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Introduction

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High-Yield Terms

  • Ischemia: refers to a condition caused by restricted blood flow to a tissue, resulting in a shortage of oxygen and glucose needed for continued metabolism

  • Hypoxia: a condition where a tissue, or the entire body, is restricted or deprived of the flow of oxygen

  • Pasteur effect: strictly speaking, this refers to the inhibition of bacterial fermentation by oxygen; with respect to eukaryotic metabolism it refers to an increased rate of glycolysis in response to hypoxia

  • Anorexigenic: causing a suppression of appetite

  • Orexigenic: causing an increase in appetite

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Major Organ Integration of Metabolism

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All tissues of the body carry out metabolic processes for survival and growth but not all of the major metabolic pathways are operational at the same time, nor at the same level in all tissues. Changes in nutritional, hormonal, physiological, and pathological status lead to alterations in metabolism in each tissue. In addition, and critical to an understanding of metabolic integration, is that changes in metabolic function in one tissue can, and often do, have potentially profound impacts on the metabolic processes of other tissues.

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In the context of overall tissue integration, it is most important to demonstrate how the metabolic processes of the brain, liver, adipose tissue, gastrointestinal tract, and skeletal muscle are interconnected. Within the context of organ integration of metabolism, the pathways that are involved include glycogen synthesis (glycogenesis) and breakdown (glycogenolysis), glycolysis, gluconeogenesis, TCA cycle, lipid synthesis (lipogenesis) and breakdown (lipolysis), lipid oxidation, ketogenesis, amino acid catabolism, urea cycle, protein synthesis, and proteolysis. Each of these processes is most often discussed, as in this text, as isolated metabolic pathways. However, in order to fully appreciate how normal and abnormal physiology and pathology affect the status of the human organism, it is imperative to fully understand their interconnection.

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With respect to metabolic integration, the best way to gain an understanding of the processes involved is to examine how these major metabolic pathways respond to everyday feeding and then fasting between meals. Prolonged absence of food results in death within a very short period of time. This fact explains why humans have evolved a complex system of regulatory circuits that were evolutionarily designed to stimulate our desire to seek and consume food. These circuits involve both neuroendocrine (Chapters 43 and 44) and endocrine (Chapters 46, 49, and 50) functions throughout the body. These regulatory pathways allow humans to survive periods of fasting by stimulating energy storage after meal intake and precisely controlling the release of this energy when needed. Unfortunately, humans have not evolved with an equally exquisite means to control the storage of energy. Caloric excess ultimately results in obesity (Chapter 45) and its many associated pathologies including diabetes (Chapter 47), cardiovascular disease (Chapter 48), and cancer (Chapter 52).

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Although overall metabolic regulation across ...

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