Endocrine system (wk4) Flashcards
Describe the overview of the endocrine system:
-In the body there are 2 types of glands;
1. Exocrine glands -> Secrete their products into duct e.g. sweat or the intestines
2. Endocrine glands -> Ductless and release hormones into the blood
-The endocrine system is one of the body’s 2 major communication systems. It consists of glands and organs that secrete hormones and a single gland may secrete multiple hormones.
-Hormones are chemical messengers carried by the blood to target cells
What are hormones and their functions?
-Released by glands to (always) elicit a response -> enhance or inhibit cellular reactions
-Present a very low concentrations (a little bit can do a lot) -> much lower than other similar molecules
-A response to a small amount is possible due to the way the cell ‘behaves’ (a targeted response allows the process to constantly occur)-> This means that although a given hormone travels throughout the body in the blood, it affects only specific target cells
-Functions;
1. Help to regulate-> chemical composition (tightly controlled to be allowed to regulate) and volume of internal environment, metabolism and energy balance, contraction of smooth and cardiac muscle fibres, glandular secretions (act on other endocrine glands to elicit responses), some immune system activities.
2. Control growth and development
3. Regulate operation of reproductive system
4. Help establish circadian rhythm (Around 24 hours? – regulate sleep-wake cycle and body temperature)
How do hormones travel and interact with target cells?
-Endocrine and nervous system
-Hormones operate in synchrony with the nervous system (sends electrical signal which is simultaneous and the endocrine system uses these messages to initiate a response from this)
1. Endocrine = chemical messengers -> Act relatively more slowly and often have longer lasting effects
> Mediator molecules – Hormones delivered to tissues throughout body by blood
> Site of mediator action – Far from site of release, usually binds to receptors on or in target cells
> Types of target cells – Cells throughout the body
2. Nervous system = electrical conduit system -> Instantaneous short-lived
-Endocrine target cells are found throughout the body. The time to onset of action is seconds/hours/days and is generally longer (seconds to days)
> Mediator molecules – NTs released locally in response to nerve impulses
> Site of mediator action – Close to site of release, at synapse; binds to receptors in the postsynaptic membrane.
> Types of target cells – Muscle (smooth, cardiac and skeletal), cells, gland cells and other neurones
Describe the 3 main structural classes of hormones:
- Amines -> Derivatives of the amine acid tyrosine and secreted by the adrenal medulla and the hypothalamus. E.g. Thyroid hormones, Dopamine, Catecholamines (epinephrine and norepinephrine)
- Peptides and proteins -> The majority of hormones are polypeptides, and many peptide hormones are synthesised as large, inactive molecules that are cleaved into active fragments. E.g. Insulin
- Steroids -> Produced from cholesterol (liquid soluble) by the adrenal cortex and the gonads. E.G. Aldosterone, Cortisol, Androgens (testosterone), Estrogen
Describe hormone transport, metabolism and excretion:
- Peptide and all catecholamine hormones are water soluble and therefore circulate and dissolve into plasma (this allows transport to be much easier). Some peptide hormones bind to plasma proteins (slower to act as they are bound to plasma proteins).
- Steroid and thyroid hormones circulate to bind to plasma proteins
- The liver and kidneys are the major organs that remove hormones from the plasma by metabolizing them or excreting them (the liver and kidney is very crucial in this process through metabolism). Liver -> major organ responsible for the metabolism. Kidneys -> filter the blood, remove waste products
- Peptide hormones and catecholamines are rapidly removed from the blood
- Steroid and thyroid hormones are removed more slowly because they circulate and bound to plasma proteins
- After their excretion, some hormones are metabolised to more active molecules in their target cells or organs
Describe the mechanisms of hormone action:
- It is the presence of specific receptors for those hormones on or in the target cells necessary for response (public transport -> blood, specific destinations -> target tissues)
- Hormone receptors -> For lipid-soluble steroid and thyroid hormones, the majority of receptors are inside target cells. This affects cell function by altering the gene expression)
- For water-soluble peptide hormones and catecholamines receptors are on the plasma membrane (cell wall). Both hormones exert both rapid (nongenomic) and slower (gene transcription_ actions on the same target cell
Describe the mechanisms of hormone action
-Responsiveness and hormonal interactions:
- The responsiveness of a target cell to a hormone depends on;
1. The hormone’s concentration in the blood
2. The abundance of the target cell’s hormone receptors
3. Influences exerted by other hormones
>Hormonal interactions can have 3 types of effects;
1. Permissive: Action of one hormone enhances the responsiveness (how big the response is) or activity of another hormone e.g. epinephrine and thyroid hormones (T3 and T4) stimulation of lipolysis
2. Synergistic: The effect of 2 hormones acting together is greater or more extensive than one hormone acting on its own e.g. follicle-stimulating hormone and estrogen (they release and work together)
3. Antagonistic: One hormone opposes the actions of another hormone e.g. insulin and glycogen
-Negative feedback systems regulate the secretion of many hormones
What is hormone secretion controlled by?
