up under scrutiny. Research has shown that neither the removal of the salivary glands nor the presence of excess salivation in dogs disrupts the animals' regulation of water intake. Studies have indicated that the amount of water consumed is somehow measured and related to the organism's water deficit. This occurs even before the water has been replaced in the person's tissues and cells. Thus, dry mouth is a symptom of the need for water.
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