AQA Syllabus focus:
'The function of the endocrine system, including glands and hormones.'
The endocrine system is the body’s slower chemical communication network. It helps regulate internal processes by releasing hormones from specialized glands, influencing behavior, growth, metabolism, and reproduction over longer periods.
What the endocrine system does
The endocrine system is one of the body’s main communication systems. Instead of sending rapid electrical impulses like the nervous system, it sends chemical messages through the blood. Its main function is to regulate body processes and maintain coordinated activity across different organs.
Endocrine system: A network of glands that secrete hormones directly into the bloodstream to regulate body processes.
Because hormones travel in the blood, endocrine effects are usually slower to start than nerve signals. However, they are often longer lasting, which makes the endocrine system especially important for controlling processes that do not need instant changes, such as growth, metabolism, reproductive development, and aspects of mood and behavior.
Hormones as chemical messengers
A hormone is the substance released by an endocrine gland.
Hormone: A chemical messenger produced by an endocrine gland and carried in the blood to target cells or organs.
Hormones circulate throughout the body, but they only affect target cells that have the correct receptors. Receptors are specialized protein sites that allow a particular hormone to bind and produce an effect. This means one hormone can travel widely in the bloodstream, yet only certain tissues will respond. Hormones can change the activity of organs, influence the release of other hormones, or alter rates of processes such as energy use or sexual maturation.
Major endocrine glands and their hormones
Endocrine glands are organs that produce and release hormones. Several important glands are commonly studied in psychology because their hormones influence behavior as well as physical functioning.
Labeled overview of the human endocrine system, showing where the major glands sit in the body (e.g., pituitary in the brain, thyroid in the neck, adrenal glands above the kidneys, and pancreas in the abdomen). Use it to link each gland’s location to the hormones it releases and the broad body systems those hormones regulate over longer timescales. Source
Pituitary gland
The pituitary gland is often called the master gland because it releases hormones that control other endocrine glands. It has a central role in coordinating the endocrine system. For example, some pituitary hormones stimulate other glands to start or stop releasing their own hormones. It also releases growth hormone, which affects physical development.
Thyroid gland
The thyroid gland releases thyroxine, a hormone that helps regulate metabolism. Metabolism refers to the rate at which the body uses energy. If thyroxine levels are high, body processes may speed up; if they are low, the body may feel slower and more tired. This shows how hormones can influence both physical activity and psychological state.
Adrenal glands
The adrenal glands, located above the kidneys, release several hormones, including adrenaline and cortisol. These hormones are associated with physiological arousal and help the body cope with demanding situations. In this subtopic, the key point is that the adrenal glands are endocrine glands and their hormones enter the bloodstream to influence organs throughout the body.
Pancreas
The pancreas has an endocrine function because it releases insulin and glucagon, hormones that regulate blood glucose levels.
Diagram showing the pancreas’ negative-feedback control of blood glucose, contrasting insulin release when blood glucose is high with glucagon release when blood glucose is low. This visual reinforces how endocrine signals travel via the bloodstream to produce coordinated, body-wide effects (e.g., changing glucose uptake and release). Source
Stable glucose levels are important because cells, especially brain cells, need a steady energy supply. Hormonal control from the pancreas therefore supports both physical functioning and mental performance.
Ovaries and testes
The ovaries and testes are also endocrine glands. The ovaries release estrogen, while the testes release testosterone. These hormones are important in sexual development, reproduction, and the emergence of secondary sexual characteristics during puberty. Hormones from these glands can also influence mood, motivation, and patterns of behavior.
Glands and secretion
An endocrine gland is an organ that makes hormones and releases them directly into the bloodstream. Endocrine glands are often described as ductless because they do not use tubes to carry secretions to a surface. Instead, hormones enter surrounding capillaries and are distributed by circulation. Different glands release different hormones, and very small amounts can have significant effects if the target tissue is sensitive to them. This helps explain why hormonal imbalance can sometimes produce noticeable changes in bodily functioning and behavior.
How endocrine communication works
The endocrine system works through a sequence of steps:
a gland detects that a hormone needs to be released
the hormone is secreted directly into the bloodstream
the hormone circulates around the body
target cells with the correct receptors respond
the activity of tissues or organs changes
This process helps explain why hormones can have widespread but selective effects. They can travel almost everywhere in the body, but only cells with matching receptors will react. Hormonal action is therefore both broad and specific. Compared with nerve transmission, endocrine communication is generally less immediate, but it is well suited to regulating ongoing bodily states over time.
Why the endocrine system matters in psychology
Psychology students study the endocrine system because behavior is affected not only by the brain and nerves, but also by chemical communication. Hormones can influence arousal, energy levels, sexual behavior, and long-term patterns of development. This is why the endocrine system provides an important biological explanation for aspects of human behavior. Understanding glands and hormones also helps explain how physical changes in the body can produce psychological effects, and why changes in hormone levels can alter how a person feels or acts.
Practice Questions
Outline what is meant by a hormone. (2 marks)
1 mark for stating that a hormone is a chemical messenger.
1 mark for stating that it is secreted by an endocrine gland and/or carried in the bloodstream to a target cell or organ.
Explain the function of the endocrine system, referring to glands and hormones. (6 marks)
1 mark for stating that the endocrine system is a system of glands.
1 mark for stating that glands secrete hormones directly into the bloodstream.
1 mark for explaining that hormones are chemical messengers.
1 mark for explaining that hormones act on target cells/organs with appropriate receptors.
1 mark for one accurate example of a gland with its hormone, such as thyroid and thyroxine or pancreas and insulin.
1 mark for linking hormonal action to regulation of body processes such as metabolism, growth, reproduction, or arousal.
FAQ
It is called the master gland because many of its hormones control other endocrine glands, such as the thyroid and adrenal glands.
However, it does not work entirely independently. It is closely linked to the hypothalamus, which is part of the brain. The hypothalamus helps monitor the body’s internal state and can signal the pituitary to release or stop releasing certain hormones.
So, the pituitary is a major control center within the endocrine system, but it is also guided by the brain.
An endocrine gland releases hormones directly into the bloodstream. These hormones travel to target organs or cells elsewhere in the body.
An exocrine gland releases substances through ducts rather than into the blood. Examples include:
sweat glands
salivary glands
glands that release digestive enzymes
The pancreas is unusual because it has both endocrine and exocrine functions. Its endocrine part releases hormones such as insulin, while its exocrine part releases digestive substances into the gut.
The speed of hormonal action depends partly on how the hormone works at the target cell.
Some hormones act by attaching to receptors on the outside of a cell and triggering a rapid response. Others enter the cell and affect gene activity, which takes longer.
The speed also depends on:
how much hormone is released
how quickly it reaches the target tissue
how sensitive the receptors are
This is why not all hormonal effects feel equally fast, even though endocrine communication is generally slower than neural communication.
A hormone’s effect depends on the type of receptor and the cell’s internal machinery.
Two tissues may both respond to the same hormone, but they can respond in different ways because:
they have different receptor subtypes
they activate different chemical processes inside the cell
they play different biological roles
So, the hormone itself is not the whole story. The nature of the target tissue also determines what happens after the hormone binds.
No. Many hormones are released in pulses or follow a daily rhythm.
For example:
cortisol is often higher earlier in the day
melatonin is associated with darkness and sleep timing
reproductive hormones can change across longer cycles
These rhythms help the body match hormone release to changing needs, such as sleeping, waking, eating, or responding to environmental demands.
This means a hormone level measured at one time of day may differ from a level measured later, even in a healthy person.
