Endocrine System: Hormone Functions and Glands

• Two systems coordinate body responses: the nervous and endocrine systems.
• Together, the nervous and endocrine systems are referred to as the neuro-endocrine system.
• The endocrine system regulates body activities by releasing hormones into the bloodstream.
• Hormonal responses can be rapid or delayed, exhibiting a variety of effects.

Hormone Functions

• Hormones regulate the chemical composition and volume of the internal environment.
• They help regulate metabolism and energy balance.
• Hormones regulate the contraction of smooth and cardiac muscle fibers, as well as secretion by glands.
• Hormones help maintain homeostasis despite disruptions like infection, and trauma.
• They regulate immune system activities.
• Hormones play a role in growth and development.
• They contribute to reproduction, including gamete production, fertilization, and fetal nourishment.

Exocrine & Endocrine Glands

• The body contains two kinds of glands: exocrine and endocrine.
• Exocrine glands secrete products into body ducts leading to cavities, organ lumens, or the body surface.
  • Examples include sudoriferous, sebaceous, mucous, and digestive glands.
• Endocrine glands secrete hormones into the extracellular space, diffusing into capillaries and circulating throughout circulatory system.

Endocrine Glands

• The endocrine system is made up of endocrine glands which include:
  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pineal glands
  • Pancreatic islets
• Many organs have cells that secrete hormones, but are not fully endocrine organs.
  • These organs include the hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, and placenta.

Hormones

• Hormones are chemical substances carried by cells that initiate specific actions.
• Hormones can have powerful effects even in low concentrations.
• Approximately 50 different hormones are produced in the body.
• Target cells are those affected by a hormone.
• Hormones influence target calls by binding to proteins or glycoproteins in the cell membrane called receptors.

The Pituitary Gland & Hypothalamus

• The pituitary gland or hypophysis is attached to the hypothalamus at the base of the brain.
• Acting as a major link between the nervous and endocrine systems, the hypothalamus integrates endocrine functions.
• The pituitary gland is called the master endocrine gland.
• Together, the hypothalamus and pituitary gland regulate growth, development, metabolism, and homeostasis.
• The pituitary gland consists of the posterior and anterior pituitary.
• It is pea-sized, weighs about 500mg, and originates from different cells.
• The anterior pituitary (adenohypophysis) is an upgrowth of glandular epithelium from the pharynx.
• The posterior pituitary (neurohypophysis) is a downgrowth of nervous tissue from the brain.

Blood Supply to Pituitary Gland

• Branches of the internal carotid artery supply arterial blood to the pituitary gland.
• The anterior lobe is indirectly supplied by blood that has passed through a capillary bed in the hypothalamus.
• The posterior lobe is supplied directly.

Pituitary Influence

• The hypothalamus controls hormone release from both the anterior and posterior pituitary.

Hormones of the Pituitary Gland

• The anterior pituitary hormones include:
  • GH
  • TSH
  • ACTH
  • PRL
  • LH
  • FSH
• The posterior pituitary hormones include:
  • Vassopressin (ADH)
  • Oxytocin

The Posterior Pituitary

• The posterior pituitary works as a unit with the hypothalamus.
• The posterior pituitary stores and releases oxytocin (OT) and antidiuretic hormone (ADH), which the hormones are produced in the hypothalamus.
• OT controls uterine contractions during delivery and milk ejection during breastfeeding.
• ADH causes retention of body water, controlling the body's water-balancing mechanism.

The Anterior Pituitary

• The anterior pituitary secretes hormones that control a wide range of bodily activities.
• The hypothalamus regulates the anterior pituitary by producing releasing and inhibiting hormones which stimulate or suppress hormone release, respectively.
• Five principle types of cells secrete seven major hormones in the anterior pituitary.

Anterior Pituitary Hormones

• GH stimulates general body growth and regulates metabolism aspects.
• Thyroid stimulating hormone (TSH) controls secretion and other thyroid gland activities.
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the secretion of estrogen and progesterone and the maturation of oocytes in the ovaries and the secretion of testosterone and sperm production in the testes.
• Prolactin (PRL) initiates milk production in the mammary glands.
• Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete glucocorticoids.
• Melanocyte-stimulating hormone (MSH) affects skin pigmentation.