Hormone secretion controlled by;
1. Plasma concentration of an ion or nutrient that the hormone regulates. E.g. insulin secretion.
2. Neuron input to the endocrine cells. The autonomic nervous system controls hormone secretion via the adrenal medulla and other endocrine glands. Neurones in the hypothalamus also secrete hormones. Neural input from the autonomic nervous system controls the secretion of many hormones.
3. Other hormones. Often the secretion of a particular hormone is directly controlled by the blood concentration of another hormone. A hormone that stimulates the secretion of another hormone is often refereed to as a tropic hormone. E.g. Thyroid-stimulating hormone (TSH) or Follicle-stimulating hormone (FSH). It is released into the anterior pituitary which targets the secretion of hormones from the thyroid gland (TSH).
What are endocrine disorders and the pharmacological effects of hormones?
-Despite varied functional consequences, all endocrine diseases can be categorised in 1 of 4 ways;
1. Too little hormones -> Hyposecretion e.g. type 1 diabetes
2. Too much hormone -> Hypersecretion e.g. gigantism such as too much growth hormone which can lead to very tall individuals
3. Decreases responsiveness of the target cells to hormones -> hypo responsiveness e.g. types 2 diabetes (the pancreas is releasing hormones but the tissue is unable to uptake the hormone and then the response is not appropriate)
4. Increases responsiveness of that target cells to hormones -> hyperresponsiveness e.g. elevated HR due to increased circulating levels of thyroid hormone
-Pharmacological administration of hormones for medical purposes can result in supraphysiological concentrations and effects not typically observed with at physiological concentrations
-The risk of side effects depends on; type of steroid, dose, length of treatment and the age of the patient
Describe the posterior and anterior pituitary gland:
-Posterior pituitary gland -> The posterior pituitary gland is a neural extension of the hypothalamus. Hormones ae synthesized in the hypothalamus, axons pass down the infundibulum, terminate in the posterior pituitary and release hormones. E.g. oxytocin and vasopressin
-Anterior pituitary gland -> Secretes growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, and 2 gonadotropic hormones -> follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
Describe the relationship between the hypothalamus and pituitary gland:
-Anterior pituitary gland and the hypothalamus -> Secretion of the anterior pituitary gland hormones is controlled mainly by hypophysiotropic hormones (hormone that is responsible for releasing another hormone) from the hypothalamus via the portal vessels connecting the hypothalamus and anterior pituitary gland
-The typical sequence is where a hypophysiotropic hormone (hormone 1 from the hypothalamus) controls the secretion of an anterior pituitary gland hormone (hormone 2), which in turn controls the secretion of a hormone by a 3rd endocrine gland (hormone 3) – which can release a response in the tissue or cell
Describe the control systems and key hormones from the anterior pituitary
-Thyroid gland
-Thyroid hormones have diverse and widespread effects throughout the body, e.g. protein synthesis in follicular epithelial cells and increases in DNA replication and cell division
-The thyroid gland sits within the neck in front of the trachea
-The thyroid gland produces thyroxine (called T4 as it contains 4 iodine’s) and triiodothyronine (called T3 as there are three iodine’s)
-Thyroid-stimulating hormone (TSH) production -> Controlled by the negative feedback action of T3 and T4 on the anterior pituitary gland, and the hypothalamus. TSH causes hypertrophy of thyroid tissue but excessive exposure of the thyroid gland to TSH can cause goiter (enlarged neck/thyroid gland)
-Actions -> Increased T3 and T4 levels are associated with increased oxidative substrate metabolism and increased mitochondrial enzyme activity. This increases carbohydrate and lipid metabolism, therefore T3 & T4 = high metabolic rate.
