Endocrine Glands and Hormonal Signals

1.1 Endocrine glands and hormones produced by them

Endocrine glands release chemical signals called hormones into the blood stream and signals through nerve to allow human body to respond with respect to changes in their environment. The secretion of gland is triggered by either nervous pathway or a change in certain substances in the blood stream (Engelking, 2012). As soon as hormones reach their target cell they start modification of the cell to eradicate the parent stimulus that trigger the hormonal release. This is called negative feedback. There are nine endocrine glands present in human body. The list of them and hormones associated with them are as follows,

1. Pituitary: Situated at the base of brain. Secretes: Adrenocorticotrophic hormone (ACTH), growth hormone (GH), follicle-stimulating hormone (FSH), melanocyte-stimulating hormone (MSH), thyroid stimulating hormone (TSH), leutinising hormone (LH), prolactin, vasopressin and oxytocin (Fox, 2015).

2. Thyroid: Situated at the root of the throat. Secretes: Thyroxine, tri-iodothyroxine, calcitonin(Fox, 2015).

3. Parathyroid: Situated in the neck behind the thyroid gland. Secretes: Parathyroid hormone.

4. Adrenal: Situated at the adrenal cortex of kidneys. Secretes: adrenaline, glucocorticoids, mineralocorticoids, epinephrine, and norepinephrine(Fox, 2015).

5. Ovary (in female): Located at the either side of fallopian tube of female. Secretes: Oestrogen and progesterone (Strauss, et al, 2013).

6. Testis (in male): Located just behind the penis of male. Secrets: Testosterone (Strauss, et al, 2013).

7. Islets of Langerhans: Situated within pancreas. Secretes: Insulin and glucagon (Islam, 2010).

8. Skin: External layer of human body. Secrets: Vitamin C (Fox, 2015)

9. Hypothalamus: Located at the base of brain. Secrets: Thyrothophin-releasing factor (TRF), corticotrophin-releasing factor (CRF), growth-hormone releasing factor (GRF), follicle-stimulating hormone releasing factor (FHRF), leutinising hormone-releasing factor (LHRF), prolactin-inhibiting factor (PIF), growth hormone-inhibiting factor (GHIR) and melanocyte-inhibiting factor). (Rosol, et. al. 2015)

1.2 Function of major hormones secreted by the endocrine system:

As noted earlier there are several kind of hormones are produced by the endocrinal glands. Among them some are very important for human physiology and they are discussed here briefly (Llyod, 2012).

Adrenocorticotrophin (ACTH): It is a peptide based hormone generated at Pars distalis region of pituitary gland. It is targeted to the cells of adrenal cortex to stimulate secretion of adrenal cortical steroids (Fox, 2015). Rapid action of ACTH stimulates the delivery of cholesterol to mitochondria where P450scc enzyme is located. It also stimulates lipoprotein uptake by cortical cells. Hyposecretion of ACTH causes secondary adrenal insufficiency whereas its lower secretion leads to diseases like Addison’s disease.

Growth Hormone: Also known as somatotropic hormone is a proteinaseous hormone formed from the acidophilic cell of Pars distalis of pituitary gland. It controls growth of bone and muscle by acting on the somatic cell. It also regulates nitrogen metabolism as well as carbohydrate and fat metabolism, elevates store of glycogen at skeletal and cardiac muscle. Hyper secretion of this causes gigantism and its deficiency causes the failure of growth for children (Raiti, et al, 2013).

Thyrotrophin: Also known as thyroid stimulating hormone is a glycoprotein generated from Pars distalis of pituitary. It regulates the rate of iodine consumption by thyroid tissue and it influences the synthesis of thyroid hormone. Its higher secretion leads to Goiter.

Leuteinsing Hormone (LH): Also known as interstitial cell stimulating hormone (ICSH) is generated from Pars distalis acts on corpus luteum or interstitial cells of testes. This is a gonadotrophic hormone. It’s stimulate the production of androgen in testes and controls formation of corpus lutea, progesterone secretion (Strauss, et al, 2013).

Follicle stimulating hormone (FSH): It is generated from Pars distalis of pituitary and acts on Graafian follicles acts on testes and ovary to control the growth of follicle. This is also a gonadotrophin (Strauss, et al, 2013).

Prolactin: It is generated from the Pars distalis and it is also protenaseous in nature. It controls proliferation of mammary gland and also initiates secretion of milk (Strauss, et al, 2013)..

Vasopressin: This polypeptide hormone is generated from neurohypophysis area of the pituitary. It increases the blood pressure by its action on arterioles. It promotes the reabsorption of water by kidney(Fox, 2015).

Oxytocin: This polypeptide hormone is also generated at neurohypophysis and work on ejection of milk at mammary gland. It also contract the uterine muscle(Fox, 2015).

Thyroxin: It is a major hormone secreted by thyroid gland to the bloodstream. It plays key role in development of brain and function of heart muscle, helps in digestion and bone maintenance (De Groot, et al, 2013).

