1 CHAPTER-1 GENERAL THEMES IN PHYSIOLOGY TOPIC 1 THE SPECTRUM OF STUDY OF PHYSIOLOGY Physiology—Definition Animal Physiology deals with the study of functions of the tissues, organs and organ systems of animals. The ultimate goal of the study of physiology is to understand the mechanisms that operate in animals at all levels, in physical and chemical terms. Physiological Mechanisms Obey Laws of Physics and Chemistry: The knowledge of Physiology is firmly rooted in the laws and concepts of physics and chemistry. Following are the examples of some physical and chemical laws and concepts that relate to various physiological processes: • Ohm’s Law—applies to blood flow and pressure; ionic current; capacitance of membranes. • Boyle’s Law and the Ideal Gas Law—apply to respiration. • Law of Gravity—applies to blood flow. • Concepts of kinetic and potential energy—apply to muscle contraction; chest movements during inhalation and exhalation. • Concepts of Inertia, momentum, velocity and drag—apply to animal locomotion. These are just a few examples. Many more physical and chemical laws and concepts are used to explain various physiological phenomena. Curiosity underlies the learning of Physiology Underlying all studies of animal physiology is the curiosity (desire to know) about how animals and their systems work. e.g. • How can a Hummingbird’s heart beat 20 times a second during hovering flight? • How do insects see in the ultraviolet spectrum? • How do kangaroo rats survive in the desert with no access to drinking water? Such questions fuel the curiosity of animal physiologists and there is no bound to this curiosity. The more we know, the more we realize how little we know about physiological systems of animals. From Animal Physiology to Human Physiology Study of animal physiology has provided an insight into the physiological processes of humans. The human species shares the same fundamental biological processes with all other animal species and has a linked evolutionary history. To illustrate this relationship let’s give few examples: 2 • The heart beat in the human body results from the same physiological mechanisms that underlie heart function in fishes, frogs, snakes, birds, or apes. • Similarly, the molecular events that produce an electrical nerve impulse in the human brain are fundamentally the same as those that produce an impulse in the nerves of a squid, crab or rat. For these reasons, the study of animal physiology has made innumerable contributions to understanding human physiology. In fact, most of what we have learned about the functions of human cells, tissues, and organs was known first through the study of various species of vertebrate and invertebrate animals. While animal physiology lays the foundation of human physiology, the human physiology is the foundation of scientific medical practice. Understanding of the functioning and malfunctioning of living tissues provides the foundation for developing effective, scientifically sound treatment for human diseases. In the modern times, animal physiology has contributed through new techniques for generating unique animal models for specific human diseases (e.g., diabetic mice, congenitally fat rats, zebra fish embryos with heart defects). These models allow a wide range of experiments that provide the insights into the underlying physiological processes of such diseases and defects. 3 TOPIC 2 CENTRAL THEMES IN ANIMAL PHYSIOLOGY Our major goals of study of animal physiology are to: • explore the physiological processes that are basic to all animal groups • show how they have been shaped by selective forces during evolution Comparing and contrasting how different organisms have adapted to survive similar environmental challenges provides useful insights about the patterns of physiological evolution and the adaptive value of physiological processes. Central themes in animal physiology: As we study animal physiology and physiological adaptations, we notice several basic themes, repeatedly emerging. We shall briefly discuss five major themes here, i.e. (i) Structure-function relationships (ii) Adaptation, Acclimatization and Acclimation (iii) Principle of homeostasis (iv) Feedback control systems (v) Conformity and Regulation 4 TOPIC-3 STRUCTURE-FUNCTION RELATIONSHIPS One of the central principles of animal physiology is that “function is based on structure”. In other words, in living organisms, structural design is matched to functional demands. In very simple manner we can say that the way something is arranged enables it to play its role and fulfill its job, within an organism. Such structure-function relationships arise through evolution and natural selection. Example: Let’s illustrate this with a comprehensive example: • A frog leaps for a prey. It