Animal physiology is, to use a common phrase, how animals work.
Animals are, in one sense, machines, and the aim of the science of physiology is to understand how these machines function—what drives them, how they operate, the interaction of the various systems they comprise, and the physical and chemical constraints on how they work.
Animals are also organisms, and this course is intended to help you understand how animals work as integrated units, i.e. as organisms. We will be concerned with how organisms’ various components work to keep an animal alive, with how these are coordinated, and how the various types of animals, despite their disparate evolutionary histories, solve common physiological problems, sometimes in remarkably innovative ways.
This course is the fourth in a series of courses that, together, would be the equivalent of a one-semester course in animal physiology. I strongly recommend that you take the first two courses in the series, Animal Physiology 1. Respiration and gas exchange, Animal Physiology 2. Blood and circulation,and Animal Physiology 3: Digestion and metabolism before you take this course. .
This course is intended for the upper-division biology student. It is also a good course for graduate students and practicing professionals looking for a brush-up course in animal physiology. I presume that you come into this course with the background in chemistry, physics, mathematics and biology that can be reasonably expected of a senior biology student.
The course consists of about six hours of video clips, parceled into seven lectures.
Temperature is an important variable in the physical processes of life, and animals go to a great deal of trouble to manage it. You will see how.
Energy lifestyle is tied closely to what we might call temperature lifestyle. The body temperatures of animals can be regulated or unregulated, can be sustained by high rates of internal heat production, or by clever exploitation of external sources of heat, or can be high, or relatively cool. Here we introduce the concept of the thermal lifestyle space, which allows us to map out the relationship between energy use and body temperature.
KW: body temperature; homeotherm; put poikilotherm; warm-blooded; cold-blooded; endothermic; endotherm; ectotherm; ectothermic;
One of the major thermal lifestyles among the animals are the endothermic homeotherms, those animals that use internally generated heat to sustain a high and regulated body temperature. These are the mammals and birds. Understanding the energetics of these creatures involves a basic understanding of the physics of heat transfer, which is explained by a simple heat transfer law known as Fourier’s law. Fourier’s law form the basis of a more general theory of the energetics of endothermic homeotherms.
KW: Fourier’s law; heat transfer; energy balance; thermal conductance;
Fourier’s law is the foundation for a more general theory of energetics of endothermic homeotherms, known as the Scholander model. This model is the foundation for understanding adaptation of endothermic homeotherms to a variety of habitats on earth. An important quantity that describes this adaptation is the thermal conductance.
KW: Fourier’s law; Scholander model; thermal conductance; endothermic homeotherm; lower critical temperature; basal metabolic rate; upper critical temperature;
The thermal conductance is affected by body size, specifically thermal conductance increases with increasing body size. The scaling of thermal conductance depends upon a complex interaction between the multiple modes of heat exchange between the environment and skin surface.
KW: Scholander model; thermal conductance; scaling; lower critical temperature; radiation; convection; energy budget; heat balance;
The thermal conductance is the result of a complex interaction between multiple modes of heat transfer, both within and external to the animal. This means that the thermal conductance is amenable to physiological adjustment, and this has implications for the energy and water costs of being an endothermic homeotherm.
KW: Scholander model; thermal conductance; core – shell model; conduction; convection; radiation; blood flow; for sickness; pelage; plumage; feathers;
Bergmann’s rule argues that animal body sizes varies systematically with latitude and elevation. Specifically, it states that body sizes tends to increase with increasing latitude, and with increasing elevation. Bergmann’s rule has commonly been explained as an adaptation to cold temperatures, supposedly through some effect of surface to volume ratios, which increase with increasing body size, and which is supposedly reduce heat losses from the body. This is manifestly false. Increasing body size, by any measure, increases the energy cost of living and thermoregulation.
KW: Bergmann’s rule; thermal conductance; basal metabolic rate; body size; surface to volume ratio; scaling; energy cost;
There is a sound energetic explanation for Bergmann’s rule that does not rely on a crude reliance on surface to volume ratios, and the Scholander model points the way. Specifically, increasing body size results in an extension of its the thermal neutral zone of animals to colder and colder temperatures. The thermal neutral zone is that range of temperatures in which thermal conductance can be regulated physiologically, so that the energy budget can be balanced without having to increase metabolic heat production or evaporative heat loss.
