Maintaining a constant internal environment by providing the cells with what they need to survive oxygen, nutrients, and removal of waste is necessary for the well-being of individual cells and of the entire body. The many processes by which the body controls its internal environment are collectively called homeostasis. The complementary activity of major body systems maintains homeostasis.
Homeostasis refers to stability, balance, or equilibrium within a cell or the body. Homeostasis is an important characteristic of living things. Keeping a stable internal environment requires constant adjustments as conditions change inside and outside the cell. The adjusting of systems within a cell is called homeostatic regulation. Because the internal and external environments of a cell are constantly changing, adjustments must be made continuously to stay at or near the set point the normal level or range.
Homeostasis can be thought of as a dynamic equilibrium rather than a constant, unchanging state. The endocrine system plays an important role in homeostasis because hormones regulate the activity of body cells. The release of hormones into the blood is controlled by a stimulus.
For example, the stimulus either causes an increase or a decrease in the amount of hormone secreted. Then, the response to a stimulus changes the internal conditions and may itself become a new stimulus.
This self-adjusting mechanism is called feedback regulation. Feedback regulation occurs when the response to a stimulus has an effect of some kind on the original stimulus. The type of response determines what the feedback is called. Negative feedback occurs when the response to a stimulus reduces the original stimulus.
Positive feedback occurs when the response to a stimulus increases the original stimulus. Negative feedback is the most common feedback loop in biological systems. The system acts to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostatic balance. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and exhale more carbon dioxide, your breathing rate increases.
Thermoregulation is another example of negative feedback. When body temperature rises, receptors in the skin and the hypothalamus sense the temperature change.
The temperature change stimulus triggers a command from the brain. This command, causes a response the skin makes sweat and blood vessels near the skin surface dilate , which helps decrease body temperature.
Figure 1: Control of blood glucose level is an example of negative feedback. Blood glucose concentration rises after a meal the stimulus. The hormone insulin is released by the pancreas, and it speeds up the transport of glucose from the blood and into selected tissues the response. Blood glucose concentrations then decrease, which then decreases the original stimulus. The secretion of insulin into the blood is then decreased. Positive feedback is less common in biological systems.
Positive feedback acts to speed up the direction of change. The thyroid follicle cells synthesize the hormone thyroxine, which is also known as T 4 because it contains four atoms of iodine, and triiodothyronine, also known as T 3 because it contains three atoms of iodine.
T 3 and T 4 are released by the thyroid in response to thyroid-stimulating hormone produced by the anterior pituitary, and both T 3 and T 4 have the effect of stimulating metabolic activity in the body and increasing energy use. A third hormone, calcitonin, is also produced by the thyroid.
Calcitonin is released in response to rising calcium ion concentrations in the blood and has the effect of reducing those levels. Most people have four parathyroid glands ; however, the number can vary from two to six.
These glands are located on the posterior surface of the thyroid gland Figure The parathyroid glands produce parathyroid hormone. Parathyroid hormone increases blood calcium concentrations when calcium ion levels fall below normal. The adrenal glands are located on top of each kidney Figure The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla.
These regions secrete different hormones. The adrenal cortex produces mineralocorticoids, glucocorticoids, and androgens. The main mineralocorticoid is aldosterone, which regulates the concentration of ions in urine, sweat, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The glucocorticoids maintain proper blood-glucose levels between meals. They also control a response to stress by increasing glucose synthesis from fats and proteins and interact with epinephrine to cause vasoconstriction. Androgens are sex hormones that are produced in small amounts by the adrenal cortex. They do not normally affect sexual characteristics and may supplement sex hormones released from the gonads.
The adrenal medulla contains two types of secretory cells: one that produces epinephrine adrenaline and another that produces norepinephrine noradrenaline.
Epinephrine and norepinephrine cause immediate, short-term changes in response to stressors, inducing the so-called fight-or-flight response. The responses include increased heart rate, breathing rate, cardiac muscle contractions, and blood-glucose levels.
They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue, and redirect blood flow toward skeletal muscles and away from skin and viscera. The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system that originate from the hypothalamus. The pancreas is an elongate organ located between the stomach and the proximal portion of the small intestine Figure It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones.
