Human endocrine system
The endocrine system consists of endocrine glands that secrete organic compounds, chemical
messengers called hormones
Different endocrine glands do not function in isolation but interact continuously through their
hormone secretion.
This contributes to the maintenance of homeostasis, in that almost all metabolic activities in the body
are controlled and coordinated by the endocrine system.
Endocrine glands:
These are ductless glands that release their secretions (hormones) directly into the blood stream. The
body transports the hormones to the target organs where they perform their functions.
Exocrine glands:
These are glands that release their secretions via ducts to a body cavity or to the outside. E.g., salivary
glands that secrete saliva into ducts that open into the mouth cavity.
The type of control and coordination brought about by hormones in the blood known as chemical
coordination.
Hormones
Þ Hormones are organic chemical messengers
Þ Most hormones are proteins
Þ Hormones are secreted in small quantities directly into the bloodstream by the endocrine glands
Þ Hormones are transported to target organs by the bloodstream
Target organs: an organ that is affected by a specific hormone. The target organ has receptors for this
hormone which enable it to respond.
Þ Hormones have a regulatory function that can be stimulating or inhibiting.
Þ Hormones are mostly regulated by negative feedback mechanisms.
Stimulating: triggering a response to increase activity.
Inhibiting: restraining a response to decrease activity.
, Negative feedback:
In a negative feedback mechanism, any deviation from the normal levels triggers a response that
restores the normal levels.
Example: Hormone A and B
- Hormone A stimulates the secretion of hormone B; therefore, hormone B levels rise.
- High levels of hormone B, however, inhibit the secretion of hormone A - therefore
hormone A levels decrease.
- The lower levels of hormones A do not stimulate the secretion of hormone B – therefore,
hormone B levels also drop.
- The lower levels of hormone B do not inhibit the secretion of hormone A anymore-
therefore hormone A levels rise and return to normal
- In this way, levels of both hormones are kept in balance through ongoing cycles.
Endocrine glands and their hormones
The pituitary gland
The pituitary gland is located at the base of the brain, attached to the hypothalamus by a short stalk.
It fits into a small bony cavity in the cranium.
The pituitary gland consists of two lobes:
Þ Anterior lobe
Þ Posterior lobe
The endocrine system consists of endocrine glands that secrete organic compounds, chemical
messengers called hormones
Different endocrine glands do not function in isolation but interact continuously through their
hormone secretion.
This contributes to the maintenance of homeostasis, in that almost all metabolic activities in the body
are controlled and coordinated by the endocrine system.
Endocrine glands:
These are ductless glands that release their secretions (hormones) directly into the blood stream. The
body transports the hormones to the target organs where they perform their functions.
Exocrine glands:
These are glands that release their secretions via ducts to a body cavity or to the outside. E.g., salivary
glands that secrete saliva into ducts that open into the mouth cavity.
The type of control and coordination brought about by hormones in the blood known as chemical
coordination.
Hormones
Þ Hormones are organic chemical messengers
Þ Most hormones are proteins
Þ Hormones are secreted in small quantities directly into the bloodstream by the endocrine glands
Þ Hormones are transported to target organs by the bloodstream
Target organs: an organ that is affected by a specific hormone. The target organ has receptors for this
hormone which enable it to respond.
Þ Hormones have a regulatory function that can be stimulating or inhibiting.
Þ Hormones are mostly regulated by negative feedback mechanisms.
Stimulating: triggering a response to increase activity.
Inhibiting: restraining a response to decrease activity.
, Negative feedback:
In a negative feedback mechanism, any deviation from the normal levels triggers a response that
restores the normal levels.
Example: Hormone A and B
- Hormone A stimulates the secretion of hormone B; therefore, hormone B levels rise.
- High levels of hormone B, however, inhibit the secretion of hormone A - therefore
hormone A levels decrease.
- The lower levels of hormones A do not stimulate the secretion of hormone B – therefore,
hormone B levels also drop.
- The lower levels of hormone B do not inhibit the secretion of hormone A anymore-
therefore hormone A levels rise and return to normal
- In this way, levels of both hormones are kept in balance through ongoing cycles.
Endocrine glands and their hormones
The pituitary gland
The pituitary gland is located at the base of the brain, attached to the hypothalamus by a short stalk.
It fits into a small bony cavity in the cranium.
The pituitary gland consists of two lobes:
Þ Anterior lobe
Þ Posterior lobe
