Animal Structure and Function

AN INTRODUCTION TO ANIMAL STRUCTURE AND FUNCTION Introduction The study of animal form and function is integrated by the common set of problems that all animals must solve. These include how to extract oxygen from the environment, how to nourish themselves, how to excrete waste products, and how to move. Animals of diverse evolutionary histories and varying complexities must solve these general challenges of life. A. Functional Anatomy: An Overview 1. Animal form and function reflects biology’s major themes Animals provide vivid examples of biology’s overarching theme of evolution. The adaptations observed in a comparative study of animals evolved by natural selection. For example, the long, tonguelike proboscis of a hawkmoth is a structural adaptation for feeding. Recoiled when not in use, the proboscis extends as a straw through which the moth can suck nectar from deep within tube-shaped flowers. While natural selection provides a mechanism for long-term adaptation, organisms also have the capacity to adjust to environmental change over the short term by physiological responses. For example, while most insects are inactive when cold, the hawkmoth, Manduca sexta, can forage for nectar when air temperatures are as low as 5oC. The moth uses a shivering-like mechanism for preflight warm up of its flight muscles. Once in flight, the waste heat of metabolic activity in the flight muscles and other adaptations maintain a muscle temperature of 30oC, even when the external environment is close to freezing. Searching for food, generating body heat and regulating internal temperature, sensing and responding to environmental stimuli, and all other animal activities require fuel in the form of chemical energy. The concept of bioenergetics—how organisms obtain, process, and use their energy resources—is another connecting theme in the comparative study of animals. Animals also show a correlation between structure and function. Form fits function at all levels of life, from molecules to organisms. Knowledge of a structure provides insight into what it does and how its works. Conversely, knowing the function of a structure provides insight about its construction. Anatomy is the study of the structure of an organism. Physiology is the study of the functions an organism performs. The distinction blurs when we apply the structure-function theme, and “anatomy-and-physiology” rolls off the tongue as though it were one big compound noun. The form-function principle is just another extension of biology’s central theme of evolution. 2. Function correlates with structure in the tissues of animals Life is characterized by hierarchical levels of organization, each with emergent properties. Animals are multicellular organisms with their specialized cells grouped into tissues. In most animals, combinations of various tissues make up functional units called organs, and groups of organs that work together form organ systems. For example, the human digestive system consists of a stomach, small intestine, large intestine, and several other organs, each a composite of different tissues. Tissues are groups of cell with a common structure and function. Different types of tissues have different structures that are especially suited to their functions. A tissue may be held together by a sticky extracellular matrix that coats the cells or weaves them together in a fabric of fibers. The term tissue is from a Latin word meaning “weave.” Tissues are classified into four main categories: epithelial tissue, connective tissue, nervous tissue, and muscle tissue. Occurring in sheets of tightly packed cells, epithelial tissue covers the outside of the body and lines organs and cavities within the body. The cells of a epithelium are closely joined and in many epithelia, the cells are riveted together by tight junctions. The epithelium functions as a barrier protecting against mechanical injury, invasive microorganisms, and fluid loss. The free surface of the epithelium is exposed to air or fluid, and the cells at the base of the barrier are attached to a basement membrane, a dense mat of extracellular matrix. Epithelia are classified by the number of cell layers and the shape of the cells on the free surface. A simple epithelium has a single layer of cells, and a stratified epithelium has multiple tiers of cells. The shapes of cells may be cuboidal (like dice), columnar (like bricks on end), or squamous (flat like floor tiles). Some epithelia, called glandular epithelia, absorb or secrete chemical solutions. For example, glandular epithelia lining tubules in the thyroid gland secrete a hormone that regulates fuel consumption. The glandular epithelia that line the lumen of the digestive and respiratory tracts form a mucous membrane that secretes a slimy solution called mucus that lubricates the surface and keeps it moist. The free epithelial surfaces of some mucous membranes have beating cilia that move the film of mucus along the surface. In the respiratory tubes, this traps dust and particles. Connective tissue functions mainly to bind and support other tissues. Connective tissues have a sparse population of cells scattered through an extracellular matrix. The matrix generally consists of a web of fibers embedding in a uniform foundation that may be liquid, jellylike, or