3.5 The Endocrine System

Endocrine System Overview

The endocrine system is a network of glands that produce hormones, chemical messengers carried through the bloodstream to regulate bodily functions. These hormones influence metabolism, growth, mood, and behavior, all of which play a role in maintaining the body's internal balance.

For psychology, the endocrine system matters because hormones directly shape how you feel and act. Stress, aggression, mood swings, energy levels: these all have hormonal components. The endocrine system also works closely with the nervous system, so understanding one helps you understand the other.

Glands of the Endocrine System

Hypothalamus — A brain structure that serves as the command center linking the nervous system to the endocrine system. It regulates the pituitary gland and controls vital functions like body temperature, hunger, thirst, sleep, and emotional responses (stress, fear, pleasure). Think of it as the bridge between what your brain detects and how your body responds hormonally.

Pituitary gland — Often called the "master gland," it sits at the base of the brain and releases hormones that control other endocrine glands. It has two parts:

• Anterior pituitary releases hormones that stimulate growth (growth hormone), thyroid function (thyroid-stimulating hormone), adrenal function (adrenocorticotropic hormone), reproductive function (follicle-stimulating hormone, luteinizing hormone), and milk production (prolactin)
• Posterior pituitary releases hormones that regulate water balance (antidiuretic hormone) and uterine contractions during childbirth (oxytocin)

The pituitary doesn't act on its own. It takes orders from the hypothalamus, which is why the hypothalamus is really the one running the show.

Thyroid gland — Located in the neck, it secretes thyroid hormones (T3 and T4) that regulate metabolism, growth, and development. It also secretes calcitonin, which lowers blood calcium levels.

Parathyroid glands — Four small glands behind the thyroid that secrete parathyroid hormone (PTH), which raises blood calcium levels. PTH and calcitonin work as opposites to keep calcium balanced.

Adrenal glands — Sit on top of the kidneys and have two distinct regions:

• Adrenal cortex (outer layer) secretes cortisol (a stress hormone), aldosterone (regulates salt and water balance), and small amounts of sex hormones (androgens)
• Adrenal medulla (inner core) secretes epinephrine and norepinephrine, the "fight or flight" hormones

Pancreas — Located in the abdomen, it secretes insulin (lowers blood sugar) and glucagon (raises blood sugar). These two hormones work together to keep blood glucose levels stable.

Gonads (ovaries and testes) — The reproductive glands:

• Ovaries secrete estrogen and progesterone, which regulate the menstrual cycle, pregnancy, and female secondary sex characteristics
• Testes secrete testosterone, which regulates sperm production, muscle growth, and male secondary sex characteristics

A note on exocrine glands: These are not part of the endocrine system. Exocrine glands (like sweat glands and salivary glands) secrete substances through ducts directly to a target area, rather than releasing hormones into the bloodstream. You should know the distinction for the exam.

Hormones and Behavioral Influence

This is where the endocrine system connects most directly to psychology. Hormones don't just regulate physical processes; they shape how you think, feel, and behave.

Cortisol is the primary stress hormone, released by the adrenal cortex in response to physical or psychological stress. It increases blood sugar, suppresses immune function, and aids metabolism. Short bursts of cortisol are helpful, but chronically elevated cortisol is linked to anxiety, depression, memory impairment, and weight gain. This is why prolonged stress takes such a toll on both body and mind.

Epinephrine and norepinephrine are released by the adrenal medulla during excitement, stress, or danger. They increase heart rate, blood pressure, and blood sugar, preparing the body for the fight-or-flight response. These hormones can produce feelings of anxiety, restlessness, and irritability, which is why your heart pounds and your hands shake when you're scared.

Thyroid hormones (T3 and T4) regulate metabolism, growth, and brain development. Their psychological effects are a good example of how hormone levels matter:

• Hypothyroidism (too little) causes weight gain, fatigue, and depression
• Hyperthyroidism (too much) causes weight loss, anxiety, and irritability

Insulin and glucagon regulate blood sugar. Insulin lowers blood sugar by promoting glucose uptake into cells; glucagon raises it by triggering glucose release from the liver. When this system breaks down (as in diabetes), it affects mood, energy levels, and cognitive function.

Sex hormones (estrogen, progesterone, testosterone) regulate reproductive functions and secondary sex characteristics, but they also influence behavior. Estrogen affects mood and cognitive function. Testosterone influences aggression, competitive behavior, and sex drive. Both males and females produce all three hormones, just in different amounts. Imbalances can contribute to mood disorders, irritability, and changes in behavior.

Endocrine System and Homeostasis

Endocrine vs. Nervous System

Both systems maintain homeostasis (the body's stable internal state), but they work differently:

The two systems interact constantly. The hypothalamus is the key link: it receives nervous system input (stress signals, emotions, sensory information) and translates that into hormonal commands sent to the pituitary gland. Some chemicals even serve double duty. Epinephrine and norepinephrine, for example, function as hormones when released by the adrenal glands and as neurotransmitters when released by neurons in the sympathetic nervous system.

Feedback Mechanisms and Hormone Action

The endocrine system uses feedback loops to keep hormone levels in the right range.

Negative feedback is the most common mechanism. It works like a thermostat: when a hormone's effect reaches a certain level, the system dials back production. For example, when blood glucose rises after a meal, the pancreas releases insulin to lower it. Once glucose drops back to normal, insulin secretion slows down. This prevents overcorrection.

Positive feedback is rarer. Instead of reducing the stimulus, it amplifies it, driving a process to completion. The classic example is childbirth: oxytocin stimulates uterine contractions, which push the baby against the cervix, which triggers even more oxytocin release. The cycle only stops once the baby is delivered.

For hormones to have any effect, they need to reach the right cells. Target cells are specific cells that respond to a particular hormone because they have the matching hormone receptors, specialized proteins on or within the cell that bind to that hormone and trigger a response. If a cell lacks the receptor for a given hormone, that hormone has no effect on it.

Endocrine Disorders

Endocrine disorders result from the over- or under-production of hormones, or from problems with hormone receptors on target cells. A few examples relevant to this course:

• Diabetes results from insufficient insulin production (Type 1) or the body's inability to use insulin effectively (Type 2), leading to unregulated blood sugar
• Thyroid disorders like hypothyroidism and hyperthyroidism produce the mood and energy symptoms described above
• Growth hormone deficiency can impair physical development in children

These disorders illustrate a core principle: even small hormonal imbalances can have wide-ranging effects on metabolism, growth, development, and behavior.