This Veterinary Physiology is intended to be useful for elementary physiology course in colleges of agriculture, zoology, veterinary and animal sciences. In particular I have tried to cover the VCI syllabus. Hope the efforts will be of some use to students. 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 nondomestic animals such as rat and rabbit have been cited. Physiology has now grown into a vast discipline. 

The word physiology initially meant for that branch of biology which deals with the function of living organisms.Physiology can be defined as the study of mechanisms of body functions. Physiology is concerned with the way the various systems of the body function and the way each contributes to the functions of the body as a whole.Thus, physiology not only seeks to define the functions of living organisms like feeding, digestion, respiration, transport of gases in the blood, circulation and functions of the heart, excretion and kidney functions, muscle and movement, reproduction etc but also seeks to answer to ‘why’ and ‘how’ all these functions are regulated.

Blood is the fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries . Blood is a body fluid in animals and human beings that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. Hematology is the science or study of blood, blood-forming organs and blood diseases.

The heart is a muscular structure that contracts in a rhythmic pattern to pump blood. The heart functions as a pump to provide a continuous circulation of blood throughout the body.The blood vessels that take blood from the heart to the various tissues are called arteries.Arterioles open into a network of capillaries which constitute the microcirculation. 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. The primary function of the heart is to transfer sufficient blood from the venous system to the arterial side of the circulation under sufficient pressure to maintain the circulatory needs of the body.

Transport systems are crucial to survival. Unicellular organisms rely on simple diffusion for transport of nutrients and removal of waste. Multicellular organisms have developed more complex circulatory systems. The circulatory system is responsible for the transport of the various essential compounds and other factors around the body, as well as the removal of the metabolic wastes that accumulate in the tissues from body activities to places of elimination.

The muscular system allows movement and necessary functions such as circulation, digestion, and breathing. It is made up of muscles and connective tissue, such as tendons. The muscular system also helps provide support and shape. In the body, muscles have primary functions to provide motion, maintain posture, and generate heat.

A neuron is the basic unit of nervous tissue. It is specialized for the function of reception, integration and transmission of information in the body. The nerve cells are excitable, i.e. capable of generation of electrical impulses at their membranes. The electrical impulses generated in the excitable tissues, in most instances, can be used to transmit signals along the membranes.

Detection of sensations allow the body to be aware of changes (or stimuli) that occur in the environment or inside the body. These senses permit the central nervous system to produce reactions for the stimuli and maintain body homeostasis. Somatic senses (“soma” means body) detect touch, pain , pressure, temperature, and tension on the skin and in internal organs. Special senses detect the sensations of taste, smell, hearing, equilibrium, and sight, only in special sense organs in the head region. All senses are detected by sensory receptors, and after integration and processing being done in the central nervous system, motor nerves produce a response.

Digestion is the process by which food is converted into substances that can be absorbed and assimilated by a living organism. In most animals it is accomplished in the digestive tract by the mechanical and enzymatic breakdown of foods into simpler chemical compounds. Digestion usually consists of two elements: a mechanical/ physical stage, e.g. mastication, churning, emulsification, in which no chemical bonds are broken and no enzymes are required; and a chemical stage requiring enzymes in which complex food molecules are broken down by hydrolysis into a state in which they can be absorbed.

The primary role of the respiratory system is to provide O2 to the tissues for metabolic needs and remove the CO2 formed by them. 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 mitochondria of cells of the various tissues.

Substances which are in excess of the body and/or are harmful need to be removed constantly. This function is carried out by the excretory organs which include skin, gastrointestinal tract and kidney. Skin excretes water and salts in the form of sweat. Gastrointestinal tract removes undigested food material as well as some heavy metals in the form of faeces. Kidneys are the main excretory organs which together with a pair of ureters, a urinary bladder and urethra constitute the excretory system. Renal function is essential for homeostasis.

A large proportion of the body of mammals including domestic animals, is made up of water. This water is an essential attribute 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 milieu interieur, and Cannon the fluid matrix.

Animal body contains several fluids other than blood and lymph which have specialized functions for specific organs. Two of these fluids are cerebrospinal fluid and synovial fluid. The cerebrospinal fluids protects and supports brain and spinal cord function. Synovial fluid protects the functioning of the movement of the body by bone-muscular joints.

Endocrine glands are organs which secrete chemical substances called hormones into the bloodstream. Hormones are chemical substances that are produced in one tissue, released into the bloodstream, and carried to other tissues, where they act through specific receptors to bring about changes in cellular activities. The term hormone is derived from the Greek root meaning to excite.

Reproduction is essential for the survival of the species but not for the survival of the individual. The female reproductive system is entrusted with the responsibility to produce female gametes ie ova and hormones that regulate and integrate the functional activity of the female reproduction.

The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that transport the sperm to the penis and glands that add secretions to the sperm to make semen. The adult mammalian testis performs two important functions, spermatogenesis and male sex hormone production. It is an organ structurally designed to produce the haploid male gametes from diploid postnatal germ-line stem cells, i.e. type A spermatogonia.

Lactation is the process of milk secretion by the mammary gland. Lactation is the final phase of the complete reproductive cycle of mammals. Newborns of most species are dependent on maternal milk during the neonatal period. Lactation is therefore essential for survival of the species.

Growth is a correlated increase in the mass of the body in definite intervals of time, in a way characteristic of the species i.e. increase in size and weight. This means growth includes the protein, fat and mineral deposition in the body. Growth is an irreversible increase in the metabolically active protoplasm. There is a characteristic rate of growth for each species and a characteristic adult size and development. Maximum size and development are fixed by heredity and nutrition is an essential factor determining whether this maximum will be reached. An optimum nutritional regime is the one which enables the organism to take full advantage of its heredity.

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.

Ecophysiology is the branch of physiology which aims to define the adaptive features of an organism which fits it to its environment. Most of the time the environment in which the animal lives is a difficult one, hence the requirement of adaptation to environment. Environment is defined as the surroundings in which the organism lives.

Behavior can be defined as an expressed course of action produced in organisms in response to stimulus from a given situation. Alternatively, behaviour may be defined as the response by the animals to internal and external stimuli. Behaviour is a significant indicator of the health of animals. The goal of all behavioral responses is common i.e. to survive and reproduce.

Appendix 1 : Syllabus (For the Degree of Bachelor Of Veterinary Science And Animal Husbandry (B.V.Sc. & A.H.)

A2 Milk 340, 341 Abiotic Environment 379, 390 Abomasum 151, 152, 153, 156, 158, 164, 344