2. The Endocrine System
• Information is sent through the endocrine (hormonal) system
as chemical substances known as hormones
• Hormones are carried by the blood and can therefore circulate
around the whole body
• Hormones transmit information from one part of the organism to
another and bring about a change (they provide a signal that triggers
a response)
• They alter the activity of one or more specific target organs
3. • Hormones are used to control functions that do not need instant
responses
• Compared to the nervous system the effects of hormones are slower but
they act for longer
• Hormones are produced by endocrine glands
• The endocrine glands that produce hormones in animals are known
collectively as the endocrine system
4. • A gland is a group of cells that produces and releases one or more
substances (a process known as secretion)
5.
6. Structures that make up the endocrine system
Important structures in the endocrine system are:
• Pituitary gland: a ‘master gland’ that makes hormones such
as FSH and LH
• Thyroid: produces thyroxine, which controls metabolic rates and
affects growth
• Pancreas: produces insulin, which regulates blood glucose levels
• Adrenal glands: produces adrenaline
• Testes (males): produce testosterone
• Ovaries (females): produce oestrogen
7.
8.
9. Pituitary Gland
• The pituitary gland in the brain is a ‘master gland’ which secretes
several hormones into the blood in response to body conditions.
• These hormones in turn act on other glands to stimulate other
hormones to be released to bring about effects
• For example, in certain conditions, the pituitary gland makes and
secretes thyroid-stimulating hormone (TSH) which stimulates the
thyroid to release thyroxine
10. Hormones are synthesized and released into the bloodstream from a
gland (such as the pituitary gland) and circulate in the bloodstream,
having an effect on target cells
11. • A negative feedback mechanism is an important type of control that
is found in homeostasis.
• A negative feedback control system responds when conditions change
from the ideal or set point and returns conditions to this set point.
• There is a continuous cycle of events in negative feedback.
12. General stages in negative feedback
In general this works by:
• if the level of something rises, control systems reduce it again
• if the level of something falls, control systems raise it again
13. Feedback control regulates hormone levels in
the blood
Feedback Mechanism
Insulin
Blood sugar
Cells absorb more blood sugar
Negative feedback
15. • The hypothalamus and pituitary gland have important roles in
detecting and controlling thyroxine levels.
• Low thyroxine levels in the bloodstream stimulate the hypothalamus
to release TRH and this causes the pituitary to release TSH so the
thyroid releases more thyroxine. So blood levels return to normal.
• Normal thyroxine levels in the bloodstream inhibit TRH release from
the hypothalamus and the production of TSH by the pituitary, so
normal blood levels are maintained.
16. • The control of thyroxine levels by the hypothalamus and pituitary
gland is an example of negative feedback.
thyroid-stimulating hormone (TSH)
