I. Endocrine System & Hormones Figure 1: Human Endocrine System Endocrine System: a) Endocrine glands are ductless since they lack specific vessels for the transport of hormones throughout the body. Instead, hormones diffuse directly into capillary beds upon their release & are transported throughout the body via the circulatory system.
Figure 2: Endocrine Glands & Target Tissues Target Tissue/Organs: a) Hormones can only bring about a response in tissues bearing protein receptors specific for the hormone. Depending on the hormone, these receptors may exist within the cell membrane or within the cytoplasm itself. Hormone Classes Figure 2: Steroid & Peptide/Monoamine Hormone Action Steroid Hormone Action Peptide/Monoamine Hormone Action Steroid Hormones: Monoamines & Peptide Hormones:
Hormone Regulation Figure 3: Negative Feedback Regulation of Hormone Secretion The secretion of most hormones is initiated by releasing hormones secreted by the hypothalamus. The secretion of a hormone, & its concentration in the body, is controlled by the following negative feedback mechanism: a) The secretion of a releasing hormone (RH) stimulates the pituitary to release a stimulating hormone (SH), which in turn stimulates the release of a hormone from a target gland. b) Eventually, the target hormone will reach concentrations that will desensitize the pituitary to the releasing factor, suppressing the secretion of the stimulating hormone. Consequently, the secretion of the target hormone will also decrease & eventually cease. III. Anatomy of the Human Endocrine System Figure 4: Hypothalamus Hypothalamus: a) The pituitary gland lies immediately below the hypothalamus in a boney recess at the base of the brain. It releases various Stimulating Hormones upon stimulation by hypothalamic RH s.
Figure 5: Pituitary Gland: Anterior Pituitary Anterior Pituitary Hormones: is comprised of endocrine tissues & produces its own hormones: Trophic Hormones are those that target other endocrine glands (i.e. all stimulating hormones); Nontrophic Hormones are those that target non-endocrine tissues. a) Growth Hormone (GH): stimulates liver to produce a growth factor called Insulin-like Growth Factor (IGF-1). This growth factor stimulates both bone & muscle growth. b) Adrenocorticotrophic Hormone (ACTH): stimulates the adrenal cortex to release glucocorticoids & mineralocorticoids to regulate blood sugar concentrations & blood pressure during fight or flight. c) Thyroid Stimulating Hormone (TSH): stimulates thyroid gland to release thyroxine/thyroid hormone (TH). d) Gonadotrophic Hormones (FSH & LH): stimulates activities of male & female gonads. e) Prolactin: stimulates production of milk by mammary glands in breast tissue. Figure 5.1: Posterior Pituitary Posterior Pituitary Hormones: stores hormones produced by the hypothalamus. a) Oxytocin: stimulates uterine muscles to contract during labor & the contraction of milk ducts during nursing. b) Antidiuretic Hormone (ADH): secreted during times of dehydration whereupon it binds to receptors in tubules of the kidney & promotes the reabsorption of water back into the bloodstream. In the absence of antidiuretic hormone, the kidney tubules are impermeable to water, which is mixed with & excreted as urine.
Figure 6: Thyroid Gland The Thyroid Gland secretes the hormone Thyroxine or TH when stimulated by TSH from the anterior pituitary. TH plays a crucial role in regulating metabolic rate (carbohydrate metabolism, protein, & fat metabolism) & thus plays an important role in growth & development. The thyroid also helps to regulate blood calcium levels when calcium levels are too high, the thyroid secretes the hormone Calcitonin, which stimulates the deposition of Ca 2+ to promote bone growth. Furthermore, calcitonin inhibits the reabsorption of Ca 2+ by the kidneys. Figure 6.1: Parathyroid Glands (Rear View of Thyroid) The Parathyroid Glands exist as paired patches of endocrine tissue embedded within the thyroid. When calcium levels in the blood are too low, the parathyroids produce & release PTH, which stimulates the reabsorption of Ca 2+ by the kidneys. Furthermore, PTH stimulates the bone cells to decompose the mineralized matrix of bone & release Ca 2+ into the blood.
