Preface This small text book on Physiology of Domestic Animals is intended to be useful for elementary animal physiology course in colleges of agriculture, zoology, veterinary and animal sciences. In all chapters, the aim has been to present a clear and concise account of the functioning of various systems of domestic animals. Where appropriate, examples from human and non domestic animals such as rat and rabbit have been cited. Physiology has now grown into a vast discipline. I hope that the book will be useful for the students. However, I request the indulgence of learned readers to send in their valuable suggestions for improvement. I am indebted to my teachers, colleagues and friends. I am especially grateful to Drs. D. Swarup, A.K. Goel, Puneet Kumar, Ms. Neha, for encouragement and useful suggestions and to my wife Mrs. Sadhana Jindal for their constant encouragement.

Introduction The animals which man has domesticated over the years to meet his own needs for food, clothing, power or companionship include cattle, buffalo, sheep, goats, horses, pigs, cats, dogs and poultry. Domestic animals like most other animal forms existing on earth including human beings, are the result of a long evolution and therefore each other and indeed to all living beings. Much has been learned or better understood about the physiology of these animals through comparative study on the other animal forms, though many a time these animals have served as experimental models for studies designed to gain knowledge about human physiology. Dog and goat for a long time yielded to such experimentation. The domestic animals have been used for obtaining information on their physiology per se only recently and most experiments have been designed to understand better the mechanisms related to production potential of these animals in terms of meat, milk and wool. Knowledge is also gained by the study of sick and abnormal individuals or through surgery. Animal ethology (study of behaviour) is a comparatively recent discipline which has contributed abundantly to our understanding of the behaviour of animals in relation to their productivity. The place of domestic animals in nature is objectively described by placing them along with other living organisms in the accepted scheme of classification as formulated by the biologists. The domestic animals belong to the Animal Kingdom, which like the plant kingdom, is divided into a number of major categories, the Phyla. The animal Phyla range from the relatively simple protozoa mostly single celled forms, to the chordata which encompasses backboned animals, including domestic animals and human beings. This scheme of classification is based on the concept of evolution of life and the animals are classified on the basis of closeness or the distance of their relationship. The life cycle of the animal (Ontogeny) is said to repeat the evolutionary development of the animal (Phylogeny). The domestic animals belong to the phylum Chordata, sub phylum Vertebrata, characterised by the presence of a back bone or vertebral column. They are further classified to belong to class Mammalia and sub class Eutheria. The cow, buffaloes, goat, sheep etc. belong to order Ungulata in sub class Eutheria. The horse and asses belong to order Perissodactyla. The cats and dogs belong to order Carnivora. The poultry belong to the order Galliformes in Class Aves.

Water : Importance of Water in Physiological Functions of the Body A large proportion of the body of mammals including domestic animals, is made up of water. This water is an essential attributes for the life of the animals. Animals deprived of food but provided water will survive much longer than the animals provided with food but deprived of water. Water is the basis of the liquid environment of the cells, which Claude Bernard called the milieur interieur, and Cannon the fluid matrix. Its most important biophysical qualities are its solvent properties, its ionization and its heat of vaporization. Water is essential for metabolic processes, as most of these reactions must occur in aquatic environment. Water provides the medium for the transport of nutrients from their sites of absorption to the site of metabolism. Water also provides the medium for the transport of waste products from their sites of production to their sites of final excretion. Together with the main electrolytes, water forms an excellent buffering medium. Moreover, water is an essential substrate or end product of many biological reactions. Water due to its high latent heat of evaporation or fusion is an excellent insulator for the loss of body heat and is essentially required for the loss of heat to maintain body temperature, through the evaporation of water from the skin and the respiratory tract. Water also acts as a vehicle for the transportation of nutrients, hormones, waste products and gases etc. Body water is derived from several sources which include the water drunk, intake of water through food and metabolic body water. Water is lost from the body through several routes which include loss of water through the feces and urine, evaporation of water from the skin and respiratory tract. The concentration of water in the body varies from one tissue to another, being least in the dentine of teeth and greatest in the grey matter of the brain. The amount of water in the body also varies considerably with the age of the animal. The younger animals contain considerably more water than the older animals. This effect is mostly due to increase in the fatty tissues in the body, which contains relatively much less water, as the relative composition of tissues of the body other than fat remains relatively constant. The proportion of water in ruminants is higher as compared to nonruminants.

