Endocrine Physiology - Human Anatomy and Physiology

Gastrin is a hormone secreted by the G cells of the stomach that acts to increase gastric acid secretion, increases the growth of gastric mucosa, increases gastric motility, and stimulates the secretion of digestive enzymes. In addition to stimulating the release of pepsinogen, gastrin stimulates the pancreas to release digestive enzymes. Bicarbonate, however, is not an enzyme and is stimulated by another hormone.

Secretin is a hormone secreted by the S cells in the duodenum that acts to increase pancreatic bicarbonate secretion and decrease gastric acid secretion.

Remember that hormones are generally slow-acting in the body. Glands will respond to the conditions of the body by elevating hormones in a negative feedback fashion. If a person has high blood glucose levels, it is expected that the body is going to increase hormones that act to lower the blood glucose levels.

Insulin is responsible for stimulating glucose sequestration for use in cell metabolism or storage in the liver as glycogen. As a result, insulin levels would be elevated in a person with high blood sugar levels.

Glucagon is functional antagonist of insulin, and serves to elevate blood glucose levels through glycogenolysis and gluconeogenesis. Cortisol is released by the adrenal cortex in response to adrenocorticotropic hormone (ACTH) and long-term stressors, such as prolonged hunger, thirst, and heat/cold exposure. Thyroxine (T4) is a thyroid hormone that mediates basal metabolic rate and is released in response to thyroid-stimulating hormone (TSH).

While testosterone is associated with the growth spurt during puberty, it is also responsible for the closing of the epiphyseal plates on long bones. This process helps stop the growth in length of the body.

Testosterone is the primary male sex hormone and is secreted by Leydig cells of the testes. Testosterone is a steroid hormone, meaning it can enter cells and act directly on DNA as a transcription factor. In this way, it mediates the production of sperm, anatomical growth, and development of secondary sex characteristics in males.

Hypersecretion, or oversecretion, of the growth hormone during childhood can typically result in gigantism. The person will be very tall and have normal face and body proportions; the growth plates are still present at this time and they all receive a larger amount of the hormone.

Dwarfism would be a result of hyposecretion, or undersecretion, of growth hormone during childhood.

Acromegaly is caused by hypersecretion of growth hormone, but during adulthood as opposed to childhood.

Erythropoietin acts on red bone marrow to increase the amount of red blood cells being produced. Aldosterone is responsible for electrolyte homeostasis via the excretory system. Cortisol is the hormone associated with stress response, and regulates blood glucose levels. Lastly, prolactin is responsible for milk production.

Vasopressin, also known as antidiuretic hormone, is released by the posterior pituitary and targets the kidney for water reabsorption. Milk production is controlled by prolactin. Milk ejection is an effect of oxytocin. Lastly, the thymosin and thymopoietin are responsible for lymphocyte development.

ADH inserts special channels into the collecting ducts of the kidney that allow for water absorption from the fluid that is destined to become urine. An increasing amount of ADH would decrease the amount of urine voided from a person as more water is being absorbed in the distal convoluted tubule and collecting ducts of the kidneys. ADH also constricts arterioles everywhere in the body, which increases the peripheral resistance and arterial pressure.

Erythropoietin is a glycoprotein that increases the production of red blood cells. Erythropoietin is secreted by the kidneys and liver, and activates red bone marrow to induce hematopoiesis.

Oxytocin has many effects in the body. First, it stimulates contraction of the myoepithelial cells which surround mammary alveoli in the mammary gland. This causes milk to be ejected. Oxytocin is also released during labor and works to enhance uterine contractions in order to facilitate birth. Lastly, oxytocin is widely known as the "bonding hormone," and studies show that its presence establishes maternal behavior. Oxytocin does not, however, form and regulate the female reproductive system; this is done by the female sex hormone, estrogen.

Steroids, when taken exogenously, diminish the production of steroids in the body. This can lead to atrophy of the adrenal glands (responsible for producing cortisol-stress steroid hormone). Thus, when patients are taking steroids for whatever reasons (sports, chemo, infection and etc) it is important for the steroids to be tapered off and not removed immediately since the body needs time to begin to reproduce the hormone again.

When thinking of hormones, it helps to remember that they are generally slow acting, indirect, and long lasting. All endocrine hormones travel through the bloodstream in order to reach their target cells. It can help to compare the activity of hormones to the activity of neurotransmitters, which are fast acting and have immediate effects on their target cell. Consider the effects of acetylcholine release at a neuromuscular junction in comparison to growth hormone release into the blood. The muscle twitch is much more transient and quick to react, whereas the effects of growth hormone can take much longer to appear.

All steriod hormones are lipid soluble and will thus bind either nuclear or cytoplasmic receptors. Most peptide hormones are water soluble and cannot diffuse through the plasma membrane of the target cell; they must bind to receptors embedded in the cell membrane. The one exception to this rule is thyroid hormone. It is a peptide hormone, but is a lipophilic tyrosine derivative and binds to nuclear receptors.

A positive feedback loop features further deviation from normal. In the case of childbirth, oxytocin produces uterine contractions, which then causes more oxytocin to be released and more uterine contractions to occur until the baby is delivered. A negative feedback loop returns the body to normal conditions.

Neural or synaptic communication occurs when a neurotransmitter (or chemical messenger) diffuses across the synaptic cleft of the presynaptic neuron and postsynaptic neuron or target cell. Examples of these neurotransmitters include acetylcholine and norepinephrine. Endocrine communication occurs when chemical messengers are released into the blood stream. These chemical messengers are called hormones.

The secretion of hormones can often be controlled by concentrations of nutrients, ions, and other molecules. The secretion of parathyroid hormone is regulated and controlled by the concentration of calcium found in the plasma. The function of parathyroid hormone is to increase the concentration of calcium in the blood. This is done by breaking down bone and increasing calcium reabsorption in the kidneys and intestine.

Calcitonin, thyroxine, triiodothyronine are the hormones that are secreted by the thyroid. The c-cells of the thyroid secrete calcitonin. The function of calcitonin is to decrease the concentration of calcium in the blood by activating osteoblasts and/or deactivating osteoclasts, ultimately storing the calcium from the blood into bone.

The correct answer is parathyroid hormone (PTH). PTH is secreted by the parathyroid gland when calcium levels become too low and acts to increase levels to normal range.

Calcitonin is also involved in calcium regulation, but is secreted by the thyroid gland when calcium levels become too high. Calcitonin reduces the action of the parathyroid gland and PTH until calcium levels are reduced to normal range. Thyroid stimulating hormone (TSH) causes the thyroid gland to grow in size in an attempt to make more hormones if iodine levels become too low. Melatonin is synthesized in the pineal gland and is important for the regulation of the sleep cycle. Cortisol is secreted by the adrenal cortex, and increases blood glucose levels, and has other effects.

Peptide hormones are large polar hormones that are able to freely float in the bloodstream. Once they reach their target cells, peptide hormones attach to transmembrane receptors on the target cell. This generally initiates a second messenger signal cascade to amplify the response, eventually driving a change in genetic expression and transcription.

In contrast, steroid hormones are small and nonpolar. These characteristics allow them to freely diffuse across the plasma membrane of the cell and enter the nucleus, where they can directly act as transcription factors to regulate genetic expression.

Parathyroid hormone (PTH) is released when blood calcium levels are low, and functions to raise blood calcium levels via a negative feedback mechanism. It does this by increasing osteoclast activity and degrading bone to release calcium into the blood. It also increases absorption of calcium in the renal tubule, increases production of 25-hydroxyvitamin D3, and increases absorption of calcium in the intestines.