Endocrine System Anatomy and Physiology

These glands are small and unremarkable to look at, but they run blood glucose, fluid balance, blood pressure, the stress response, and reproduction. When you hang insulin, watch a thyroid storm, manage Addisonian crisis, or titrate DDAVP, you are working this system. Know the glands, their hormones, and what each hormone does.

Functions of the Endocrine System

Despite the variety of hormones, cells respond through only two mechanisms (direct gene activation and the second messenger system). The system as a whole handles:

1. Water equilibrium. Regulates the solute concentration of the blood.
2. Growth, metabolism, and tissue maturation. Controls tissue growth (bone, muscle), the metabolic rate (supporting normal body temperature and mental function), and the maturation that produces adult features and behavior.
3. Heart rate and blood pressure. Helps manage both and prepares the body for physical activity.
4. Immune control. Helps regulate the production and function of immune cells.
5. Reproductive function. Regulates development and function of the male and female reproductive systems.
6. Uterine contractions and milk release. Controls uterine contractions during delivery and stimulates milk release in lactating women.
7. Ion management. Regulates Na+, K+, and Ca2+ concentrations in the blood.
8. Blood glucose. Controls blood glucose and other nutrient levels.
9. Direct gene activation. Lipid-soluble steroid hormones diffuse through the target cell's plasma membrane, enter the nucleus, and bind a receptor protein. The hormone-receptor complex binds the cell's DNA, activating genes to transcribe mRNA, which is translated in the cytoplasm into new proteins.
10. Second messenger system. Water-soluble nonsteroidal hormones (protein and peptide hormones) cannot enter the cell, so they bind receptors on the plasma membrane and act through a second messenger.

Anatomy of the Endocrine System

The endocrine organs are small but functionally enormous in maintaining homeostasis.

Hypothalamus

The major endocrine organs are the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands, the pancreas, and the gonads.

• Hypothalamus. Part of the nervous system but also a major endocrine organ, producing several hormones. It is the autonomic and endocrine control center of the brain, inferior to the thalamus.
• Mixed function. Some glands are purely endocrine; others (pancreas and gonads) are mixed, both endocrine and exocrine.

Pituitary Gland

About the size of a pea.

• Location. Hangs by a stalk from the inferior hypothalamus, cradled in the "Turk's saddle" of the sphenoid bone.
• Lobes. Two functional lobes: the anterior pituitary (glandular tissue) and posterior pituitary (nervous tissue).

Anterior Pituitary Hormones

• Growth hormone (GH). A general metabolic hormone aimed mainly at growth of skeletal muscle and long bones. Protein-sparing and anabolic: builds amino acids into proteins and drives target cells to grow and divide.
• Prolactin (PRL). A protein hormone structurally similar to GH. Its only known human target is the breast, where after childbirth it stimulates and maintains milk production.
• Adrenocorticotropic hormone (ACTH). Regulates the endocrine activity of the adrenal cortex.
• Thyroid-stimulating hormone (TSH, thyrotropin). Influences growth and activity of the thyroid gland.
• Gonadotropic hormones. Regulate the hormonal activity of the gonads (ovaries and testes).
• Follicle-stimulating hormone (FSH). Stimulates follicle development in the ovaries; maturing follicles produce estrogen and eggs for ovulation. In men, FSH stimulates sperm development.
• Luteinizing hormone (LH). Triggers ovulation and makes the ruptured follicle produce progesterone and some estrogen. In men, LH stimulates testosterone production by the interstitial cells of the testes.

Posterior Pituitary Hormones

The posterior pituitary stores hormones made by hypothalamic neurons rather than making them itself.

• Oxytocin. Released in significant amounts during childbirth and nursing. Drives powerful uterine contractions during labor and triggers milk ejection (let-down reflex).
• Antidiuretic hormone (ADH). Makes the kidneys reabsorb more water, so urine volume falls and blood volume rises. In larger amounts it raises blood pressure by constricting arterioles, hence the name vasopressin.

Thyroid Gland

• Location. At the base of the throat, just inferior to the Adam's apple; easily palpated on exam.
• Lobes. Two lobes joined by a central isthmus.
• Composition. Hollow follicles that store a sticky colloidal material.
• Thyroid hormone. The body's major metabolic hormone, actually two iodine-containing hormones: thyroxine (T4) and triiodothyronine (T3).
• Thyroxine. The major hormone secreted by the thyroid follicles.
• Triiodothyronine. Mostly formed at target tissues by conversion of thyroxine.
• Function. Controls the rate at which glucose is oxidized into body heat and chemical energy; also important for normal tissue growth and development.
• Calcitonin. Lowers blood calcium by depositing calcium in bone. Made by the parafollicular cells between the follicles.

Parathyroid Glands

• Location. On the posterior surface of the thyroid gland.
• Parathyroid hormone (PTH, parathormone). The most important regulator of blood calcium homeostasis. PTH is hypercalcemic (raises blood calcium); calcitonin is hypocalcemic. PTH also stimulates the kidneys and intestines to absorb more calcium.

Adrenal Glands

Structurally and functionally two endocrine organs in one.

Adrenal Cortex Hormones

The cortex produces three groups of steroid hormones (corticosteroids): mineralocorticoids, glucocorticoids, and sex hormones.

• Mineralocorticoids. Primarily aldosterone, from the outermost cortical layer. Regulate the salt content of the blood, especially sodium and potassium, and help control water and electrolyte balance.
• Renin. An enzyme the kidneys release when blood pressure drops; it triggers formation of angiotensin II, a potent stimulator of aldosterone release.
• Atrial natriuretic peptide (ANP). Blocks aldosterone release to reduce blood volume and blood pressure.
• Glucocorticoids. From the middle layer; include cortisone and cortisol. Promote normal cell metabolism and help the body resist long-term stressors, mainly by raising blood glucose (a hyperglycemic hormone). They also reduce pain and inflammation by inhibiting prostaglandins.
• Sex hormones. Produced in small amounts throughout life from the innermost layer; mostly androgens, with some estrogens.

