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MCAT Content / Biological Bases Of Behavior / The Endocrine System

The Endocrine System

Topic: Biological Bases Of Behavior

The endocrine and nervous systems work together to act as a communication system for the human body controlling homeostatic functions and behaviour.

The endocrine system acts as a communication tool within the human body, working in tandem with the nervous system to communicate with the body’s other internal systems. Both the nervous and endocrine systems send messages everywhere inside the human body some of these messages have an impact on behaviour.

Hormones are chemicals within the endocrine system that affect physiological activity. They are secreted by one tissue and conveyed by the bloodstream to another tissue. Hormones have high levels of specificity, which means they only react with certain receptor sites in the body. The best way to describe hormones is to think of a lock and a key: only a certain hormone (lock) can create a certain response within your body’s receptive tissue (key).

There are eight major endocrine glands, each with a different function.

– Pituitary gland: the “brain” of the endocrine system; regulates all seven of the other glands and secretes growth hormone.
– Thyroid: regulates a person’s metabolic rate, which is the amount of energy expended daily by a person at rest.
– Thymus: assists in the development of a person’s immune system.
– Adrenal gland: regulates fluid and sodium balance within the body, and secretes epinephrine (“adrenaline”) when the body is under stress, producing the fight-or-flight response.
– Ovaries (in females) and testes (in males): control the development of secondary sex characteristics.
– Pancreatic islets: regulate blood sugar.
– Pineal gland: regulates biorhythms and mood, and stimulates the onset of puberty.

The endocrine system affects behaviour by controlling key functions in the body. For example, the hypothalamus controls the basic drives for hunger, thirst and sexual attraction, determining behaviour and responses to stimuli. The testes secrete testosterone which in men and women is both linked to the sexual arousal. The hormone serotonin in the brain determines mood levels. Estrogen in females is important in the mood changes of women particularly in the menopause where it can cause low arousal, depression and irritability. The best example is secreted by the adrenal glands, norepinephrine and epinephrine are hormones which cause the body to initiate the fight or flight response causing fear and aggression.

MCAT The Endocrine System 

Practice Questions

Khan Academy

Physiological stress response to cuss words


MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Passage 6 Question 37

Biology Question Pack, Vol. 2 Passage 13 Question 87

Sample Test B/B Section Passage 8 Question 41

Key Points

• The endocrine system acts as a communication tool for the human body, working in tandem with the nervous system to communicate with the body’s other internal systems.

• The endocrine system differs from the nervous system in that its chemical signals are slower-moving and longer-lasting.

• Hormones act as chemical messengers within the body, telling it to perform specific physical and mental functions.

• There are eight major endocrine glands, each performing a different function: the pituitary gland, the thyroid, the thymus gland, the adrenal gland, the ovaries (female) and testes (male), the pancreatic islets, and the pineal gland

• Hormones can alter human behaviour by driving survival functions (fight or flight) and processes such as hunger and excretion.

Key Terms

Epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress

Hormone: a chemical that is made by specialist cells, usually within an endocrine gland, and it is released into the bloodstream

Endocrine system: the collection of glands that produce hormones that regulate bodily processes

Gland: an organ that synthesizes a substance, such as hormones or breast milk, and releases it, often into the bloodstream or into cavities inside the body or on its outer surface

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