contracts the powerful skeletal muscles attached to the bones of its limbs. • As the prey is swallowed, it reaches the stomach where the smooth muscles grind and mix the food contents. • After digestion, the nutrients are absorbed into the blood. The blood flows due to the beating of cardiac muscles of the heart. Throughout this process in frog’s body, three structurally distinct forms of muscles carry out three distinct functions. Thus our basic point is elaborated: • The skeletal muscles are evolved and adapted for movement of the bones. • The smooth muscles of digestive tract are adapted for such contractions that help grinding and mixing the food materials. • Cardiac muscles are specialized to pump and circulate blood throughout the body. Structure-Function Relationships are demonstrated at all levels of biological organization: The structure-function relationships are not restricted only to the muscles but are found in every tissue of an animal’s body. And more correctly speaking, these relationships are demonstrated at all levels of biological organization down to the molecular and atomic levels. Example: To illustrate this, let’s have a look at the figure. This figure illustrates the structure-function relationships of muscle tissue at all levels of biological organization. Biological function at each level of organization depends on the structure of that level and more microscopic levels below it. Beginning with the whole animal, this principle can be traced down from muscles through cells to molecular levels. 5 SYSTEM LEVEL: Groups of skeletal muscles form a system that helps to move frog’s limbs. CELLULAR LEVEL: Skeletal muscles themselves are composed of muscle cells. MOLECULAR LEVEL: The muscle cells are formed from thousands of macromolecular assemblages known as sarcomeres. These sarcomeres form the basic unit of muscle contraction. The sarcomeres are formed from a pair of contractile proteins—actin and myosin. Conclusion: We can precisely say that the principle that function depends on structure holds true across the whole range of physiological processes. 6 TOPIC-4 ADAPTATION, ACCLIMATIZATION AND ACCLIMATION Adaptation: Adaptation is an evolutionary process that occurs extremely slowly in a species over thousands of generations as a result of which the physiology of the members of that species becomes very well matched to the environment in which it lives. This phenomenon ensures the survival of the species. Adaptations are generally not reversible. Acclimatization: Acclimatization is a physiological, biochemical, or anatomic change within an individual animal that results from the animal’s chronic exposure to new, naturally occurring environmental conditions. Acclimation: Acclimation refers to the same process as acclimatization, but the changes are induced experimentally in the laboratory or field by the investigator. Acclimatization and acclimation are acquired characters that are restricted to only one or few members of a species. The adaptations acquired are not inheritable, so have no evolutionary significance. Generally, both acclimation and acclimatization are reversible. Examples: (a) Acclimatization: Let’s consider an animal that voluntarily migrates from a valley to a high mountain i.e. a voluntary change happens in its natural environment in which oxygen partial pressure is low. Effects: • The lung ventilation rate will increase initially to acquire adequate oxygen. • However, within few days, lung ventilation will begin to drop back towards normal rates as the other physiological mechanisms that facilitate gas exchange at high altitude begin to operate. • This individual animal is said to have acclimatized to the new high-altitude conditions. (b) Acclimation: Now consider another condition in which an animal physiologist places that same animal in a hypobaric chamber (that is at low atmospheric pressure) simulating high-altitude conditions. Effects: The animal breathing rate will react in the same way as during acclimatization but we shall call that it has acclimated to the experimental conditions. 7 (c) Adaptation: In contrast to these short-term responses let’s consider the bar-headed goose, which is able to fly above the peaks of Mount Everest. This species of goose has become adapted to high altitude due to natural selection that has operated for thousands of years on the species. 8 TOPIC-5 PRINCIPLE OF HOMEOSTASIS Walter Cannon coined the term homeostasis in 1929 to describe the tendency of organisms to maintain relative internal stability despite of significant external environmental changes. Fluctuations in Environmental parameters are challenging for animals Although many animals seem to live comfortably in their environment, most habitats are actually quite hostile to animal