Regulation and Body Plans

Regulation: Mechanisms of homeostasis moderate changes in the internal environment The internal environment of vertebrates is called the interstitial fluid. This fluid exchanges nutrients and wastes with blood contained in microscopic vessels called capillaries. While a pond-dwelling hydra is powerless to affect the temperature of the fluid that bathes its cells, the human body can maintain its “internal pond” at a more-or-less constant temperature of about 370C. Similarly, our bodies control the pH of our blood and interstitial fluid to within a tenth of a pH unit of 7.4. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

There are times during the course of the development of an animal when major changes in the internal environment are programmed to occur. For example, the balance of hormones in human blood is altered radically during puberty and pregnancy. Actually the internal environment of an animal always fluctuates slightly. Homeostasis is a dynamic state, outside forces tend to change the internal environment and internal control mechanisms oppose such changes. The stability of the internal environment is remarkable. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2. Homeostasis depends on feedback circuits Any homeostatic control system has three functional components: a receptor, a control center, and an effector. The receptor detects a change in some variable in the animal’s internal environment, such as a change in temperature. The control center processes the information it receives from the receptor and directs an appropriate response by the effector. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Regulating and Conforming A regulator uses internal control mechanisms to moderate internal change in the face of external, environmental fluctuation A conformer allows its internal condition to vary with certain external changes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(temperature conformer) Fig. 40-7 40 River otter (temperature regulator) 30 Body temperature (°C) 20 Largemouth bass (temperature conformer) 10 Figure 40.7 The relationship between body and environmental temperatures in an aquatic temperature regulator and an aquatic temperature conformer 10 20 30 40 Ambient (environmental) temperature (ºC)

Homeostasis Organisms use homeostasis to maintain a “steady state” or internal balance regardless of external environment In humans, body temperature, blood pH, and glucose concentration are each maintained at a constant level Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Mechanisms of Homeostasis Mechanisms of homeostasis moderate changes in the internal environment For a given variable, fluctuations above or below a set point serve as a stimulus; these are detected by a sensor and trigger a response The response returns the variable to the set point Animation: Negative Feedback Animation: Positive Feedback Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Response: Heater turned off Room temperature decreases Stimulus: Fig. 40-8 Response: Heater turned off Room temperature decreases Stimulus: Control center (thermostat) reads too hot Set point: 20ºC Figure 40.8 A nonliving example of negative feedback: control of room temperature Stimulus: Control center (thermostat) reads too cold Room temperature increases Response: Heater turned on