solid. In most cases, the connective tissue cells secrete the matrix. There are three kinds of connective tissue fibers, which are all proteins: collagenous fibers, elastic fibers, and reticular fibers. Collagenous fibers are made of collagen. Collagenous fibers are nonelastic and do not tear easily when pulled lengthwise. Elastic fibers are long threads of elastin. Elastin fiber provides a rubbery quality. Reticular fibers are very thin and branched. Composed of collagen and continuous with collagenous fibers, they form a tightly woven fabric that joins connective tissue to adjacent tissues. The major types of connective tissues in vertebrates are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage, bone, and blood. Each has a structure correlated with its specialized function. Loose connective tissue binds epithelia to underlying tissues and functions as packing materials, holding organs in place. Loose connective tissue has all three fiber types. Two cell types predominate in the fibrous mesh of loose connective tissue. Fibroblasts secrete the protein ingredients of the extracellular fibers. Macrophages are amoeboid cells that roam the maze of fibers, engulfing bacteria and the debris of dead cells by phagocytosis. Adipose tissue is a specialized form of loose connective tissues that store fat in adipose cells distributed throughout the matrix. Adipose tissue pads and insulates the body and stores fuel as fat molecules. Each adipose cell contains a large fat droplet that swells when fat is stored and shrinks when the body uses fat as fuel. Fibrous connective tissue is dense, due to its large number of collagenous fibers. The fibers are organized into parallel bundles, an arrangement that maximizes nonelastic strength. This type of connective tissue forms tendons, attaching muscles to bones, and ligaments, joining bones to bones at joints. Cartilage has an abundance of collagenous fibers embedded in a rubbery matrix made of a substance called chondroitin sulfate, a protein-carbohydrate complex. Chondrocytes secrete collagen and chondroitin sulfate. The composite of collagenous fibers and chondroitin sulfate makes cartilage a strong yet somewhat flexible support material. The skeleton of a shark is made of cartilage and the embryonic skeletons of many vertebrates are cartilaginous. We retain cartilage as flexible supports in certain locations, such as the nose, ears, and vertebral disks. The skeleton supporting most vertebrates is made of bone, a mineralized connective tissue. Osteoblasts deposit a matrix of collagen. Then, calcium, magnesium, and phosphate ions combine and harden within the matrix into the mineral hydroxyapatite. The combination of hard mineral and flexible collagen makes bone harder than cartilage without being brittle. The microscopic structure of hard mammalian bones consists of repeating units called osteons. Each osteon has concentric layers of mineralized matrix deposited around a central canal containing blood vessels and nerves that service the bone. Blood functions differently from other connective tissues, but it does have an extensive extracellular matrix. The matrix is a liquid called plasma, consisting of water, salts, and a variety of dissolved proteins. Suspended in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells) and cell fragments called platelets. Red cells carry oxygen. White cells function in defense against viruses, bacteria, and other invaders. Platelets aid in blood clotting. Nervous tissue senses stimuli and transmits signals from one part of the animal to another. The functional unit of nervous tissue is the neuron, or nerve cell. It consists of a cell body and two or more extensions, called dendrites and axons. Dendrites transmit nerve impulses from their tips toward the rest of the neuron. Axons transmit impulses toward another neuron or toward an effector, such as a muscle cell. Muscle tissue is composed of long cells called muscle fibers that are capable of contracting when stimulated by nerve impulses. Arranged in parallel within the cytoplasm of muscle fibers are large numbers of myofibrils made of the contractile proteins actin and myosin. Muscle is the most abundant tissue in most animals, and muscle contraction accounts for most of the energy-consuming cellular work in active animals. There are three types of muscle tissue in the vertebrate body: skeletal muscle, cardiac muscle, and smooth muscle. Attached to bones by tendons, skeletal muscle is responsible for voluntary movements. Skeletal muscle is also called striated muscle because the overlapping filaments give the cells a striped (striated) appearance under the microscope. Cardiac muscle forms the contractile wall of the heart. It is striated like cardiac muscle, but cardiac cells are branched. The ends of the cells are joined by intercalated disks, which relay signals from cell to cell during a heartbeat. Smooth muscle, which lacks striations, is found in the walls of the digestive tract, urinary bladder, arteries, and other internal organs. The cells are spindle-shaped. They contract more slowly than skeletal muscles but can remain contracted longer. Controlled by different kinds of nerves than those controlling skeletal muscles, smooth muscles are responsible for involuntary body activities. These include churning of the stomach and constriction of arteries. 