GH: Growth Hormone

• It is the anterior pituitary's most abundant hormone.
• GH stimulates growth of most body cells particularly in bones and skeletal muscles.
• Body growth in response to GH secretion is evident in childhood and adolescence.
• GH secretion maintains the mass of bones and skeletal muscles thereafter.
• It also regulates metabolism in organs like the liver, intestines, and pancreas.
• Growth hormone-releasing hormone (GHRH) stimulates its release, while growth hormone release-inhibiting hormone (GHRIH), also known as somatostatin, suppresses it. Both originate in hypothalamus.
• GH secretion is greater at night during sleep and is also stimulated by hypoglycemia (low blood sugar), exercise, and anxiety.
• Secretion peaks in adolescence and then declines with age.

Thyroid Stimulating Hormone (TSH)

• Thyrotrophin releasing hormone (TRH) from the hypothalamus stimulates the release of this hormone.
• TSH stimulates growth and activity of the thyroid gland, which secretes the hormones thyroxine (T4) and tri-iodothyronine (T3).
• Release is lowest in the early evening and highest during the night.

Adrenocorticotrophic Hormone (ACTH, Corticotrophin)

• Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of ACTH by the anterior pituitary.
• ACTH increases the concentration of cholesterol and steroids within the adrenal cortex
• ACTH levels are highest at about 8 a.m. and fall to their lowest about midnight.

Prolactin

• Prolactin is secreted during pregnancy to prepare the breasts for lactation after childbirth.
• Prolactin releasing hormone (PRH) stimulates prolactin blood level; prolactin inhibiting hormone (PIH, dopamine) and increased prolactin blood level lower it.

Gonadotrophins

• Just before puberty, two gonadotrophins (sex hormones) are secreted in gradually increasing amounts by the anterior pituitary in response to luteinising hormone releasing hormone (LHRH), also known as gonadotrophin releasing hormone (GnRH).
• Rising levels of these hormones promote mature functioning of the reproductive organs.
• Males and females secrete follicle stimulating hormone (FSH) and luteinising hormone (LH).
  • FSH in females regulates the development of sex organs and development of immature ovarian follicles.
  • FSH secretes oestrogen & progesterone during menstrual cycle.
  • FSH in males initiates spermatogenesis.
  • In females LH stimulates ovulation
  • LH maintains the corpus luteum
  • LH stimulates progesterone secretion.
  • In males LH regulates testosterone regulation.

Thyroid Gland

• The thyroid gland sits up front of the neck at the level of the 5th, 6th and 7th cervical vertebrae together with 1st thoracic vertebrae.
• The thyroid id a vascular gland that weighs about 25g and is surrounded by a fibrous capsule.
• This butterfly shaped gland containing two loves on either side of the thyroid cartilage and upper cartilaginous rings of the trachea.
• The lobes are joined by a narrow isthmus, lying in front of the trachea.
• The lobes are roughly cone shaped, about 5 cm long and 3 cm wide.
• The arterial blood supply to the gland is through the superior and inferior thyroid arteries. -The superior thyroid artery is a branch of the external carotid artery -The inferior thyroid artery is a branch of the subclavian artery.
• The venous return travels by the thyroid veins into the internal jugular veins.
• The gland is composed of largely spherical follicles formed from cuboidal epithelium.
  • They secrete and store colloid, a thick sticky protein material.
  • Between the follicles are parafollicular cells also called C-cells which secrete the hormone calcitonin.

Thyroid Hormones

• Essential to iodine for thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3) formation.
• The thyroid selectively takes up iodine from the bloodstream through a process called iodine trapping.
• The thyroid stimulating hormone (TSH) from the anterior pituitary stimulates T3 and T4 release into the blood.
• Thyrotrophin releasing hormone (TRH) from the hypothalamus stimulates TSH secretion; secretion is also affected by exercise, stress, malnutrition, blood glucose levels, and sleep.
• T3 and T4 are essential for growth and development particularly in the skeletal system.
• Physiological effects of T3 and T4 on the heart, skeletal muscles, skin, digestive and reproductive systems are more evident when there is thyroid gland underactivity or overactivity
• The effects can be profound in childhood.

Calcitonin

• Secreted by parafollicular or C-cells in the thyroid gland. Calcitonin function is to lower raid blood calcium (Ca2+) levels.
• bone cells promoting their storage of calcium
• kidney tubules inhibiting the reabsorption of calcium. Release of calcitonin is stimulated by blood calcium levels.

The Parathyroid Glands

• These are four glands, each around 50g, containing connective tissues with spherical cells arranged in columns with blood-containing sinusoids.
• The main function is to secrete parathyroid hormones (PTH, parathormone) which increase the blood calcium levels.
• This happens through increasing the absorption from the small intestine and reabsorption from the renal tubules.

