Endocrine System - Biology

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.

The adrenal medulla secretes epinephrine (adrenaline), which is involved in the body's "fight of flight" response. Epinephrine is released in response to direct neural stimulation during periods of short-term stress and acts to stimulate the sympathetic nervous system.

The pituitary gland is known as the "master" endocrine gland because it secretes several hormones that control other endocrine glands. These are known as tropic hormones. Adrenocorticotropic hormone, thyroid-stimulating hormone, and follicle-stimulating hormone are some examples of pituitary tropic hormones.

Steroid hormones are the only ones derived from cholesterol and are always characterized by a four ring molecular structure. Some examples include sex hormones such as androgens and estrogens as well as some adrenal hormones like cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid).

Increased potassium levels will stimulate aldosterone to increase potassium secretion. The main regulators of aldosterone are potassium and the renin-angiotensin pathway.

The pituitary gland is known as the "master" endocrine gland because it secretes several hormones that control other endocrine glands. These are known as tropic hormones. Adrenocorticotropic hormone, thyroid-stimulating hormone, and follicle-stimulating hormone are some examples of pituitary tropic hormones.

Steroid hormones are the only ones derived from cholesterol and are always characterized by a four ring molecular structure. Some examples include sex hormones such as androgens and estrogens as well as some adrenal hormones like cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid).

Increased potassium levels will stimulate aldosterone to increase potassium secretion. The main regulators of aldosterone are potassium and the renin-angiotensin pathway.

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.

The adrenal medulla secretes epinephrine (adrenaline), which is involved in the body's "fight of flight" response. Epinephrine is released in response to direct neural stimulation during periods of short-term stress and acts to stimulate the sympathetic nervous system.

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.