Human Physiology - Chemical Coordination and Integration (Endocrine Glands and Hormones)

Endocrine glands are ductless glands that secrete hormones directly into the bloodstream to reach target organs. Unlike exocrine glands, they do not utilize a duct system.

Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.

The Hypothalamus contains neurosecretory cells called nuclei which produce hormones like $GnRH$ (Gonadotropin-releasing hormone) and Somatostatin to regulate the pituitary gland.

The Pituitary gland (Hypophysis) is divided into the Adenohypophysis (Anterior) and Neurohypophysis (Posterior). It secretes $GH$, $PRL$, $TSH$, $ACTH$, $LH$, $FSH$, $MSH$, Oxytocin, and $Vasopressin$ ($ADH$).

The Thyroid gland produces $T_3$ (Triiodothyronine) and $T_4$ (Tetraiodothyronine/Thyroxine). These require Iodine ($I_2$) for synthesis and regulate the Basal Metabolic Rate ($BMR$).

The Parathyroid gland secretes Parathyroid Hormone ($PTH$), a hypercalcemic hormone that increases blood $Ca^{2+}$ levels by acting on bones and renal tubules.

The Adrenal Medulla secretes Catecholamines: Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine), often called 'Emergency Hormones' or 'Fight or Flight' hormones.

The Pancreas acts as a composite gland. The Islets of Langerhans contain $\alpha$-cells (secreting Glucagon) and $\beta$-cells (secreting Insulin) to maintain glucose homeostasis.

Water-soluble hormones (e.g., Insulin, Epinephrine) interact with surface receptors and generate secondary messengers like $cAMP$, $IP_3$, or $Ca^{2+}$.

Lipid-soluble hormones (e.g., Steroids, Iodothyronines) pass through the cell membrane and bind to intracellular receptors to regulate gene expression via the $Hormone-Receptor$ complex.