1. Explain the characteristics of a regulatory mechanism.
2. Describe the fluid compartments of the body.
3. Explain the control of osmometric thirst and volumetric thirst and the role of angiotensin.
4. Describe the neural control of thirst.
5. Describe the characteristics of the two nutrient reservoirs and the absorptive and fasting phases of metabolism.
6. Discuss social and environmental factors that begin a meal.
7. Discuss the head, gastric, and intestinal factors responsible for stopping a meal.
8. Discuss research on the role of the brain stem and hypothalamus in hunger and satiety.
9. Discuss physiological factors that may contribute to obesity.
10. Discuss physiological factors that may contribute to anorexia nervosa and bulimia nervosa.

Claude Bernard (1813-1878) said, "The constancy of the internal milieu is a necessary condition for a free life."

• Homeostasis (home ee oh stay sis)—The process by which the body’s substances and characteristics (such as temperature and glucose levels) are maintained at their optimal level.
• Ingestive behavior (in jess tiv)—Eating or drinking.

A physiological regulatory mechanism is one that maintains the constancy of some internal characteristic of the organism in the face of external variability. A regulatory mechanism contains four essential features: the system variable (the characteristic to be regulated), a set point (the optimal value of the system variable), a detector that monitors the value of the system variable, and a correctional mechanism that restores the system variable to the set point.

• System variable—A variable that is controlled by a regulatory mechanism; for example, temperature in a heating system.
• Set point—The optimal value of the system variable in a regulatory mechanism.
• Detector—In a regulatory process, a mechanism that signals when the system variable deviates from its set point.
• Correctional mechanism—In a regulatory process, the mechanism that is capable of changing the value of the system variable.
• Negative feedback—A process whereby the effect produced by an action serves to diminish or terminate that action; a characteristics of regulatory systems.
• Satiety mechanism—A brain mechanism that causes cessation of hunger or thirst, produced by adequate and available supplies of nutrients or water.
• Intracellular fluid—The fluid contained within cells.
• Extracellular fluid—All body fluids outside cells: interstitial fluid, blood plasma, and cerebrospinal fluid.

• The body contains four major fluid compartments: one compartment of intracellular fluid and three compartments of extracellulr fluid. Approximately two-thirds of the body’s water is contained in the intracellular fluid, the fluid portion of the cytoplasm of cells. The rest is extracellular fluid, which includes the intravascular fluid (the blood plasma), the cerebrospinal fluid, and the interstitial fluid. Interstitial means "standing between"; indeed, the interstitial fluid stands between our cells—it is the "seawater" that bathes them.
• Two of the fluid compartments of the body must be kept within precise limits: the intracellular fluid and the intravascular fluid. The intracellular fluid is controlled by the concentration of solutes in the interstitial fluid. Normally, the interstitial fluid is isotonic with the intracellular fluid. That is, the concentration of solutes in the cells and in the interstitial fluid that bathes them is balanced, so that water does not tend to move into or out of the cells. If the interstitial fluid loses water (becomes more concentrated, or hypertonic), water will move out of the cells through osmosis. On the other hand, if the interstitial fluid gains water (becomes more dilute, or hypotonic), water will move into the cells.
• The volume of the blood plasma must be closely regulated because of the mechanics of the operation of the heart. If the blood volume falls too low, the heart can no longer pump the blood effectively; if the volume is not restored, heart failure will result.

• As you will see, the two important characteristics of the body fluids—the solute concentration of the intracellular fluid and the volume of the blood—are monitored by two different sets of receptors. A single set of receptors would not work, because it is possible for one of these fluid compartments to be changed without affecting the other.

• Intravascular fluid—The fluid found within the blood vessels.
• Interstitial fluid—The fluid that bathes the cells, filling the space between the cells of the body [the "interstices"].
• Isotonic—Equal in osmotic pressure to the contents of the cell. A cell placed in an isotonic solution neither gains nor loses water.
• Hypertonic—The characteristic of a solution that contains enough solute that will draw water out of the cell placed in it, through the process of osmosis.
• Hypotonic—The characteristic of a solution tha contains so little solute that a cell placed in it will absorb water, through the process of osmosis.