Body needs oxygen to oxidise the food nutrients and derive energy. In this process carbon dioxide is produced in the body, which is toxic and must be removed quickly and efficiently. The oxygen utilization & carbon dioxide production takes place within the mitochondrion of cells of the various tissues. The transport of O2 and CO2 to and from the cells of various tissue is done by the blood. But the actual uptake of oxygen from the air and discharge of CO2 into the environment takes place at the level of lungs. This exchange of O2 and CO2 between the organism and the environment is defined as respiration. Air is moved in and out of the lungs by their bellows action (ventilation). It is then distributed within each lung, so that alveoli are adequately ventilated and also perfused by the blood. Oxygen and carbon dioxide are then exchanged between the gas mixture in the alveoli and the blood in the pulmonary capillaries (gas transfer). The gases are then transported in the blood to the tissue.

Whenever we get a cut, a red fluid comes out of the wound. This red fluid is called blood. Blood is very important for the body as is evident from the fact that the loss of blood from the body makes it weak and may even cause death. Physiologically the blood is a red tissue composed of cells suspended in a pale yellow fluid, plasma. Unicellular organisms that live in immediate contact with the external environment obtain nutrients directly from that environment and eliminate unused or unwanted materials. Blood and lymph are important connecting fluids among the diverse anatomical structures of the mammalian organisms. Thus blood is the liquid vehicle by which the major organic nutrients are transported from the intestine where they are absorbed, to the liver where they are processed and thence to other organs. Similarly blood is the liquid vehicle for the transport of waste materials from various organs to the kidney and gastrointestinal tract from where they are eliminated. Thus the blood is the major transport carrier of nutrients, metabolites, oxygen, hormones, waste products etc. inside the body. Blood is an important medium to maintain homeostasis of various electrolytes in the body. Besides blood provides the army against any external attack on the body by harmful microorganisms. This army is in the form of cells, called white blood or corpuscles or leucocytes. In a normal adult human being, about 5 to 6 litre of blood is present which is about 8 percent of the body weight. In domestic animals generally 5 to 8 per cent of the body weight is blood (say 30-40 litre in a cow). The blood is composed of two parts, the cells and the fluid part or plasma. The cellular constituents of blood include erythrocytes or red blood corpuscles, leucocytes or white blood corpuscles and thrombocytes or platelets. The cellular constituents of blood varies with the species as well as with the age and physiological stage of the animal. The extracellular constituents or plasma is composed of water, electrolytes, proteins, lipids, glucose, enzymes and hormones etc. The blood undergoes coagulation on exposure to air leaving clear watery fluid, called serum. The serum is nothing but plasma minus fibrinogen and other clotting factors. The serum is a clear yellowish fluid. The yellow colour is due to the presence of small quantities of bilirubin, a bile pigment and of carotenoids. The blood serum of cattle is yellow due to presence of more of carotenoids whereas that of buffalo is white due to the absence of carotenoids. The blood clot consists of a network of fibrous strands of fibrin in which are enmeshed the cellular elements of the blood. Blood clotting is a protective mechanism against excessive loss of blood from the body. The lymph also clots, although, somewhat more slowly than blood.

The circulatory system includes the heart, blood and lymph vessels, blood, lymph, cerebrospinal and tissue fluids. Circulation of blood in the body is important for the transport of nutrients to all parts of the body and for the transport of gases and waste products. In fact most of the functions of blood could not have been possible, if there were no circulation of blood. In order to constantly circulate the blood to all parts of the body and back, the body needs the services of an efficient pump. The pump or heart is based on the ability of muscle to contract or shorten. The blood is carried in a system of elastic tubes of pipes (arteries, capillaries and veins). The circulation of blood in the domestic animals or mammals is referred to as closed circulation because the blood returns to the heart without leaving this system of tubes. The heartbeat frequency or pulse rate is usually given as the number of heart beats per minute. The heart rate increases several times during exercise or thermal stress. The average pulse rate of various domestic animals is given in Table 1.