KW: Bergmann’s rule; thermal conductance; thermal neutral zone; metabolic energy cost; evaporation water costs;
Temperature is important because it affects the kinetics of the innumerable chemical reactions that sustain life. The effect of temperature must be coordinated. We'll see how
Temperature affects the ability of animals to live in environments by affecting the kinetics of the enzyme mediated reactions that make up life’s chemistry. In particular, temperature adaptation involves multiple enzymes that must work together to mediate biochemical pathways. At the heart of this understanding why some basic principles of enzyme kinetics.
KW: temperature adaptation; enzyme kinetics; Q10; thermodynamic temperature; Michaelis-Menten equation;
Enzymes can be engineered to perform well at almost any temperature where water is in its liquid phase. We can demonstrate this by looking at the temperature adaptation of a crucial enzyme, acetylcholinesterase, that is an important feature of the regulation of nervous transmission. Animals that are adapted to different temperatures have acetylcholinesterase enzymes that are structured to perform as well at cold temperatures as they do at warm temperatures. This variation is amenable to natural selection.
KW: temperature adaptation; enzyme kinetics; Michaelis-Menten equation; ice fish; trout; temperature acclamation; protein structure; secondary structure; alpha helix; beta sheet; disulfide bridge; random thermal motion; stabilization; Michaelis constant;
Engineering an enzyme to work at any given temperature involves building stabilizing structures into a protein that can resist the destabilizing forces of random thermal motion. This balancing act is evident in the stabilization of proteins that operate at different temperatures. As temperatures go from cool to warm, the abundance of strong stabilizing forces in an enzyme increases.
KW: temperature adaptation; enzyme kinetics; Michaelis-Menten equation; thermal breadth; Michaelis constant;
Adaptation of single enzymes to temperature is a simple matter of selection for variance of particular iso-enzymes that function well at a particular temperature. Enzymes rarely work alone, however but as part of a larger suite of physiological functions that make up that organisms biology. Evaluating temperature adaptation involves, therefore, the ability to measure the effect of temperature on an ecologically relevant organismal function, like running speed. Doing so produces ecological performance curves, which can be used to evaluate an animal’s so-called thermal niche.
KW: temperature adaptation; Sprint speed; lizard; ecological performance curve; thermal niche; reaction pathway; organismal function;
Evaluating the ecological performance curve requires a meaningful measure of environmental temperature. This treats temperature as part of an overall thermal energy balance for an animal, which results in a rational metric: for environmental temperature called the operative temperature. Evaluating the operative temperature and is variation through the day and through the year can be combined with the ecological performance curve to fully characterize and animals thermal niche. This shows that poikilotherms adapt to varying temperatures and varying habitats through behavior rather than genetic adaptation.
KW: temperature adaptation; ecological performance curve; Sprint speed; lizard; thermal niche; behavior;
Life exists over a broad range of temperatures, from hot springs to glaciers. This is where adaptation is pushed to its limits. We'll see some examples of these adaptations.
Adapting to extremes of temperature involves special manipulations of the animal’s energy budget. In particular, adapting to extreme high temperatures requires some manipulation of the evaporation heat loss term of net metabolism. Although life can exist at temperatures much hotter than this, animals with complex nervous systems appear to be limited to about 40°C or so.
KW: temperature adaptation; evaporation; body temperature tolerance; energy budget;
Water evaporates from the body through two principal routes: through the skin, and from the lungs. For most animals, evaporation is divided roughly half and half between cutaneous and respiratory water losses. Evaporation from the skin is largely a passive process, limited by diffusion from the wet internal layers of the skin through the meshwork of keratin cells that make up the epidermis. Evaporation heat loss to the skin can be manipulated by variations of skin thickness, the presence of impermeable diffusion barriers like scales, or by the presence of oils in the skin.
KW: evaporation; skin; dermis; epidermis; scales; pelage; plumage; diffusion barrier; water vapor pressure; partial pressure of water vapor; waterproof frog; Chiromantis; Phyllomedusa;
Water loss in the lungs is tied into the predominant function of the lungs, respiratory gas exchange. Options for manipulating respiratory water loss are typically limited by it being tied to respiration and metabolism.
KW: evaporation; respiratory water loss; water vapor pressure; partial pressure of water vapor; saturation;
Even though it is difficult to manipulate respiratory water loss independent of respiratory gas exchange, it is possible to reduce respiratory water loss by manipulating the temperature of the exhaled air. Doing so involves a complicated pattern of heat and mass exchange between inhaled and exhaled air and complex exchange surfaces within the nasal cavities. This is exemplified by the control of respiratory water vapor loss in the camel’s nose.