Feedback Loops in Homeostasis The dynamic equilibrium of homeostasis is maintained by negative feedback, which helps to return a variable to either a normal range or a set point Most homeostatic control systems function by negative feedback, where buildup of the end product shuts the system off Positive feedback loops occur in animals, but do not usually contribute to homeostasis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Negative-feedback is analogous to a system that controls the temperature in a room. In this case, the control center, called a thermostat, also contains the receptor, a thermometer. When room temperature falls, the thermostat switches on the heater, the effector. Fig. 40.9a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Our own body temperature is kept close to a set point of 37oC by the cooperation of several negative- feedback circuits that regulate energy exchange with the environment. Fig. 40.9b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In contrast to negative feedback, positive feedback involves a change in some variable that trigger mechanisms that amplify rather than reverse the change. During childbirth, the pressure of the baby’s head against sensors near the opening of the uterus stimulates uterine contractions. These cause greater pressure against the uterine opening, heightening the contractions, which cause still greater pressure. Positive feedback brings childbirth to completion, a very different sort of process from maintaining a steady state. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Alterations in Homeostasis Set points and normal ranges can change with age or show cyclic variation Homeostasis can adjust to changes in external environment, a process called acclimatization Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Homeostatic processes for thermoregulation involve form, function, and behavior Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range Endothermic animals generate heat by metabolism; birds and mammals are endotherms Ectothermic animals gain heat from external sources; ectotherms include most invertebrates, fishes, amphibians, and non-avian reptiles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Endothermy is more energetically expensive than ectothermy In general, ectotherms tolerate greater variation in internal temperature, while endotherms are active at a greater range of external temperatures Endothermy is more energetically expensive than ectothermy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Variation in Body Temperature The body temperature of a poikilotherm varies with its environment, while that of a homeotherm is relatively constant Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Balancing Heat Loss and Gain Organisms exchange heat by four physical processes: conduction, convection, radiation, and evaporation Radiation Evaporation Convection Conduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Heat regulation in mammals often involves the integumentary system: skin, hair, and nails Epidermis Sweat pore Dermis Muscle Nerve Sweat gland Hypodermis Adipose tissue Oil gland Blood vessels Hair follicle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Five general adaptations help animals thermoregulate: Insulation Circulatory adaptations Cooling by evaporative heat loss Behavioral responses Adjusting metabolic heat production Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insulation Insulation is a major thermoregulatory adaptation in mammals and birds Skin, feathers, fur, and blubber reduce heat flow between an animal and its environment Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Circulatory Adaptations Regulation of blood flow near the body surface significantly affects thermoregulation Many endotherms and some ectotherms can alter the amount of blood flowing between the body core and the skin In vasodilation, blood flow in the skin increases, facilitating heat loss In vasoconstriction, blood flow in the skin decreases, lowering heat loss Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The arrangement of blood vessels in many marine mammals and birds allows for countercurrent exchange Countercurrent heat exchangers transfer heat between fluids flowing in opposite directions Countercurrent heat exchangers are an important mechanism for reducing heat loss Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Canada goose Bottlenose dolphin Blood flow Artery Vein Vein Artery Fig. 40-12 Canada goose Bottlenose dolphin Blood flow Artery Vein Vein Artery 35ºC 33º Figure 40.12 Countercurrent heat exchangers 30º 27º 20º 18º 10º 9º

Some bony fishes and sharks also use countercurrent heat exchanges Many endothermic insects have countercurrent heat exchangers that help maintain a high temperature in the thorax Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cooling by Evaporative Heat Loss Many types of animals lose heat through evaporation of water in sweat Panting increases the cooling effect in birds and many mammals Sweating or bathing moistens the skin, helping to cool an animal down Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Behavioral Responses Both endotherms and ectotherms use behavioral responses to control body temperature Some terrestrial invertebrates have postures that minimize or maximize absorption of solar heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Adjusting Metabolic Heat Production Some animals can regulate body temperature by adjusting their rate of metabolic heat production Heat production is increased by muscle activity such as moving or shivering Some ectotherms can also shiver to increase body temperature Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

While some aspects of the internal environment are maintained at a set point, regulated change is essential to normal body functions. In some cases, the changes are cyclical, such as the changes in hormone levels responsible for the menstrual cycle in women. In other cases, a regulated change is a reaction to a challenge to the body. For example, the human body reacts to certain infections by raising the set point for temperature to a slightly higher level, and the resulting fevers helps fight infection. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Over the short term, homeostatic mechanisms can keep a process, such a body temperature, close to a set point, whatever it is at that particular time. But over the longer term, homeostasis allows regulated change in the body’s internal environment. Internal regulation is expensive and animals use a considerable portion of their energy from the food they eat to maintain favorable internal conditions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Acclimatization in Thermoregulation Birds and mammals can vary their insulation to acclimatize to seasonal temperature changes When temperatures are subzero, some ectotherms produce “antifreeze” compounds to prevent ice formation in their cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Physiological Thermostats and Fever Thermoregulation is controlled by a region of the brain called the hypothalamus The hypothalamus triggers heat loss or heat generating mechanisms Fever is the result of a change to the set point for a biological thermostat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Sweat glands secrete sweat, which evaporates, cooling the body. Fig. 40-16 Sweat glands secrete sweat, which evaporates, cooling the body. Thermostat in hypothalamus activates cooling mechanisms. Blood vessels in skin dilate: capillaries fill; heat radiates from skin. Body temperature decreases; thermostat shuts off cooling mechanisms. Increased body temperature Homeostasis: Internal temperature of 36–38°C Body temperature increases; thermostat shuts off warming mechanisms. Decreased body temperature Figure 40.16 The thermostatic function of the hypothalamus in human thermoregulation Blood vessels in skin constrict, reducing heat loss. Thermostat in hypothalamus activates warming mechanisms. Skeletal muscles contract; shivering generates heat.