Describe the control systems and key hormones from the anterior pituitary
-Cortisol
-Cortisol secretion during stress is mediated by the hypothalamus-anterior pituitary gland system
-Physiological function (non-stressful situations) -> Cortisol affects the responsiveness of smooth muscle to epinephrine and norepinephrine (permissive action), helping to maintain normal blood pressure. It’s required to maintain the certain enzymes concentration involved in metabolic homeostasis, therefore preventing plasma glucose concentration dropping too far below normal level. It has anti-inflammatory and anti-immune functions (overactive immune system can be caused by increased cortisol levels)
-Physiological function (stressful situations) -> Cortisol effects on metabolism;
1. Stimulation of protein catabolism (breaks down molecules into smaller ones) in bone, lymph, muscle etc
2. Stimulation of liver uptake of amino acids and their conversion to glucose (utilise carbohydrate sources as quickly as possible)
3. Maintenance of plasma glucose concentrations
4. Stimulation of triglycerides catabolism in adipose tissue, with release of glycerol and fatty acids into the blood
-Enhanced vascular reactivity, improving CV performance
-Inhibition of non-essential functions e.g. reproduction and growth
-Cortisol and recovery -> massage decreases circulating cortisol levels and therefore supports recovery as; cortisol at rest inhibits immune system and inflammatory response and decreases capillary permeability in injured areas
-Fight or flight response is initiated from the endocrine and nervous system working in synchrony
-Cortisol has the highest levels at 9am and hits the lowest values at 9pm to 12am – this is the normal diurnal rhythm
Describe the control systems and key hormones from the anterior pituitary
-Growth hormone
-The hormones important to human growth are; growth hormone, insulin-like growth factors 1 and 2, T3 (essential for growth during childhood and adolescence), insulin (meainlu during foetal life) testosterone and estradiol
-Growth hormone secretion is stimulated by growth hormone-releasing hormone (GHRH) and inhibited by somatostatin (SST)
-Growth hormone is the major stimulus of postnatal growth. It stimulates the release of IGF-1 from the liver and many other cells, IGF-1 then acts locally and stimulates cell division. Growth hormones act directly on cells to stimulate protein synthesis and the secretion is highest during adolescence. It decreases our adipose tissue but increases our bone growth and muscle mass
Describe the control systems and key hormones from the anterior pituitary
-Sex hormones
- Testosterone -> Main source in males and is produced by the testes. In females it is produced in smaller quantities in the ovaries and the adrenal cortex. Testosterone promotes muscle growth and development of male sex characteristics. It can provide confidence, increased muscle mass and strong bones
- Oestrogen and Progestogen -> The main source in females and is produced by the ovaries. Important source in post-menopausal females as some oestrogen is produced in smaller amounts by other tissues e.g. liver, pancreas and skin. In males estrogen is produced when FSH binds to FSH receptors. It promotes the development of female sex characteristics and regulates menstrual cycle and adipose tissue growth. Oestrogens also promote endothelia function which have protective effects such as decreased vascular injury (and enhance adipose tissue growth)
-A study found that an increase in sex hormone levels helps builds muscle and reduce body fat
-DHEA (Dehydroepiandrosterone) and DHEAS (Dehydroepiandrosterone sulphate) are hormones produced by the adrenal cortex. DHEAS are precursors for sex hormones e.g. testosterone. DHEA/S affect various systems of the body, such as to be anti-ageing. DHEA/S production peaks at 20-30 and then declines with age
-Hormone replacement therapy (HRT) found in oestrogen in women can improve muscle function, maintain muscle mass and prevent fat infiltration into the muscle compartment. Exercise and HRT can be considered counteractive treatments to age-related changes in muscle phenotype. HRT in testosterone in men can preserve muscle tissue and offset age-related muscle loss.