KW: evaporation; respiratory water loss; camel’s nose; turbinate bone; nasal conchae; inhalation; exhalation;
Another means of adapting to high environmental temperature allows parts of the body to vary with environmental temperature. Camels, for example, will allow their body temperatures to fluctuate as much as 7°C throughout the day. This allows the camel to save considerable quantities of metabolic energy and water. This mechanism is tied to being able to protect the brain temperature, however, which is much more vulnerable to high temperatures and is the rest of the body. Camels can use the cooling of the nasal cavity in conjunction with a countercurrent heat exchanger to keep the brain cool well the temperature of the rest of the body fluctuates.
KW: evaporation; carotid rete; countercurrent exchanger; nasal vein; nasal cavity; camel’s nose; regional heterothermy; brain temperature; body temperature;
In cold environments, evaporation cannot balance the energy budget as it does in hot environments. In the cold, another suite of adaptations is called for. We'll see what those are.
Adapting to cold employ several different strategies, including reconfiguring cell membranes to keep them fluid, manipulating the thermodynamics of ice crystal formation to prevent the body from freezing, and manipulations of body temperature regulation, like torpor or hibernation. Maintaining membrane fluidity involves manipulating the structure of phospholipid molecules to balance needs for membrane fluidity versus membrane breakdown.
KW: temperature adaptation; cold; melting point; phospholipid; fluidity; viscosity; saturation;
Membranes have to balance fluidity versus integrity. Cell membranes appear to do this by manipulating the saturation levels of the hydrocarbon chains in the membrane phospholipids. Specifically, animals that are adapted to cold conditions load the membranes with phospholipid molecules that are relatively unsaturated. Adaptation to cold involves reconfiguring membranes to increase the proportion of unsaturated phospholipids, helping maintain a relatively constant membrane viscosity.
KW: temperature adaptation; cold; melting point; fluidity; viscosity; saturated; unsaturated; Arctic sculpin; pupfish;
Freezing of the body water can damage cells substantially, because the expansion of ice disrupts the cells mechanical integrity. Annotations to avoid freezing include using hydroxyl rich molecules as anti-freezes, synthesis of specialized molecules called anti-freeze glycoproteins, using ice nucleation proteins to allow body water to freeze in a controlled manner that helps prevent cell damage, and super cooling the body tissues to below the freezing points.
KW: temperature; adaptation to cold; anti-freeze; freezing point depression; van’t Hoff equation; glycoprotein; antifreeze; super cooling; ice nucleation protein;
Endothermic homeotherms prevent freezing of the body water by using metabolic energy to keep them warm. This comes at a metabolic cost, however, which can sometimes be too heavy for the animal to bear. Torpor is one of those mechanisms, which involves periodic relaxation of body temperature regulation to reduce metabolic energy costs, and still help prevent freezing of the body during cold conditions.
KW: temperature adaptation; torpor; metabolic energy cost; Hummingbird; energy use rate; energy stores;
Hibernation is another mechanism employed by endothermic homeotherms for manipulating body temperature regulation. It involves deep reductions in the level of the regulated temperature, and the metabolic energy cost needed to keep the body warm. Hibernation is a phenomenon of small animals, and this limitation can be explained by the effects of body size on the heavy cost of rewarming the body following a bout of hibernation.
KW: temperature adaptation; cold; hibernation; pocket mouse; body size; scaling; rewarming; metabolic rate;
Managing water and salt content of the body is an essential component of physiological function. This means managing a water budget.
Water balance is a physiological problem of managing flows, just like heat balance is a problem of managing the flows of heat. There are several avenues for water to flow between animals and their environments, and there are also different types of hydric environments that call for different kinds of adaptations for managing an animal’s water balance.
KW: water; evaporation; urine; seawater; freshwater; osmosis; metabolic water; water budget; hydric environment; terrestrial; desiccation; formed water;
Osmosis is the flow of water from regions of low sided concentration to regions of high sided concentration. Osmosis is a confusing concept, because it is often taught badly, and mixes up concepts of osmotic pressure with a potential energy that actually moves watered by osmosis. Osmosis is also confusing, because the actual physical process of osmosis is frequently misrepresented.
KW: osmosis; osmotic pressure; osmotic potential; water potential; hydrostatic pressure; osmolarity; molality; Morse equation; colligative property; number line; entropy; orderliness;
Kidneys are the principal water balance organs of vertebrates, but the kidneys are composite organs that comprise numerous functional subunits called nephrons. Nephrons are the sites where body water is filtered and otherwise modified to produce urine. The nephron is a complex interface between the blood and the external environment. The function of the nephron is best understood as the product of three processes. First is filtration, which produces a liquid called filtrate. Next are the two processes of reabsorption and secretion, which act to modify the filtrate into urine.
