Endocrine System and Hormones

The endocrine system works with the nervous system to coordinate body functions.
Endocrine glands release hormones into the bloodstream.
The blood carries hormones to cells throughout the body.
Learning goals include understanding how hormones maintain body homeostasis and describing the function of the hypothalamus and pituitary hormones.

Hormones are chemical regulators (proteins, lipids) produced by cells that affect other cells.
They facilitate cell communication, but this process is slower than nervous signals.
Endocrine glands secrete hormones directly into the bloodstream.

The human body produces approximately 50 hormones.
Hormones affect only specific types of cells with matching hormone receptors.
Hormones travel through the body in the blood and affect specific target cells.
Hormones influence target cells by binding to specific protein or glycoprotein receptors.
Only target cells with receptors that bind and recognize a specific hormone are affected; for example, TSH binds to thyroid gland cells but not to ovary cells.

One hormone can have multiple actions; testosterone, for example, influences sperm formation, male reproductive tract development, and secondary sex characteristics.
Many hormones can regulate a single process; for example, glucose conversion to glycogen is regulated by insulin, thyroid hormones, epinephrine, and cortisol.

Steroid hormones are derived from cholesterol (e.g., estrogen, progesterone).
They diffuse through the lipid bilayer and bind to protein receptors within target cells.
Receptors are located in the cytosol or nucleus.
Binding alters gene expression by turning specific genes of nuclear DNA on or off.

Also called protein hormones.
Cannot diffuse through the lipid bilayer.
Receptors are integral proteins in the plasma membrane that protrude into the interstitial fluid.
Hormone binding to receptors on the cell surface activates another membrane protein, which converts ATP into cyclic AMP (cAMP) in the cytosol.
cAMP acts as a messenger, activating certain enzymes within the cell.
These enzymes catalyze reactions that produce a physiological response.

Hormones are released in short bursts via exocytosis.
Increased stimulation leads to more frequent bursts of hormone release.
Decreased stimulation reduces the frequency of bursts.
Hormone secretion is regulated by:
- Signals from the nervous system
- Chemical changes in the blood
- Other hormones ('chain reaction')

Faulty control of secretion:
- Hyposecretion (too little)
- Hypersecretion (too much)
Faulty or Inadequate Hormone Receptors

The hypothalamus is a small region of the brain that controls the pituitary gland.
It serves as a major link between the nervous system and the endocrine system.
It controls the autonomic nervous system and regulates body temperature, thirst, hunger, and defensive reactions such as fear and rage.

Together, the hypothalamus and pituitary gland regulate virtually all aspects of growth, development, metabolism, and homeostasis.
The hypothalamus secretes 9 different hormones.
The pituitary gland secretes 7 hormones.
The pituitary gland is divided into two lobes: anterior and posterior.

The anterior pituitary gland secretes hormones that regulate a wide range of bodily activities, from growth to reproduction.
Hormones released from the hypothalamus regulate the release of pituitary hormones.

Human Growth Hormone (hGH):
- Stimulates tissues to secrete insulin-like growth factors (IGFs).
- IGFs stimulate general body growth and regulate aspects of metabolism.
Thyroid-stimulating Hormone (TSH):
- Controls the secretions and other activities of the thyroid gland.
Adrenocorticotropic Hormone (ACTH):
- Stimulates the adrenal cortex to secrete glucocorticoids that regulate glucose metabolism.
Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH):
- Act on the gonads (testes and ovaries).
- Stimulate the secretion of estrogen and progesterone and the maturation of oocytes in the ovaries.
- Stimulate the secretion of testosterone and sperm production in the testes.
Prolactin (PRL):
- Initiates milk production in the mammary glands.

hGH acts indirectly on tissues by promoting the synthesis and secretion of small IGF protein hormones.
IGFs cause cells to grow and multiply by increasing the rate at which amino acids enter cells and are used to synthesize proteins.

Increases the growth rate of the skeleton and skeletal muscles during childhood and the teenage years.
In adults, hGH and IGFs help maintain muscle and bone mass and promote the healing of injuries and tissue repair.

Hyposecretion of hGH during the growth years.
Slows bone growth and causes the epiphyseal plates to close before normal height is reached.
Other organs of the body fail to grow.
Treatment requires the administration of hGH during childhood, before the epiphyseal plates close.

Hypersecretion of hGH during childhood.
Abnormal increase in the length of long bones.
A person with this condition is unusually tall but has approximately normal body proportions.

Hypersecretion of hGH during adulthood.
Further lengthening of the long bones cannot occur because the epiphyseal plates are closed.
Instead, the bones of the hands, feet, cheeks, and jaw thicken.

Does not synthesize hormones.
Stores and releases two hormones: oxytocin and antidiuretic hormone (ADH).
These hormones are produced in the hypothalamus and packed into vesicles, which are then transported to and stored in the posterior pituitary.

Targets two tissues during and after the delivery of a baby: the mother’s uterus and breasts.
Enhances contractions of smooth muscle cells in the wall of the uterus.
Stimulates milk ejection from the mammary glands.
Oxytocin is also called the 'love hormone.'
Oxytocin plays an important role in reproduction, feelings of attraction and bonding with our young.
Oxytocin is thought to eliminate fear and encourage TRUST.
Oxytocin was the first polypeptide to be synthesized. It's synthetic version is known today as "Pitocin" and is used in labor inductions.
Oxytocin released at birth and during breastfeeding helps contract the uterus and stop bleeding.
Oxytocin actually means "quick birth."

Antidiuretic Hormone
Is released in response to a high blood osmotic pressure.
Decreases urine production by causing the kidney to return more water to the blood.
Decreases water loss through sweating.
Increases blood pressure by causing the arterioles to constrict.
Hyposecretion of ADH or non functioning ADH receptors causes diabetes insipidus.