Chapter 9: Internal Regulation - Temperature, Thirst, and Hunger
Behavioral Adaptations in Temperature Regulation
Garter Snake Strategy: A small male garter snake produces female pheromones to attract larger males. These larger males attempt to copulate with him, and in the process, their body contact warms the smaller snake.
Avian Thermoregulation: Some bird species stand on one leg during cold weather to keep the other leg tucked under their body for warmth.
Excretory Cooling in Vultures: Vultures defecate on their legs on hot days; the evaporation of the moisture from the waste provides a cooling effect.
Toucan Bill Regulation: Toucans possess disproportionately large bills that allow them to direct blood flow toward the bill. This mechanism helps them either radiate heat to cool down or conserve heat to warm up.
Lizard Huddling: Lizards huddle together as a behavioral defense against rapidly changing environmental temperatures.
Concepts of Homeostasis and Allostasis
Basal Metabolism: This is defined as the energy used to maintain a constant body temperature while at rest. It accounts for approximately of an individual's total daily energy or kilocalorie expenditure.
Homeostasis: Refers to biological processes that keep specific body variables within a fixed range. In mammals, variables like temperature regulation, thirst, and hunger are nearly homeostatic (though not perfectly so).
Set Point: The specific level at which a homeostatic process maintains a variable.
Negative Feedback: These are processes that function to reduce discrepancies or deviations from the established set point.
Allostasis: The adaptive manner in which the body alters its set point in response to changes in life circumstances or the external environment.
Controlling Body Temperature: Ectotherms and Endotherms
Biological Priority: Temperature regulation is considered a high biological priority. Twice as much energy is dedicated to temperature regulation as all other bodily activities combined.
Ectothermic (Poikilothermic): * Definition: Animals whose body temperatures are determined by their environment (e.g., fish and lizards). * Mechanism: These organisms lack internal, physiological mechanisms for regulation and must regulate temperature by choosing specific locations in their environment. * Note: The term "cold-blooded" is commonly used but technically inaccurate.
Endothermic (Homeothermic): * Definition: Animals with physiological mechanisms that maintain a near-constant body temperature (approximately or ) regardless of environmental variations. * Physics of Heat: These animals generate heat in proportion to their total mass but radiate heat in proportion to their surface area. * Cooling Mechanisms: Sweating, licking the skin, and panting. * Heating Mechanisms: Shivering, fluffing fur/feathers, increasing metabolic rate, and decreasing blood flow to the skin. * Muscle Efficiency: Muscle activity benefits from being kept as warm as possible, making the animal ready for vigorous movement. * Constraints: Proteins in the body can lose useful properties and break bonds at high temperatures. Furthermore, reproductive cells require cooler temperatures than the rest of the body.
Responses to Temperature Variation
Internal Temperature Too High (Hyperthermia): * Problem: Needs faster heat loss to the external environment. * Behavioral Response: Reduce physical activity, seek shade, stretch out to increase surface area, and reduce clothing or fur/feather cover. * Functional Response: Dilate surface blood vessels (radiation), sweat or pant (evaporation), and lie on cool surfaces or swim (conduction).
External Temperature Too High: * Problem: Heat gain from the environment is too high. * Behavioral Response: Similar to hyperthermia (shade, reduced activity). * Functional Response: If behavior is insufficient, physiological cooling mechanisms (sweating/dilation) begin.
Internal Temperature Too Low (Hypothermia): * Problem: Heat production is lower than heat loss. * Behavioral Response: Increase voluntary activity (hand rubbing) or involuntary activity (shivering), huddle to reduce surface area, and add clothing. * Functional Response: Constrict surface blood vessels to reduce radiation and raise hairs (goosebumps) to trap warm air.
External Temperature Too Low: * Problem: Heat loss to the environment is too great. * Behavioral Response: Increase activity and huddling; if insufficient, physiological warming mechanisms begin.
Survival in Extremes and Endothermic Advantages
Extreme Cold Survival: Some insects and fish possess antifreeze-like compounds in their blood during winter to prevent freezing.