Figure 6.2: Thyroid & Parathyroid Regulation of Blood Ca 2+ Levels Figure 7: Adrenal Glands (Cortex vs Medulla) Adrenal Gland: a) Adrenal Medulla: controlled by the sympathetic nervous system. In response to sudden stress (+or-), it releases the hormone Epinephrine & Norepinephrine to mediate the body s Fight or Flight response. Functions to make energy more available to the body in times of stress by stimulating the release of glucose from the liver & fatty acids from adipose cells. Also serves to increase heart rate & direct blood to organs & muscles needed most to deal with the stress (heart, brain, skeletal muscles). b) Adrenal Cortex: controlled by ACTH released by the pituitary gland. Glucocorticoids convert fats & proteins to glucose in times of prolonged stress (starvation). Also secretes mineralocorticoids such as aldosterone to stimulate the reabsorption of Na + from filtrate by nephrons to maintain proper blood volume & pressure at times when blood pressure drops due to blood loss or dehydration.
Dual Glands of Human Endocrine System Figure 8: Pancreas (Dual Gland): Islet Cells Pancreas: a) Glucagon: produced by alpha cells & acts (along w/epinephrine) to stimulate the conversion of glycogen to glucose by the liver when blood sugar levels are low (as they are between meals). b) Insulin: produced by beta cells & acts to stimulate the diffusion of glucose into body cells when blood sugar levels are high (as they are following a meal). Also stimulates the uptake & conversion of blood glucose to glycogen by liver cells. Figure 8.1: Pancreas (Islet Cells) & Blood Sugar Regulation
Figure 9: Testis (Dual Gland) Testicle Structure Pituitary-Testis Axis Figure 9.1: Ovaries (Dual Gland)
IV. Hormonal Control of Female Reproductive Cycles Ovarian Cycle The ovarian cycle is a roughly 28-day cycle during which a Follicle within the ovary grows, matures, & releases an immature egg (primary oocyte) into the oviduct. It consists of a follicular & luteal stage, each regulated by hormones secreted by the hypothalamus & pituitary glands. Figure 10: Ovarian Cycle Menstrual Cycle The menstrual cycle is a roughly 28-day cycle in which the uterine lining, or Endometrium, grows & becomes increasingly glandular, changes that make it prepared for the potential arrival & implantation of an early embryo (blastula). The menstrual cycle can be organized into the following phases: flow phase, proliferative phase, & secretory phase. Figure 10.1: Menstrual Cycle
Figure 10.2: Hormonal Synchronization of Ovarian & Menstrual Cycles Ovarian Follicular Stage & Menstrual Proliferative Stage During the Follicular Stage of the ovarian cycle, the pituitary gland (in response to GnRH from the hypothalamus) secretes FSH & LH. During the early stages of this phase, follicular cells are only receptive to FSH, which cause it to grow & secrete the hormone estrogen. Estrogen from the developing follicle causes the endometrium in the uterus to thicken. Thus, by way of the hormone estrogen, the follicular stage of the ovarian cycle is synchronized w/the Proliferative Stage of the menstrual cycle. As estrogen levels rise throughout the follicular phase, it has a positive feedback effect on FSH & LH secretion from the anterior pituitary. This results in both an FSH & LH Surge around day 14. The LH surge causes the primary oocyte to complete meiosis I secondary oocyte, upon which it is Ovulated as the follicle ruptures & is swept into the oviduct during ovulation. The FSH surge stimulates the development of other follicles within the ovary for future cycles. Ovarian Luteal Stage & Menstrual Secretory Stage
During the Luteal Stage of the ovarian cycle, the increased LH levels stimulate the remaining follicular tissue to form the Corpus Luteum ( yellow body ). LH stimulates the corpus luteum to secrete progesterone & small amounts of estrogen. These hormones stimulate the further thickening & vascularization of the endometrium & the growth of endometrial glands that secrete a fluid (rich in glycogen) that nourishes an embryo prior to implantation. Thus, by way of progesterone, the luteal stage of the ovarian cycle is synchronized with the Secretory Phase of the menstrual cycle. Estrogen & progesterone levels ultimately inhibits the secretion of FSH & LH by the pituitary (-Feedback). In response to decreased LH levels, the corpus luteum begins to degenerate, causing levels of estrogen & progesterone to crash (this liberates the pituitary from the effects of these hormones, allowing it to once again release FSH & LH to start a new cycle). The rapid drop in estrogen & progesterone levels initiates the Flow Phase of the menstrual cycle causes spasms of the arteries in the uterine lining that deprives the endometrium of blood. This results in the degeneration of the endometrium. When the endometrium is shed, blood vessels break & bleeding results. A mixture of blood & discarded tissue leaves the vagina in a process called Menstruation, which usually occurs about 14 days after ovulation. Figure 11: Role of HCG Should pregnancy occur, the embryo makes an LH-like hormone called HCG to prevent the corpus luteum from disintegrating. Thus sufficient levels of progesterone are produced to maintain the endometrium.