Form and Function The comparative study of animals reveals that form and function are closely correlated. What an animals does (function) is closely related to the structure of its body (form). By selecting, over many generations, what works best among the available variations in a population, natural selection adapts an organisms’ anatomy and physiology to suit its environment.

Size and shape are constrained by physical laws How well an organism performs an action (e.g. swims, flies, runs) depends on the animal’s shape and size, which are strongly influenced by physical laws. For example, a major challenge for swimming animals is overcoming drag. As a result, a wide variety of organisms have evolved similar streamlined body shapes and control structures (fins and flippers) that enable them to move smoothly and powerfully through water.

Convergent evolution Different species have converged on similar solutions to the same evolutionary challenge. (a) Tuna (b) Shark (c) Penguin (d) Dolphin Figure 40.2a–e (e) Seal

Size and shape are constrained by physical laws Physical laws and the need to exchange materials with the environment place limits on the range of animal forms. An animal’s size and shape have a direct effect on how the animal exchanges energy and materials with its surroundings. Organisms that depend on exchanging gases directly across their surface must have a large surface area relative to their volume (e.g. an amoeba).

Size and shape are constrained by physical laws A single-celled protist living in water has a sufficient surface area of plasma membrane to service its entire volume of cytoplasm Diffusion (a) Single cell Figure 40.3a

Size and shape are constrained by physical laws Surface to volume ratio is a critical limiting factor in how large a single celled organism can grow. As a cell’s linear dimensions (e.g. length) are increased,its volume increases as a cube function of that linear dimension. However, the surface area by which the cell is supplied with its requirements increases only as a square function. Thus, the volume a cell can attain is limited by its surface area.

Size and shape are constrained by physical laws Multi-cellular organisms can also depend on diffusion to supply them with their needs, but they must have thin body walls for diffusion to work. Mouth Gastrovascular cavity Diffusion Diffusion Figure 40.3b (b) Two cell layers

Size and shape are constrained by physical laws Insects have an impermeable exoskeleton and do not exchange gases across it. To get gases to the tissues they depend on a dense network of tubes called the tracheal system that connect to the outside via pores. The system still depends on passive diffusion and tubes must reach to within a few cells to transfer gases. As a result, the size insects can attain is limited (despite what Hollywood movies show).

Size and shape are constrained by physical laws Organisms with more complex body plans solve the surface area problem by having highly folded internal surfaces specialized for exchanging materials (e.g. villi of gut). The folding greatly increases the surface area. In addition, active transportation of materials to and from sites far from the surface takes place by means of a circulatory system.

Figure 40.4 External environment Mouth Food CO2 O2 Animal body Respiratory system Blood 50 µm 0.5 cm Cells A microscopic view of the lung reveals that it is much more spongelike than balloonlike. This construction provides an expansive wet surface for gas exchange with the environment (SEM). Heart Nutrients Circulatory system 10 µm Interstitial fluid Digestive system The lining of the small intestine, a diges- tive organ, is elaborated with fingerlike projections that expand the surface area for nutrient absorption (cross-section, SEM). Excretory system Anus Inside a kidney is a mass of microscopic tubules that exhange chemicals with blood flowing through a web of tiny vessels called capillaries (SEM). Unabsorbed matter (feces) Metabolic waste products (urine) Figure 40.4

Form and function are correlated at all levels of organization Animals are composed of cells Groups of cells with a common structure and function make up tissues Different tissues make up organs which together make up organ systems.

Tissue Structure and Function Different types of tissues have different structures that are suited to their functions Tissues are classified into four main categories: Epithelial, connective, muscle, and nervous.

Epithelial Tissue Epithelial tissue Covers the outside of the body and lines organs and cavities within the body Contains cells that are closely joined

Connective Tissue Connective tissue Functions mainly to bind and support other tissues Contains sparsely packed cells scattered throughout an extracellular matrix

Muscle Tissue Muscle tissue Is composed of long cells called muscle fibers capable of contracting in response to nerve signals Is divided in the vertebrate body into three types: skeletal, cardiac, and smooth

Nervous Tissue Nervous tissue Senses stimuli and transmits signals throughout the animal

Organs and Organ Systems In all but the simplest animals (i.e. sponges) different tissues are organized into organs. Organs are made up of multiple tissues each of which performs a function that contributes to the overall functioning of the organ.

Arrangement of tissues in an organ (the stomach) Lumen of stomach Mucosa. The mucosa is an epithelial layer that lines the lumen. Submucosa. The submucosa is a matrix of connective tissue that contains blood vessels and nerves. Muscularis. The muscularis consists mainly of smooth muscle tissue. 0.2 mm Serosa. External to the muscularis is the serosa, a thin layer of connective and epithelial tissue. Figure 40.6

Organ systems Representing a level of organization higher than organs, organ systems carry out the major body functions of most animals. These include: respiratory, circulatory, digestive, excretory, and immune systems among others.

Organ systems in mammals Table 40.1

Energetics Animals use the chemical energy in food to sustain form and function All organisms require chemical energy for growth, repair, physiological processes, regulation, and reproduction.

Energy Sources and Allocation Animals harvest chemical energy from the food they eat Once food has been digested, the energy-containing molecules are usually used to make ATP (adenosine triphosphate), which powers cellular work.

