The pancreas, illustrated in Figure 1, is an elongated organ that is located between the stomach and the proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both endocrine and exocrine functions.
The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans, as visible in the micrograph shown in Figure 2. The pancreatic islets contain two primary cell types: alpha cells, which produce the hormone glucagon, and beta cells, which produce the hormone insulin. These hormones regulate blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise the blood glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver. Together, glucagon and insulin regulate blood glucose levels.
The pineal gland produces melatonin. The rate of melatonin production is affected by the photoperiod. Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin is produced; however, melatonin production increases during the dark photoperiod (night). In some mammals, melatonin has an inhibitory affect on reproductive functions by decreasing production and maturation of sperm, oocytes, and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms, particularly circadian rhythms such as the sleep-wake cycle and eating habits.
The gonads—the male testes and female ovaries—produce steroid hormones. The testes produce androgens, testosterone being the most prominent, which allow for the development of secondary sex characteristics and the production of sperm cells. The ovaries produce estradiol and progesterone, which cause secondary sex characteristics and prepare the body for childbirth.
|Table 1. Endocrine Glands and their Associated Hormones|
|Endocrine Gland||Associated Hormones||Effect|
|Hypothalamus||releasing and inhibiting hormones||regulate hormone release from pituitary gland; produce oxytocin; produce uterine contractions and milk secretion in females|
|antidiuretic hormone (ADH)||water reabsorption from kidneys; vasoconstriction to increase blood pressure|
|Pituitary (Anterior)||growth hormone (GH)||promotes growth of body tissues, protein synthesis; metabolic functions|
|prolactin (PRL)||promotes milk production|
|thyroid stimulating hormone (TSH)||stimulates thyroid hormone release|
|adrenocorticotropic hormone (ACTH)||stimulates hormone release by adrenal cortex, glucocorticoids|
|follicle-stimulating hormone (FSH)||stimulates gamete production (both ova and sperm); secretion of estradiol|
|luteinizing hormone (LH)||stimulates androgen production by gonads; ovulation, secretion of progesterone|
|melanocyte-stimulating hormone (MSH)||stimulates melanocytes of the skin increasing melanin pigment production.|
|Pituitary (Posterior)||antidiuretic hormone (ADH)||stimulates water reabsorption by kidneys|
|oxytocin||stimulates uterine contractions during childbirth; milk ejection; stimulates ductus deferens and prostate gland contraction during emission|
|Thyroid||thyroxine, triiodothyronine||stimulate and maintain metabolism; growth and development|
|calcitonin||reduces blood Ca2+ levels|
|Parathyroid||parathyroid hormone (PTH)||increases blood Ca2+ levels|
|Adrenal (Cortex)||aldosterone||increases blood Na+ levels; increase K+ secretion|
|cortisol, corticosterone, cortisone||increase blood glucose levels; anti-inflammatory effects|
|Adrenal (Medulla)||epinephrine, norepinephrine||stimulate fight-or-flight response; increase blood gluclose levels; increase metabolic activities|
|Pancreas||insulin||reduces blood glucose levels|
|glucagon||increases blood glucose levels|
|Pineal gland||melatonin||regulates some biological rhythms and protects CNS from free radicals|
|Testes||androgens||regulate, promote, increase or maintain sperm production; male secondary sexual characteristics|
|Ovaries||estrogen||promotes uterine lining growth; female secondary sexual characteristics|
|progestins||promote and maintain uterine lining growth|