12.6 - The Endocrine System

Endocrine System of Humans

The endocrine system consists of a number of glands that release chemical messengers called hormones in the body. The endocrine glands are ductless glands, which means they secrete the hormones directly into the bloodstream. The hormones produced only affect specific target cells despite circulating throughout the body. The hormones bind to specific receptors on the surface of the target cells, causing the cells to respond accordingly. The hormones are involved in 3 primary functions – reproduction, growth and homeostasis.

Figure 14 - The human endocrine system

The Functions of Hormones Secreted by Each Endocrine Glands

The pituitary gland is the 'master' endocrine gland because it secretes a number of hormones that control other endocrine glands. Despite so, the pituitary gland is controlled by the hypothalamus. It consists of 2 parts – the anterior pituitary gland and the posterior pituitary gland.

Functions of the hormones

Stimulating hormones are hormones that control the secretion of other hormones.

• Anterior pituitary gland
  • Hormones: Luteinizing hormone (LH)
    • Target tissues/organs: Gonads
      • Functions: Stimulates ovulation, development of corpus luteum and secretion of estrogen and progesterone in females.
      • Functions: Stimulates testosterone secretions in males.
  • Hormones: Growth hormone (GH)
    • Target tissues/organs: Soft tissues and bones
      • Functions: Stimulates growth, protein synthesis and fat metabolism.
  • Hormones: Adrenocorticotropic hormone (ACTH)
    • Target tissues/organs: Adrenal glands
      • Functions: Stimulates the adrenal cortex to release hormones.
  • Hormones: Thyroid-stimulating hormone (TSH)
    • Target tissues/organs: Thyroid gland
      • Functions: Stimulates the thyroid gland to release thyroxine.
  • Hormones: Follicle-stimulating hormone (FSH)
    • Target tissues/organs: Gonads
      • Functions:
        • Stimulates the development of follicles in the ovaries in females.
        • Stimulates spermatogenesis in males.
• Posterior pituitary gland
  • Hormones: Antidiuretic hormone (ADH)
    • Target tissues/organs: Kidneys
      • Functions: Stimulates water reabsorption by the renal tubules in the kidneys.
  • Hormones: Oxytocin
    • Target tissues/organs: Uterus, Mammary glands
      • Functions:
        • Stimulates contractions of the uterine muscles during childbirth.
        • Stimulates the release of milk in females.
• Adrenal gland
  • Hormones: Aldosterone
    • Target tissues/organs: Kidney
      • Functions: Increases the reabsorption of mineral salts in the kidneys.
  • Hormones: Adrenaline
    • Target tissues/organs: Muscle, liver and heart
      • Functions:
        • Increase the level of sugar and fatty acids in the blood.
        • Increase heart activity and the rate and depth of breathing.
        • Increase metabolism rate and constrict blood vessels.
• Thyroid gland
  • Hormones: Thyroxine
    • Target tissues/organs: All tissues
      • Functions:
        • Increase the rate of metabolism.
        • Increase body temperature.
        • Control growth and development.
• Pancreas
  • Hormones: Insulin
    • Target tissues/organs: Liver, muscles and adipose tissues
      • Functions: Decrease the blood glucose level and stimulate the conversion of glucose to glycogen.
  • Hormones: Glucagon
    • Target tissues/organs: Liver, muscles and adipose tissues
      • Functions: Increase the blood sugar level and promote the conversion of glycogen to glucose.
• Testis
  • Hormones: Testosterone
    • Target tissues/organs: Gonads, skin, muscles and bones
      • Functions:
        • Stimulate the development of male secondary sexual characteristics.
        • Stimulate spermatogenesis.
• Ovary
  • Hormones: Estrogen
    • Target tissues/organs: Gonads, skin, muscles and bones
      • Functions:
        • Stimulate the development of female secondary sexual characteristics.
        • Stimulate maturation of ova.
        • Stimulate the repair of uterine lining.
  • Hormones: Progesterone
    • Target tissues/organs: Gonads
      • Functions: Stimulate the development of uterine lining and the formation of placenta.

Involvement of the Nervous System and Endocrine System in a 'fight or flight' Situation

During a life-threatening situation, for example, someone chases you with a knife, your hypothalamus will send nerve impulses directly to the adrenal medulla. The adrenal medulla contains neurons from the sympathetic division of the autonomic nervous system (ANS).

1. In 'fight or flight' situations, the nerve impulses from the hypothalamus
2. stimulates the cells of the adrenal medulla to secrete adrenaline and noradrenaline.
3. Adrenaline and noradrenaline cause the heartbeat and breathing rates, blood pressure, blood glucose level and metabolic activity to increase.

Nervous system and Endocrine system**

• Similarities

  • Both contain target tissues/organs
  • Both react to stimuli by giving appropriate response
  • Both are involved in controlling activities in the body

• Differences

  • Nervous system

    • Components: Made up of tremendous amounts of neurons
    • Duration of effect: Short
    • Signals: Electrical impulses
    • Response: Immediate
    • Effector: Involved the specific organ

  • Endocrine system

    • Components: Consists of endocrine glands
    • Duration of effect: Long
    • Signals: Hormones (chemical substances)
    • Response: Slow, takes time
    • Effector: Involved several organs