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A gland is an organ in an animal's body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).

Types of gland

Glands can be divided into two groups:

  • Endocrine glands - are glands that secrete their product directly onto a surface rather than through a duct.
  • Exocrine glands - secrete their products via a duct, the glands in this group can be divided into three groups:

- Apocrine glands - a portion of the secreting cell's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands.
- Holocrine glands - the entire cell disintegrates t- secrete its substances (e.g., sebaceous glands)
- Merocrine glands - cells secrete their substances by exocytosis (e.g., mucous and serous glands). Also called "eccrine."

The type of secretory product of an Exocrine gland may also be one of three categories:

  • Serous glands - secrete a watery, often protein-rich product
  • Mucous glands - secrete a viscous product, rich in carbohydrates (eg. glycoproteins)
  • Sebaceous glands - secrete a lipid product

Exocrine glands

Typical exocrine glands include sweat glands, salivary glands, mammary glands, and many glands of the digestive system.

Endocrine glands and the hormones secreted

Hypothalamus produces - Thyrotropin-releasing hormone (TRH)
- Gonadotropin-releasing hormone (GnRH)
- Growth hormone-releasing hormone (GHRH)
- Corticotropin-releasing hormone (CRH)
- Somatostatin (SS; also GHIH, growth factor-inhibiting hormone) - Dopamine (DA)
  • Pineal Gland produces
  • - Dimethyltryptamine
    - Melatonin
    • Pituitary gland (hypophysis) produces
    • - Anterior pituitary lobe (adenohypophysis)
      + Growth hormone (GH)
      + Prolactin (PRL)
      + Adrenocorticotropic hormone (ACTH, corticotropin)
      + Thyroid-stimulating hormone (TSH, thyrotropin)
      + Follicle-stimulating hormone (FSH, a gonadotropin)
      + Luteinizing hormone (LH, a gonadotropin)
      - Posterior pituitary lobe (neurohypophysis)
      + Oxytocin (ocytocin)
      + Arginine vasopressin (AVP; also ADH, antidiuretic hormone)
      + Lipotropin
      • Thyroid gland produces
      • - Triiodothyronine (T3), the potent form of thyroid hormone
        - Thyroxine (T4), a less active form of thyroid hormone
        - Calcitonin
        • Parathyroid gland produces
        • - Parathyroid hormone (PTH)
          • Heart produces
          • - Atrial-natriuretic peptide (ANP)
            • Stomach and intestines produce
            • - Cholecystokinin (CCK)
              - Gastrin
              - Ghrelin
              - Neuropeptide Y (NPY)
              - Secretin
              - Somatostatin
              • Liver produces
              • - Insulin-like growth factor (IGF)
                - Angiotensinogen
                - Thrombopoietin
                • Islets of Langerhans in the pancreas produce
                • - Insulin
                  - Glucagon
                  - Somatostatin
                  • Adrenal glands produce
                  • - Adrenal cortex
                    + Glucocorticoids (chiefly cortisol)
                    + Mineralocorticoids (chiefly aldosterone)
                    + Androgens (including DHEA and testosterone)
                    - Adrenal medulla
                    + Adrenaline (epinephrine)
                    + Noradrenaline (norepinephrine)
                    - Testosterone
                    • Kidney produces
                    • - Renin
                      - Erythropoietin (EPO)
                      - Calcitriol (the active form of vitamin D3)
                    • Skin produces
                    • - Vitamin D3 (calciferol)
                    • Adipose tissue
                    • - Leptin
                      - Oestrogens (mainly oestrone)

                      In males only

                      • Testes
                      • - Androgens (chiefly testosterone)

                        In females only

                        • Ovarian follicle
                        • - Oestrogens (mainly oestradiol)
                          • Corpus luteum
                          • - Progesterone
                            - Oestrogens (mainly oestradiol)
                            • Placenta (when pregnant)
                            • - Progesterone
                              - Oestrogens (mainly oestriol)
                              - Human chorionic gonadotropin (HCG)
                              - Human placental lactogen (HPL)


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