Endocrine System - Biology

When thinking of hormones, it helps to remember that they are generally slow acting, affect all types of tissues at once, and can last for long periods of time in the body. As a result, hormones would not be described as fast-acting in the body.

In contrast, neurotransmitters are generally considered fast-acting, as they are released to a small, targeted area and elicit an immediate response.

Endocrine glands only secrete their products into the blood. Sweat glands secrete sweat outside the body and never touch the blood. Similarly, anything inside the gastrointestinal tract is technically outside the body! For something (broken-down food molecules, water, salts etc.) to enter the body, it must be absorbed across the walls of the gastrointestinal tract. Remember, humans are like hollow cylinders in the sense that we have a tube from mouth to anus, which is considered outside the body.

Endocrine glands are glands of the endocrine system that secrete hormones (their products) directly into the blood rather than through a duct (exocrine organs use ducts). The major glands of the endocrine system include: adrenal glands, hypothalamus, parathyroid glands, thyroid gland, testes, ovaries, pancreas, pituitary gland, and pineal gland. The hypothalamus and pituitary gland are considered neuroendocrine organs. The pineal gland is located in the brain and secretes the hormone melatonin, which helps regulate the circadian (sleep-wake) cycle, especially before sleep.

The adrenal cortex has three zones. The first zone releases aldosterone, the second releases cortisol, and the third releases androgens, which includes testosterone. Vasopressin (antidiuretic hormone) is synthesized by the hypothalamus and released by the posterior pituitary.

The adrenal medulla is very different from the adrenal cortex and has a different origin as well. It is made of chromaffin cells, which are neuroendocrine cells, and release catecholamines (epinephrine and norepinephrine), in response to sympathetic stimulation.

Adrenocorticotropic hormone (ACTH) helps stimulate the release of cortisol from the adrenal glands. Vasopressin and oxytocin are produced by the hypothalamus and secreted by the posterior pituitary. Growth hormone and prolactin have no direct effect on the adrenal glands.

Hormones will be elevated in response to the conditions of the body. In other words, the body's hormones will be elevated in order to provide a negative feedback response. When blood glucose is high, insulin is released to lower these levels to the normal range. In contrast, when blood glucose is low, glucagon is released to raise these levels back to normal.

Thyroxine (T4) is released by the thyroid in order to increase the basal metabolic activity of the body, causing it to produce more ATP energy. Parathyroid hormone is released to stimulate osteoclast activity and raise blood calcium levels when they are too low.

TSH is secreted by the anterior pituitary in response to TRH (thyrotropin-releasing hormone), which is secreted by the hypothalamus. TSH then acts on the thyroid to stimulate the release of thyroid hormone into the blood. Adequate levels of thyroid hormone then cause negative feedback on the hypothalamus and pituitary to decrease the release of TRH and TSH.

In primary hypothyroidism, the problem lies in the thyroid-- the thyroid is not releasing enough thyroid hormone and it can be due to a variety of causes. What's important here is that there is no negative feedback on the pituitary to decrease its synthesis of TSH and therefore, the TSH levels are higher. It's just not able to function to increase the level of thyroid hormone and bring it to normal.

Thyrotropin-releasing hormone leads to the stimulation of prolactin at the same time as thyroid-stimulating hormone. Oxytocin and antidiuretic hormone are produced by the hypothalamus and stored and secreted by the posterior pituitary. Somatostatin and growth hormone are not influenced by thyrotropin-releasing hormone.

Oxytocin and antidiuretic hormone are released from the posterior pituitary. All other choices are released from the anterior pituitary.

Antidiuretic hormone (ADH) is a hormone released by the posterior pituitary when there is an imbalance of water in the body. Its function is the same as aldosterone, which also helps regulate water levels in the body. ADH causes channels to open in the collecting duct for water to exit the filtrate and enter the blood, increasing blood volume and retaining water.

In contrast, aldosterone causes channels to open for sodium to exit the filtrate and enter the blood. The blood becomes more concentrated, which draws water out of the filtrate to help dilute the increased sodium levels. This also leads to increased blood volume and water retention.

Parathyroid hormone is responsible for increasing blood calcium levels. Calcitonin has the opposite effect, and lowers blood calcium levels. These two hormones act in a negative feedback loop to keep calcium levels relatively constant. When calcium levels are high, calcitonin is released. When calcium levels are low, parathyroid hormone is released.

Insulin serves to low blood glucose levels, while glucagon acts to increase blood glucose. Antidiuretic hormone (ADH) helps conserve water in the body by increasing water reabsorption in the kidneys.

Insulin stimulates the reuptake of glucose from the blood into the cells. Thus, the glucose levels in the blood decrease, as the glucose is taken into cells. The cells may either store it as glycogen (in liver and skeletal muscle) or use it in glycolysis to make ATP.

Epinephrine is the hormone that gets you ready for a "fight or flight" response. This means increasing heart rate, increasing the force of heart contractions, increasing blood flow to the muscles, and opening the windpipes. This would help you if you needed to run away or fight by increasing the amount of oxygen thats delivered to the blood. Also, during a fight, flight, or freight response, the pupils dilate to allow more light to enter, making it easier to see in the dark.

Parathyroid hormone is secreted by the parathyroid gland in response to low levels of serum calcium. It acts to increase the resorption of calcium from the bone, kidneys, and intestines. Note that vitamin D is also a hormone that shares this function.

Insulin functions to decrease blood sugars. It does so by acting on adipose tissue to uptake glucose. It causes increased glucose utilization by the muscles. It also inhibits the release of glucagon, which has the opposite effects as insulin.

Gluconeogenesis by the liver occurs in response to very low blood glucose, when the body is in need of sugar. Glucagon causes gluconeogenesis in the liver.

Steroid hormones and tyrosine derivatives are largely nonpolar, and can enter the nucleus of target cells. Peptide hormones are unable to cross the cell membrane, and must attach to membrane-bound receptors in order to affect target cells. Remember that the membrane is only permeable to small, nonpolar molecules. Peptide hormones are proteins, meaning they are usually large and polar. In order to affect the cell, these hormones cannot cross the membrane, and must instead bind to a receptor at the surface.

Steroid hormones include testosterone, estrogen, and aldosterone. Tyrosine derivatives include the thyroid hormones (T3 and T4) and epinephrine. Most other hormones are peptide hormones.

Insulin is an example of an endocrine hormone because it is secreted into the blood and transported to distant areas of the body. Insulin is released from the pancreas, but acts on numerous regions of the body, including the liver and muscle cells.

Autocrine and paracrine signaling involve signaling close to the cell that secreted the hormone. In paracrine signaling, molecules are secreted by one cell and bind to receptors on an adjacent cell to elicit a response. In autocrine signaling, the secreted compounds bind to receptors on the surface of the same cell from which they were released, eliciting a response from the same cell. Exocrine secretions are released into ducts designed to connect outside of the body, such as the digestive tract or sweat glands. This is in contrast to endocrine secretions, which enter the blood or interstitium.

The thyroid gland is responsible for secreting thyroid hormones (T3 and T4), which are responsible for setting the basal metabolic rate (BMR). The concentrations of these hormones are what tell the cells which metabolic pathways to undergo. Since body heat is a byproduct of metabolism, thyroid hormone also determines body temperature.