Mineralocorticoids: Most important mineralocorticoid is aldosterone. It regulates the salt and water level of body to maintain the blood pressure. Lowering of aldosterone level causes loss of excessive salt and water through kidney causing dehydration and hypotension (Katsu, et al, 2016).

Glucocorticoids: These are basically cortisol and involved in the reaction to illness and regulation of metabolic behaviour of human body. It encourages the production of glucose by pushing out fat and muscles to produce glucose. It also has anti-inflammatory behaviour (Katsu, et al, 2016)..

Adrenal androgens: These are the sex hormones of male and they are of two types, dehydroepiandrosteron and testosterone. They have small effect but plays vital role in initial development of sex organs of a male during childhood. They also help to produce body hair in female during their puberty (Fox, 2015) .

Oestrogen: It is secreted by ovary and primarily work it the development of sexual characteristics that are secondary, for example breast development, menstrual cycle regulations etc. The thickness of vaginal wall is also maintained by the oestrogen level (Strauss, et al, 2013).

Progesterone: It is produced by the ovary in the corpus luteum. It plays major role during the menstrual cycle and also the maintenance of pregnancy. Thickening of uterus wall lining is maintained by this hormone for making the bed of fertilized egg (Strauss, et al, 2013).

Testosterone: It helps in the sexual characteristic development of male. Prostrate and testes development are caused by this hormone. Bone mass is maintained by it and drive for sex in male can be regulated by this hormone (Strauss, et al, 2013).

Glucagon, Insulin and somatostatin: These three hormones are produced by islet of Langerhans. Role of insulin is to promote the energy production from the carbohydrate source (glucose) stored in our body. It maintains the sugar level in blood to maintain high (hyperglycaemia) or low (hypoglycaemia) condition. Glucagon maintain the sugar level in blood to prevent its rapid lowering down. The stored glycogen converted to glucose by this hormone. It regulates the release of other hormones by acting on different glands and organs. Gastric acid secretion is regulated by this hormone in stomach. (Devlin, 2011)

3. Explain the bloodstream’s role in the distribution of hormones to target organs.

Hormones after secretion from the definite gland or organs they will transmit to the effector organ by the blood stream. Few of the hormones dissolved in the blood stream and then circulated to different part via capillary, but most of them are carried by bounding to plasma protein in the blood flow. Say for example hydrophobic steroid hormones are transported through plasma protein bound manner (Emmens, 2013).

3.1. Describe, in detail, the two molecular processes in steroid and peptide action

The action and molecular process of steroid hormone starts with the specific receptor present in the cell termed as steroid hormone receptors. These receptors regulates the transduction of signal through the action of genomes or in absence of the genome depending upon the context. The complex forming interaction between steroid hormones and their specific receptors can activate a diverse kind of signals to activate the receptor and produce the hormone receptor complex. It will interact with DNA sequence which will express the protein necessary for molecular process. (Idler, 2012) Peptide hormones molecular process starts with their synthesis at rough endoplasmic reticulum. Once released they interact with the plasma membrane of the cell which is specific for it. Upon binding with target it regulate physiological reactions like growth, metabolism, reproduction and stress by intracellular signalling. (Pincus, 2013)

3.2 Outline the need for specificity and relate this to the action of the above

The main feature of steroid hormones and peptide hormone function is their specificity. The binding mode of both kind of hormones leads to different kinds of triggers which regulates specific cellular action. Interestingly in both cases this hormone receptor complexes are key intermediates which has specific method of orchestrating physiological processes (Leavitt, 2013).

3.3. Explain the need for different mechanisms for fat-soluble and non-fat soluble hormones and relate to named examples

The fat soluble hormones generally permeate the plasma membrane of the cell and worked by binding with intercellular receptor and after entry binds to the receptor protein. On the other hand non-fat soluble hormones bind to the receptor protein on cell surface plasma membrane and activated accordingly (Eick, et al, 2011) . Epinephrine is a water soluble hormone released by adrenal cortex. This hormone when present in the concentration at 10-8 to 10-10 M concentration in blood stream, get binds to the epinephrine binding cites present in the outer surface of liver cells. This binding makes a change in the conformation of the membrane protein and leads to the activation of an enzyme called adenylate cyclase inside the cell. This enzyme in turns activates the release of glucose outside the cell. (Sharara-Chami, et. al. 2012) Oestrogen is a fat soluble sex hormone act to regulate the uterine actions and its growth. It is permeable to the cell membrane. Oestrogen receptor proteins are present in the cytosol of uterine cells. They bind with oestrogen to form oestrogen-receptor complex which in turn goes to the nucleus to bind with the chromatin. (Thomas, et. al. 2013)

