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Temperature, Osmotic Regulation and the Urinary System Chapter 50 Regulating Body Temperature The rate of any chemical reaction is affected by temperature -Thus, the effect of temperature is mainly on the enzymes involved in metabolism 2 Regulating Body Temperature Body temperature is determined by internal factors, such as metabolism, external factors that affect heat transfer, as well as behavior Body heat = heat produced + heat transferred -Note that the heat transferred can be either positive or negative -Can be used for both heating and cooling 3 1 Regulating Body Temperature Four mechanisms of heat transfer are relevant to biological systems -Radiation = By electromagnetic radiation -Conduction = Directly between two objects -Convection = By the movement of a gas or liquid -Evaporation = Conversion of water to gas 4 Regulating Body Temperature Heat transfer also depends on other factors, that influence these four physical processes -Surface area to mass ratio -Difference between ambient and body temperature -Specific heat conduction 5 Classification of Organisms For many years, animals were classified according to whether they maintained a constant body temperature -Homeotherms = Regulate their body temperature about a set point -Also called “warm-blooded” -Poikilotherms = Allow their body temperature to conform to the environment -Also called “cold-blooded” 6 2 Classification of Organisms Limitations to this dichotomy led to another view based on how body heat is generated -Endotherms = Use metabolism to generate body heat and maintain temperature above ambient temperature -Ectotherms = Do not use metabolism to produce heat and have body temperature that conforms to ambient temperature Heterotherms fall between these extremes 7 Ectotherms Ectotherms regulate temperature using behavior 8 Ectotherms Many marine animals, such as killer whales, limit heat loss in cold water using countercurrent heat exchange -Warm blood pumped from within the body in arteries warms the cooler blood returning from the skin within veins 9 3 Ectotherms Reptiles place themselves in varying locations of sunlight and shade -Some can maximize the effect of behavioral regulation by also controlling blood flow In general, ectotherms have low metabolic rates, which have the advantage of low energy intake -However, they are not capable of sustained 10 high-energy activity Endotherms A high metabolic rate can be used to warm the endotherm if it is cold The simplest way to regulate body temperature is by the control of blood flow to the surface of the animal -Vasodilation increases blood flow, thereby increasing heat dissipation -Vasoconstriction decreases blood flow, thus limiting heat loss 11 Endotherms When ambient temperatures rise, many endotherms take advantage of evaporative cooling in the form of sweating or panting The advantage of endothermy is that it allows sustained high-energy activity -The tradeoff is that the high metabolic rate requires constant and high energy intake (food) 12 4 Endotherms In animal physiology, size does matter! -Smaller animals have much higher metabolic rates per unit body mass relative to larger animals -Small endotherms in cold environments require significant insulation to maintain their body temperature -Large endotherms in hot environments usually have little insulation 13 14 Endotherms When temperatures fall below a threshold, animals resort to thermogenesis, or use of normal energy metabolism to produce heat -Shivering thermogenesis uses muscles to generate heat, without producing useful work -Nonshivering thermogenesis alters fat metabolism to produce heat instead of ATP -Brown fat is utilized 15 5 Control of Body Temperature Pyrogens are substances that cause a rise in temperature -Act on the hypothalamus to increase the normal set point to a higher temperature -Produce the state we call fever -A normal response to infection 16 Control of Body Temperature Torpor is a state of dormancy produced by a reduction in both metabolic rate and body temperature -Allows an animal to reduce the need for food intake Hibernation is an extreme state in which torpor lasts for weeks or months -Practiced usually by mid-sized animals 17 Osmolarity and Osmotic Balance To maintain osmotic balance, the extracellular compartment of an animal’s body must be able to take water from and excrete excess water into the environment -Inorganic ions must also be exchanged to maintain homeostasis -These exchanges occur across specialized epithelial cells, and, in most vertebrates, through the kidney 18 6 Osmolarity and Osmotic Balance Osmotic pressure is the measure of a solution’s tendency to take in water by osmosis Osmolarity is the number of osmotically active moles of solute per liter of solution Tonicity is the measure of a solution’s ability to change the volume of a cell by osmosis -Solutions may be hypertonic, hypotonic, 19 or isotonic Osmolarity and Osmotic Balance Osmoconformers are organisms that are in osmotic equilibrium with their environment -Include most marine invertebrates, and cartilaginous fish (sharks and relatives) All other vertebrates are osmoregulators -Maintain a relatively constant blood osmolarity despite different concentrations in their environment 20 Osmolarity and Osmotic Balance Freshwater vertebrates are hypertonic to their environment -Have adapted to prevent water from entering their bodies, and to actively transport ions back into their bodies Marine vertebrates are hypotonic to their environment -Have adapted to retain water by drinking seawater and eliminating the excess ions through kidneys and gills 21 7 Osmoregulatory Organs In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system A variety of mechanisms have evolved to accomplish this -Single-celled protists use contractile vacuoles 22 Osmoregulatory Organs Invertebrates use specialized cells & tubules -Flatworms use protonephridia which branch into bulblike flame cells -Open to the outside of the body, but not to the inside -Earthworms use nephridia -Open both to the inside and outside of the body 23 Osmoregulatory Organs Insects use Malpighian tubules, which are extensions of the digestive tract -Waste molecules and K+ are secreted into tubules by active transport -Create an osmotic gradient that draws water into the tubules by osmosis -Most of the water and K+ is then reabsorbed into the open circulatory system through hindgut epithelium 24 8 Osmoregulatory Organs The kidneys of vertebrates consist of thousands of repeating units, nephrons -Create a tubular fluid by filtering the blood under pressure through the glomerulus -Filtrate contains many small molecules, in addition to water and waste products -Most of these molecules and water are reabsorbed into the blood -Waste products are eliminated from the body in the form of urine 25 Evolution of the Vertebrate Kidney Cartilaginous fish, including sharks and rays, reabsorb urea from the nephron tubules -Maintain a blood urea concentration that is 100 times higher than that of mammals -Blood is isotonic to surrounding sea -These fishes do not need to drink seawater or remove large amounts of ions from their bodies 26 Evolution of the Vertebrate Kidney The amphibian kidney is identical to that of freshwater fish The kidneys of reptiles are very diverse -Marine reptiles drink seawater and excrete an isotonic urine -Eliminate excess salt via salt glands -Terrestrial reptiles reabsorb much of the salt and water in their nephron tubules -Don’t excrete urine, but empty it into cloaca 27 9 Evolution of the Vertebrate Kidney Mammals and birds are the only vertebrates that can produce urine that is hypertonic to body fluids -Accomplished by the loop of Henle Birds have relatively few or no nephrons with long loops, and so cannot produce urine as concentrated as that of mammals -Marine birds excrete excess salt from salt glands near the eyes 28 Nitrogenous Wastes When amino acids and nucleic acids are catabolized, they produce nitrogenous wastes that must be eliminated from the body -First step is the removal of the amino (-NH2) group, and its combination with H+ to form ammonia (NH3) in the liver -Toxic to cells, and thus it is only safe in dilute concentrations 29 Nitrogenous Wastes Bony fishes and amphibian tadpoles eliminate most of the ammonia by diffusion via gills Elasmobranchs, adult amphibians, and mammals convert ammonia into urea, which is soluble in water Birds, terrestrial reptiles, and insects convert ammonia into the water-insoluble uric acid -Costs most energy, but saves most water 30 10 Nitrogenous Wastes Mammals also produce uric acid, but from degradation of purines, not amino acids -Most have an enzyme called uricase, which convert uric acid into a more soluble derivative called allantoin -Humans lack this enzyme -Excessive accumulation of uric acid in joints causes gout 31 11