The nervous and endocrine systems are the two major regulatory systems in the body.

• The nervous system controls structures by sending action potentials along axons, which release chemicals signals at their ends near the cells they control, whereas the endocrine system releases chemicals into the circulatory system, which carries the chemicals to the cells they control.

• The endocrine system has a more general effect, acts more slowly, and has a longer lasting effect than the nervous system.

Functions of the Endocrine System

The main functions of the endocrine system include regulation of

• Water balance; uterine contractions during parturition: milk release from the breasts; metabolism; tissue maturation; sodium, potassium, and calcium ion concentration of blood; heart rate; blood pressure; preparation for physical activity; blood glucose concentration; immune cell production; and reproductive functions in males and females.

Chemical Signals

• Chemical signals bind to receptor sites on receptor molecules.

• Intracellular chemical signals are produced in one part of a cell and travel to another part of the same cell and bind to receptors.

• Intercellular chemical signals are released from one cell, are carried in the intercellular fluid, and bind to receptors in other cells.

• Intercellular chemical signals can be classified as autocrine, paracrine, hormone, neurohormone, neuromodulator, neurotransmitter, or pheromone chemical signals.

Receptors

• Chemical signals bind to receptor sites on receptor molecules to produce a response.

• Intracellular receptors are located in the cytoplasm or nuclei and can regulate enzyme activity or regulate the synthesis of specific messenger RNA.

• Membrane-bound receptors can produce a response by directly opening ion channels, activating G proteins, or activating enzymes that synthesize intracellular chemical signals, or by phosphorylating proteins inside of the cell.

Hormones

• Endocrine glands produce hormones that are released into the circulatory system and travel some distance, where they act on target tissues to produce a response.

• A target tissue for a given hormone has receptor molecules for that hormone.

Chemistry

• Hormones are basically proteins, peptides, or lipids.

• Protein and peptide hormones bind to receptors on the cell membrane and cause permeability changes or the production of a second messenger inside the cell. Lipid-soluble hormones, such as the steroids and thyroid hormones, enter the cell and bind to receptors inside the cell.

• The combining of hormones with their receptors results in a response.

Regulation of Hormone Secretion

• The secretion of hormones is controlled by negative-feedback mechanisms.

• Secretion of hormones from a specific gland is controlled by blood levels of some chemical, another hormone, or nervous system.

• The endocrine system consists of ductless glands.

• Some glands of the endocrine system perform more than one function.

The Endocrine Glands and Their Hormones

The Pituitary and Hypothalamus

• The pituitary is connected to the hypothalamus of the brain by the infundibulum. It is divided into the anterior and posterior pituitary.

• Secretions from the anterior pituitary are controlled by hypothalamic-releasing hormones that pass through the hypothalamic-pituitary portal system from the hypothalamus.

• Hormones secreted from the posterior pituitary are controlled by nervous impulses that pass from the hypothalamus through the infundibulum.

• The hormones released from the anterior pituitary are growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and melanocyte-stimulating hormone (MSH).

• Hormones released from the posterior pituitary include antidiuretic hormone (ADH) and oxytocin.

The Thyroid Gland

• The thyroid gland secretes thyroid hormones, which control the metabolic rate of tissues, and calcitonin, which helps regulate blood calcium levels.

The Parathyroid Glands

• The parathyroid glands secrete parathyroid hormone, which helps regulate blood levels of calcium. Active vitamin D also helps regulate blood levels of calcium.

The Adrenal Glands

• The adrenal medulla secretes primarily epinephrine and some norepinephrine. These hormones help prepare the body for physical activity.

• The adrenal cortex secretes three classes of hormones.

• Glucocorticoids (cortisol) reduce inflammation and break down fat and protein, making them available as energy sources to other tissues.

• Mineralocorticoids (aldosterone) help regulate sodium and potassium levels and water volume in the body. Renin, secreted by the kidneys, helps regulate blood pressure by increasing angiotensin II and aldosterone production. These hormones cause blood vessels to constrict and enhance sodium and water retention by the kidney.

• Adrenal androgens increase female sexual drive; normally adrenal androgens have little effect in males.

The Pancreas, Insulin, and Diabetes

• The pancreas secretes insulin in response to elevated levels of blood glucose and amino acids. Insulin increases the rate at which many tissues, including adipose tissue, liver, and skeletal muscles, take up glucose and amino acids.

• The pancreas secretes glucagon in response to reduced blood glucose and increases the rate at which the liver releases glucose into the blood.

The Testes and Ovaries

• The testes secrete testosterone, and the ovaries secrete estrogen and progesterone. These hormones help control reproductive processes.

• LH and FSH from the pituitary gland control hormone secretion from the ovaries and testes.

The Thymus Gland

• The thymus gland secretes thymosin, which enhances the ability of the immune system to function.

The Pineal Body

• The pineal body secretes melatonin, which may help regulate the onset of puberty by acting on the hypothalamus.

Other Hormones

• Hormones secreted by cells in the stomach and intestine help regulate stomach, pancreatic, and liver secretions.

• The prostaglandins are hormones that have a local effect, produce numerous effects on the body, and play a role in inflammation.

• Erythropoietin from the kidney stimulates erythrocyte production.

• The placenta secretes human chorionic gonadotropin, estrogen, and progesterone, which are essential to the maintenance of pregnancy.