3. The organ systems of an animal are interdependent In all but the simplest animals (sponges and some cnidarians) different tissues are organized into organs. Many vertebrate organs are suspended by sheets of connective tissues called mesenteries in body cavities moistened or filled with fluid. Mammals have a thoracic cavity housing the lungs and heart that is separated from the lower abdominal cavity by a sheet of muscle called the diaphragm. In some organs the tissues are arranged in layers. For example, the vertebrate stomach has four major tissues layers. A thick epithelium lines the lumen and secretes mucus and digestive juices into it. Outside this layer is a zone of connective tissue, surrounded by a thick layer of smooth muscle. Another layer of connective tissue encapsulates the entire stomach. Organ systems carry out the major body functions of most animals. Each organ system consists of several organs and has specific functions. The efforts of all systems must be coordinated for the animal to survive. For instance, nutrients absorbed from the digestive tract are distributed throughout the body by the circulatory system. The heart that pumps blood through the circulatory system depends on nutrients absorbed by the digestive tract and also on oxygen obtained from the air or water by the respiratory system. Any organism, whether single-celled or an assembly of organ systems, is a coordinated living whole greater than the sum of its parts. B. Body Plans and the External Environment 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. 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. Similarly, the laws of hydrodynamics constrain the shapes that are possible for aquatic organisms that swim very fast. Tunas, sharks, penguins, dolphins, seals, 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. 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. 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. 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. 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. 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. Most animals are more complex and are 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. 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, the animal’s organ systems can control the composition of the solution bathing its cells. C. Regulating the Internal Environment 1. Mechanisms of homeostasis moderate changes in the internal environment More than a century ago, Claude Bernard made the distinction between external environments surrounding an animal and the internal environment in which the cells of the animal actually live. The internal environment of vertebrates is called the interstitial fluid. This fluid exchanges nutrients and wastes with blood contained in microscopic vessels called capillaries. Bernard also recognized that many animals tend to maintain relatively constant conditions in their internal environment, even when the external environment changes. 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. 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. Still, the stability of the internal environment is remarkable. Today, Bernard’s “constant internal milieu” is incorporated into the concept of homeostasis, which means “steady state,” or internal balance. Actually the internal environment of an animal always fluctuates slightly. Homeostasis is a dynamic state, an interplay between outside forces that tend to change the internal environment and internal control mechanisms that oppose such changes. 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. One type of control circuit, a negative-feedback system, can control 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. In a negative-feedback system, a change in the variable being monitored triggers the control mechanism to counteract further change in the same direction. Owing to a time lag between receptor and response, the variable drifts slightly above and below the set point, but the fluctuations are moderate. Negative-feedback mechanisms prevent small changes from becoming too large. Most homeostatic mechanisms in animals operate on this principle of negative feedback. 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. One mechanism by which humans control body temperature involves sweating as a means to dispose of metabolic heat and cool the body. If the thermostat in the brain detects a rise in the temperature of the blood above the set point, it sends nerve impulses directing sweat glands to increase their production of sweat. This lowers body temperature by evaporative cooling. When body temperature drops below the set point, the thermostat in the brain stops sending the signals to the glands and the body retains more of the heat produced by metabolism. • In contrast to negative feedback, positive feedback involves a change in some variable that triggers mechanisms that amplify rather than reverse the change. For example, 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. 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 fever helps fight infection. 