Module 1_Thirst (1) PDF

PHYSIOLOGICAL/BIOLOGICAL PSYCHOLOGY Kalat, J. W. (2019). Biological Psychology 13th Edition. Boston, MA, USA: Cengage Module: INTERNAL REGULATION Major Topics: 1. Temperature Regulation 2. Thirst 3. Hunger Module 2: Thirst Mechanisms of Water Regulation Water regulation is critical for mai...

PHYSIOLOGICAL/BIOLOGICAL PSYCHOLOGY Kalat, J. W. (2019). Biological Psychology 13th Edition. Boston, MA, USA: Cengage Module: INTERNAL REGULATION Major Topics: 1. Temperature Regulation 2. Thirst 3. Hunger Module 2: Thirst Mechanisms of Water Regulation Water regulation is critical for maintaining homeostasis, and organisms exhibit different strategies based on their environment. 1. Strategies Across Species A. Aquatic Animals (e.g., Beavers): Beavers and other animals living in water-rich environments drink abundant water and eat moist foods. They excrete dilute urine to balance the excess water intake, ensuring their salt and electrolyte levels remain stable. B. Desert Animals (e.g., Gerbils): Desert animals like gerbils, which face water scarcity, adopt strategies to conserve water: Dry feces and concentrated urine: This minimizes water loss through excretion. Burrowing behavior: They avoid high temperatures during the day, reducing the need for sweating (which they cannot do). Specialized nasal passages: These structures recapture moisture from exhaled air, a crucial adaptation in arid climates. C. Humans: Humans employ flexible strategies depending on water availability: o Excess water availability: Humans drink more water than necessary and excrete the excess through dilute urine. Water scarcity: To conserve water, humans: Excrete more concentrated urine. Reduce sweating. Employ hormonal mechanisms like vasopressin release. 2. Hormonal Regulation of Water Balance A key hormonal mechanism regulating water in humans is the release of vasopressin (also called antidiuretic hormone or ADH). A. Function of Vasopressin: - Released by the posterior pituitary gland, vasopressin helps conserve water by: - Constriction of blood vessels: This raises blood pressure to counteract the effects of reduced blood volume during dehydration. - Kidney function modulation: Vasopressin enables the kidneys to reabsorb water from urine, producing more concentrated urine. B. Circadian Rhythm of Vasopressin Secretion: - Secretion increases during sleep, reducing the need to urinate at night. This mechanism preserves water during periods of fasting (when drinking is not possible). 3. Drinking Behavior in Humans A. Regulation of Water Intake: Humans typically drink water: - During meals: Water aids digestion. - In social situations: Drinking often occurs without physiological thirst due to social or cultural practices. B. Excessive Water Intake: - Humans tend to drink more water than necessary under normal circumstances, similar to aquatic animals. However, drinking excessively without consuming salts (e.g., during alcohol abuse) can deplete the body's salt levels, potentially leading to harmful conditions like hyponatremia (low sodium). 4. Adaptive Benefits - These mechanisms reflect evolutionary adaptations to diverse environmental conditions. - Humans retain the flexibility to function in both water-abundant and water-scarce environments by dynamically switching between strategies akin to those of aquatic and desert animals. OSMOTIC THIRST Osmotic thirst arises from an imbalance in solute concentration between the inside and outside of cells. 1. What is Osmotic Thirst? - Osmotic thirst is triggered by the movement of water across cell membranes due to differences in solute concentration, primarily sodium. Osmotic Pressure: - Osmotic pressure drives water movement from areas of lower solute concentration to higher solute concentration through a semipermeable membrane. - This occurs when solutes, like sodium, accumulate outside cells after eating salty foods, leading to dehydration of cells as water moves out to balance the solute concentration. Set Point for Solutes: The concentration of solutes in body fluids is maintained at approximately 0.15 M (molar). Any deviation from this set point triggers mechanisms to restore balance. 2. How Does the Body Detect Osmotic Pressure? The brain and specific sensory systems monitor and respond to changes in osmotic pressure: Receptors in the Brain: Located around the third ventricle, the key receptors include: - OVLT (Organum Vasculosum Laminae Terminalis): Detects osmotic pressure and sodium levels. - Subfornical Organ (SFO): Has neurons that either stimulate or suppress thirst. - These areas lack a strong blood-brain barrier, allowing them to monitor blood content effectively. Input Sources: The OVLT receives input from: - The digestive tract, enabling anticipatory adjustments for osmotic changes. - Osmotic pressure sensors in the bloodstream. 