Kidney is one of the main excretory organs of the body. By secreting urine the kidney serves as the outstanding guardian of the milieu interieur. The main functions of the kidney in the maintenance of body homeostasis are listed below. Maintenance of proper osmotic concentrations. Maintenance of blood volume. Maintenance of ion concentration. Removal of nitrogenous end products of protein metabolism primarily urea, creatinine and ammonia. Removal of foreign substances and/or their metabolic end products. Thus, kidney control the concentration of most of the constituents of body fluids. In case of birds, the primary end product of protein metabolism is uric acid instead of urea. So, the major excretory product through urine in case of birds is uric acid. Briefly, the kidney form urine in the following way. Endowed with a relatively huge blood supply, the renal glomeruli filter about 200 litres per 24 hours of protein free fluid from the blood in human being ( ie 50-60 times the plasma volume). This filtrate, which contains the soluble non protein components of the plasma, is submitted to extensive reabsorption by the renal tubules. The relative control on the reabsorption process by the kidney, is the key mechanism by which the kidney is able to maintain the composition of the milieu interieur. Kidney also releases several hormones like renin, erythropoietin and prostaglandins, which have the capacity to influence cells and alter their physiological processes.

Ruminants have evolved to consume and subsist on roughage - grasses and shrubs built predominantly of cellulose.The rumen is a fermentation vat par excellance, providing an anaerobic environment, constant temperature and pH, and good mixing. Well-masticated substrates are delivered through the esophagus on a regular schedule, and fermentation products are either absorbed in the rumen itself or flow out for further digestion and absorption downstream. Rumen gases, particularly methane, are increasingly in the news because of their contribution to greenhouse gas and climate change. The ruminant stomach is composed of four parts; the rumen, reticulum, omasum and abomasum. These compartments gives the cow and other ruminants a decided advantage in digesting and utilizing those parts of the plants and other compounds which are practically useless to animals with simple stomach. Substances such as cellulose, a major constituent of plant tissues and urea, a non protein nitrogen compound, are of limited use of non ruminants. However ruminants through fermenting and synthesizing action of microorganisms in their complex stomach, can efficiently utilize these substances for productive purposes. The dairy cow converts this feed into milk and meat, two highly nutritious and palatable food for humans. This ability to make use of feed which otherwise would go waste because it is not directly consumable for humans is one of the most compelling justifications for the existence of cattle and buffaloes, in a world which must make use of all possible food resources.

Body needs energy for the functioning of various processes for the nutrition and repair of damage tissue. This energy comes from the food nutrients. The food nutrients undergo a series of chemical reactions in the body to release the necessary energy. The higher animals are dependent on plants and other animals for their nutrients. They can classified into two groups. Carnivores : Those animals which obtain most of their food by eating other animals are carnivores in these species digestion is mainly enzymatic in nature and microbial digestion is minimal. Herbivores : These animals obtain most of their food nutrients from herbage and other plant parts. In the herbivores, microbial digestion is of great significance and allows utilization of plant materials that are not broken down by the enzymes present in the digestive secretions. Most of the domesticated animals except cat & dog fall in the category of herbivores. The domesticated herbivores can be divided into three groups: Ruminants : Those possessing a rumen (cow, buffalo, sheep, goat, camel etc.) in which there is extensive microbial fermentation of the herbage diet in a specialized region of the digestive tract, prior to its digestion by the alimentary enzymes. Psedoruminants eg members of the camelidae family eg camel, Ilama, alpaca, vicuna etc. In pseudoruminants animal third compartment of the compound stomach is absent. Non-Ruminant : The second group is those with simple stomachs (Horse, Pig) in which fermentation takes place in the posterior part of the digestive tract.