The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans. Among the cell types in each pancreatic islet are the alpha cells, which produce the hormone glucagon, and the beta cells, which produce the hormone insulin. These hormones regulate blood-glucose levels. Alpha cells release glucagon as blood-glucose levels decline. When blood-glucose levels rise, beta cells release insulin. The gonads—the male testes and female ovaries—produce steroid hormones.
The testes produce androgens, testosterone being the most prominent, which allow for the development of secondary sex characteristics and the production of sperm cells. The ovaries produce estrogen and progesterone, which cause secondary sex characteristics, regulate production of eggs, control pregnancy, and prepare the body for childbirth.
Oxytocin triggers milk release from breast tissue when infants nurse and causes muscle contractions in the uterus during labor. The thyroid gland has two lobes connected by an isthmus small connecting stalk and is in the lower part of the neck just below the larynx. The thyroid gland produces three hormones:. T3 and T4 are collectively called thyroid hormone and are produced in the follicles hollow spherical structures of the thyroid gland.
Thyroid hormone affects body growth, metabolic rates, and the development of bones and skeletal muscle. Thyroid hormone also increases the sensitivity of the cardiovascular system to sympathetic nervous activity. This effect helps maintain a normal heart rate. Parafollicular cells C cells between the thyroid gland follicles produce calcitonin. Calcitonin lowers blood calcium levels. The parathyroid glands are embedded in back of the thyroid gland and secrete PTH parathyroid hormone.
PTH increases blood calcium by stimulating bone calcium release into the bloodstream and by increasing the calcium absorption rate in the gastrointestinal tract and kidneys. The suprarenal adrenal glands are on top of each kidney. Each gland has a cortex outer region and a medulla inner region. The cortex secretes glucocorticoids such as cortisol, mineralocorticoids, and small amounts of androgens and estrogens responsible for some secondary sex characteristics.
Glucocorticoids raise blood sugar levels by increasing gluconeogenesis synthesis of glucose from amino acid. This action ensures glucose supplies for the body when it is under stress. Mineralocorticoids such as aldosterone promote sodium salt reabsorption by stimulating the kidneys to absorb more sodium from the blood. The medulla "emergency gland" develops from nervous tissue; the autonomic nervous system controls its secretions. The medulla secretes epinephrine adrenaline and norepinephrine noradrenaline , chemicals that raise the blood levels of sugar and fatty acids.
These hormones also increase the heart rate and force of contraction. These effects prepare the body for the "Fight or Flight" response instant physical activity , enabling the individual to think quicker, fight harder, and run faster.
These hormones also constrict the blood vessels supplying the skin, kidneys, gastrointestinal tract, and other areas of the body not needed for the response. The ovary is the site of estrogen and progesterone synthesis.
Estrogen is required to form the ovum egg during oogenesis and prepares the uterus for implanting a fertilized egg. Progesterone prepares the breasts for lactation during pregnancy and works with estrogen to regulate the menstrual cycle.
Secondary endocrine organs include the gonads, kidneys, and thymus. The hypothalamus and the pituitary gland are part of the diencephalon region of the brain.
The hypothalamus connects the nervous system to the endocrine system. It receives and processes signals from other brain regions and pathways and translates them into hormones, the chemical messengers of the endocrine system.
These hormones flow to the pituitary gland, which is connected to the hypothalamus by the infundibulum. Some hypothalamic hormones are stored in the pituitary stores for later release; others spur it to secrete its own hormones.
The hormones released by the pituitary gland and the hypothalamus control the other endocrine glands and regulate all major internal functions. The pineal gland is small and pinecone-shaped, located at the posterior of the diencephalon region in the brain. As a seemingly unique, unpaired structure near the center of the brain, the pineal gland has been an object of historical fascination.
At night, in the absence of light, the pineal gland secretes the hormone melatonin. In the morning, when light hits the eye, photo receptors in the retina send signals to the pineal gland, which then decreases melatonin production.
The thyroid gland sits in the throat region, just below the larynx, served by large arteries with many branches and a dense network of capillaries.
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