KW: nephron; filtration; reabsorption; secretion; coelom; glomerulus; Bowman’s capsule; proximal convoluted tubule; distal convoluted tubule; coelomostome; vasa recta; filtrate; tubular nephron; vascular nephron; capillary plexus;
The first step in the production of urine is filtration at the junction between the glomerulus and Bowman’s capsule. This junction is a highly selective filter which allows water and small sites to cross, but holds back the plasma proteins and blood cells. Filtration is a passive process driven by gradients of hydrostatic pressure, and induced gradients of osmotic potential that develop during the process of filtration. Filtration rate, known precisely as the glomerular filtration rate, is driven by the net filtration pressure, which is the balance between hydrostatic pressure driving filtrate into the Bowman’s capsule, and osmotic potential drawing water from the Bowman’s capsule back into the blood. That filtration pressure is delicately poised between these two drivers of water flow.
KW: filtration; hydrostatic pressure; osmotic potential; net filtration pressure; NFP; glomerular filtration rate; GFR; colloids; colloid osmotic potential; capsule pressure; blood pressure; podocyte; slit diaphragm;
Filtrate is produced at a daily rate that exceeds the total available water in the body. Filtrate must therefore be modified considerably to keep the body in water balance. Reabsorption across the convoluted tubules does this, recovering nearly 2/3 of the water and small solutes in the filtrate, and nearly all of the solutes like glucose and amino acids. Reabsorption in the convoluted tubules is a process of solutes linked transport, whereby active transport of sodium ions from the tubule into the capillaries drives a flow of water from the tubule into the capillaries through porous proteins known as aquaporins. This arrangement is highly adaptable, by altering the transport rate of sodium, and the degree of openness of the aquaporins.
KW: active transport; sodium transport; solutes linked water transport; SLWT; aquaporin; osmotic potential; sodium recovery; solute recovery; water recovery; filtrate; glomerular filtration rate;
Secretion is the next step in the modification of filtrate to urine. In secretion, various materials such as mineral salts and nitrogenous wastes are secreted from the blood and extracellular fluid into the tubule, ready for export via the urine. The three processes of filtration, reabsorption and secretion together convert filtered body fluids into urine, and manages the solute and water balance of the body. These three together make the nephron a highly adaptable structure, which is governed by what might be called the logic of the nephron.
KW: secretion; mineral salts; magnesium; phosphate; urea; ammonia; uric acid; urine to plasma ratio; U/P ratio;
In the modification of filtrate by reinforcing in secretion produces urine. The same basic nephron design produces a wide range of urine production rates and compositions. This adaptability is part structural, and it’s part physiological.
KW: urine production; fishes; hyper osmotic; hypo osmotic; anadromous; catadromous; aquaporin;
Urine production and the physiological balance of the body salt and water are under broad hormonal control. The principal hormones include pituitary hormones that modulate blood pressure and sodium transport, as well as controlling the presence and activities of aquaporins these mechanisms operate through the proxy variable of blood pressure.
KW: urine production; blood volume; blood pressure; antidiuretic hormone; ADH; neurosecretion; pituitary; aldosterone; kidney tubule;
Sodium transport is regulated through the endocrine hormone aldosterone. Secretion of aldosterone is part of a complex system of endocrine feedbacks known as the renin angiotensin system. The components of the renin angiotensin system include the liver, parts of the kidney tubule, and the adrenal gland.
KW: blood volume; blood pressure; sodium transport; renin angiotensin system; angiotensin; angiotensin converting enzyme; angiotensinogen; liver; macula densa;
Particularly in terrestrial and marine systems, excess salt in the body is a problem that must be handled separately from the production of urine. The vertebrates have developed a remarkable range of accessory salt balance organs known as salt glands. These salt glands can be part of the gills, or can be modified secretory tissues that developed around the head.
KW: salt concentration; chloride cell; salt gland; countercurrent multiplier; lacrimal glands; lingual glands; salivary glands;
Water is essential to life, and conserving water in terrestrial environments is essential. Living on land requires specialized adaptations for limiting water loss through kidneys.