3. Brain Areas Involved in Osmotic Thirst The signals from osmotic pressure detectors are integrated in the hypothalamus, which coordinates drinking behavior and hormonal responses. Key Hypothalamic Structures: - Lateral Preoptic Area: Controls the initiation of drinking. - Supraoptic and Paraventricular Nuclei: Regulate the release of vasopressin by the posterior pituitary gland. Role of Vasopressin: - Increases water reabsorption in the kidneys. - Reduces water loss via concentrated urine.  - Stimulates thirst when needed. 4. Anticipatory Mechanisms and Allostasis The concept of allostasis plays a critical role in osmotic thirst: Anticipation of Future Needs: - Instead of waiting for cells to become dehydrated, the body acts in advance. - For example: o Drinking after consuming salty foods. o Increased vasopressin secretion before sleep, reducing the urge to urinate and retaining water. Stopping Mechanisms: - Thirst is quenched before water reaches the blood or cells, preventing overhydration: - SFO Activity Suppression: Drinking water suppresses thirst-sensitive neurons in the SFO within a minute. - Cooling the Tongue: Provides additional signals to stop drinking. HYPOVOLEMIC THIRST AND SODIUM-SPECIFIC HUNGER Hypovolemic Thirst Hypovolemic thirst occurs when the body loses a significant amount of fluid, which could happen due to bleeding, diarrhea, or sweating. Unlike osmotic thirst, this form of thirst results from a reduction in the volume of blood plasma and extracellular fluid rather than an imbalance in solute concentration. 1. Causes and Mechanisms Fluid Loss and Decreased Blood Pressure: - Loss of fluid reduces blood volume, leading to a drop in blood pressure. This impairs the heart's ability to pump blood and hampers nutrient transport into cells. Role of the Kidneys: - The kidneys detect the reduction in blood pressure and release renin, an enzyme that converts angiotensinogen (a protein in the blood) into angiotensin I. - Enzymes then convert angiotensin I into angiotensin II, a hormone with two key roles: - Constriction of Blood Vessels: Helps restore blood pressure. - Stimulation of Thirst: Signals the brain to initiate drinking behavior. 2. Brain's Role in Hypovolemic Thirst Angiotensin II and the Brain: - Angiotensin II stimulates neurons around the third ventricle and sends signals to the hypothalamus. - These neurons release angiotensin II as a neurotransmitter, further amplifying the thirst signal. Thirst Trigger: - The brain areas involved direct the body to seek fluids, but pure water is not sufficient in this case. Drinking pure water would dilute the remaining solutes in the body, further disrupting fluid balance. 3. Preference for Salty Water - To restore both fluids and electrolytes, the body prefers salty water over pure water. - This preference ensures that both fluid volume and solute concentration return to normal. Sodium-Specific Hunger - When sodium levels are depleted, the body develops a targeted craving for salt, known as sodium-specific hunger. This phenomenon is crucial for maintaining electrolyte balance. 1. Causes and Hormonal Regulation Low Sodium Levels: - Decreased sodium triggers the release of the hormone aldosterone from the adrenal glands. Aldosterone's Role: - Promotes sodium retention in the kidneys, salivary glands, and sweat glands.  - Alters taste receptors to increase the appeal of salty foods. Synergy with Angiotensin II: - Angiotensin II and aldosterone together enhance the response to salt: - Taste Receptors: Salt tastes more appealing. - Brain Neurons: Increased sensitivity to salty flavors in areas like the nucleus of the tractus solitarius. 2. Immediate Behavioral Response Preference for Salt: - When sodium is depleted, animals and humans develop a strong preference for salt, even for highly concentrated solutions that they would usually avoid. - This instinctive preference ensures rapid replenishment of sodium reserves. Examples in Humans: - Salty snacks may taste better to individuals with sodium deficits, such as after heavy sweating or menstruation. 3. Learned vs. Instinctive Preferences Instinctive Craving: - Sodium hunger is an innate mechanism, unlike cravings for other nutrients (e.g., vitamins or minerals), which are learned through trial and error. - This reflects the critical importance of sodium for physiological functions like nerve conduction and muscle contraction. You may have thought that temperature regulation happens automatically and that water regulation depends on your behavior. You can see now that the distinction is not entirely correct. You control your body temperature partly by automatic means, such as sweating or shivering, but also partly by behavioral means, such as choosing a warm or a cool place. You control your body water partly by the behavior of drinking but also by hormones that alter kidney activity. If your kidneys cannot regulate your water and sodium adequately, your brain gets signals to change your drinking or sodium intake. In short, keeping your body’s chemical reactions going depends on both skeletal and autonomic controls.