Before the turn of present century, it was considered that carbohydrates, fats, proteins and minerals were sufficient for fulfilling the nutritional needs of animals. Later it was recognised that several other factors also need to be provided in the diet in small quantities, the absence of which resulted in diseases like beriberi, scurvy, pellagra and rickets. Funk in 1912 proposed the term vitamine for such factors, vita for being essential and amine because these factors were thought to be amines in chemical structure. Later it was discovered that not all such factors were amines, hence the last ‘e’ from the word vitamine was dropped. The quantities of vitamins needed by the body are small, in the range of milligrams or micrograms. For this reason these compounds are also known as accessory foodstuffs. Thus vitamins came to be regarded as essential dietary factors which are required by an organism in small quantities and whose absence results in deficiency diseases. Many of these vitamins acts as coenzymes in important metabolic reactions of the body. The vitamins have been broadly classified on the basis of their solubility. Fat soluble vitamins - A, D, E and K. Water soluble vitamins- B complex and C. This classification has proved useful from the functional point of view also for an extended period of time and is now an established classification scheme. The water soluble vitamins cannot be stored in the body over extended periods of time during periods of opulence and used during periods of scarcity. Most vitamins cannot be synthesized by the body hence the need for dietary supplement. However, ruminants, have an advantage in that the rumen microorganisms can manufacture most of the B-complex vitamins which are utilized and required by the animal. Vitamin C is required only by human beings and guinea pig, hence is not of importance as a dietary ingredient for the domestic animals. Most other animals are able to synthesize adequate amounts of vitamin C required by them. The site of synthesis of vitamin C is not well established but it seems that the intestinal tract, liver and adrenal glands are actively involved. At this stage it should be clarified about the dietary and physiological requirements. All vitamins are physiologically required by the domestic animals but only a few are required to be a essential dietary ingredient.

Endocrine glands are organs which secrete chemical substances called hormones into the blood stream. Hormones are substances produced by specific glands, secreted directly into the blood and transported to various organs and tissues where they exert their effects. The term hormone is derived from the Greek root meaning to excite. The organs/cells where there hormones bring about their actions are called target organs/cells. The target organs (hormone binding receptors) posses specific receptors for bringing about the action of these substances. The term neurosecretion signifies the production and reiose of hormones by nerve cells that have the cytological characteristics of secretory cells. Several hormones are the product of nervous mechanisms hence the term neurosecretion is used and neurohumour for such substances. Hormones act as a mode of transmitting information in the body in addition to the nervous system. Their mode of action is slow but more prolonged. Chemically the hormones are either proteins or steroids. Hormones are secreted in very small amount but their actions are profound. Early investigators were hampered by the non availability of suitable methods for their estimation. The availability of radioimmunoassay, radio-receptor assay, enzyme linked immunological assay have greatly facilitates researches in the field of endocrinology. The mode of action of these hormones has been elucidated in several instances. Involvement of membrane permeability and cyclic AMP has been implicated in several cases. Most of these hormones have been of therapeutic interest for several decades. The mode of administration of steroid hormones can be both oral or parenteral while protein hormones are active only when injected. Several of these hormones are used for increasing the production potential of meat animal, for improving the reproductive efficiency of domestic animal, hence are of direct economic importance to the farmers. Chemical characterization and their synthesis including their more potent analogues in several cases, have greatly facilitated their widespread application in the animal production and animal health strategies. Endocrinology is the study of the structure and function of the endocrine glands and their secretory products, including the consequences of excessive or deficient production of these hormones.

Reproduction in the males culminates in the production of semen containing fertile spermatozoa and deposition of these in the female reproductive tract at an opportune time. Male Reproductive Organs The reproductive organs in male domestic animals comprise the testes, epididymides, and accessory glands. The testes produce the spermatozoa and the male sex hormones (androgens). The spermatozoa pass from the testis into the epididymis where they acquire motility and attributes essential for fertilization and are then stored there. At ejaculation they are squeezed along the ductus deferens into the urethra and mixed with the secretions of the accessory glands to constitute the ejaculated semen. Testis The testis are composed of long tubes called seminiferous tubules and interstitial tissue. The spermatozoa are produced within the seminiferous tubules. This process is known as spermatogenesis. The interstitial tissue contains Leydig cells which are responsible for the production of the male hormone called androgens. The androgens are produced under the influence of hormones from the anterior pituitary namely ICSH (LH) and FSH. The principal androgens produced by the testis are testosterone and dihydrotestosterone. The testis are suspended from the body in the loose sacs called scrotum, present in the inguinal region. The scrotum in addition to its container function, also acts as a regulator of temperature required for spermatogenesis.

