Human Physiology, 14th edition (2016)

676 Chapter 19

thyroxine, and other hormones ( fig.  19.6 ). For example, dur-

ing prolonged fasting there is increased glucagon secretion and

decreased secretion of insulin (discussed in section 19.3). There

is also reduced thyroxine secretion (which slows the metabolic

rate); increased growth hormone secretion (which causes the

release of fatty acids from adipose tissue); increased ACTH and

cortisol secretion (which also promotes the release of circulat-

ing energy substrates); and decreased secretion of sex steroids

(which reduces the chances of pregnancy). The specific meta-

bolic effects of these hormones are summarized in table 19.4, and

some of their actions are illustrated in figure 19.7.

| CHECKPOINT

4. Distinguish between the energy reserves and
   circulating energy substrates, identifying the
   molecules in each category.


5. Explain the functions of adipose tissue and the
   dangers of obesity.


6. Identify the brain regions involved in hunger and
   describe the regulation of hunger by specific
   neurotransmitters and hormones.


7. Identify the hormones that increase or decrease
   blood glucose and those that stimulate lipogenesis
   or lipolysis.

Figure 19.6 The regulation of metabolic
balance. The balance of metabolism can be tilted toward
anabolism (synthesis of energy reserves) or catabolism
(utilization of energy reserves) by the combined actions of
various hormones. Growth hormone and thyroxine have both
anabolic and catabolic effects, so these hormones are shown in
parentheses on both sides of the balance.

Metabolism

Anabolism Catabolism

Glycogen Glucose Glycogen

Triglycerides Fatty acids Triglycerides

Protein Amino acids Protein

Insulin

Sex steroids

(Growth hormone)

(Thyroxine)

Glucagon

Epinephrine

Glucocorticoids

(Growth hormone)

(Thyroxine)

Table 19.4 | Endocrine Regulation of Metabolism

Hormone Blood Glucose Carbohydrate Metabolism Protein Metabolism Lipid Metabolism

Insulin Decreased ↑ Glycogen formation ↑ Protein synthesis ↑ Lipogenesis

↓ Glycogenolysis ↓ Lipolysis

↓ Gluconeogenesis ↓ Ketogenesis

Glucagon Increased ↓ Glycogen formation No direct effect ↑ Lipolysis

↑ Glycogenolysis ↑ Ketogenesis

↑ Gluconeogenesis

Growth hormone Increased ↑ Glycogenolysis ↑ Protein synthesis ↓ Lipogenesis

↑ Gluconeogenesis ↑ Lipolysis

↓ Glucose utilization ↑ Ketogenesis

Glucocorticoids

(hydrocortisone)

Increased ↑ Glycogen formation

↑ Gluconeogenesis

↓ Protein synthesis ↓ Lipogenesis

↑ Lipolysis

↑ Ketogenesis

Epinephrine Increased ↓ Glycogen formation No direct effect ↑ Lipolysis

↑ Glycogenolysis ↑ Ketogenesis

↑ Gluconeogenesis

Thyroid hormones No effect ↑ Glucose utilization ↑ Protein synthesis No direct effect