Endothermy Advantages: * The constant high temperature () ensures mammals are ready for rapid movement even in cold conditions. * Higher temperatures were likely not developed because proteins become unstable at temperatures exceeding . * Cooler environments are necessary for reproductive cell health.
Brain Mechanisms and Fever
Critical Regions: The anterior hypothalamus and the preoptic area are essential for temperature control. Because they are adjacent (near the optic chiasm), they are collectively called the Preoptic Area/Anterior Hypothalamus (POA/AH).
Monitoring: The POA/AH monitors its own temperature and receives input from the immune system and temperature-sensitive receptors in the skin and spinal cord.
Fever Mechanism: * Infection by bacteria or viruses triggers leukocytes to release small proteins called cytokines. * Cytokines attack intruders and stimulate the vagus nerve. * The vagus nerve signals the hypothalamus to initiate a fever, which is an increased set point for body temperature.
Fever Purpose: Certain bacteria grow less vigorously at high temperatures, and fever enhances immune system activity.
Fever Thresholds: * Fever above () generally does more harm than good. * Fever above () is considered life-threatening.
Behavioral Fever: Newborn rabbits with immature hypothalami will move to warm rooms to develop a fever by behavioral means to fight infection.
Functional Nuclei of the Hypothalamus
Suprachiasmatic Nucleus (SCN): Regulates circadian rhythms.
Preoptic Area: Regulates temperature.
Organum Vasculosum Laminae Terminalis (OVLT): Detects osmotic pressure.
Supraoptic and Paraventricular Nuclei: Responsible for the release of vasopressin.
Lateral Preoptic Area: Controls drinking behavior.
Subfornical Organ (SFO): Stimulated by Angiotensin II to induce drinking.
Lateral Nucleus: Increases eating behaviors.
Ventromedial Nucleus: Decreases eating behaviors.
Ventral Noradrenergic Bundle: Decreases eating behaviors.
Paraventricular Nucleus (PVN): Involved in decreasing eating.
Thirst and Water Regulation
Body Composition: Water constitutes approximately of the mammalian body.
Chemical Concentration: The concentration of chemicals in water determines the rate of all chemical reactions in the body.
Conservation Mechanisms: Excreting concentrated urine or decreasing sweat.
Primary Regulation: Drinking more water than needed and excreting the excess.
Hormonal Control: When the body needs water, the posterior pituitary releases Vasopressin (Antidiuretic Hormone/ADH). * Function 1: Raises blood pressure by constricting vessels. * Function 2: Enables kidneys to reabsorb water and secrete concentrated urine.
Types of Thirst
Osmotic Thirst: * Cause: Increase in solute concentration. * Mechanism: The body maintains a combined solute concentration at a fixed level of (molar). Water flows across semipermeable membranes from areas of low concentration to high concentration (osmotic pressure). * Detection: Neurons detect osmotic pressure via receptors around the third ventricle, including the OVLT and the subfornical organ. Peripheral receptors in the stomach/intestines detect sodium. * Signaling: Signals reach the supraoptic and paraventricular nuclei to release vasopressin and the lateral preoptic area to initiate drinking. * Satiety: To inhibit thirst, the body monitors swallowing and water content in the stomach/intestines.
Hypovolemic Thirst: * Cause: Low blood volume (low blood pressure), preventing nutrients/water from reaching cells. * Detection: Baroreceptors attached to large veins detect pressure of blood returning to the heart. * Hormonal Cascade: Low blood volume triggers the kidneys to release Renin, which splits angiotensinogen into Angiotensin I, which is kemudian converted into Angiotensin II. * Angiotensin II Functions: Constricts blood vessels to reverse volume loss and stimulates the subfornical organ to increase drinking. * Preference: Animals with osmotic thirst prefer pure water; those with hypovolemic thirst prefer slightly salty water (pure water would dilute fluids too much).
Sodium-Specific Cravings: Caused by the release of aldosterone, which makes kidneys, salivary glands, and sweat glands conserve sodium and excrete watery fluids.
Hunger and the Digestive System
Caloric Example (Big Mac, ): * Protein: (). * Fat: (). * Carbohydrates: (). * Total: .