The reproductive system of female domestic animals consists of (1) A pair of ovaries attached with ligaments to the uterus (2) A pair of fallopian tubes the distal ends of which (infundibula) serve to receive ova expelled from the ovary and the proximal ends enter the uterus (3) The uterus proper with its outer smooth myometrium and inner mucous endometrium which either sloughs off periodically or develops in part into a placenta, depending on whether pregnancy occurs (4) Vagina which serves to receive sperm. Ovaries The two ovaries are connected to the back of the female with ligaments inside the main body cavity. When puberty, or sexual maturity is reached, the ovaries begin to function with a rhythm or cycle of activity, the most evident manifestation of which is estrus or heat. The estrus period or heat is the period when the female is receptive to the male. The duration of heat in cattle is around 19 hours. The interval between heats in 19½ to 20 days in cattle. The duration of estrus and cycle is variable in different individuals and in different seasons of the year. The ovarian parenchyma contains a large number of potential egg forming cells. There are many more of these than are required for the breeding life of female. New follicles begin to grow each estrus cycle one of which finally ovulates. The ovary is covered by the peritoneum except the attached border where the vessels and nerves enter. In most animals the ovary is some what divided into a cortex, in which the follicles with the developing ova are located and a medulla which is rich in blood vessels, lymphaticus and nerves. Each Graafian follicle normally contains only one developing ovum. The follicle vary in size according to their stage of development, the more advanced project the surface of the ovary slightly.

Lactation is the process of milk secretion by the mammary gland. Milk is one of the most fascinating biological fluid whose role as a nearly complete food in sustaining life processes is well known. Milk provides an essential and highly digestible form of nutrients for the mammalian young during the critical period after birth. Milk also forms the first meal of the mammalian youngs. Most mammals produce milk according to the requirements for their young one’s. However, man for centuries have selectively bred several animals, so that they produce milk, far in excess of the requirements of their youngs. The excess milk is harvested by man for his own and his children’s nurture. The physiology of the mammary gland or the mechanism of milk secretion forms an important part of the man’s strategy to manipulate it to its own advantage. Milk forms the raw material for the multi million dollar dairy industry all over the world. In India, dairying is the second largest agriculture based industry. The most important animal domesticated by man for production of milk is undoubtedly the cow. Buffalo as a milk producer is important only in Asia & parts of Africa. In India its importance is as much or even more as compared to the cow in milk production ability. The other important milk producing animals are goat, sheep, camel, yak, and mithun. The ability to produce milk is dependent on several factors including breed, season, feed etc. Majority of our animals do not conform to any descript type, some 20 per cent of them belong to 26 cattle breeds and 7 buffalo breeds. The major milk producing breed of buffaloes are Murrah and Nili Ravi. The season affects the milk producing ability of domestic animals. In Northern India, cow’s milk production is highest during March and lowest during September. However, the buffalo milk production is highest during October and lowest during June.

All animals, including domestic animals and humans, depend entirely on their sense organs and nervous system for information about external environment and the internal status of their bodies. Sensory inputs, together with the genetically inherited organization and properties of the nervous system is responsible for the all knowledge and determines how each animal behaves. Nervous system may be viewed as an anatomical arrangement of neurons with a large number of different kinds of synapses through which sensory information from the periphery of the body is transmitted through numerous sensory neurons to a set of neurons in a central region of the body, where the information is integrated and from which impulses are then directed out through numerous motor neurons to effectors. The central nervous system is distinguished from the peripheral nervous system in that it is made up of a concentration of tracts, ganglia and nuclei. Thus nervous system is a structural and functional mechanism involved in the coordination of an animals responses to internal and external environment. Neurons : The central and peripheral portion of the nervous system are comprised of nerve cells (neurons) and supporting tissues. Supporting tissues : The supportive tissues of the peripheral nervous elements consist primarily of connective tissue and specialized cells called neurolemma or Schwann cells. The supportive tissues of central nervous system are glia cells or neuroglia. These cells have functional and nutritive roles in addition to their roles as supportive tissue.

