The endocrine system releases hormones into the blood that control the bodies' activities. The endocrine system works more slowly than the nervous system. Most hormones enter the interstitial fluid and then the bloodstream. They exert their effects by binding to the target cells. There are two chemical classes of hormones, lipid-soluble and water-soluble. The receptors for lipid-soluble hormones are located inside the cell and water-soluble receptors are part of the plasma membrane. Lipid soluble hormones take longer to affect the body than water-soluble but the affects last longer.
The pituitary gland has two sections, the anterior pituitary made up of epithelial cells and the posterior pituitary made up of neuronal cells which are both controlled by the hypothalamus. The anterior pituitary hormones are stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus. These hormones reach the pituitary through the hypophyseal portal system. A portal system is when blood flows from one capillary network through a portal vein and then to another capillary network without passing through the heart. The hormones of the anterior pituitary are human growth hormone (Hgh), Thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), adrenocorticotropic hormone (ACTH), and melanocyte-stimulating hormone (MSH). The posterior pituitary does not synthesize hormones but it does store and release two hormones, oxytocin (OT) and antidiuretic hormone (ADH). The hypothalamus, neurosecretory cell produce these hormones. All these pituitary hormones are water-soluble.
The thyroid gland consists of the right and left lobes and the isthmus connecting the two lobes. The thyroid produces and secretes Triiodothyronine (T₃), Thyroxine (T₄), and calcitonin (CT). The production of T₃ and T₄ is controlled by TSH released by the anterior pituitary gland. Calcitonin is controlled by the levels of Ca²⁺ in the blood through a negative feedback system. When Ca²⁺ levels are high calcitonin inhibits the action of osteoclasts. The parathyroid glands are embedded in the posterior surface of the lateral lobe of the thyroid. The hormone they produce and secrete is the parathyroid hormone (PTH). Low Ca²⁺ levels in the blood stimulate the parathyroid gland to release PTH. This has the opposite effects of calcitonin on the Ca²⁺ levels in the blood.
The adrenal glands consist of the adrenal cortex and the adrenal medulla. The adrenal cortex is divided into three zones, the zona glomerulosa, zona fasciculate, and zona reticularis. The zona glomerulosa produces and secrets aldosterone to increase reabsorption of Na⁺ and H₂O and stimulate the excretion of K⁺. Zona fasciculate produces and secretes cortisol and cortisone. This increases protein breakdown, stimulates gluconeogenesis and lipolysis, and depresses immune system. The zona reticularis secreates androgens (mainly dehydroepiandrosterone or DHEA). We do not know what induces DHEA but it is turned into estrogen after menopause in females. The adrenal medulla hormones are epinephrine and Norepinephrine which are released by post-ganglionic neurons of the sympathetic division of the ANS.
Pancreas have alpha cells that secrete the hormone glucagon, beta cells that secrete insulin and delta cells that secrete somatostatins. Glucagon raise blood glucose levels, insulin lowers glucose levels and somatostatins regulate levels of glucagon and insulin.
The ovaries produce estrogen and progesterone and the testes produce testosterone. Estrogen and progesterone along with gonadotropic hormones regulate the female reproductive cycle, maintain pregnancy, prepare the mammary glands for lactation and promote development and maintenance of female secondary sex characteristics. Testosterone stimulates decent of testes before birth, regulates spermatogenesis, and promotes development and maintenance of male secondary sex characteristics.
