I know it's been about a week since my last post but I thought I'd wait a bit so I could make this post more interesting!!
Good news by-the-way! We've finally moved onto the more exciting stuff - Life Sciences!! xD Yey!! haha!
In this post, as you can probably guess from the title ^^, I will be discussing the all-important and absolutely astonishing endocrine system! (Quick note: I was originally going to be talking about Endocrine, Renal *AND* Respiratory in this one post but... I started writing and realised that there was just *SO* much to write about on each system - why does the human body have to be so complex?!?!? Ahaa - So I've decided to split this post into three A&P parts. So, A&P part two and three will follow soon! Enjoy!!)
The Endocrine System
This vital system in our body consists of widely separated endocrine glands which secrete hormones. Hormones, as you may recall from GCSE biology, are 'messenger' substances produced in one part of the body but regulates the activity of cells in other parts of the body. Most hormones enter interstitial fluid and then the bloodstream. The circulating blood then delivers hormones to cells throughout the body. Hormones exert their effects by binding to receptors on or in their 'target' cells and their levels are greatly influenced by factors such as stress, infection and changes in balance of fluid and minerals in the blood. If, however, a hormone is present in excess, the number of target-cell receptors may decrease. This is called 'down-regulation'. In contrast, when a hormone is deficient, the number of receptors may increase. This is known as 'up-regulation'.
The Endocrine system is vital for regulating mood, growth and development, tissue function and metabolism, sexual function and reproductive processes. Responses of the Endocrine system are also slower than the responses of the nervous system. Although it may take seconds for a hormone to act, most take several minutes or more to cause a response. And while the nervous system acts on specific muscles and glands, the influence of the Endocrine system is much broader as it helps regulate virtually all types of body cells.
Endocrine Glands
Endocrine glands secrete hormones into the interstitial fluid surrounding the secretory cells. The hormones then diffue into the blood capillaries and the blood carries them to target cells throughout the body. Because they depend so much on the cardiovascular system to distribute products, the endocrine glands are some of the most vascular tissues in the body.
The Endocrine Glands & the hormones they produce...
Pituitary Gland
Human Growth Hormone (HGH)
-Chemical Nature: Protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Stimulates protein synthesis and release of energy from fats.
Thyroid Stimulating Hormone (TSH)
-Chemical Nature: glyco-protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Stimulates production and release of thyroid hormones.
Adrenocorticotrophic hormone (ACTH)
-Chemical Nature: Peptide.
-Mode of Action: Cyclic AMP.
-Important Roles: Stimulates production and release of adrenal cortex hormones.
Follicle stimulating hormone (FSH)
-Chemical Nature: glyco-protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Maturation of follicles in females and production of sperm in males.
Luteinizing hormone (LH)
-Chemical Nature: glyco-protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Triggers ovulation and development of corpus luteum.
Prolactin (PR)
-Chemical Nature: Protein.
-Mode of Action: --
-Important Roles: Stimulates milk production by mammary glands.
Melanocyte stimulating hormone (MSH)
-Chemical Nature: Peptide.
-Mode of Action: Cyclic AMP.
-Important Roles: Increases skin pigmentation.
Anti-diuretic hormone (ADH)
-Chemical Nature: Peptide.
-Mode of Action: Cyclic AMP.
-Important Roles: Stimulates reabsorption of water by kidney tubules.
Oxytocin
-Chemical Nature: Peptide.
-Mode of Action: --
-Important Roles: Stimulates contraction of the uterus.
Pineal Gland
Melatonin
-Chemical Nature: Amine.
-Mode of Action: --
-Important Roles: Possible inhibitory action on ovaries.
Thyroid Gland
Thyroxin
-Chemical Nature: Amino Acid.
-Mode of Action: Cyclic AMP.
-Important Roles: Increases metabolic rate, stimulates growth in infants.
Thyrocalcitonin
-Chemical Nature: Peptide.
-Mode of Action: --
-Important Roles: Promotes calcium absorption by bones.
Parathyroid Glands
Parathyroid hormone (PTH)
-Chemical Nature: Protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Promotes calcium absorption from intenstine, stimulates calcium release from bones.
Thymus Gland
Thymosin
-Chemical Nature: Peptide.
-Mode of Action: --
-Important Roles: Possible influence on B-lymphocytes.
Pancreas (Islets of Langerhans)
Insulin
-Chemical Nature: Protein.
-Mode of Action: Cyclic AMP.
-Important Roles: Stimulates absorption of glucose into liver and muscle cells, formation of glycogen.
Glucagon
-Chemical Nature: Peptide.
-Mode of Action: Cyclic AMP.
-Important Roles: Increases blood glucose level, breakdown of glycogen.
Adrenal Glands
The Cortex:
Mineralocorticoids, aldosterone
-Chemical Nature: Steroid.
-Mode of Action: Gene activation.
-Important Roles: Stimulates reabsorption of sodium ions by kidney tubules, reduces reabsorption of potassium ions.
Glycocorticoids: Hydrocortisone, Corticosterone, Cortisone
-Chemical Nature: Steroid.
-Mode of Action: Gene activation.
-Important Roles: Reduce effects of stress responses.
The Medulla:
Adrenalin (80%), Noradrenalin (20%)
-Chemical Nature: Amine.
