Physiology Of Endocrine System
Endocrine System
- Classes of hormones
- Plasma hormone concentration
- Feedback control of hormone
secretion
- Hormone receptors and
intracellular signaling
1. The endocrine system
Hormone is a chemical that is produced by the body and has a specific regulatory effect on a target cell or organ.
Certain diseases commonly encountered in general medical practice are caused by a deficiency or an excess of specific hormones.
Anatomical loci of the principal endocrine glands and tissues of the body
The hormones are produced by the major endocrine organs (e.g., pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, testes and ovaries).
Endocrine cells may be dispersed throughout the body (e.g., in the hypothalamus, placenta, heart, kidney,
gut mucosa).
1. Hypothalamus
Hormone(s) Produced | Overview: Major Function(s) |
1. Releasing hormones | Stimulate releasing of anterior pituitary gland hormones. For example, Growth hormone-releasing hormone (GHRH) stimulates growth hormone (GH) release |
2.Inhibitory | Inhibit releasing of anterior pituitary gland hormones. For example, Prolactin-inhibiting hormone Inhibits prolactin release |
2. Anterior pituitary gland
Hormone(s) Produced | Overview: Major Function(s) |
Thyrotrophin-stimulating hormone (TSH) | Stimulates thyroid gland (stimulates synthesis and secretion of thyroid hormones) |
Adrenocort-icotropic hormone (ACTH) | Stimulates synthesis and secretion of adrenal cortex hormones |
Follicle-stimulating hormone (FSH) | Stimulates gonads:
|
Luteinizing hormone (LH) | Stimulates gonads:
|
Growth hormone (GH) | Chronic growth-promoting effect via insulin-like growth factor 1 (IGF-1) |
Prolactin | • Required in lactation for mammary growth, initiation of milk secretion, and maintenance of milk production |
3. Posterior pituitary gland
Hormone(s) Produced | Overview: Major Function(s) |
Antidiuretic hormone (ADH) | • Vasoconstricts arterioles→ increases blood pressure |
Oxytocin | • Stimulates uterine contractions during labor
|
4. Pineal gland
Hormone(s) Produced | Overview: Major Function(s) |
Serotonin | Modulates sleep patterns (it is produced in the daytime). Hormone of well-being and happines |
Melatonin | Modulates sleep patterns. patterns (sleep-wake timing). Hormone of depression |
5. Thyroid gland
Hormone(s) Produced | Overview: Major Function(s) |
Triiodothyronine (T3) & thyroxine (T4) | • Required for normal growth and development (especially development of the central
nervous system) |
Calcitonin | Reduce serum [Ca2+] (decreases Ca2+ resorption from bone by inhibition of osteoclast activity, , decreases renal Ca2+ reabsorption, decreases intestinal Ca2+ absorption) |
6. Parathyroid gland
Hormone(s) Produced | Overview: Major Function(s) |
Parathyroid hormone (PTH) | • Increases serum [Ca2+] (increases Ca2+ resorption from bone by stimulation of osteoclast activity, , increases renal Ca2+ reabsorption, increases intestinal Ca2+ absorption) |
7. Adrenal gland
The adrenal cortex and the adrenal medulla are distinct structures visible in a cross-section of the adrenal gland.
8. Pancreas
Hormone(s) Produced | Overview: Major Function(s) |
Insulin | The main anabolic hormone. Decreases blood glucose (promotes the absorption of glucose from the blood into body cells, storage of glucose as glycogen in liver, muscle and adipose tissue; increase of DNA replication and protein synthesis) |
Glucagon (α cell) | Increases blood glucose |
9. Testes
Hormone(s) Produced | Overview: Major Function(s) |
Testosterone | •Plays a key role in the development of male reproductive organs in fetus
|
10. Ovaries
Hormone(s) Produced | Overview: Major Function(s) |
Estrogens | •Plays a key role in the development of female reproductive organs in fetus
|
Progesterone | •Important in maintenance of pregnancy |
Other Endocrine Organs
1. Peptides are the largest group of hormones (growth hormone , prolactin, cortisol , insulin et al. ). Peptide hormones are generally water soluble and do not require carrier molecules in the blood.
2. Amines are a small group of hormones that includes the catecholamines (dopamine, epinephrine, and norepinephrine) and the thyroid hormones. Catecholamines are water-soluble hormones that do not require carrier proteins in the plasma. Thyroid hormones are poorly soluble in water and do require carrier proteins in the blood.
3. Steroid hormones are synthesized from cholesterol and include cortisol, aldosterone, testosterone, estrogen, and progesterone. Steroids generally require carrier proteins in the blood due to their low water solubility.
The magnitude of a response to a hormone depends on how many
receptors are occupied at the target cell, which in turn depends
on the free hormone concentration in the extracellular fluid. The
plasma free hormone concentration is affected by:
1. The rate of hormone secretion.
2. The rate of hormone elimination.
3. The extent of hormone binding to plasma proteins.
For some hormones, the plasma hormone concentration is strongly influenced by a rhythmic pattern of secretion. For example, the steroid hormone cortisol has a distinctive circadian (day/night) pattern of secretion, with the highest hormone concentration in the early morning hours and less concentration during late afternoon and evening.
Plasma hormone concentration is strongly influenced by the rate of hormone elimination. The half-life of a hormone is the time it takes to reduce the plasma hormone concentration by one half.
The half-life of a hormone
Hormones can be removed from plasma by the following processes:
Simple negative feedback occurs when a hormone, or a response to a hormone, directly inhibits further secretion of that hormone. For example, insulin secretion by the β-cells in the pancreas causes a decrease in the blood glucose concentration, which directly inhibits further insulin release.
complex negative feedback
In some cases, hormone secretion is under hierarchical control (complex negative feedback); for example, hormone secretion from a primary target gland that is controlled by the anterior pituitary hormones, which in turn are controlled by hypothalamic factors.
For example, the hierarchical negative feedback control of thyroid hormone secretion.
In a few cases, the rate of hormone secretion may be controlled by positive feedback, in which the effects of the hormone result in further hormone secretion. For example, the surge in the plasma luteinizing hormone concentration, which occurs just prior to ovulation, is due to positive feedback stimulation by estrogen.
Hormone receptors and intracellular signaling
A response to a particular hormone is seen only in cells with specific receptors for that hormone. Receptors are proteins that may be in the surface membrane (e.g., peptide hormones and catecholamines), in the cell cytoplasm (e.g., steroid hormones), or in the nucleus (e.g., thyroid hormones).
In most cases, activation of a receptor by hormone binding changes the target cell activity either through the generation of intracellular second messengers or via changes in gene transcription and translation.
SECOND MESSENGER SYSTEMS FOR PEPTIDES AND CATECHOLAMINES
The hormonereceptor complex associates with intracellular G proteins. The different G proteins activate different second messenger pathways. Second messengers are intracellular signaling molecules released by the cell to trigger physiological changes. The cell releases second messenger molecules in response to exposure to extracellular signaling molecules—the first messengers. First messengers are extracellular factors, often hormones or neurotransmitters.
STEROID AND THYROID HORMONE SIGNALING
Most steroid hormone receptors are present in the cytoplasm and are accessed when steroids diffuse through lipid membranes to enter target cells. Once a steroid receptor binds to its hormone, it enters the nucleus to interact with DNA. Binding to DNA is the starting point for gene transcription. Thyroid hormone receptors are widely expressed among the body tissues and function in the same manner as steroid receptors.