Bone Bones provide the rigid framework of the body of animals and man. The skeletal system serves three main functions. (1) It supports and protects the soft tissues and organs of the body. (2) It helps to maintain, through homeostatic regulation, the constancy of the ionic environment within the organism. (3) It helps in locomotion with the help of muscles and tendons. The ends of long bone is called the epiphysis: the long, compact shaft, the diaphysis where it joins the epiphysis, the metaphysis. The diaphysis is composed of a hollow cylinder which contained the medullary cavity and bone marrow. During postnatal life, erythropoiesis occurs mainly in the bone marrow. In the adult, bone contains on wet weight basis approximately 25% water, 45 Percent ash and 30 percent organic matter. With increasing age, the ash content progressively increases as the water content decreases; the organic fraction remains relatively constant. Calcium constitutes about 37% of the ash content, and phosphorus about 18.5%.

Domestic animals including chicken and mammals, belong to the category of homeotherms, as opposed to poikilotherms. Homeotherms or warm blooded animals are those which are able to maintain constant body temperature in the face of varying environmental temperature. The poikilotherms or the cold blooded animals change their body temperature according to the temperature of the surroundings. Most invertebrates and lower vertebrates belong to the category of poikilotherms. Why do animals have to maintain constant body temperature? As we are aware that enzymatic reactions in the body are temperature sensitive. In fact, a 10 degree centigrade increase in temperature can result in doubling of the reaction rates. In order to maintain optimum functioning of the enzymatic reactions in the body a constant temperature is required. This constant body temperature is not very constant in the sense that it varies somewhat during exercise and also when the animal is exposed to extremes of temperature of the surroundings. That means that the temperature regulation mechanisms in the body are not 100 percent perfect. At this stage it is desirable to clarify the distinction between surface temperature and core temperature. The surface temperature, in contrast to the core temperature rises and falls with the temperature of the surroundings. It is the temperature of the deep tissues or internal organs which remains constant. The temperature of the rectum is usually taken as a useful indicator of core temperature because of the ease of measurement. The rectal temperature of several domestic animals is given in Table 1.

Appendix 1 : Selected textbooks in Physiology Reece, William O. (2004) (Ed.) Duke’s Physiology of Domestic Animals, 12th Edition, Comstock Publishing Assoc. Ithaca, New York. Phillis, J.W. (1976) (Ed.) Veterinary Physiology, John Wright, Bristol. pp 892. Breazile, J.E. (1972) A Textbook of Veterinary Physiology. Harcourt, Philadelphia. Rowen D. Frandson, W. Lee Wilke, Anna Dee Fails (2009) Anatomy and Physiology of Farm Animals. 7th Edition. Wiley, John & Sons. Bone, J.F. (1999) Animal Anatomy and Physiology. 3rd Edition Reston Pub. Co. Reston, Virginia, U.S.A. Svendson, Par. (1984) An Introduction to Animal Physiology. 2nd Editions, MTP Press (Lancaster Lancashire and Boston) Ralph, C.L. (1978) Introductory Animal Physiology, McGrawhill Book Company, N.Y. Schmidt-Nielson, K. (1994) Animal Physiology, 3rd Ed. Cambridge University Press, New York. Ruch, T.C. and Patton, H.B. (1973) (Ed.) Physiology and Biophysics, 20th Ed. 3 Vols. W.B. Saunders Company, Philadelphia. Prosser, C.L. (1973) (Ed.) Comparative Animal Physiology, 3rd Edition. W.B. Saunders Company, Philadelphia. Hoar, W.S. (1983) General and Comparative Physiology, 3rd Edition, Prentice Hall Inc., Englewood Cliffe, N.J.