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. D. Introduction to the Bioenergetics of Animals 1. Animals are heterotrophs that harvest chemical energy from the food they eat All organisms require chemical energy for growth, physiological processes, maintenance and repair, regulation, and reproduction. Plants use light energy to build energy-rich organic molecules from water and CO2, and then use those organic molecules for fuel. In contrast, animals are heterotrophs and must obtain their chemical energy in food, which contains organic molecules synthesized by other organisms. Food is digested by enzymatic hydrolysis, and energy-containing food molecules are absorbed by body cells. Most fuel molecules are used to generate ATP by the catabolic processes of cellular respiration and fermentation. The chemical energy of ATP powers cellular work, enabling cells, organs, and organ systems to perform the many functions that keep an animal alive. Since the production and use of ATP generates heat, an animal must continuously loose heat to its surroundings. After energetic needs are met, any remaining food molecules can be used in biosynthesis. This includes body growth and repair, synthesis of storage material such as fat, and production of reproductive structures, including gametes. Biosynthesis requires both carbon skeletons for new structures and ATP to power their assembly. 2. Metabolic rate provides clues to an animal’s bioenergetic “strategy” The flow of energy through an animal - an animal’s bioenergetics - ultimately sets the limits on the animal’s behavior, growth, and reproduction and determines how much food it needs. An understanding of energetics tells us a great deal about an animal’s adaptations and how it fits into its environment as an energy consumer. Physiologists measure the rates at which animals use chemical energy to meet its basic needs and how these rates change in different circumstances. The amount of energy an animal uses in a unit of time is called its metabolic rate - the sum of all the energy-requiring biochemical reactions occurring over a given time interval. Energy is measured in calories (cal) or kilocalories (kcal). A kilocalorie is 1,000 calories. The term Calorie, with a capital C, as used by many nutritionists, is actually a kilocalorie. Metabolic rate can be determined several ways. Because nearly all the chemical energy used in cellular respiration eventually appears as heat, metabolic rate can be measured by monitoring an animal’s heat loss. A small animal can be placed in a calorimeter, which is a closed insulated chamber equipped with a device that records the animals heat loss. A more indirect way to measure metabolic rate is to determine the amount of oxygen consumed or carbon dioxide produced by an animal’s cellular respiration. These devices may measure changes in oxygen consumed or carbon dioxide produced as activity changes. Over long periods, the rate of food consumption and the energy content of food can be used to estimate metabolic rate. A gram of protein or carbohydrate contains about 4.5-5 kcal and a gram of fat contains 9 kcal. This method must account for the energy in food that cannot be used by the animal (the energy lost in feces and urine). There are two basic bioenergetic “strategies” used by animals. Birds and mammals are mainly endothermic, maintaining their body temperature at a certain level with heat generated by metabolism. Endothermy is a high-energy strategy that permits intense, long-duration activity of a wide range of environmental temperatures. Most fishes, amphibians, reptiles, and invertebrates are ectothermic, meaning they do not produce enough metabolic heat to have much effect on body temperature. The ectothermic strategy requires much less energy than is needed by endotherms, because of the energy cost of heating (or cooling) an endothermic body. However, ectotherms are generally incapable of intense activity over long periods. 3. Metabolic rate per gram is inversely related to body size among similar animals One of animal biology’s most intriguing, but largely unanswered questions has to do with the relationship between body size and metabolic rate. Physiologists have shown that the amount of energy it takes to maintain each gram of body weight is inversely related to body size. For example, each gram of a mouse consumes about 20 times more calories than a gram of an elephant. The higher metabolic rate of a smaller animal demands a proportionately greater delivery rate of oxygen. A smaller animal also has a higher breathing rate, blood volume (relative to size), and heart rate (pulse) and must eat much more food per unit of body mass. One hypothesis for the inverse relationship between metabolic rate and size is that the smaller the size of an endotherm, the greater the energy cost of maintaining a stable body temperature. The smaller the animal, the greater its surface to volume ratio, and thus the greater loss of heat to (or gain from) the surroundings. However, this hypothesis fails to explain the inverse relationship between metabolism and size in ectotherms. Nor is it supported by experimental tests. Researchers continue to search for causes underlying this inverse relationship. 