HESI A2 Exam: Endocrine System
Hormone Regulation
The endocrine system consists of multiple organs that secrete hormones directly into the bloodstream to control body processes and maintain homeostasis. Hormones then travel through the bloodstream and bind to receptors, triggering a response by the target tissue.
Steroid hormones are water-soluble and bind to receptors on the surface of target cells. They induce a signaling cascade that leads to a rapid response.
Peptide hormones are fat-soluble and can pass freely through the plasma membrane and bind to receptors inside the cell. They induce changes in cellular gene expression. Their effects are longer lasting (compared to steroid hormones).
Hypothalamus and Pituitary
The main regulatory gland of the endocrine system is the hypothalamus, which is located in the brain under the thalamus.
The hypothalamus is a bridge between the endocrine and nervous systems. As part of the nervous system, it sends electrical signals to the adrenal gland to release adrenaline. The hypothalamus contains centers for the regulation of hunger and thirst.
The hypothalamus controls hormone production via the anterior pituitary and produces hormones that are then stored and released by the posterior pituitary.
The hypothalamus produces two hormones that are stored and released in the posterior pituitary: oxytocin and vasopressin. Oxytocin stimulates uterine contractions during labor.
Anti-diuretic hormone (ADH) is made by special nerve cells found in the hypothalamus. The nerve cells transport the hormone down their nerve fibers (axons) to the pituitary gland where the hormone is released into the bloodstream. ADH helps to control blood pressure by acting on the kidneys and the blood vessels.
The posterior pituitary releases oxytocin and vasopressin (also known as antidiuretic hormone, or ADH). The release of oxytocin stimulates the contraction of the uterus and the mammary glands during labor.
The anterior pituitary releases direct hormones, which stimulate a response directly at the target hormone, and tropic hormones, which are hormones that have other endocrine glands as their target.
GnRH (gonadotropin RH) stimulates the anterior pituitary to release LH and FSH.
GHRH (growth hormone RH) stimulates the anterior pituitary to release GH.
GHIH (somatostatin) inhibits the release of GH from the anterior pituitary.
CRH (cortiocotropin RH) stimulates the anterior pituitary to release ACTH.
PIH inhibits the release of prolactin from the anterior pituitary.
Hormones Released by the Anterior Pituitary
Follicle stimulating hormone (FSH) stimulates gonads to produce gametes. FSH induces the maturation of an ovarian follicle in females and stimulates spermatogenesis in males.
Luteinizing hormone (LH) stimulates the ovaries and testes. The hormone LH induces ovulation in females and stimulates testosterone production in males.
ACTH stimulates the adrenal cortex to release hormones.
Thyroid-stimulating hormone (TSH) stimulates the thyroid to release thyroxin. Thyroxin controls the rate of metabolism.
Thyroid hormones (T3 and T4) regulate the metabolic rate. The thyroid releases calcitonin in response to high blood calcium levels and induces the storage of calcium in the bone.
Growth hormone (GH) stimulates bone growth.
Prolactin induces milk production at the mammary glands in females.
Adrenal Glands
The adrenal glands are located just above the kidneys. They are composed of an outer cortex and an inner (adrenal) medulla.
The adrenal medulla secretes epinephrine, which includes the "fight or flight" response. The adrenal medulla controls cardiac function, raises blood sugar and controls the size of blood vessels. However, the parathyroid gland regulates calcium levels in the blood.
The adrenal cortex secretes hormones that regulate salt and sugar balance in the body, including aldosterone.
Other glands
The pineal gland secretes melatonin to regulate sleep cycles. Of the endocrine organs, the function of the pineal gland was the last discovered. Located deep in the center of the brain, the pineal gland was once known as the “third eye.” The pineal gland produces melatonin, which helps maintain circadian rhythm and regulate reproductive hormones.
The thyroid and parathyroid are located in the neck and control metabolism and calcium balance.
Blood Glucose
The pancreas is responsible for regulating blood glucose levels. The pancreas functions as both an endocrine gland by releasing hormones and as an exocrine gland by releasing digestive enzymes.
Glucagon, along with insulin, is released by the pancreas.
Insulin is released in response to high blood glucose and leads to a decrease in blood glucose.
Diabetes is associated with abnormally high blood glucose levels and is caused by insufficient insulin secretion.
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