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Chapter 8: Endocrine Physiology

GENERAL PRINCIPLES OF ENDOCRINOLOGY

  1. A hormone is a chemical produced by the body that has a specific regulatory effect on a target cell or organ.

  2. Chemical signaling can occur in three modes (Figure 8-1).

    1. Endocrine: The hormone is carried a large distance via the blood.

    2. Paracrine: The hormone only diffuses in local extracellular fluid.

    3. Autocrine: The hormone acts at receptors on the cell that secreted it (e.g., growth factors made by cancer cells).

  3. The seven classical endocrine glands are the pituitary, adrenal, thyroid, parathyroid, pancreas, testis, and ovary (Table 8-1).

  4. Other organs also secrete hormones (e.g., hypothalamus, heart, kidney, gastrointestinal tract, liver, adipose tissue, bone).

  5. Endocrine diseases may be caused by excess or deficiency of a hormone or by defects in hormone receptors or downstream intracellular signaling.

  6. Certain neoplasms (e.g., small cell lung cancer) may secrete hormones, causing paraneoplastic syndromes.

Figure 8-1

Modes of intercellular communication.

Five illustrations depict various modes of intercellular communication. The first illustration shows the neural mode of intercellular communication. A neuron with a cell body, dendrites and an axon is shown, the axon terminates to form a synapse releasing neurotransmitters that communicate with the post synaptic cell. The second illustration shows the endocrine mode of intercellular communication. It shows three cells stacked one on the top of the other, and a blood capillary running close to these cells. Peptide hormones are released from the storage vesicles of the cells; these peptide hormones diffuse through the blood vessel wall into the blood circulation. The third illustration shows the neuroendocrine mode of intercellular communication. A neuron with a cell body, dendrites and an axon is shown. Next to the distal end of the axon is a blood vessel. Diffusion of chemical transmitter from the axon into blood occurs. The fourth illustration shows the paracrine mode of intercellular communication. Four cells are shown stacked one above the other. The hormone diffuses locally to reach target cell. The last illustration depicts the autocrine mode of intercellular communication. It shows a cell that secretes a hormone that acts on the same cell.
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Table 8-1Summary of Hormones Produced by the Major Endocrine Organs
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Table 8-1 Summary of Hormones Produced by the Major Endocrine Organs

Endocrine Gland

Hormone(s) Produced

Chemical Structure

Overview: Major Function(s)

Hypothalamus

Corticotropin-releasing hormone (CRH)

Peptide

  • Stimulates adrenocorticotropic hormone (ACTH)

Growth hormone-releasing hormone (GHRH)

Peptide

  • Stimulates growth hormone (GH) release

Growth hormone inhibitory hormone (GHIH; also known as somatostatin)

Peptide

  • Inhibits GH release

Gonadotropin-releasing hormone (GnRH)

Peptide

  • Stimulates luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release

Prolactin-inhibiting factor (PIF; also known as dopamine)

Peptide

  • Inhibits prolactin release

Thyrotropin-releasing hormone (TRH)

Peptide

  • Stimulates thyroid-stimulating hormone (TSH) and prolactin release

Anterior pituitary

ACTH

Peptide

  • Trophic to adrenal cortex

  • Stimulates synthesis and secretion of cortisol, aldosterone, and androgens

FSH

Peptide

  • Promotes sperm maturation via Sertoli cells in testes

  • Stimulates ovarian follicle development

GH

Peptide

  • Acute metabolic effects oppose insulin

  • Chronic growth-promoting effect via insulin-like growth factor 1 (IGF-1)

LH

Peptide

  • Stimulates Leydig cells of testes to secrete testosterone

  • LH surge important for ovulation and formation of corpus luteum

Prolactin

Peptide

  • Required in lactation for mammary growth, initiation of milk secretion, and maintenance of milk production

TSH

Peptide

  • Stimulates synthesis and secretion of thyroid hormones

Posterior pituitary

Antidiuretic hormone (ADH) (also called arginine vasopressin, or AVP)

Peptide

  • Increases water retention at kidney

  • Vasoconstricts arterioles

Oxytocin

Peptide

  • Stimulates uterine contractions during labor

  • Milk ejection in lactation

Thyroid

Triiodothyronine (T3) and thyroxine (T4)

Amine

  • Increase metabolic rate

  • Required for normal growth and development

Adrenal cortex

Aldosterone

Steroid

  • Decreases urinary Na+ excretion

  • Increases urinary K+ and H+ excretion

Cortisol

Steroid

  • Released in response to stress

  • Multiple metabolic actions

Adrenal medulla

Epinephrine and norepinephrine

Amine

  • Produces effects similar to actions of the sympathetic nervous system

Pancreas

Insulin (β cell)

Peptide

  • Promotes storage of glucose as glycogen in liver and muscle

  • Promotes uptake of glucose and storage as triglyceride in adipose tissue and liver

Glucagon (α ...

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