The Adrenal Glands

• The paired adrenal (suprarenal) glands lie superior to the kidneys enclosed within the renal fascia and area about 4 cm long and 3 cm thick.
• The adrenal glands are made up of the outer adrenal cortex and inner adrenal medulla.
• The adrenal cortex produces about 40 different hormones collectively known as corticosteroids.
• Death can happen a few days to a week with complete loss due to dehydration and electrolyte imbalances.
• The glands contain have two parts which have different functions.
• Adrenal cortex is essential to life; adrenal medulla is not.

Hormones of the Adrenal Gland

• Adrenal cortex are: cortisol, corticosterone, cortisone, aldosterone, androgens
• Adrenal medulla are: epinephrine, Nor- epinephrine

Adrenal Cortex

• Produces three groups of steroid hormones from cholesterol known as adrenocorticoids which are glucocorticoids, mineralocorticoids, and sex hormones.
• Hormone groups have different actions but are structurally similar so their actions may overlap.
• Cortisol (hydrocortisone) is the main glucocorticoid; small amounts of corticosterone and cortisone are also produced.
• Glucocorticoids are essential for life regulating metabolism and responses to stress also known as 'steroids'.
  • ACTH from the anterior pituitary and by stress stimulates cortisone.
• Mineralocorticoids regulate minerals and electrolytes in the balance
  • Aldosterone stimulates reabsorption of sodium (Na+) by the renal tubules and excretion of potassium (K+) in theurine.
  • Sodium reabsorption is also accompanied by retention of water which helps with regulating blood pressure and blood volume.
• Blood potassim regulates aldosterone by the cortes of the adrenal glands. Low blood potassium has stimulate the release of aldosterone.

Renin-angiotensin-aldosterone system

• When there is a renal blood flow reduction renin is secreted by kidney cells.
• Renin then stimulates angiotension.

Sex Hormones

• Adrenal cortex secretes androgens (male sex hormones) and estrogen while they aren insignificant for the amount of sex hormones released in the ovaries and testies during adulthood.

Adrenal Medulla

• Surrounded by cortex in the embryo and is part of the sympathetic nervous system.
• Releases adrenaline (epinephrine, 80%) and noradrenaline (norepinephrine, 20%). when stimulated.
• Noradrenaline is sympathetic division of the autonomic nervous system that the blood helps releases to the body from the simulation of the sympathetic nervous system. This prolongs and increases the stimulation of the simpatico nervous system.

The Pancreas

• Pancreas is classified as both of and exocrine cells for digestive system.
• The three types of cells in the pancreatic islets are:
  • (alpha) cells that secrete glucagon
• beta cells that secrete insulin
• delta cells which regulate somatostatin (GHRIH) with a blood glucose level is between 3.5 and 8 mmol/litre (63 to 144 mg/100 mL).

Insulin and Glucagon

• Insulin is low raised nutrient levels which includes amino acids and fatty acids.
• When nutrients are the storage and breakdown happen from:
  • acting on cell membranes and stimulating uptake and use of glucose by muscle and connective cells
  • increasing conversion of glucose to glycogen especially in the liver and skeletal muscles
  • accelerates uptake of amino acids and cells
  • promotes synthesis of fatty acids and fats
• Decreases which is a breakdown of glucose into glucagon.

Glucagon function

• Increases the liver stimulation for glycogenolysis.
• Decreases the low blood sugar and exercise and somatosin with insulin.

Somatostatin (GHRIH)

• This helps the the glucagon in hypothalamus and inhibiting LH with anterior pituitrary, and it helps to inhibit the secretion.

Pineal Gland

• Attached to the roof of the third ventricle and terminates in the hypothalamus.
• It is reddish brown in color about 10mm in longitude and in a capsule
• Tends to atrophy during puberty and in old age it calcified. The gland produces melatonin for regulation of sleep and effects on human activity.
• This is also know to regulate the body's bilogical clock.

The Ovaries and Testes

• The oval shape in the gonads.
• The ovaries are connected to progesterone and estrogen which helps with sexual charatceristics for regulation during reproduction.

Hormones

• Ovaries create relaxin and connective tissue for child birth.
• Testes help with characteristics along with inhibin-inhibiting of follicle production with stimulating hormones

The Thymus

• Proliferation of WBC and foreign substances with hormones promote and aging.

Local Hormones

• Histamine is synthesised in mass cells, and basaphils in blood and the inflammatory espically with the allergy reactions. Serotonin is platelets found in brain and intestinal where intestines cause contraction. Influences blood in theCNS

Prostaglandins

• The PG cells for are short term lipid substances
• Helps with the the wide and ranging inflammation
• Helps with potentially pain
• Controls regulating fever
• Regulates blood clotting