• Hypovolemia (hy poh voh lee mee a)—Reduction in the volume of the intravascular fluid

• Most of the time, we ingest more water and sodium than we need and the kidneys excrete the excess. However, if the levels of water or sodium fall to low, correctional mechanisms—drinking water or ingesting sodium—are activated.
• Because loss of water from either the intracellular or intravascular fluid compartments stimulates drinking, researchers have adopted the terms osmometric thirst and volumetric thirst to describe them.
• Thirst simply means a tendency to seek water and to ingest it.
• Eventually, the loss of water from the cells and the blood plasma will be great enough that both osmometric thirst and volumetric thirst will be produced.

• Osmometric thirst occurs when the tonicity (solute concentration) of the interstitial fluid increases. This increase draws water out of the cells, and they shrink in volume.
• Neurons that respond to changes in the solute concentration of the interstitial fluid—osmoreceptors.
• Osmoreceptors responsible for osmometric thirst are located in the region of the anterior hypothalamus that borders the anteroventral tip of the third ventricle (the AV3V).
• The OVLT (if you really want to know, that stands for the organum vasculosum of the lamina terminalis), like the other circumventricular organs, is located on the blood side of the blood-brain barrier. That means the substances dissolved in the blood pass easily into the interstitial fluid within this organ.

• Osmometric thirst—Thirst produced by an increase in the osmotic pressure of the interstitial fluid relative to the intracellular fluid, thus producing cellular dehydration.
• Osmoreceptor—A neuron that detects changes in the solute concentration of the interstitial fluid that surrounds it.

1. Water is lost through evaporation
2. Concentration of interstitial fluid increases

Volumetric Thirst

• Volumetric thirst, when we lose water through evaporation, we lose it from all three fluid compartments: intracellular, interstitial, and intravascular.
• Loss of blood is the most obvious cause of pure volumetric thirst. Should damage occur to the osmometric system, volumetric thirst provides a second line of defense against a loss of water, and it provides the means for the loss of water, and it provides the means for the loss of isotonic fluid to instigate drinking.
• OVLT (organum vasculosum of the lamina terminalis). A circumventricular organ located anterior to the anterventral portion of the third ventricle; served by fenestrated capillaries and thus lacks a blood-brain barrier.
• Volumetric thirst. Thirst produced by hypovolemia.
• The Role of Angiotensin. The kidneys contain cells that are able to detect decreases in the flow of blood to the kidneys. When the flow of blood to the kidneys decreases, these cells secrete an enzyme called renin. Renin enters the blood, where it catalyzes the conversion of a protein called angiotensinogen into a hormone called angiotensin.
• Renin. A hormone secreted by the kidneys that causes the conversion of angiotensinogen in the blood into angiotensin.
• Angiotensin. A peptide hormone that constricts blood vessels, causes the retention of sodium and water, and produces thirst and a salt appetite.
• Angiotensin has several physiological effects: It stimulates the secretion of hormones by the posterior pituitary gland and the adrenal cortex that cause the kidneys to conserve water and sodium, and it increases blood pressure by causing muscles in the small arteries to contract. It has two behavioral effects: It initiates both drinking and a salt appetite.
• Atrial Baroreceptors. The second set of receptors for volumetric thirst lies within the heart. Physiologists had long known that the atria of the heart. The atria are positively filled with blood being returned from the body by the veins. The more blood that is present, the fuller the atria become just before each contraction of the heart. Thus, when the volume of the blood plasma falls, the atria become less full, and the stretch receptors within them will detect this change.

• Sensory information from the baroreceptors located in the atria of the heart is sent to a nucleus in the medulla: the nucleus of the solitary tract.
• This nucleus sends efferent axons to many parts of the brain, including the region around the AV3V.
• Nucleus of the solitary tract. A nucleus of the medulla that receives information from visceral organs and from the gustatory system.
• The second signal for volumetric thirst is provided by angiotensin, located in one of the circumventricular organs. The subfornical organ (SFO), is the site at which blood angiotensin acts to produce thirst. This structure gets its name from its location, just below the commissure of the ventral fornix.
• Subfornical organ (SFO). A small organ located in the confluence of the lateral ventricles, attached to the underside of the fornix; contains neurons that detect the presence of angiotensin in the blood and excite neural circuits that initiate drinking.
• Neuron in the subfornical organ send their axons to the median preoptic nucleus, a small nucleus wrapped around the front of the anterior commissure, a fiber bundle that connect the amygdala and anterior temporal lobe.
• The median preoptic nucleus receives information from angiotensin-sensitive neurons in the SFO. In addition, this nucleus receives information from the OVLT (which contains osmoreceptors) and from the nucleus of the solitary tract (which receives information from the atrial baroreceptors). According to Thrasher and his coleagues, the median preoptic nucleus integrates the information it receives and, through its efferent connections with other parts of the rbain, controls drinking.
• The region of the AV3V seems to play a critical role in fluid regulation in humans.