Adrenal Medulla Hormones

The medulla develops from nervous tissue, like the posterior pituitary.

• Catecholamines. When sympathetic neurons stimulate the medulla, it releases epinephrine (adrenaline) and norepinephrine (noradrenaline) into the blood.
• Function. Catecholamines raise heart rate, blood pressure, and blood glucose and dilate the lung passageways. They prepare the body for short-term stress and produce the alarm stage of the stress response.

Pancreatic Islets

The pancreas, near the stomach in the abdominal cavity, is a mixed gland.

• Islets of Langerhans (pancreatic islets). Masses of hormone-producing tissue scattered among the enzyme-producing acinar tissue.
• Hormones. The islets produce insulin and glucagon, acting as fuel sensors during fed and fasting states.
• Beta cells. High blood glucose stimulates insulin release from the beta cells.
• Alpha cells. Low blood glucose stimulates glucagon release from the alpha cells.
• Insulin. Acts on nearly all body cells, increasing their uptake of glucose across the plasma membrane. It clears glucose from the blood, so its effect is hypoglycemic.
• Glucagon. Insulin's antagonist. Hyperglycemic; its main target is the liver, which it drives to break stored glycogen into glucose and release it into the blood.

Pineal Gland

• Location. Hangs from the roof of the third ventricle.
• Melatonin. The only hormone secreted in substantial amounts by the pineal gland. Levels rise and fall over the day, peaking at night to make us drowsy. It is believed to be the sleep trigger that sets the day-night cycle.

Thymus Gland

Large in infants and children, shrinking through adulthood.

• Location. Upper thorax, posterior to the sternum.
• Thymosin. Helps the normal development of T-lymphocytes (T cells) and the immune response.

Gonads

The gonads produce sex hormones identical to those made by the adrenal cortex; the difference is the source and amount.

Hormones of the Ovaries

• Location. A pair of almond-sized organs in the pelvic cavity.
• Steroid hormones. Besides eggs, the ovaries produce estrogen and progesterone.
• Estrogen. Drives the development of female sex characteristics at puberty; with progesterone, promotes breast development and the cyclic changes of the menstrual cycle.
• Progesterone. Works with estrogen in the menstrual cycle. In pregnancy it quiets the uterine muscle so an implanted embryo is not aborted and helps prepare breast tissue for lactation.

Hormones of the Testes

• Location. Paired oval organs suspended in the scrotum, outside the pelvic cavity.
• Androgens. Besides sperm, the testes produce male sex hormones (androgens), of which testosterone is the most important.
• Testosterone. At puberty, promotes growth and maturation of the reproductive organs, brings out secondary sex characteristics, and stimulates sex drive. Also required for continuous sperm production.

Other Hormone-Producing Tissues and Organs

Pockets of hormone-producing cells also sit in fatty tissue and in the walls of the small intestine, stomach, kidneys, and heart. Notably, the kidneys produce erythropoietin, which stimulates red blood cell development in the bone marrow; without it, anemia results.

Placenta

A temporary organ formed in the pregnant uterus.

• Function. Serves as the fetus's respiratory, excretory, and nutrition system and produces proteins and steroid hormones that maintain the pregnancy and prepare for delivery.
• Human chorionic gonadotropin (hCG). Produced in early pregnancy by the embryo and then the fetal placenta; keeps the ovaries producing estrogen and progesterone so the uterine lining is not shed.
• Human placental lactogen (hPL). Works with estrogen and progesterone to prepare the breasts for lactation.
• Relaxin. Relaxes the mother's pelvic ligaments and the pubic symphysis to ease birth passage.

Physiology of the Endocrine System

Hormones mainly control reproduction, growth and development, mobilizing defenses against stressors, maintaining electrolyte, water, and nutrient balance, and regulating cellular metabolism and energy balance.

Chemistry of Hormones

• Hormones. Chemicals secreted by endocrine cells into the extracellular fluid that regulate the metabolic activity of other cells.
• Classification. Nearly all are either amino acid-based (proteins, peptides, amines) or steroids.
• Steroid hormones. Made from cholesterol; include the gonadal sex hormones and the adrenal cortex hormones.
• Amino acid-based hormones. All the rest, nonsteroidal amino acid derivatives.

Mechanisms of Hormone Action

Hormones circulate everywhere but affect only certain cells or organs.

• Target cells. A target cell must carry specific protein receptors, on its membrane or inside, for the hormone to bind and act.
• Effect. Hormones alter cellular activity, speeding up or slowing a normal metabolic process rather than starting a new one.
• Changes after binding. Depending on the hormone and target, one or more of the following occurs:

1. Changes in plasma membrane permeability or electrical state.
2. Synthesis of proteins or regulatory molecules (such as enzymes).
3. Activation or inactivation of enzymes.
4. Stimulation of mitosis.
5. Promotion of secretory activity.

Control of Hormone Release

Negative feedback is the main control of blood hormone levels. The stimuli that activate endocrine organs fall into three categories:

• Hormonal stimuli. The most common: other hormones prod a gland into action. Hypothalamic hormones stimulate the anterior pituitary, and many anterior pituitary hormones stimulate other endocrine organs.
• Humoral stimuli. Changing blood levels of ions or nutrients trigger release. Falling blood calcium prompts the parathyroid glands to release PTH.
• Neural stimuli. In a few cases nerve fibers drive release; the classic example is sympathetic stimulation of the adrenal medulla to release norepinephrine and epinephrine during stress.