Summary

Summary

The endocrine and nervous systems maintain homeostasis. The endocrine system is a network of glands that secrete hormones. These hormones, which are quite potent, travel through the bloodstream to affect the functioning of target cells. The nervous and endo-crine systems both exert very precise effects. Hor-mones include steroids, amines, peptides, proteins, and glycoproteins. Steroid hormones enter target cells and bind receptors, forming complexes in the nuclei. Nonsteroid hormones bind receptors in target cell membranes. The concentration of each hormone in the body fluids is regulated.

The pituitary gland has an anterior lobe and a pos-terior lobe. Its secretions are mostly controlled by the hypothalamus. The anterior pituitary gland secretes GH, PRL, TSH, ACTH, FSH, and LH. The posterior pituitary gland secretes ADH and oxytocin. The thyroid gland in the neck consists of two lobes and secretes thy-roxine and triiodothyronine. The parathyroid glands, on the posterior thyroid gland, secrete PTH.

The adrenal glands, located atop the kidneys, consist of a medulla and a cortex, each with differ-ing functions. The medulla secretes epinephrine and ­norepinephrine and the cortex secretes aldosterone, cortisol, and sex hormones. The pancreas secretes diges-tive juices as well as hormones and is vital for normal balancing of glucagon and insulin. Other endocrine glands include the pineal gland, thymus, ovaries, tes-tes, digestive glands, and hormone-producing organs. The pineal gland is located deep in the cerebral hemi-spheres and secretes the hormone melatonin. The thy-mus is located deep inside the mediastinum posterior to the sternum, shrinks with aging, and is important in early immunity. The ovaries secrete estrogens and pro-gesterone, whereas the testes secrete the testosterone. The digestive glands secrete gastrin, ghrelin, secretin, cholecystokinin, and incretins.

Hormone-producing organs include the heart, kidneys, and skin. The adipose tissue and skeleton also secrete hormones. The heart’s atria secrete atrial natriuretic peptide, which stimulates urinary sodium excretion. The kidneys secrete erythropoietin, a red blood cell GH. The skin secretes cholecalciferol, an inactive form of vitamin D₃ . The adipose tissue cells release leptin, resistin, and adiponectin. The bones, via their osteoblasts, secrete osteocalcin, which func-tions in insulin secretion.

Stress occurs when the body responds to stressors that threaten homeostasis. Stress responses include increased sympathetic nervous system action and increased adrenal hormone secretion. Physical factors and psychological factors both can produce stress. The hypothalamus controls the stress response, also known as general adaptation syndrome. The resis-tance response involves the release of CRH to stimu-late the secretion of ACTH, which stimulates cortisol secretion. This prepares the body for physical action to alleviate the stress.