Endocrine glands:

Definition:
Endocrine glands are organs that produce and release hormones directly into the bloodstream to
regulate various body functions.
– The Endocrine system and the Nervous system work together to integrate in the brain and
complement each other, but they tend to work at different speeds.
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– Nerves respond within split-seconds but their action soon fades
– Some hormones have longer lasting effects and act over hours, weeks, and years.
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Hormones regulate processes such as:
– The breakdown of chemical substances in metabolism of what we eat and drink
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– Fluid balance and urine production
– The body’s growth and development
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– Sexual reproduction
General Characteristics of hormones:
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. Hormones are chemicals released into the bloodstream by endocrine glands (like the
thyroid or pancreas), which do not use ducts, unlike exocrine glands (e.g., sweat glands).
. While major glands (like the pituitary and thyroid) are part of the endocrine system, many
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other organs also produce hormones.
. Hormones are released in response to specific signals, which can be:
–
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Humoral: Changes in blood substance levels (like glucose).
– Neural: Nervous system signals (like stress).
–
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Hormonal: Other hormones trigger their release.
. Endocrine glands can release several hormones with different roles, and they can be
secreted independently of one another.
. Hormones affect only specific cells with the right receptors, even though they travel
through the entire body.
. Hormones can change cell activity by altering the cell membrane, metabolic processes, or
cell division rate. They can have different effects on different cells.
. Hormones can work together (synergistic), oppose each other (antagonistic), or prepare
cells for the action of another hormone (permissive).
. Hormones don’t stay in the blood long; they’re quickly used or broken down by the liver
and kidneys to prevent excess effects.
Major Endocrine Glands:
Pituitary Gland (Hypophysis):
The pituitary gland is a small but crucial structure located at the base of the brain, connected to
the hypothalamus. It’s housed in a bony area called the sella turcica. It has two parts: the anterior
pituitary and posterior pituitary, each with distinct roles.
Anterior Pituitary Gland (Master Gland):
The anterior pituitary is called the “master gland” because it produces hormones that control
other glands in the body. These include:
. Thyroid Stimulating Hormone (TSH): Stimulates the thyroid to produce hormones.
. Adrenocorticotropic Hormone (ACTH): Stimulates growth and function of the adrenal
glands.
. Follicle Stimulating Hormone (FSH):
– In women, it helps follicles grow before ovulation.
– In men, it helps sperm cells develop.
. Luteinizing Hormone (LH):
– In women, it helps the follicle mature and triggers the production of progesterone for
pregnancy.
– In men, it stimulates testosterone production.
. Growth Hormone (GH): Promotes growth of bones and tissues. Too much GH in childhood
causes gigantism; too much in adulthood leads to acromegaly (enlarged hands, feet, and
face). Too little GH in childhood causes dwarfism.
. Prolactin (PRL): Stimulates breast development during pregnancy and milk production
after childbirth. It has no major role in men.
If the pituitary is damaged (e.g., by surgery or radiation), hormone treatment may be needed for
life to keep other glands functioning.
Posterior Pituitary Gland:
The posterior pituitary doesn’t make its own hormones but stores and releases them in response
to signals from the brain. These include:
. Antidiuretic Hormone (ADH): Helps regulate water balance in the body. When the body is
dehydrated, ADH is released to reduce urine output and conserve water.
. Oxytocin: Oxytocin helps with childbirth by triggering uterine contractions, aids
breastfeeding by releasing milk, and promotes social bonding and trust. Low levels may be
linked to autism.

Pineal Gland:
The pineal gland helps regulate sleep and biological rhythms by producing melatonin, which is
triggered by light. It affects cycles like the menstrual cycle, puberty, and may be linked to
conditions like seasonal affective disorder (SAD).

Thymus:
The thymus is a gland that shrinks with age. It produces thymosin, which helps develop the
immune system and mature T-cells, crucial for fighting infections.

