Introduction 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 theory (drive theories) proposed by Clark L. Hull
in 1943. The lack of some factor, such as water or food, causes the body to feel unpleasant. This is turn 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 Bradford 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.
Water Regulation Process When a human being’s water intake is lower than its level of water loss, two bodily processes are set in motion. First, the person becomes thirsty and drinks water (provided it is available). Second, the kidneys start to retain water by reabsorbing it and concentrating the urine. Thus, the kidneys can conserve the water that is already in the body. These processes are set in motion by the central nervous system (CNS).
The CNS responds to two primary internal bodily mechanisms. One is cellular dehydration thirst, and the other is hypovolemic thirst (a change in the volume of water in the body). To understand these mechanisms, one must realize that the body contains two main supplies of water. One supply, the intracellular fluid, is in the cells; the other supply consists of the extracellular fluid surrounding the cells and tissues and the fluid in the circulatory system. Water moves between these two areas by means of a process called osmosis, which causes it to move from an area of higher concentration to an area of lower concentration.
A person who is deprived of water will experience cellular dehydration thirst as a result of water loss caused by perspiration and excretion through the urine. This increases the salt concentration in the extracellular fluid, thereby lowering the water concentration. Thus, the cells lose their water to the surrounding extracellular fluid. The increasing salt concentration triggers specialized osmoreceptors located in the hypothalamic region of the brain. Two events occur: First, drinking is stimulated; second, antidiuretic hormone (ADH) is secreted from the pituitary gland in the brain. The ADH helps to promote the reabsorption of water into the kidneys.
The second kind of thirst, hypovolemic thirst, occurs when there is a decrease in the volume of the extracellular fluid as a result of bleeding, diarrhea, or vomiting. This produces a decrease in the salt concentration of the extracellular fluid, which lowers the blood pressure, which in turn stimulates the kidney cells to release a chemical. Eventually, the thirst receptors in the hypothalamus are stimulated; these cause the organism to consume water. In addition, ADH is secreted in this process, which promotes the conservation of water.
The regulation of water intake in humans is thus related to a number of factors and is quite complex. Though cellular dehydration thirst and hypovolemic thirst play a role, it appears that in humans, peripheral factors such as dry mouth play an even larger role. Humans can drink rapidly, replacing a twenty-four-hour water deficit in two to three minutes. This occurs even before the cellular fluid has replaced the water, which takes approximately eight to twelve minutes.
Motivational Factor Thirst is a strong motivational factor. The importance of replacing lost water is underscored by the fact that a person can survive for a month without food but for only several days without water. It appears that both thirst processes help to promote drinking. Researchers have estimated that 64 to 85 percent of the drinking following water loss is caused by cellular dehydration thirst. Hypovolemic thirst accounts for 5 to 27 percent of the drinking, and the remainder is caused by peripheral factors.
The two types of thirst are independent of each other. The receptors for both thirsts are located in the hypothalamic region of the brain, but they are at different locations. Research has shown that lesions in one region will have no effect on thirst regulation in the other region.
Although the motivation to drink in humans is under conscious control by peripheral factors, unconscious control does exert a large influence. A study of cellular dehydration thirst using goats showed that the injection of a saline solution that has a salt concentration of more than 0.9 percent salt (body fluids have a salt concentration of 0.9 percent salt) into the area in which the osmoreceptors are located will produce a drinking response within sixty seconds. Similar results have been found regarding hypovolemic thirst; injecting angiotensin II (a converted protein found in the blood) into the hypothalamus causes a drinking response. This occurs even in animals that are fully hydrated. These animals will consume in direct proportion to the amount of angiotensin II injected into the hypothalamus.
Diet can have a profound effect on water balance in humans. Eating salty foods will produce cellular dehydration thirst despite adequate fluid levels, because water will flow out of the cells into the extracellular fluid. In contrast, salt-free diets will produce hypovolemic thirst by causing water to flow into the cells. Other factors also cause thirst. As stated previously, diarrhea, vomiting, and blood loss will cause hypovolemic thirst as a result of the loss of extracellular fluid. Therefore, significant blood loss will cause a person to become thirsty.
Impact of Diseases Diseases can also have an impact on thirst. An interesting example of such a disease is diabetes. Diabetes is a condition in which the body cannot process blood glucose (a type of sugar) properly. Improper diet or medication can cause diabetic ketoacidosis, which causes the levels of glucose and ketone bodies (derivatives from fat) in the blood to rise. This creates a major shift in the water balance of the body. Water leaves the cells and enters the blood system, causing the volume of blood to increase. This extra fluid (along with potassium and sodium) is excreted from the body in the urine, which causes the body to suffer dehydration and triggers a tremendous thirst. Since fluid is lost from both cells and extracellular fluid, this causes both types of thirst. Excessive thirst is still a symptom of diabetes, but it has become rare as a result of education and improved treatment.
Impact of Exercise Thirst motivation also operates during exercise. In short-term exercise, thirst motivation does not come into play because the body usually maintains its temperature. During long-term exercise, however, water intake at intervals facilitates athletic performance by helping to maintain body temperature. The motivation to drink occurs as a result of sweating, which causes the salt concentration in the body to rise during exercise, thereby causing cellular dehydration thirst. Interestingly, voluntary thirst and peripheral factors do not motivate one to take in water during prolonged exercise in the heat until it is too late. Thus, coaches should insist that athletes drink water as they perform.
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