Gastrointestinal Functions: Movement of food, secretion of juices, absorption of water/electrolytes, blood circulation, and control by nervous/hormonal systems.
Digestive Path: * Mouth: Saliva enzymes break down food. * Stomach: Hydrochloric acid and enzymes digest proteins. The pyloric sphincter controls food entry into the intestines. * Small Intestine (Duodenum/Jejunum/Ileum): Main site for nutrient absorption. Excess is stored as glycogen, protein, or fat. * Large Intestine: Absorbs water and minerals; lubricates for excretion.
Lactose Intolerance: Many adults lose the ability to metabolize lactose due to decreased levels of the enzyme lactase.
Regulation of Food Intake
Feeding Strategies: Strategies include preferring sweet, avoiding bitter, preferring familiar foods, and conditioned taste aversions.
Satiety Signals: * Mouth: Chewing and tasting provide some motivation, but sham feeding (where food leaks out of a tube) does not produce satiety. * Stomach Distention: The primary signal to stop eating. The Vagus Nerve (Cranial Nerve X) conveys information about stomach wall stretching. * Nutrient Contents: The Splanchnic Nerves convey information about nutrient content in the stomach to the spinal cord. * Duodenum: Glucose in the duodenum produces satiety. It also releases Cholecystokinin (CCK).
Cholecystokinin (CCK): Closes the sphincter between the stomach and duodenum to fill the stomach faster and stimulates the vagus nerve to send satiety messages to the hypothalamus.
Insulin: Facilitates glucose entry into cells. High insulin levels normally decrease hunger, but chronically high levels (as in obesity) cause more fat storage and a quicker return of hunger.
Glucagon: Stimulates the liver to convert stored glycogen back into glucose.
Diabetes: Cells receive little glucose due to low insulin; blood glucose stays high while cells "starve," leading to weight loss despite high food intake.
Long-Term Weight Regulation and Brain Mechanisms
Leptin: A peptide produced by fat cells. * Low levels increase hunger. * High levels increase physical/immune activity but don't always decrease hunger (leptin resistance). * Puberty is triggered by specific leptin levels during adolescence.
Arcuate Nucleus: Located in the hypothalamus, it contains hunger-sensitive and satiety-sensitive neurons. * Hunger-Sensitive Input: Taste pathways and Ghrelin (released by the stomach and brain). * Satiety-Sensitive Input: CCK (intestine distention), Insulin (blood glucose/fat), and Leptin.
Paraventricular Nucleus (PVN): Inhibits the lateral hypothalamus. Output from the arcuate nucleus excites the PVN to stop eating. * Inhibitory transmitters (blocking satiety/promoting eating): GABA, Neuropeptide Y (NPY), and Agouti-Related Peptide (AgRP).
Lateral Hypothalamus: Controls insulin secretion, alters taste, and releases Orexin (which increases food seeking and general reinforcement). * Damage results in refusal of food/water.
Ventromedial Hypothalamus (VMH): Damage leads to overeating and weight gain. Subjects eat normal-sized but more frequent meals.
Eating Disorders
Obesity: * Influences: Genetics by environment interaction (e.g., Native American Pimas), lifestyle, portion sizes, and high fructose intake. * Treatments: Lifestyle changes, exercise, and drugs like Sibutramine (blocks serotonin/norepinephrine reuptake) or Orlistat (prevents fat absorption). * Gastric Bypass: Closing off part of the stomach to allow faster distention/satiety. * Prader-Willi Syndrome: Genetic condition with ghrelin levels five times higher than normal.
Bulimia Nervosa: * Cycle: Binge-purge cycle (vomiting, laxatives, diuretics) often triggered by low-calorie dieting. * Health Risks: Electrolyte imbalance, irregular heartbeat, kidney damage, throat damage, and tooth decay. * Prevalence: Estimates suggest up to of adolescent females may be bulimic.
Anorexia Nervosa: * Characteristics: Starvation, preoccupation with thinness, distorted body image. * Physiological effects: Loss of menstrual cycle, downy hair growth (lanugo), heart/kidney failure. * Mortality: Up to die from starvation or suicide.