4. Animals adjust their metabolic rates as conditions change Every animal has a range of metabolic rates. Minimal rates power the basic functions that support life, such as cell maintenance, breathing, and heartbeat. The metabolic rate of a nongrowing endotherm at rest, with an empty stomach, and experiencing no stress is called the basal metabolic rate (BMR). The BMR for humans averages about 1,600 to 1,800 kcal per day for adult males and about 1,300 to 1,500 kcal per day for adult females. • In ectotherms, body temperature changes with temperature of the surroundings, and so does metabolic rate. Therefore, the minimal metabolic rate of an ectotherm must be determined at a specific temperature. The metabolic rate of a resting, fasting, nonstressed ectotherm is called its standard metabolic rate (SMR). For both ectotherms and endotherms, activity has a large effect on metabolic rate. Any behavior consumes energy beyond the BMR or SMR. Maximal metabolic rates (the highest rates of ATP utilization) occur during peak activity, such as lifting heavy weights, all-out running, or high-speed swimming. In general, an animal’s maximum possible metabolic rate is inversely related to the duration of activity. Both an alligator (ectotherm) and a human (endotherm) are capable of intense exercise in short spurts of a minute or less. These “sprints” are powered by the ATP present in muscle cells and ATP generated anaerobically by glycolysis. Neither organism can maintain their maximum metabolic rates and peak activity levels over longer periods of exercise, although the endotherm has an advantage in endurance tests. The BMR of a human is much higher than the SMR of an alligator. Both can reach high levels of maximum potential metabolic rates for short periods, but metabolic rate drops as the duration of the activity increases and the source of energy shifts toward aerobic respiration. Sustained activity depends on the aerobic process of cellular respiration for ATP supply. An endotherm’s respiration rate is about 10 times greater than an ectotherm’s. Only endotherms are capable of long-duration activities such as distance running. Between the extremes of BMR or SMR and maximal metabolic rate, many factors influence energy requirements. These include age, sex, size, body and environmental temperatures, the quality and quantity of food, activity level, oxygen availability, hormonal balance, and time of day. Diurnal organisms, such as birds, humans, and many insects, are usually active and have their highest metabolic rates during daylight hours. Nocturnal organisms, such as bats, mice, and many other mammals are usually active at night or near dawn and dusk, and have their highest metabolic rates then. Metabolic rates measured when animals are performing a variety of activities give a better idea of the energy costs of everyday life. For most terrestrial animals, the average daily rate of energy consumption is 2-4 times BMR or SMR. Humans in most developed countries have an unusually low average daily metabolic rate of about 1.5 times BMR - an indication of relatively sedentary lifestyles. 5. Energy budgets reveal how animals use energy and materials Different species of animals use the energy and materials in food in different ways, depending on their environment, behavior, size, and basic energy “strategy” of endothermy or ectothermy. For most animals, the majority of food is devoted to the production of ATP, and relatively little goes to growth or reproduction. However, the amount of energy used for BMR (or SMR), activity, and temperature control varies considerably between species. For example, the typical annual energy budget of four vertebrates reinforces two important concepts in bioenergetics. First, a small animal has a much greater energy demand per kg than does a large animal of the same class. Second, an ectotherm requires much less energy per kg than does an endotherm of equivalent size. Further, size and energy strategy has a great influence on how the total annual energy expenditure is distributed among energetic needs. The human female spends a large fraction of her energy budget for BMR and relatively little for activity and temperature regulation. The cost of nine months of pregnancy and several months of breast feeding amounts to only 5-8% of the mother’s annual energy requirements. Growth amounts to about 1% of her annual energy budget. A male penguin spends a much larger faction of his energy expenditures for activity because he must swim to catch his food. Because the penguin is well insulated and fairly large, he has relatively low costs of temperature regulation despite living in the cold Antarctic environment. His reproductive costs, about 6% of annual energy expenditures, mainly come from incubating eggs and bringing food to his chicks. Penguins, like most birds, do not grow once they are adults. The deer mouse spends a large fraction of her energy budget on temperature regulation. Because of the high surface-to-volume ratio that goes with small size, mice lose body heat rapidly to the environment and must constantly generate metabolic heat to maintain body temperature. Female deer mice spend about 12% of their energy budgets on reproduction. In contrast to endotherms, the ectothermic python has no temperature regulation costs. Like most reptiles, she grows continuously throughout life. For example, in one year she can add 750 g of new body tissue and produce about 650 g of eggs. Through the python’s economical ectothermic strategy, she expended only 1/40 of the energy expended by the same-sized endothermic penguin.