Thyroid Gland:
The thyroid is the largest endocrine gland, located in the neck, and has a butterfly shape. It
produces hormones that regulate metabolism and body functions.
A) Thyroid Hormones (T3, T4):
These hormones, activated by TSH, regulate metabolism, energy production, and development.
Lack of iodine can cause a goiter.
– Hypersecretion: Leads to Graves’ disease with symptoms like weight loss and irritability.
– Hyposecretion: In children, it causes cretinism (growth and mental delays); in adults, it
causes myxedema (low energy, weight gain).
B) Calcitonin: This hormone helps lower blood calcium by promoting bone formation, mainly
affecting children.
– Hyposecretion of calcitonin: Lack of it can cause rickets, usually due to calcium or vitamin
D deficiencies.

Parathyroid gland:
The parathyroid glands are small, round glands located on the back of the thyroid. There are
usually 4 or 5 of them.
Parathyroid Hormone (PTH):
PTH helps regulate blood calcium levels, working opposite to calcitonin (which lowers calcium).
It raises calcium by:
– Increasing calcium absorption in the kidneys and intestines.
– Stimulating osteoclasts to break down bone, releasing calcium and phosphorus into the
blood.
Maintaining calcium balance is important for:
– Muscle and nerve function
– Blood clotting
– Activating certain enzymes
– Cell function
If all the parathyroid glands are removed (like during thyroid surgery), PTH replacement therapy
is needed to keep calcium levels stable.

Pancreas:
The pancreas is both an exocrine and endocrine gland. Most of the pancreas is exocrine (98%),
where it produces digestive enzymes. The small 2% of endocrine part contains Islets of
Langerhans, which are clusters of cells that regulate blood sugar.
Insulin and Glucagon:
– Insulin: After you eat, insulin helps lower blood sugar by moving glucose (sugar), amino
acids, and fatty acids from the blood into cells (except liver cells). It lowers blood sugar
levels. Only the brain, liver, and red blood cells don’t need insulin to take in glucose.
– Glucagon: Between meals, glucagon helps raise blood sugar. It mainly works in the liver to
create and release glucose into the blood.
These two hormones work together to keep blood sugar levels steady, ensuring that your brain
and other cells have enough energy to function.
– After eating: High blood sugar triggers insulin.
– While fasting: Low blood sugar triggers glucagon.

Diabetes:
Diabetes is a group of diseases causing high blood sugar. It includes Diabetes Insipidus (due to
lack of ADH) and Diabetes Mellitus (Type 1 and Type 2).
– Diabetes Insipidus: Caused by low ADH, leading to excessive urination and dehydration.
– Diabetes Mellitus: High blood sugar from Type 1 (low insulin) or Type 2 (insulin
resistance). Type 1 requires insulin, Type 2 is linked to obesity and inactivity.
Complications: Immediate issues include high blood sugar and ketoacidosis; long-term
problems include heart disease, kidney damage, blindness, and nerve damage.

Adrenal Glands:
Located on top of the kidneys, divided into two parts: the adrenal cortex (outer layer) and
adrenal medulla (inner layer), each with distinct functions.
Adrenal cortex:
Vital for life, producing steroid hormones from cholesterol, regulated by ACTH. It includes:
. Mineralocorticoids (aldosterone): Regulate salt and water balance.
– Addison’s Disease (low aldosterone) causes skin darkening, kidney issues, fatigue, weight
 – Addison’s Disease (low aldosterone) causes skin darkening, kidney issues, fatigue, weight
loss, and difficulty handling stress.
– Cushing’s Syndrome (too much cortisol) weakens skin and muscles.
. Glucocorticoids (cortisol): Increase blood sugar, reduce inflammation, and manage
stress.
. Sex Hormones: Affect protein synthesis and sexual development.
Adrenal Medulla:
Not essential for life but helps respond to stress, producing epinephrine and norepinephrine,
which enhance the “fight or flight” response.

Ovaries & Testes:

Minor/Temporary Endocrine Glands:

Liver:
Releases some erythropoietin to support red blood cell production.

Heart:
Secretes ANP to lower blood pressure and sodium.

Stomach & Duodenum:

Adipose Tissue:
Releases leptin, signaling fullness after eating.

Placenta:
Releases hormones during pregnancy, like CGH, estrogens, and progesterone.