07 The Endocrine System
Learn how endocrine glands and hormones coordinate target cells, feedback regulation, growth, metabolism, reproduction, and interactions with other body systems.
Endocrine glands and their hormones
The is a network of glands and -secreting cells that coordinates body functions. Endocrine glands release hormones into the blood rather than sending them through ducts. Hormones travel to target cells and influence processes such as growth, energy use, reproduction, and responses to changing conditions.
Endocrine cells are also found in organs such as the heart and digestive tract, so the major glands listed here are examples rather than a complete inventory.
Major glands and their hormones
Hypothalamus: Releasing and inhibiting hormones direct many pituitary secretions and connect information from the nervous system with endocrine responses.
Pituitary gland: Growth (GH), thyroid-stimulating (TSH), adrenocorticotropic (ACTH), follicle-stimulating (FSH), luteinizing (LH), and prolactin help regulate growth, other endocrine glands, reproduction, and milk production. The posterior pituitary releases oxytocin and , both made in the hypothalamus.
Thyroid gland: Thyroxine (T4) and triiodothyronine (T3) affect metabolic activity, growth, and development. Calcitonin helps regulate blood calcium.
Parathyroid glands: Parathyroid (PTH) raises blood calcium and helps regulate calcium balance with the kidneys, intestines, and bones.
Adrenal glands: Cortisol and aldosterone support stress responses, blood pressure, and salt balance. Epinephrine and norepinephrine support rapid fight-or-flight responses.
Pancreas: Insulin generally lowers blood glucose, while glucagon raises it; together, they help keep blood glucose within a useful range.
Ovaries: Estrogens and progesterone help regulate reproductive cycles, pregnancy, and female reproductive development.
Testes: Testosterone supports sperm production and male reproductive development.
Pineal gland: Melatonin helps coordinate daily sleep–wake rhythms.
How hormones affect target cells
A affects a cell only if that cell has a compatible . Receptors can be on the cell surface or inside the cell. When a binds to its , it changes the target cell’s activity—for example, by changing enzyme activity, opening or closing channels, or influencing which proteins the cell makes.
Hormones include peptides and proteins, steroid hormones, and amino-acid derivatives. Their chemical properties help determine how they reach and act on target cells. A can affect several organs at once, but only cells with the appropriate receptors respond.
signals also differ in timing. A rapid change in blood glucose can prompt pancreatic release, whereas some effects, such as changes in gene activity, develop more gradually.
Feedback and endocrine regulation
Most endocrine systems use : when a ’s level or effect is sufficient, signals promoting further release decrease. This helps maintain by keeping internal conditions within workable ranges.
Thyroid feedback
The hypothalamus releases thyrotropin-releasing (TRH), which prompts the pituitary to release TSH. TSH stimulates the thyroid to release T4 and T3. As thyroid levels rise, they reduce stimulation from the hypothalamus and pituitary, limiting further release.
release can also respond directly to changing conditions. After a meal, rising blood glucose stimulates insulin release. Between meals, falling glucose helps stimulate glucagon release. These responses connect the digestive system’s supply of nutrients with the body’s use and storage of energy.
Growth and development
Growth reflects interacting signals rather than a single acting alone. Growth from the anterior pituitary promotes growth and affects how the body uses nutrients. It also stimulates production of insulin-like growth factor 1 (IGF-1), especially in the liver; together, these signals support tissue and bone growth. Thyroid hormones are also important for normal growth and development.
and energy balance
is the set of chemical processes by which the body obtains, uses, and stores energy. Thyroid hormones influence the rate of many metabolic processes.
Insulin promotes movement of glucose from the blood into many cells and supports energy storage. Glucagon helps raise blood glucose when it is low. Cortisol also affects how the body uses fuels, particularly during stress. These hormones link endocrine activity with the digestive system, liver, muscles, and other tissues.
Reproductive pathways
Reproductive regulation involves a pathway between the hypothalamus, pituitary, and gonads. The hypothalamus releases , prompting the pituitary to release FSH and LH. These hormones act on the ovaries or testes, supporting reproductive-cell development and production of sex hormones. Sex hormones feed back to the brain and pituitary, helping regulate the pathway.
Connections with other body systems
Hormones travel through the circulatory system to reach targets throughout the body. Endocrine signals coordinate the work of multiple organ systems:
Insulin and glucagon connect nutrient absorption with blood-glucose control.
PTH links the parathyroid glands with the bones, kidneys, and intestines to regulate calcium.
Reproductive hormones act on reproductive organs and other tissues.
The hypothalamus integrates information from the brain with endocrine signals, illustrating how the nervous and endocrine systems work together.
Through these connections, endocrine activity works alongside the circulatory, digestive, skeletal, and reproductive systems as well as the nervous system.