The nephrons of fishes, amphibians, and most reptiles are incapable of forming a urine that is more concentrated than the blood. The kidneys of mammals and birds, in contrast, produce a urine that can be concentrated many times over the concentration of the blood. The nephrons of mammals and birds are, for the most part, the same design and physiology as the nephrons of the fishes in amphibians. The kidneys of mammals and birds can produce a more concentrated urine, because the nephrons are organized into a larger structure, the kidney, which produces a local environment that enables the concentration of urine.
KW: Animal physiology; nephron; kidney; loop of Henle; hyper osmotic; mammals; birds;
In the kidneys of mammals and birds, the multitudinous nephrons work together to produce a large-scale environment contained within the kidney. Specifically, the nephrons work to impose a large-scale gradient of osmotic potential from the cortex to the medulla of the kidney. This large-scale osmotic potential gradient is what enables the kidneys of mammals and birds to produce a highly concentrated urine. It is not the nephrons that produce the concentrated urine, rather it is the passage of fluids through the collecting duct through this large-scale osmotic potential gradient.
KW: Animal physiology; nephron; kidney; loop of Henle; hyper osmotic; mammals; birds; osmotic potential; ascending limb; descending limb; collecting duct; salt transport; aquapor
The kidneys of mammals and birds are remarkably adaptable, and this adaptation does not involve the evolution of new physiology in the nephron. The kidneys of mammals and birds can concentrate urine by as little as four times, and is much as 25 times the concentration of the blood from which it’s derived. This physiological flexibility is what underscores the ability of mammals and birds to live in desiccated and terrestrial environments. The flexibility arises from fairly simple modifications of the kidney’s loops of Henle.
KW: nephron; kidney; loop of Henle; hyper osmotic; mammals; birds; osmotic potential; renal cortex; renal medulla; U/P ratio; energy cost; osmotic potential gradient;
The nephrons of reptiles are no different in design from the nephrons of amphibians and fishes. This means that the nephrons of reptiles are incapable of producing a urine that is more concentrated than the blood. Nevertheless, reptiles are fully terrestrial, and they have adaptations that enable them to conserve water even if the kidneys are not capable of producing concentrated urine. Water recovery among reptiles takes place in the cloaca, and it exploits the reptiles tendency to handle nitrogenous waste by forming it into uric acid.
KW: nephron; reptile; cloaca; uric acid; water balance; osmotic potential; water recovery;
Insects do not maintain high pressures and their circulatory systems, so they cannot use blood pressure to produce filtrate like vertebrates do. Nevertheless, insects are highly successful terrestrial animals, and they must have ways to concentrate urine, and reduce water loss just as terrestrial vertebrates must. Insects use a solute transport system concentrated in the Malphighian tubules, which are tubular structures that’s radiate into the sea loan at the junction between the midgut and the hindgut. The simple structure is amenable to physiological flexibility and evolutionary adaptation, just as the kidneys of mammals and birds are. Insects start from an entirely different structure of water balance, however.
KW: Malphighian tubule; insect; solute linked water transport; uric acid; precipitation; osmotic potential;
Terrestrial environments are uniformly desiccated, and animals that live in terrestrial environments must stringently conserve water. One of the major sources of water loss is the need to flush nitrogenous wastes from the body. Nitrogenous wastes can come in the form either of toxic ammonia, or less toxic urea or uric acid. Animals can produce all three, and the form of nitrogenous waste depends upon the hydric environment.
KW: nitrogen metabolism; ammonia; urea; uric acid; water balance; hydric environment;
The form of nitrogenous waste that is eliminated from the body depends upon hydric environment. Animals that live in freshwater, or areas with abundant fresh water tend to excrete nitrogenous waste as ammonia, well animals that live in more desiccated environments secrete their nitrogenous wastes in the form either of urea or uric acid.
KW: nitrogen metabolism; ammonia; urea; uric acid; water balance; hydric environment;
Review and recap the understanding of heat, temperature and water balance, and the nature of adaptation.
I am a Professor of Biology at the State University of New York College of Environmental Science and Forestry in Syracuse, New York.
I am a physiologist by training but with a deep interest in the interface of physiology, ecology, adaptation and evolution. You can read some of my thoughts in two books I have published: The Extended Organism: The Physiology of Animal-Built Structures (2007) and The Tinkerer's Accomplice: How Design Emerges from Life Itself (2007), both published by Harvard University Press. I have completed a third book, Purpose and Desire: Biology's Second Law, which I hope will be published soon.
My current research focuses on the problem of emergent physiology in social insect colonies. specifically the mound building termites of southern Africa.