The endocrine system helps to keep our bodies working in a balance, called homeostasis. While our nervous system uses electrical impulses, the endocrine system releases chemicals called hormones. Hormones work more slowly than nerves, but can have longer lasting effects.
The endocrine system consists of nine major glands located throughout the body. Together, these glands produce dozens of hormones, and release them directly into their surrounding blood vessels. In this way, the endocrine system regulates metabolic and growth rates, and our body’s development. Lab tests are used to diagnose and manage health conditions that are associated with hormonal imbalances.
A gland is a group of cells that produces a substance, and releases it via a process called secretion. There are two types of glands. Exocrine glands have ducts or channels which secrete chemicals such as saliva or sweat. Endocrine glands do not have ducts; they secrete hormones directly into the bloodstream.
The hypothalamus is located in the brain, and connects the nervous and endocrine systems. Based on information sent by the brain, it sends signals to the pituitary gland to release hormones. In addition, the hypothalamus has a role in regulating mood, body temperature, hunger, and sleep.
The pineal gland is a small, pine-cone shaped endocrine gland in the brain. It detects daylight and darkness to control the release of melatonin, a derivative of serotonin that is important for sleep.
The pituitary gland, also called the hypophysis, is an endocrine gland about the size of a pea. Although it weighs less than an ounce, it is one of the most important glands in the body. Located at the base of the brain and directly connected to the hypothalamus, the pituitary gland secretes nine hormones. All are important for maintaining homeostasis, and stimulating other endocrine glands to produce and secrete their own hormones.
This particular gland has two distinct areas: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The former is larger, and releases the majority of the hormones. The smaller, posterior lobe stores hormones, but does not synthesize any. The pituitary serves as a conduit between the endocrine system and nervous systems by way of the hypothalamus.
The pituitary is known as the “master” endocrine gland because its hormones stimulate glands throughout the body to produce additional hormones. There are eight different pituitary hormones. These include thyroid stimulating hormone (TSH), necessary for thyroid hormone production, and adrenocorticotrophic hormone (ACTH) that promotes adrenal gland responses to stress.
The thyroid gets its name from the Greek word for “shield” because of its shape. This butter-fly shaped gland is one of the largest endocrine glands. It sits in the front of the neck, just below the thyroid cartilage or “Adam’s apple”. The isthmus bridges the two lobes of the thyroid over the trachea.
The thyroid gland controls how quickly the body uses energy (metabolism), blood calcium levels, protein synthesis, and how the body responds to other hormones. The primary thyroid hormones are triiodothyronine (T3), tetraiodothyronine (T4), and calcitonin. T4 doesn’t perform any specific functions, but is converted to T3, by various organs. T3 regulates heart rate, growth, metabolic rate, and other functions. Iodine and tyrosine are important for the formation of T3 and T4. Calcitonin ensures that an appropriate amount of calcium remains in the bloodstream. It slows down the rate at which bone is broken down to decrease the amount of calcium that is released into the blood.
The secretion of these hormones is regulated by thyroid-stimulating hormone (TSH) from the anterior lobe of the pituitary gland. TSH is regulated by thyrotropin-releasing hormone (TRH), produced by the hypothalamus. The most common thyroid problems include an overactive thyroid gland, known as hyperthyroidism, or an under-active one, called hypothyroidism.
The parathyroid glands produce a hormone that specifically controls calcium levels in the blood. They are four small glands located near the thyroid gland, and produce parathyroid hormone or PTH. This hormone increases the rate at which bone is mineralized. As a result, more calcium gets released into the blood. PTH also assists in calcium absorption from the intestines, and prevents the kidneys from losing too much calcium into urine. Parathyroid hormone works in conjunction with calcitonin from the thyroid gland, and their combined effects keep blood calcium levels stable.
The thymus is a specialized organ of the immune system. It “educates” a group of white blood cells called T-lymphocytes (T cells) to fight infections, prevent cancers, and keep the cells of the body healthy. It is critical to the adaptive immune system.