• A regulatory system contains four features: a system variable (the variable that is regulated), a set point (the optimal value of the system variable), a detector to measure the system variable, and a correctional mechanisms to change it. Physiological regulatory systems, such as control of body fluids and nutrients, require a satiety mechanism to anticipate the effects of the correctional mechanism, because the changes brought about by eating and drinking occur only after a considerable period of time.
• The body contains three major fluid compartments: intracellular, interstitial, and intravascular. Sodium and water can easily pass between the intravascular fluid and the interstitial fluid, but sodium cannot penetrate the cell membrane. The solute concentration of the interstitial fluid must be closely regulated. If it becomes hypertonic, cell lose water; if it becomes hypotonic, they gain water. The volume of the intravascular fluid (blood plasma) must also be kept within bounds.
• Osmometric thirst occurs when the interstitial fluid becomes hypertonic, drawing water out of cells. This event, which can be caused by evaporation of water from the body or by ingestion of a salty meal, is detected by osmorecpetors in the region of the anteroventral third ventricle (the AV3V). The receptors are located both in the OVLT, a circumventricular organ, and in adjacent regions of the brain. Activation of the circumreceptors stimulates drinking.
• Volumetric thirst occurs along with osmometric thirst when the body loses fluid through evaporation. Pure volumetric thirst is caused by blood loss, vomiting, and diarrhea. One stimulus for volumetric thirst is provided by a fall in blood flow to the kidneys. This event triggers the secretion of renin, which converts plasma angiotensinogen to angiotensin. Angiotensin acts on neurons in the brain and stimulates thirst. The hormone also increases blood pressure and stimulates the secretion of pituitary and adrenal hormones that inhibit the secretion of water and sodium by the kidneys and induce a sodium appetite. (Sodium is needed to help restore the plasma volume.) Volumetric drinking can also be stimulated by a set of baroreceptors in the atria of the heart that detect decreases blood volume and send this information to the brain.
• The region of the AV3V detects and integrates signals that produce both osmometric and volumetric thirst. Volumetric thirst stimulated by angiotensin involves another circumventricular organ: the subfornical organ. Volumetric thirst stimulated by the atrial stretch receptor system reaches the AV3V region via a relay in the nucleus of the solitary tract. Neurons in the SFO, the AV3V region, and the OVLT (which, you will remember, contains osmoreceptors) all send axons to the median preoptic nucleus. Neurons in this nucleus stimulate drinking through their connections will other parts of the brain.

• We can achieve water balance by the intake of two ingredients: water and sodium chloride. When we eat, we must obtain adequate amounts of carbonhydrates, fats, amino acids, vitamins, and minerals other than sodium. Our food-ingestive behaviors are more complex, as are the physiological mechanisms that control them.
• To stay alive, our cells must be supplied with fuel and oxygen.

• The short-term reservoir is located in the cells of the liver and the muscles, and it is filled with a complex, insoluble carbohydrate called glycogen. Cells in the liver convert glucose (a lated to do so by the presence of insulin, a peptide hormone produced by the pancreas. Thus, when glucose and insulin are present in the blood, some of the glucose been obsorbed from the digestive tract, the level of glucose in the blood begins to fall.
• The fall in glucose is detected by cells in the brain, which cause an increase in the activity of sympathetic axons that innervate the pancreas. This activity inhibits the secretion of insulin and causes another set of cells of the pancreas to begin secreting a different peptide hormone, glucagon. The effect of glucagon is opposite that of insulin: It stimulates the conversion of glycogen into glucose. Thus, the liver soaks up excess glucose and stores it as glycogen when plenty of glucose is available, and it releases glucose in the blood begins to fall.
• The carbohydrate reservoir in the liver is reserved primarily for the central nervous system. When you wake in the morning, your brain is being fed by your liver, which is in the process of converting glycogen to glucose and releasing it into the blood. The glucose reaches the CNS, where it is absorbed and metabolized by the neurons and glia. This process can continue for a few hours, until all of the carbohydrate reservoir in the liver is used up. (The average liver holds approximately 300 calories of carbohydrate.)