12.6 - The Endocrine System

Human Endocrine System

• Endocrine glands are ductless glands that secrete hormones directly into the bloodstream.
• Hormones are organic chemical substances transported throughout the body but affect only specific target cells.
• Hormones bind to specific receptors on target-cell membranes and produce specific responses.
• The three main hormone functions are reproduction, growth and homeostasis.
• Endocrine glands: hypothalamus, pituitary, thyroid, adrenal, pancreas, ovary and testis.

Endocrine Glands and Hormones

Hypothalamus

• Gonadotrophin-releasing hormone (GnRH): Stimulates the pituitary gland to secrete FSH and LH into the blood.

Pituitary Gland

• Main endocrine gland; located at the base of the hypothalamus.
• Antidiuretic hormone (ADH) — kidney tubule: Stimulates water reabsorption.
• Oxytocin — uterine muscles and mammary glands: Stimulates uterine muscle contraction during birth and milk production by mammary glands.
• Luteinising hormone (LH) — ovary and testis: Stimulates ovulation, corpus luteum development, oestrogen and progesterone secretion, and testosterone secretion.
• Growth hormone (GH) — soft tissues and bones: Stimulates growth, protein synthesis and fat metabolism.
• Adrenocorticotropic hormone (ACTH) — adrenal cortex: Stimulates hormone secretion by the adrenal cortex.
• Thyroid-stimulating hormone (TSH) — thyroid gland: Stimulates thyroxine secretion.
• Follicle-stimulating hormone (FSH) — ovary and testis: Stimulates ovarian follicle development and spermatogenesis.

Thyroid Gland

• Thyroxine — all tissues: Increases metabolic rate and body temperature; regulates growth and development.

Adrenal Gland

• Aldosterone — kidney: Increases salt reabsorption in the kidneys.
• Adrenaline — muscles, liver and heart: Increases blood glucose and fatty-acid levels, respiratory rate, heartbeat and metabolic rate; contracts blood vessels.

Pancreas

• Insulin — liver, muscles and adipose tissues: Reduces blood glucose level and promotes conversion of excess glucose to glycogen.
• Glucagon — liver, muscles and adipose tissues: Increases blood glucose level and promotes conversion of glycogen to glucose.

Ovary

• Oestrogen — gonads, skin, muscles and bones: Stimulates female secondary sexual characteristics and follicle maturity; repairs and stimulates thickening of the uterine wall.
• Progesterone — gonad: Stimulates development of the uterine wall and formation of the placenta.

Testis

• Testosterone — gonads, skin, muscles and bones: Stimulates male secondary sexual characteristics and spermatogenesis.

Nervous and Endocrine Systems in a “Fight or Flight” Situation

1. The hypothalamus sends nerve impulses directly to the adrenal medulla.
2. Adrenal medulla cells secrete adrenaline and noradrenaline.
3. Heart rate, respiratory rate, blood pressure, blood glucose level and metabolic activity increase.
4. The heart delivers more oxygen and glucose to the brain and skeletal muscles for the additional energy needed to fight or flee.
5. When the emergency is controlled, bodily changes return to the normal range.

Comparison Between Nervous and Endocrine Systems

• Similarities: Respond to stimuli, regulate body activities and involve target tissues or organs.
• Nervous system: Network of millions of neurones; electrical impulses travel through neurones; response is rapid and immediate; effect is short and usually limited to one organ.
• Endocrine system: Ductless endocrine glands; organic chemical signals called hormones travel in blood; response is slow and prolonged; effect may involve several organs.