Task 2

2.1 Describe the principle of homeostasis

Homeostasis is a process which mitigates the need of body whenever an internal and external pathophysical process drives it out of equilibrium. The word is derived from Greek homoios (similar) and stasis (standing). It was first demonstrated by Claude Bernard, a French physiologist on 1865. An organism needs to work cooperatively to connect various biological processes. Human bodily organisation is composed of several varieties of cells with myriads of different morphological and physiological properties. When they get connected with each other then they need to work together to achieve well sustained process. This process always need to stay in equilibrium to make proper balance in organism. There are a number of homeostatic processes and each work to regulate some variables of internal environment (Torday, 2015). In human body different imbalance of internal, physical constraints leads to variation in bodily behaviour. Here homeostasis play very important role to minimize these changes to sustain its optimum condition back. Bodily conditions such as pH, temperature, level of potassium and calcium, blood sugar level have to be within optimize level. Any kind of perturbation in their upper or lower limits needs to be maintained by homeostatic processes(Torday, 2015). Broadly homeostatic processes are composed of three prime components namely 1. A receptor, 2. Controller and 3. Effector. The receptor accepts and assess the changes in environment (internal/external). It is basically sensory nervous terminal accepts stimuli (Torday, 2015). These acceptors are:

Photoreceptors: React to light stimuli (Torday, 2015).

Olfactory receptors: Present in olfactory epithelium of nose and it reacts to odours (Torday, 2015).

Gustation receptors: Receptor for taste (Torday, 2015).

Thermo receptor: This receptor is a bunch of sensory cells that are sensitive to temperature gradient (Torday, 2015).

Mechano receptor: Present in skin and reacts to mechanical stimuli (Torday, 2015).

Interoceptors: They respond to internal stimuli(Torday, 2015).

Nociceptors: This receptors are responsible for detecting pain(Torday, 2015).

Peripheral chemoreceptors: These are responsive to changes in chemical compositions of blood. As for example partial pressure of oxygen in blood. (Davis, 2016)

Homeostatic mechanisms acting in response to internal or external perturbations work in a mechanism of looping. This looping can be positive or negative depending upon the condition. Positive mechanism work in the forward direction of stimuli in order to enhance it. On the other hand negative feedback mechanism work to lower down the stimuli. The example of positive feedbacks are blood clotting, labour contractions and generation of action potential. In negative feedback the responsiveness try to minimize metabolic process. Thermo regulation, change in blood pressure, blood sugar, calcium and potassium homeostasis are examples of negative feedback (Torday, 2015).

2.2 Feedback mechanism

Example of positive feedback mechanism: Role of oxytocin in labour

During labour pain the onset of labour acts as a stimuli and with it the pituitary gland releases oxytocin. It will be used as the factor of contraction of muscle. Increased concentration of oxytocin help to open up or dilate the cervix. During the labour and birth the baby tend to give pressure against cervix and the floor of pelvis. This processes stimulates again the secretion of oxytocin. Now this higher concentration of oxytocin again helps to contract the uterus to promote child birth. Basically this is a cyclic process where the cycle is like pushing by baby against cervix leading to transmission of nerve impulse from cervix to brain. Then the pituitary gland will be stimulated by brain and secretes oxytocin. This secreted oxytocin, carried out by blood stream go to uterus. Again oxytocin stimulates uterine contraction and pushes baby towards cervix. This pushing again sends nerve impulse to complete the cycle. This oxytocin level should be high enough during pregnancy. Therefore the mother should rest properly and should remain calm during labour. (Leng, et. al. 2017)

Example of negative feedback mechanism: Control of glucose level in the body

Human body constantly maintain a particular concentration of glucose in blood. A variation of this level leads to various ailments. This is why an almost constant level of glucose is found even after fasting (i.e. without consumption of glucose). The work of insulin (a hormone secreted by islets of Langerhans cells in pancreas) is to transport glucose to cells. Cells convert excess glucose into glycogen and this prevents the excess the glucose to harm the organisms. This ultimately bring down the glucose level of blood and insulin is secreted to prevent high blood sugar level. Another hormone glucagon do the reverse job by converting glycogen to glucose and stimulates the production of glucose (gluconeogenesis) to increase blood sugar level (Kuo, et al, 2015). Approximately 75% of cells in islets of Langerhans are beta cells whereas another 20% of cells are alpha cells. Beta cells release insulin whereas alpha cells release glucagon. At high level of blood sugar (100 mg/dL) glucose enter into beta cells and trigger the release of insulin by acting on ATP and ion channels. This released insulin then work to lower the glucose level. On the other hand lowering of glucose level in plasma causes the hypothalamus to stimulate alpha cells to release glucagon. This glucagon then triggers the gluconeogenesis to produce glucose from glycogen. This whole process is a homeostatic process with negative feedback mode, where enhanced level of homeostat induces lowering of its level. (Röder, 2016)

Flowchart for Positive feedback (oxytocin during labour)

Flowchart of negative feedback: (Control of glucose level in body)

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References

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