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Animal coordination, control and homeostasis, subtopic 1 of 5Spec 7.1–7.3

Hormones and the endocrine system

Where hormones are made and how they travel, plus adrenalin and thyroxine (Higher tier).

3 sections, with a quick check at the end.

Download notes PDF4 pages

The endocrine systemSpec 7.1

Hormones are chemical messengers. They are made in endocrine glands and released straight into the blood. The blood plasma carries them around the whole body, but each hormone only affects its target organs.

  1. An endocrine gland secretes a hormone into the blood.
  2. The blood carries the hormone around the body.
  3. The hormone reaches its target organ and causes a response.
Endocrine glands you need to know
GlandWhere it isHormone made
Pituitary glandAt the base of the brainReleases hormones that control other glands and organs
Thyroid glandIn the neckThyroxine
PancreasIn the abdomen, below the stomachInsulin
Adrenal glandsOne on top of each kidneyAdrenalin
OvariesIn the lower abdomen of a femaleOestrogen and progesterone
TestesIn the scrotum of a maleTestosterone
Higher tier
  • The pituitary gland releases FSH and LH, which act on the ovaries, and TSH, which acts on the thyroid gland.
  • The pancreas also makes a second hormone, glucagon.
Female and male body outlines showing the pituitary gland, thyroid gland, pancreas, adrenal glands on top of the kidneys, ovaries and testes, each labelled with the hormone it makes.Tap to enlarge
The endocrine glands and the main hormone each one makes.

Adrenalin and fight or flightSpec 7.2Higher tier

Adrenalin is produced by the adrenal glands. It prepares the body for fight or flight, for example when you are frightened or in danger. It makes the body ready to use a lot of energy quickly.

The effects of adrenalin
EffectWhy it helps
Increased heart rateBlood is pumped around the body faster
Increased blood pressureBlood is pushed to the organs more quickly
Increased blood flow to the musclesMuscles get more oxygen and glucose for respiration, so they can release more energy
Raised blood sugar levelAdrenalin stimulates the liver to change glycogen into glucose, which is released into the blood

Thyroxine and negative feedbackSpec 7.3Higher tier

Thyroxine is made by the thyroid gland. It controls metabolic rate, which is the rate of the chemical reactions in the body. Its level is kept steady by negative feedback.

Negative feedback
A change in a level causes a response that reverses the change and brings the level back to normal.
  1. A low level of thyroxine stimulates the hypothalamus to produce TRH.
  2. TRH causes the pituitary gland to release TSH.
  3. TSH acts on the thyroid gland to produce thyroxine.
  4. The thyroxine level rises back to normal.
  5. When the thyroxine level is normal, thyroxine inhibits the release of TRH and the production of TSH. This stops the level rising too far.
Flow diagram: a low thyroxine level stimulates the hypothalamus to release TRH, TRH acts on the pituitary gland to release TSH, TSH acts on the thyroid gland to make thyroxine, and dashed arrows show the normal thyroxine level inhibiting TRH release and TSH production.Tap to enlarge
Thyroxine is controlled by negative feedback: when its level is back to normal it inhibits TRH and TSH.

Quick check

  1. How are hormones transported from endocrine glands to target organs?

    Show answer

    In the blood (plasma).

  2. Which gland produces insulin?

    Show answer

    The pancreas.

  3. Name the four effects of adrenalin. (Higher)

    Show answer

    Increased heart rate, increased blood pressure, increased blood flow to the muscles, and raised blood sugar level.

  4. Which hormone does the pituitary release to make the thyroid produce thyroxine? (Higher)

    Show answer

    TSH.