Hormones are chemical signals secreted by cells of the endocrine system.

Endocrine cells: cells that secrete hormones

Target cells: cells that have receptors for the hormones

Circulating hormones diffuse into the blood and can activate target cells far from the site of release.

Paracrine hormones: affect only target cells near the site of release.

Autocrine hormones: affect the cells that released the hormones.

Figure 41.1 Chemical Signaling Systems (Part 1)

Figure 41.1 Chemical Signaling Systems (Part 2)

Some endocrine cells are single cells (e.g., in the digestive tract.)

Endocrine glands: aggregations of secretory cells. Hormones are secreted to the extracellular space.

Exocrine glands: ducts carry products to the outside of the body.

Chemical communication arose early in evolution.

Plants, sponges, protists all use chemical signals.

In arthropods, hormones control molting and metamorphosis.

Arthropods: the rigid exoskeleton is shed during molts to allow growth.

Growth stages between molts are called instars.

The nervous system and hormonal system are linked.

Nervous system (brain) controls the endocrine gland (prothoracic gland), which produces the ecdysone that orchestrates the physiological response.

Juvenile hormone: also released from the brain—prevents maturation to adult form.

Control of development by juvenile hormone important in insects with complete metamorphosis.

Figure 41.3 Complete Metamorphosis

Three types of hormones:

Peptides or polypeptides: water-soluble, transported in blood but not across membranes.

Steroid hormones: lipid-soluble; must be bound to carrier proteins to be carried in blood.

Amine hormones: derivatives of tyrosine

Hormone receptors:

Lipid soluble hormones: receptors are inside the cell

Water-soluble hormones cannot readily pass cell membrane—receptors are on the outside

Receptors are glycoproteins with three domains:

Binding domain: projects outside plasma membrane

Transmembrane domain

Cytoplasmic domain: extends into cytoplasm—initiates target cell response

One hormone can trigger different responses in different types of cells.

Example: epinephrine (amine), fight-or-flight response

Figure 41.4 Epinephrine Stimulates “Fight or Flight” Responses

Figure 41.5 The Endocrine System of Humans (Part 1)

Figure 41.5 The Endocrine System of Humans (Part 2)

The pituitary gland is attached to the hypothalamus of the brain.

Posterior pituitary secretes neurohormones (synthesized by neurons in the hypothalmus):

1. oxytocin (uterine contractions)

2. ADH (water retention)

Figure 41.6 The Posterior Pituitary Releases Neurohormones

The anterior pituitary secretes:

Tropic hormones: control other endocrine glands

1. ACTH – adrenal cortex

2. TSH – thyroid gland

3. FSH & LH - gonads Growth hormone: promotes growth (mitosis) Prolactin: breast development and milk

production MSH: melanin production

Figure 41.7 The Anterior Pituitary Produces Many Hormones

Hormones from the hypothalamus control the anterior pituitary.

The hypothalamus produces releasing hormones—carried to the anterior pituitary by portal blood vessels.

Negative feedback loops control hormone secretion from the anterior pituitary.

Figure 41.8 Multiple Feedback Loops Control Hormone Secretion

The thyroid gland produces thyroxine (T4)

and triiodothyronine (T3)

Goiter is an enlarged thyroid gland. It can result from either hyperthyroidism (Grave’s Disease) (thyroxine excess) or hypothyroidism (myxedema) (thyroxine deficiency).

Figure 41.9 The Thyroid Gland Consists of Many Follicles (C)

The thyroid gland also produces calcitonin.

Regulation of calcium levels in the blood controlled by calcitonin, parathyroid hormone, and vitamin D.

Calcitonin inhibits osteoclasts: more Ca2+ is put into bone by osteoblasts; levels of Ca2+ in blood decrease.

Figure 41.10 Hormonal Regulation of Calcium (Part 1)

Figure 41.10 Hormonal Regulation of Calcium (Part 2)

The parathyroid glands secrete parathyroid hormone (PTH).

PTH raises blood calcium levels:

Stimulates osteoclasts Stimulates kidneys to reabsorb

calcium Activates vitamin D, which stimulates

digestive tract to absorb calcium.

Insulin binds to receptors on target cells and allows uptake of glucose.

Lack of insulin: Type I diabetes

Lack of insulin receptors on target cells: Type II diabetes (Diabetes mellitus)

Islets of Langerhaans: clusters of endocrine cells in the pancreas.

cells produce insulin

cells produce glucagon: stimulates liver to convert glycogen back to glucose

Adrenal medulla: epinephrine and norepinephrine

On target cells: -adrenergic and -adrenergic receptors.

-blockers block -adrenergic receptors.

Figure 41.11 The Adrenal Gland Has an Outer and an Inner Portion

Adrenal cortex produces corticosteroids from cholesterol:

Glucocorticoids: cortisol

Mineralocorticoids: aldosterone

Sex steroids: androgens

Figure 41.12 The Corticosteroid Hormones are Built from Cholesterol

Aldosterone stimulates kidneys to retain sodium and excrete potassium.

Cortisol: mediates reaction to stress; cells not critical for action decrease use of blood glucose; blocks immune system reactions.

Androgens: testosterone

Gonads produce sex steroids:

Testosterone

Estrogens (estradiol) and progesterone

In development, sex hormones determine whether fetus will become male or female.

Figure 41.13 The Development of Human Sex Organs (Part 1)

Figure 41.13 The Development of Human Sex Organs (Part 2)

Figure 41.13 The Development of Human Sex Organs (Part 3)

Melatonin is released in the dark; light inhibits release.

Involved in photoperiodicity: seasonal changes in light trigger physiological changes.

Figure 41.14 The Release of Melatonin Regulates Seasonal Changes

Hormones occur in extremely small concentrations.

Immunoassay techniques are used to measure concentration in the blood.

Half-life: time required for one half of the hormone molecules to be depleted.