Body Plan An animal’s size and shape, often called body plans or designs, are fundamental aspects of form and function that significantly affect the way an animal interacts with its environment. The terms plan and design do not mean that animal body forms are products of a conscious invention. The body plan or design of an animal results from a pattern of development programmed by the genome, itself the product of millions of years of evolution due to natural selection. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

1. Physical laws constrain animal form Physical requirements constrain what natural selection can “invent,” including the size of single cells. This prevents an amoeba the size of a pro wrestler engulfing your legs when wading into a murky lake. An amoeba the size of a human could never move materials across its membrane fast enough to satisfy such a large blob of cytoplasm. In this example, a physical law - the math of surface-to- volume relations - limits the evolution of an organism’s form. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Similarly, the laws of hydrodynamics constrain the shapes that are possible for aquatic organisms that swim very fast. Tunas, sharks, penguins, dolphins, seal, and whales are all fast swimmers and all have the same basic shape, called a fusiform shape. This shape minimizes drag in water, which is about a thousand times denser than air. Fig. 40.6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The similar forms of speedy fishes, birds, and marine mammals are a consequence of convergent evolution in the face of the universal laws of hydrodynamics. Convergence occurs because natural selection shapes similar adaptations when diverse organisms face the same environmental challenge, such as the resistance of water to fast travel. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2. Body size and shape affect interactions with the environment An animal’s size and shape have a direct effect on how the animal exchanges energy and materials with its surroundings. As a requirement for maintaining the fluid integrity of the plasma membrane of its cells, an animal’s body must be arranged so that all of its living cells are bathed in an aqueous medium. Exchange with the environment occurs as dissolved substances diffuse and are transported across the plasma membranes between the cells and their aqueous surroundings. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

For example, a single-celled protist living in water has a sufficient surface area of plasma membrane to service its entire volume because it is so small. A large cell has less surface area relative to its volume than a smaller cell of the same shape. These considerations place a physical constraint on cell size. Fig. 40.7a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Multicellular animals are composed of microscopic cells, each with its own plasma membrane that acts as a loading and unloading platform for a modest volume of cytoplasm This only works if all the cells of the animal have access to a suitable aqueous environment. For example, a hydra, built on the sac plan, has a body wall only two cell layers thick. Because its gastrovascular cavity opens to the exterior, both outer and inner layers of cells are bathed in water. Fig. 40.7b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Another way to maximize exposure to the surrounding medium is to have a flat body. For instance, a tapeworm may be several meters long, but because it is very thin, most of its cells are bathed in the intestinal fluid of the worm’s vertebrate host, from which it obtains nutrients. While two-layered sacs and flat shapes are designs that put a large surface area in contact with the environment, these solutions do not lead to much complexity in internal organization. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Most animals are more complex and made up of compact masses of cells, producing outer surfaces that are relatively small compared to their volume. Most organisms have extensively folded or branched internal surfaces specialized for exchange with the environment. The circulatory system shuttles material among all the exchange surfaces within the animal. Fig. 40.8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 40.4 Internal exchange surfaces of complex animals External environment CO2 Food O2 Mouth Animal body Respiratory system Blood 50 µm 0.5 cm Lung tissue Nutrients Cells Heart Circulatory system 10 µm Interstitial fluid Digestive system Figure 40.4 Internal exchange surfaces of complex animals Lining of small intestine Excretory system Kidney tubules Anus Unabsorbed matter (feces) Metabolic waste products (nitrogenous waste)

Although exchange with the environment is a problem for animals whose cells are mostly internal, complex forms have distinct benefits. Because the animal’s external surface need not be bathed in water, it is possible for the animal to live on land. Also, because the immediate environment for the cells is the internal body fluid (interstitial fluid), the animal’s organ systems can control the composition of the solution bathing its cells. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings