Metabolism and Endocrine Control: Study Guide for Human Physiology

Chapter 22: Metabolism and Energy Balance

Introduction to Metabolism

Metabolism refers to all chemical reactions that occur within living organisms to maintain life. In human physiology, metabolism encompasses the processes by which the body obtains and uses energy from food, regulates energy storage, and maintains homeostasis.

• Energy Balance: The relationship between energy intake (food) and energy expenditure (activity and basal metabolic rate).

• Fed State vs. Fasted State: The body alternates between storing energy after meals (fed state) and mobilizing stored energy during fasting (fasted state).

• Metabolic Pathways: Include glycolysis, gluconeogenesis, glycogenolysis, and lipolysis.

Example: After eating, glucose is stored as glycogen in the liver and muscles; during fasting, glycogen is broken down to maintain blood glucose levels.

Energy Storage and Utilization

• Glycogen: The primary storage form of glucose in the body, found mainly in the liver and skeletal muscle.

• Triglycerides: Stored in adipose tissue and serve as a long-term energy reserve.

• Protein: Used for energy only during prolonged fasting or starvation.

Key Equation:

$\text{Energy Balance} = \text{Energy Intake} - \text{Energy Output}$

Additional info: Energy output includes basal metabolic rate, thermic effect of food, and physical activity.

Metabolic States

• Fed (Absorptive) State: Occurs after eating; anabolic processes dominate (glycogenesis, lipogenesis).

• Fasted (Postabsorptive) State: Occurs between meals; catabolic processes dominate (glycogenolysis, gluconeogenesis, lipolysis).

Example: Insulin is the primary hormone active in the fed state, promoting glucose uptake and storage.

Hormonal Regulation of Metabolism

• Insulin: Lowers blood glucose by promoting uptake and storage.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown and gluconeogenesis.

• Other Hormones: Cortisol, epinephrine, and growth hormone also influence metabolism, especially during stress or fasting.

Additional info: The balance between insulin and glucagon is crucial for maintaining blood glucose homeostasis.

Diabetes Mellitus

Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia due to defects in insulin secretion, insulin action, or both.

• Type 1 Diabetes: Autoimmune destruction of pancreatic beta cells leads to insulin deficiency.

• Type 2 Diabetes: Characterized by insulin resistance and relative insulin deficiency.

Example: In diabetes, the inability to regulate blood glucose leads to symptoms such as polyuria, polydipsia, and weight loss.

Chapter 23: Endocrine Control of Growth and Metabolism

Hormonal Regulation of Metabolism

Endocrine hormones play a central role in regulating metabolism, growth, and energy balance. The hypothalamus and pituitary gland coordinate the release of hormones that affect metabolic processes throughout the body.

• Glucocorticoids (e.g., cortisol): Increase blood glucose by stimulating gluconeogenesis and inhibiting glucose uptake in tissues.

• Thyroid Hormones: Increase basal metabolic rate and promote growth and development.

• Growth Hormone: Stimulates protein synthesis, lipolysis, and increases blood glucose.

Key Equation:

$\text{Metabolic Rate} = \text{Energy Expended per Unit Time}$

Additional info: Metabolic rate is influenced by age, sex, genetics, and hormonal status.

Integration of Hormonal Effects

• Adrenal Cortex: Releases glucocorticoids and mineralocorticoids, which regulate metabolism and electrolyte balance.

• Thyroid Gland: Releases thyroxine (T4) and triiodothyronine (T3), which increase metabolic activity in cells.

• Pituitary Gland: Releases growth hormone, which affects growth and metabolism.

Example: During stress, cortisol levels rise, leading to increased blood glucose and altered metabolism.

Feedback Mechanisms

• Negative Feedback: Most endocrine pathways are regulated by negative feedback to maintain homeostasis.

• Example: High blood glucose stimulates insulin release; as glucose levels fall, insulin secretion decreases.

Outline of Key Topics

• Feeding and satiety

• Energy balance and output

• Regulation of body temperature

• Metabolic rate

• Carbohydrate metabolism

• Lipid metabolism

• Protein metabolism

• Growth and metabolism (continued)

• Calcium balance

Table: Comparison of Hormonal Effects on Metabolism

Hormone	Main Effect	Target Tissues
Insulin	Lowers blood glucose, promotes storage	Liver, muscle, adipose tissue
Glucagon	Raises blood glucose, mobilizes energy stores	Liver
Cortisol	Increases blood glucose, stress response	Liver, muscle, adipose tissue
Thyroid Hormones	Increase metabolic rate	Most tissues
Growth Hormone	Stimulates growth, increases blood glucose	Liver, muscle, bone

Summary

Metabolism and its endocrine regulation are central to human physiology, affecting energy balance, growth, and adaptation to stress. Understanding the roles of key hormones and metabolic pathways is essential for mastering topics in Anatomy & Physiology.