Physical aspects of heat storage in the ox, (Bos taurus

The term 'heat balance' implies a closed-circuit system of heat flow. That is, if heat flows are entirely governed by some relationship containing n variables, then only n-1 of these can be independent; the nth being totally determined by all the others. This is true of an animal since the heat production via metabolism (M) must be balanced by a net exchange of heat to the environment (H), by heat storage within the body (U) or by a combination of both. Experimental achievement of heat balance requires that evaluation of heat flows be sufficient to en-sure that the relationship M = H + U holds to within instrumental errors, In most situations the quantities M and H are large and similar in magnitude so that heat storage (U) is close to zero. This thesis presents a description of the development of means for measuring these 3 variables and the progress made in studying the relationship between direct measurements of U (temperature changes) and indirect measurements of U (from M - H) during dynamic changes. The subjects under study were Ayrshire bullocks taken from the Hannah Research Institute herd. Animals between the ages of 12 and 28 months were used since this represented a period during which growth could easily by regulated by adjusting food in take. The thesis essentially contains 5 sections of which the constituents are; a) an introduction (chapter 1); b) a description of the development of apparatus and techniques used to make experimental measurements (chapters 2, 3, 5); c) a quantitative description of heat balance and the dynamics of heat storage under conditions of different constant environments (chapters 6 & 7); d) a description of a statistical approach to the modelling of heat-transfer phenomena (chapter 8); e) a general discussion (chapter 9). The contents of each chapter are outlined in the following paragraphs. Chapter 1 contains an introduction to all aspects of heat balance, presently conceived, which are relevant to the subject matter of the thesis. Many examples of problems relating to measurement and interpretation of results of heat storage investigations are discussed. Chapters 2 and 3 describe the indirect and direct-calorimeter systems that were used for measurement of heat production and loss. An analysis is given of a method for obtaining fast response information from slowly responding metabolic chamber data. Chapter 4 outlines the methods used for measurement, of body temperature in the carotid blood stream, in the rumen and on the skin surface, A theoretical analysis of heat transfer across a hair layer is discussed in relation to the observed measurements of temperature on a skin sample under controlled conditions, A brief summary of the organisation of 2 series of experiments on heat balance is given in chapter 5 together with a description of the recording equipment and of methods for data processing. Chapter 6 discusses the results of an analysis of 24-hr heat balance in calves subjected to a constant environmental temperature of 20°C, Sources of error over and within a 24-hr period are analysed and crude patterns of heat imbalance established, The dynamics of heat imbalance in the short term are discussed in this chapter (briefly) and in the following chapter (chapter 7) The time course of core temperature, mean body temperature and skin temperature estimates are analysed in response to circadian cycles, feeding, watering and postural changes. The variation in response, due to 3 different constant ambient temperatures, is also examined. Chapter 8 presents an original approach to the study of dynamically changing heat balance, This involves the conception of continually sampled records of heat production and heat loss as time series. Mathematical representation of such series enables the evaluation of a transfer function which describes the transfer of heat across body tissues, A possible physical interpretation of the transfer function is presented but fails to account for the observed phenomena. Possible extensions of the approach are discussed. Finally, chapter 9 discusses the achievements of the experimental investigation into heat balance and tries to relate the results to current knowledge and to gaps and inconsistencies in the field of thermal physiology.