Medical brain notes 27

  The Endocrine System W e have already seen how the nervous system   regulates body functions by means of nerve impulses and integration of information by the spinal cord and brain. The other regulating system of the body is the  endocrine system , which consists of endocrine glands that secrete chemicals called  hormones . These glands, and the names of the hormones they secrete, are shown in Fig. 10–1.   Endocrine glands  are ductless; that is, they do not   have ducts to take their secretions to specific sites. Figure 10–1.  The endocrine system. Locations of many endocrine glands. Both male   and female gonads (testes and ovaries) are shown. QUESTION:  Why is the location of the thyroid gland not really important for its function? Instead, hormones are secreted directly into capillaries and circulate in the blood throughout the body. Each hormone then exerts very specific effects on certain organs, called  target organs  o...

  CHEMISTRY OF HORMONES   With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.   1.   Amines —these simple hormones are structural   variations of the amino acid tyrosine. This group includes thyroxine from the thyroid gland and epi-nephrine and norepinephrine from the adrenal medulla.   2.   Proteins —these hormones are chains of amino   acids. Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins. Short chains of amino acids may be called  peptides . Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.   3.   Steroids —cholesterol is the precursor for the   steroid hormones, which include cortisol and aldos-terone from the adrenal cortex, estrogen and pro-gesterone from the ovaries, and testosterone from the testes.

  REGULATION OF HORMONE SECRETION   Hormones are secreted by endocrine glands when there is a need for them, that is, for their effects on their target organs. The cells of endocrine glands respond to changes in the blood or perhaps to other hormones in the blood. These stimuli are the infor- mation they use to increase or decrease secretion of their own hormones. When a hormone brings about its effects, the stimulus is reversed, and secretion of the hormone decreases until the stimulus reoccurs. You may recall that this is a negative feedback mechanism, and the mechanism for thyrox-ine was depicted in Fig. 1–3. Let us use insulin as a dif-ferent example here.   Insulin is secreted by the pancreas when the blood glucose level is high; that is, hyperglycemia is the stimulus for secretion of insulin. Once circulating in the blood, insulin enables cells to remove glucose from the blood so that it can be used for energy production and enables the liver to store glucose as gly...

  THE PITUITARY GLAND   The  pituitary gland  (or  hypophysis ) hangs by a short stalk (infundibulum) from the hypothalamus and is enclosed by the sella turcica of the sphenoid bone. Despite its small size, the pituitary gland regulates many body functions. Its two major portions are the posterior pituitary gland ( neurohypophysis ), which is an extension of the nerve tissue of the hypothalamus, and the anterior pituitary gland ( adenohypophysis ), which is separate glandular tissue. All of the hormones of the pituitary gland and their target organs are shown in Fig. 10–2. It may be helpful for you to look at this summary picture before you begin reading the following sections. POSTERIOR PITUITARY GLAND   The two hormones of the  posterior pituitary gland  are actually produced by the hypothalamus and simply stored in the posterior pituitary until needed. Their release is stimulated by nerve impulses from the hypo-thalamus (Fig. 10–3). Figure 10–3...

  POSTERIOR PITUITARY GLAND   The two hormones of the  posterior pituitary gland  are actually produced by the hypothalamus and simply stored in the posterior pituitary until needed. Their release is stimulated by nerve impulses from the hypo-thalamus (Fig. 10–3).   Figure 10–3.  Structural relationships of hypothalamus and pituitary gland. ( A ) Posterior   pituitary stores hormones produced in the hypothalamus. ( B ) Releasing hormones of the hypothalamus circulate directly to the anterior pituitary and influence its secretions. Notice the two networks of capillaries. QUESTION:  In part A, name the hormones of the posterior pituitary. In part B, what stim-ulates secretion of anterior pituitary hormones? Antidiuretic Hormone   Antidiuretic hormone  ( ADH , also called  vaso-pressin ) increases the reabsorption of water by kidney   tubules, which decreases the amount of urine formed. The water is reabsorbed into the blood, so a...

  ANTERIOR PITUITARY GLAND   The hormones of the  anterior pituitary gland  regulate many body functions. They are in turn regulated by  releasing hormones  from the hypothalamus. These releasing hormones are secreted into capillaries in the hypothalamus and pass through the  hypophy-seal portal  veins to another capillary network in the   anterior pituitary gland. Here, the releasing hormones are absorbed and stimulate secretion of the anterior pituitary hormones. This small but specialized path-way of circulation is shown in Fig. 10–3. This pathway permits the releasing hormones to rapidly stimulate the anterior pituitary, without having to pass through general circulation.   Figure 10–3.  Structural relationships of hypothalamus and pituitary gland. ( A ) Posterior   pituitary stores hormones produced in the hypothalamus. ( B ) Releasing hormones of the hypothalamus circulate directly to the anterior pituitary and influence ...