Endocrine System

Endocrine System Overview

  • The endocrine system is composed of glands and cells that produce hormones.

  • Hormones are molecules transported via blood that regulate the activity of particular cells or tissues.

  • Target cells must have receptors for specific hormones to respond to them.

  • Not all glands are part of the endocrine system (e.g., mucous glands do not secrete hormones).

Types of Glands

Endocrine Glands

  • Secrete hormones directly into the bloodstream.

Exocrine Glands

  • Secrete non-hormone products onto a surface (e.g., sweat glands).

Feedback Control Mechanisms

Negative Feedback

  • The body's response reduces or stops the initial stimulus.

  • Involves processes like thermoregulation and blood glucose regulation to maintain homeostasis.

Positive Feedback

  • The body's response amplifies the initial stimulus.

  • Examples include sexual response, childbirth, and blood clotting.

  • Drives processes to completion rather than stabilization.

Simple Endocrine Pathway

  • Involves only the endocrine system.

  • Endocrine cells respond directly to stimuli; negative feedback halts the hormone response when conditions normalize.

Simple Neuroendocrine Pathway

  • Neurons respond to stimuli, causing neurosecretory cells to secrete hormones.

  • Neurosecretory cells can act as neurons and hormone-secreting cells.

  • Positive feedback in cases like suckling continues hormone production as the stimulus increases.

Antagonistic Hormone Pairs

  • Two hormones with opposite effects work together for homeostasis.

  • Example: Calcitonin promotes calcium absorption by bones while parathyroid hormone releases calcium from bones.

Diabetes Types

Type I Diabetes

  • No insulin production; high blood glucose levels as cells cannot take it up.

Type II Diabetes

  • Some insulin is produced, but cells do not respond adequately.

Pituitary Gland and Hormones

Hypothalamus Connection

  • Neurosecretory cells extend into the posterior pituitary; anterior pituitary receives hormone signals via tropic hormones.

Posterior Pituitary Hormones

  • Antidiuretic Hormone (ADH): affects kidney tubules to limit urine output.

  • Oxytocin: stimulates uterine muscles and mammary glands during childbirth and lactation.

Anterior Pituitary Hormones

  • Produces tropic hormones that regulate other endocrine glands.

  • Growth Hormone (GH): Has both tropic and non-tropic effects.

  • Too little GH can result in proportional dwarfism, while excess GH can lead to gigantism.

Melanocyte Stimulating Hormone (MSH)

  • Stimulates melanin production in response to UV radiation; pale skin lacks MSH receptors.

Hormonal Evolution

  • Prolactin is vital for milk production in mammals and regulates fat metabolism in birds.

  • Fish use prolactin for water/salt balance.

  • Many hormones are shared across species but can perform different functions depending on receptors.

Thyroid Gland Hormones

  • Triiodothyronine (T3) and Thyroxine (T4) regulate metabolism.

  • Calcitonin encourages calcium storage in bones.

Hormone Cascade (Tropic Cascade)

  • Involves the hypothalamus and anterior pituitary in response to a stimulus (e.g., thyroid hormones).

  • If thyroid hormone levels drop, the hypothalamus secretes Thyroid Releasing Hormone (TRH), promoting TSH production in the pituitary.

Hypothyroidism

  • Symptoms include weight gain, lethargy, and cold intolerance.

  • Can be primary (thyroid fails to produce hormones) or secondary (the pituitary's failure to stimulate the thyroid).

Hyperthyroidism

  • Excess thyroid hormone production can raise body temperature, cause weight loss, and elevate blood pressure.

  • Iodine is critical for hormone production; deficiency can impair thyroid function.

Parathyroid Glands

  • Produce parathyroid hormone, an antagonist to calcitonin for calcium regulation.

Pineal Gland and Melatonin

  • Releases melatonin at night, leading to drowsiness and triggering seasonal adaptations in coloration and behavior.

  • Insomnia can lead to decreased melatonin and paler skin.

Adrenal Glands

  • Located above the kidneys; respond to stress with hormones.

  • Medulla (inner part) handles short-term stress; cortex (outer part) manages long-term stress.