Anatomy & Physiology

Anatomy & Physiology Study Guide suckling at the breast, and sensory nerves innervating the nipples take the information to the hypothalamus. Then, Oxytocin is released into the circulation at the neurohypophysis, and the myoepithelial cells respond by squeezing milk from the secretory alveoli into large collecting ducts. 17.3 Thyroid Gland The thyroid gland curves across the anterior surface of the trachea. The two lobes of the thyroid gland are united by a slender connection, called the isthmus . You can feel the gland with your fingers. When a complication arises with the thyroid gland, it typically becomes visible as it enlarges and distorts the surface of the neck. The thyroid gland contains large numbers of thyroid follicles, which are hollow spheres lined by simple cuboidal epithelium. Follicle cells synthesize a globular protein called thyroglobulin . Thyroglobulin molecules house the amino acid tyrosine, the building block of thyroid hormones. Thyroid Hormones Thyroid hormones enter target cells using an energy-dependent transport system, and they affect almost every cell in the body. Inside a target cell, they bind to receptors in the cytoplasm, on the surfaces of mitochondria and in the nucleus. Thyroid hormones also activate genes that code for the synthesis of enzymes involved in glycolysis and ATP production. This effect, coupled with the direct effect of thyroid hormones on mitochondria, increases the metabolic rate of the cell. Thus, the cell consumes more energy and generates increased heat. This effect is called the calorigenic effect of thyroid hormones. 17.4 Parathyroid Glands There are normally two pairs of parathyroid glands embedded in the posterior surfaces of the thyroid gland. The cells of the two adjacent glands are separated by the dense capsule that surrounds each parathyroid gland. The parathyroid cells (also termed chief cells or principal cells) produce parathyroid hormone. Like the C cells of the thyroid gland, the parathyroid cells monitor the circulating concentration of calcium ions. When the Ca2+ concentration in the blood falls below normal, the parathyroid cells secrete parathyroid hormone (PTH) , or parathormone. The net result of PTH secretion is an increase in Ca2+ concentration in body fluids. Parathyroid hormone has three major effects: it mobilizes calcium from bone by affecting osteoblast and osteoclast activity, it enhances the reabsorption of Ca2+ at the kidneys, reducing urinary losses, and it stimulates the formation and secretion of calcitriol at the kidneys. 17.5 Suprarenal Glands A yellow, pyramid-shaped suprarenal gland or adrenal gland , sits on the superior border of each kidney. Each suprarenal gland lies at roughly the level of the twelfth rib and is firmly attached to the superior portion of each kidney by a dense fibrous capsule. Like other endocrine glands, the Achieve Page 211 of 368 ©2018