These small, triangular glands (also known as suprarenal glands) sit on top of the kidneys. Each is divided into two distinct anatomic and functional components: the outermost adrenal cortex and the inner medulla. The adrenal cortex secretes corticosteroid hormones: glucocorticoids, mineralocorticoids, and androgens. Glucocorticoids are important for metabolism (how food is stored and used), and responses to stress. They also affect growth and blood pressure. Mineralocorticoids act on the kidneys to manage the body’s electrolyte balance. Androgens are the building blocks of hormones produced by the ovaries and testes. The smaller, inner region of the adrenal gland is called the medulla. It is contains epinephrine and norepinephrine which stimulate the nervous system, and affect blood pressure. These chemicals are the body’s first line of defense against physical and emotional stress. The adrenal glands help us deal with stress as well as maintain homeostasis.
Microscopically, the adrenal cortex is divided into three distinct zones. The outermost zone secretes aldosterone, This hormone inhibits the amount of sodium excreted in the urine to maintain blood pressure and volume. The inner and middle zones together secrete several glucocorticoids such as cortisol and corticosterone as well as small amounts of androgen hormones. The frequency and amount of their secretion is controlled by corticotropin releasing hormone (CRH) and adrenocorticotropin hormone (ACTH) from the hypothalamus and pituitary respectively.
The adrenal medulla is similar to nervous system tissue, and secretes epinephrine and norepinephrine in response to stimulation by sympathetic nerves. These nerves are most active at times of stress. The release of these hormones increases the heart rate in order to pump more blood to the large skeletal muscles. The airways in the lungs expand so that more oxygen can be taken in. As more blood is sent to the active organs, less blood travels to the stomach, intestines, and bladder.
The pancreas is a gland of both the digestive and endocrine systems. As an endocrine gland, it produces several important hormones, including insulin, glucagon, somatostatin, and pancreatic polypeptide. It secretes pancreatic enzymes that facilitate digestion, and the absorption of nutrients in the small intestine. These enzymes help to break further down carbohydrates, proteins, and fats. In medical conditions such as diabetes and cystic fibrosis, the pancreas does not produce these hormones properly.
The ovary is the egg-producing organ of the female reproductive system. Ovaries in women are analogous to testes in men; they are both gonads and endocrine glands. The ovaries produce estrogen, progesterone, relaxin and inhibin. Relaxin increases the laxity and flexibility of the pelvis and joints to support pregnancy and childbirth. Inhibin helps keep the other reproductive hormones in balance.
The testicle is the male gonad. Like the ovaries in women, the testes are components of both the reproductive and endocrine systems. The primary functions of the testes are to produce inhibin, sperm (spermatogenesis), and testosterone.
Hormones are powerful “chemical messengers” through which our endocrine system controls various processes inside our body. Some hormones are fat-soluble (i.e. estrogen, testosterone), while others are water-soluble such as insulin and epinephrine. Endocrine glands secrete hormones into nearby blood vessels. These hormones then travel through the bloodstream until they reach their destination, or target cell. Upon arrival, they lock onto this cell’s receptor site to cause chemical reactions within the organ or structure. For example, in a stressful situation, cortisol can adjust the rate at which skeletal muscle fibers contract.
Hormones may have one or multiple destinations. Some only work on specific organs, while others have various functions throughout the body. The amount of circulating hormones is controlled by feedback. This means that, when an increased amount of hormone is present in the bloodstream, signals are sent for the glands to reduce its secretion. It is important that the hormones in our body are kept at the right level. If they become too high or low, they can make us sick. Although hormones come in contact with many cells throughout the body, they only react with their target cells. In addition, one single hormone can have more than one target cell, and different effects on multiple targets.
This is a pituitary hormone that stimulates the production of milk in the breast. Estrogen and progesterone prepare the milk glands for breastmilk production before the birth of a baby. Breast-feeding further stimulates the pituitary gland to release prolactin so that milk is produced as long as the baby breastfeeds.
