Porth's Essentials of Pathophysiology, 4e

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Endocrine System

U N I T 9

Source of releasing factors

Source of ADH and oxytocin

Primary capillary plexus

Superior hypophyseal artery

To dural venous sinuses

FIGURE 31-4. The hypothalamus and the anterior and posterior pituitary.The hypothalamic releasing or inhibiting hormones are transported to the anterior pituitary through the portal vessels. Antidiuretic hormone and oxytocin are produced by nerve cells in the supraoptic and paraventricular nuclei of the hypothalamus and then transported through the nerve axon to the posterior pituitary, where they are released into the circulation. ADH, antidiuretic hormone; ACTH, adrenocorticotropic hormone; FSH, follicle stimulating hormone; GH, growth hormone; LH, luteinizing hormone; MSH, melanocyte stimulating hormone;TSH, thyroid stimulating hormone.

Artery of trabecula

Posterior pituitary

Long portal veins Anterior pituitary Secretory cells

Hormones stored at fiber linings

GH TSH ACTH FSH LH Prolactin MSH

ADH

Oxytocin

Sinusoids

Interior hypophyseal artery

Feedback Regulation The level of many of the hormones in the body is reg- ulated by negative feedback mechanisms (Fig. 31-5). The function of this type of system is similar to that of the thermostat in a heating system. In the endocrine system, sensors detect a change in the hormone level and adjust hormone secretion so that blood levels are maintained within an appropriate range. When the sensors detect a decrease in blood levels, they initi- ate changes that cause an increase in hormone pro- duction; when blood levels rise above the set point of the system, the sensors cause hormone production and release to decrease. For example, an increase in thyroid hormone is detected by sensors in the hypo- thalamus or anterior pituitary gland, and this causes a reduction in the secretion of TSH, with a subsequent decrease in the output of thyroid hormone from the thyroid gland. The feedback loops for the hypotha- lamic-pituitary feedback mechanisms are illustrated in Figure 31-6. Exogenous forms of hormones (given as drug prepa- rations) can influence the normal feedback control of hormone production and release.One of themost common examples of this influence occurs with the administration of the corticosteroid hormones, which causes suppression

of the hypothalamic-pituitary–target cell system that reg- ulates the production of these hormones. Although the blood levels of most hormones are regulated by negative feedback mechanisms, a small number are under positive feedback control, in which rising levels of a hormone cause another gland to release a hormone that is stimulating to the first. There must, however, be a mechanism for shutting off the release of the first hormone, or its production would continue unabated. An example of such a system is that of the female ovarian hormone estradiol. Increased estradiol production during the follicular stage of the menstrual cycle causes increased gonadotropin (FSH) production by the anterior pituitary gland. This stimulates further increases in estradiol levels until the demise of the fol- licle, which is the source of estradiol, results in a fall in gonadotropin levels. In addition to positive and negative feedback mech- anisms that monitor changes in hormone levels, some hormones are regulated by the level of the substance they regulate. For example, insulin levels normally are regulated in response to blood glucose levels, and those of aldosterone in response to blood levels of sodium and potassium. Other factors such as stress, environmental temperature, and nutritional status can alter feedback regulation of hormone levels.