Energetics After the energetic needs of staying alive are met any remaining molecules from food can be used in biosynthesis Organic molecules in food External environment Animal body Digestion and absorption Heat Energy lost in feces Nutrient molecules in body cells Energy lost in urine Carbon skeletons Cellular respiration Heat ATP Biosynthesis: growth, storage, and reproduction Cellular work Heat Figure 40.7 Heat

Bioenergetic strategies Birds and mammals are mainly endothermic, meaning that their bodies are warmed mostly by heat generated by metabolism. They typically have high metabolic rates. Invertebrates, fishes, amphibians, and reptiles other than birds are ectothermic, meaning that they gain their heat mostly from external sources and have lower metabolic rates.

Thermoregulation Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range. In general, ectotherms tolerate greater variation in internal temperature than endotherms

Figure 40.12 40 River otter (endotherm) 30 Body temperature (°C) 20 Largemouth bass (ectotherm) Ambient (environmental) temperature (°C) Body temperature (°C) 40 30 20 10 Figure 40.12

Endothermy is more energetically expensive than ectothermy because energy must be expended to maintain a higher body temperature than the surrounding environment. However, endothermic animals can remain active under a wider range of conditions than ectothermic ones.

Organisms exchange heat by four physical processes Modes of Heat Exchange Organisms exchange heat by four physical processes Radiation: the emission of electromagnetic energy. Evaporation: loss of energy by loss of liquid molecules as gas. Conduction: direct transfer of heat energy between objects in contact. Convection: transfer of energy by movement of air or liquid past an object.

Modes of Heat Exchange Figure 40.13 Radiation is the emission of electromagnetic waves by all objects warmer than absolute zero. Radiation can transfer heat between objects that are not in direct contact, as when a lizard absorbs heat radiating from the sun. Evaporation is the removal of heat from the surface of a liquid that is losing some of its molecules as gas. Evaporation of water from a lizard’s moist surfaces that are exposed to the environment has a strong cooling effect. Convection is the transfer of heat by the movement of air or liquid past a surface, as when a breeze contributes to heat loss from a lizard’s dry skin, or blood moves heat from the body core to the extremities. Conduction is the direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock. Figure 40.13

Balancing Heat Loss and Gain Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss.

Insulation Insulation, which is a major thermoregulatory adaptation in mammals and birds, reduces the flow of heat between an animal and its environment. Insulating materials include hair, feathers and blubber. Hair and feathers trap air which is an excellent insulator.

Circulatory Adaptations 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.

Countercurrent heat exchangers Because heat can be lost rapidly in water by convection many marine mammals and birds have arrangements of blood vessels called countercurrent heat exchangers in their extremities that help limit heat loss. In these heat exchangers arteries and veins run very close to each other. As a result, heat can flow from one to the other.

Countercurrent heat exchangers Blood in the core of the body is warm and flows through the arteries out to the extremities. In the counter current heat exchanger the warm arterial blood flows next to cooler venous blood returning from the extremity and as a result heat is transferred from the warm arterial blood to the cooler venous blood.

Countercurrent heat exchangers Even though the arterial blood is cooling as it flows towards the extremity it remains warmer than the venous blood it is flowing next to. As a result, heat continues to flow from the arterial to the venous blood, and by the time the arterial blood reaches the extremity most of the heat in the arterial blood has been transferred to the venous blood and the heat retained in the core of the animal.

Countercurrent heat exchangers are found in the legs of birds and flippers of dolphins and whales. They also are found in some ectotherms such as tuna and moths, which maintain a core temperature warmer than the rest of their body.

Figure 40.15 Pacific bottlenose dolphin Canada goose Blood flow Artery In the flippers of a dolphin, each artery is surrounded by several veins in a countercurrent arrangement, allowing efficient heat exchange between arterial and venous blood. Canada goose Artery Vein 35°C Blood flow 30º 20º 10º 33° 27º 18º 9º Pacific bottlenose dolphin 2 1 3 Arteries carrying warm blood down the legs of a goose or the flippers of a dolphin are in close contact with veins conveying cool blood in the opposite direction, back toward the trunk of the body. This arrangement facilitates heat transfer from arteries to veins (black arrows) along the entire length of the blood vessels. Near the end of the leg or flipper, where arterial blood has been cooled to far below the animal’s core temperature, the artery can still transfer heat to the even colder blood of an adjacent vein. The venous blood continues to absorb heat as it passes warmer and warmer arterial blood traveling in the opposite direction. As the venous blood approaches the center of the body, it is almost as warm as the body core, minimizing the heat lost as a result of supplying blood to body parts immersed in cold water. 1 3 Figure 40.15

Cooling by Evaporative Heat Loss Many types of animals lose heat through the evaporation of water in sweat or use panting to cool their bodies. Bathing in water or mud also helps to cool animals down.

Behavioral Responses Both endotherms and ectotherms use a variety of behavioral responses to control body temperature. For example, they move into the sun or shade to gain heat or cool down. They may also adopt certain body postures (e.g. birds and insects spread their wings) to warm up or cool down.

Adjusting Metabolic Heat Production Some animals can regulate body temperature by adjusting their rate of metabolic heat production. For example, many species of flying insects use shivering to warm up before taking flight.

Time from onset of warmup (min) PREFLIGHT WARMUP FLIGHT Thorax Abdomen Temperature (°C) Time from onset of warmup (min) 40 35 30 25 2 4 Figure 40.20

Feedback Mechanisms in Thermoregulation Mammals regulate their body temperature by a complex negative feedback system that involves several organ systems. In humans, a specific part of the brain, the hypothalamus contains a group of nerve cells that function as a thermostat. If the hypothalamus detects an increase in temperature, for example, it signals sweat glands to become more active and increase evaporative cooling and blood vessels in the skin to vasodilate facilitating heat loss.