-Mode of Action: Gene activation.
-Important Roles: Increased heart and breathing rates, other 'fight or flight' responses.
Ovary
Oestrogen
-Chemical Nature: Steroid.
-Mode of Action: Gene activation.
-Important Roles: Development of female sexual characteristics, repair of uterus lining.
Progesterone
-Chemical Nature: Steroid.
-Mode of Action: Gene activation.
-Important Roles: Development o uterus ready for implantation.
Testis
Testosterone
-Chemical Nature: Steroid.
-Mode of Action: Gene activation.
-Important Roles: Development of male sexual characteristics.
The Master Gland
Attached to the hypothalamus in the brain, hangs the pituitary gland. This gland is nicknamed 'the master gland' because it stimulates all other hormone-producing glands to produce their own hormones. And while it is vital in the endocrine system, amazingly it is only the size of a pea!
It is comprised on two parts - the anterior and posterior lobe (see first picture in post). The anterior lobe secretes six hormones (which are named above) and is influenced by the hormones from the hypothalamus. The posterior lobe stores hormones that are from the hypothalamus, they are released when needed. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary.
Feedback Mechanisms
The endocrine system uses cycles and negative feedback to regulate physiological functions. Negative feedback regulates the secretion of almost every hormone. Cycles of secretion maintain physiological and homeostatic control. These cycles can range from hours to months in duration.
This 'feedback' mechanism involves the hypothalamus, the pituitary gland and the target gland to control hormone production. A feedback system promotes to release of another hormone (positive feedback) or can inhibit its release (negative release). This mechanism helps to maintain the body's balanced functioning.
Example of how it works...
1. Responding to levels of the Thyroid hormone, the hypothalamus make TRH (Thyrothropin-releasing hormone). This stimulates the anterior pituitary gland to release TSH (Thyroid-stimulating hormone). The thyroid gland is then triggered to produce hormones.
2. If the Thyroid hormone levels are too high, negative feedback alters the hypothalamus so that it produces less TRH. A lower level of TRH results in a reduced level of TSH. So the Thyroid responds by producing less hormone.
3. If the Thyroid hormone levels fall too low, the feedback mechanism is weakened. In response, the hypothalamus makes more TRH; TSH rises so that the levels of Thyroid hormone also rise.
Chemical classes of hormones
OK, so above you can see that I have (in the description of glands and the hormones they produce) mentioned whether hormones were steroids, amines, peptides or proteins. Well, now I'm going to explain what all this means. So, here goes...
There are two main types of hormones
* Lipid (fatty acids) soluble hormones: i.e. steroid hormones which are derived from cholesterol.
* Water soluble hormones:
Amine hormones: synthesised by removing the CO2 molecule and modifying amino acids. Example: Tyrosine.
Peptides and proteins: Amino acid polymers (large molecules with high melting and boiling points). Smaller peptide hormones consist of 3-49 amino acids while the larger consist of 50-200. Examples of peptides: oxytocin/ADH. Examples of proteins: HGH and insulin.
Adding to this, hormones can either be...
* Circulating hormones: Pass from secretory cells that make them, then into interstitial fluid, then into the bloodstream. Circulating hormones may linger in the bloodstream but are finally excreted by the kidneys.
* Local hormones: Act locally on neighbouring cells (or on the same cell that secreted them) without first entering the bloodstream. In comparison to circulating hormones, local hormones are inactivated quickly. Hormones acting on neighbouring cells are called 'paracrines'. Those acting on the same cell that secreted them are called 'autocrines'.
Hormone Transportation
*Water Soluble: Molecules circulate in blood plasma and are not attached to other molecules.
*Lipid soluble: Bind to transport proteins.
These transport proteins (which are made by cells in the liver) have three functions...
1. Make lipid soluble hormones temporarily water soluble which increases their solubility in the blood.
2. Delay passage of small hormone molecules through the filtering system in the kidneys which slows the rate of hormone loss in the urine.
3. provide ready reserve of hormone already present in the bloodstream.
Actually about 0.1-10% of molecules of lipid soluble hormones don't bind to transport proteins. This is called 'free fraction'. It is the free fraction that diffuses out of capillaries, binds to receptors and triggers responses.
Clinical connection: Peptide/protein hormones such as Insulin need to be taken by injection. Taken orally, the digestive enzymes destroy them by breaking their peptide bonds. Steroid/thyroid hormones both are effective when taken orally as they are not split apart during digestion and easily cross the intestial lining because they are lipid soluble.
Mechanism of hormone action
The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Receptors on target cell membranes bind only to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone changes the shape of the receptor causing the response to the hormone. There are two mechanisms of hormone action on all target cells.
Water Soluble
Water soluble hormones do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.
Lipid soluble
The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins.
Phew! I know this was a long one, guys! Probably the longest one yet! And I have to tell ya - we're not even finished yet!! There's so much that I haven't said in this post!! Plus we still have Renal and Respiratory!! O_o IKR! LOL!
Anyway, I hope you are all well and I hope you enjoyed this post!
I find all this fascinating! I love learning about how the human body works. It's truely amazing that it all goes on without us consciously knowing about it!!
Anyway, ttyl!!
Emily
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