The endocrine and nervous systems maintain homeostasis. The endocrine system is a network of glands that secrete hormones.
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 Ânorepinephrine 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.
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