Oxytocin is a pituitary hormone that stimulates muscle contractions in the uterus during childbirth. These contractions also cause the release of more oxytocin in a positive feedback cycle that continues until the baby is born. This hormone also stimulates the breasts to release milk for breastfeeding, and promotes maternal bonding with the baby.
The hormone glucagon increases blood sugar levels, and plays a vital role in maintaining its stability. Glucagon is made in the pancreas which secretes glucagon when the blood sugar levels decline. This causes cellular release of glucose in response. Glucose is the primary sugar the body uses for energy.
Glucagon also helps convert glycogen in the liver into its usable form, glucose. As a result, glucose is released into the bloodstream, and the blood sugar level rises. Without this process, the bloodstream only has enough glucose to keep you alive for 15 min. As glucose is used by the body, more is released to replace it. Glycogen can also be found in muscles.
Insulin is a hormone, produced by the pancreas, that reduces the amount of sugar in the blood. It is released when blood glucose levels increase, and reduces it in two ways. It promotes glucose absorption into muscles and fat cells, and helps the liver convert it into its storage form, glycogen. The effects of insulin counter that of glucagon. Together they form a feedback system that keeps blood sugar at an appropriate level. In patients with diabetes, this control system does not work properly. They made may need daily injections of insulin to keep their blood glucose levels within safe limits.
Reproductive hormones control the development of boys and girls. This includes the primary and secondary characteristics as well as sperm or egg production. Primary characteristics are the reproductive organs present at birth. Secondary characteristics develop at puberty. There are three primary reproductive hormones: androgens, estrogen, and progesterone.
Estrogen and Progesterone
The majority of estrogen forms in the ovaries, with smaller amounts produced by the adrenal glands and fat cells. This hormone stimulates the female reproductive organs to develop, and controls the monthly menstrual cycle. Progesterone prepares the uterus for pregnancy every month. Some contraceptive pills contain estrogen to prevent the ovaries from releasing an egg.
The male reproductive system consists of the penis, scrotum, and the two testes. The hormone testosterone is produced by the testes, and, to a lesser extent, the adrenal glands and female ovaries. It promotes sperm production as well as the male voice changes, body hair, and increased muscle mass.
Luteininzing Hormone (LH)
This is a pituitary hormone that helps regulate the function of the reproductive organs. It stimulates ovulation to release an egg from a woman’s ovaries. In men, it stimulates the testes to produce testosterone.
Follicle-stimulating Hormone (FSH)
This hormone, along with lutenizing hormone, is an important component of the reproductive system. In females, FSH promotes ovulation and estrogen production. Sperm production is dependent on this hormone in males.
Epinephrine, or adrenaline, is a hormone that works in conjunction with the nervous system to prepare our bodies to cope with danger or stress. It is a very fast acting hormone that prepares the body for emergency action, also called the “fight or flight reflex.” It increases our breathing and heart rates, and diverts extra blood to the muscles. At the same time, it slows down digestion, and releases glucose from the liver into the bloodstream so more fuel is available for muscle contractions.
Growth hormone is produced by the pituitary gland. It controls the body’s growth, and helps to sustain bones and muscles. It also increases blood glucose levels. If a child has too little growth hormone, he or she will fail to grow normally. Alternatively, too much growth hormone can cause the body to grow excessively.
The thyroid stimulating hormone is released by the pituitary gland. It stimulates the thyroid gland to produce thyroid hormones.
This pituitary hormone stimulates the adrenal glands to produce cortisol and androgens.
This pituitary hormone helps to maintain a normal blood volume, and prevent dehydration. It increases the amount of water that the kidneys return to the bloodstream, and also elevates blood pressure by constricting blood vessels. As a result, more fluid is squeezed into a smaller space, and blood pressure rises.
A diuretic is a substance that stimulates the body to remove water from the bloodstream, sending it into the urine. Caffeine and some blood pressure medications are examples. The antidiuretic hormone or vasopressin, has the opposite effect.