Introduction to endo Flashcards
Homeostasis
the maintenance of a steady state/internal environment
Homeostatic control mechanisms
what are the components of the feedback system?
What is intrinsic and extrinsic control?
Examples of controlled variables?
- Challenge produces a change in body status
- Physiology responds to maintain homeostasis
- Components of a feedback system:
o Regulated factor – set point, operating range, “error signal”
o Detector/sensor – afferent pathway
o Comparator/Control centre – determines the set point of the variable, comparing and maintaining the variable at set point; can be:
- Intrinsic – locally regulated by cells or tissues
- Extrinsic – endocrine system, nervous system
o Effector – returns the variable to the set point (efferent pathway)
o Response
• Examples of controlled variables:
o Chemical substances – ions, nutrients and hormones
o Physical entities – blood pressure, core temperature
Control of Core body temperature
what are the responses to raise and lower the temperature?
What happens to the set point when we are infected? what raise it?
What are the benefits of the raised temp?
How is the temp increased?
why is regulation less precise during a fever?
• Responses include:
o Shivering, vasoconstriction, increased metabolism – raise temperature
o Vasodilation, sweating – lower temperature
• When the body is infected, the set point is changed and body temperature rises:
o Pyrogens (bacterial or viral infections) change the set point to a higher level resulting in fever
o Benefits of a high temperature include:
- Inhibition of bacterial growth
- Speeding up of metabolic reactions
- Increased delivery of WBCs to infection sites
o Temperature is increased by:
- Bloody flow shifted to core of the body to conserve heat
- Increased muscle activity (shivering)
- Chills stop when high temperatures are reached
o The actual body temperature lags behind the rapid shift in set point, regulation is maintained during the fever but it is less precise (hence why they stay for a while)
Control of blood pressure – the baroreceptor reflex
Where are the baroreceptors found? what nerves do they innervate? where do these go?
What can reset the sensitivity of baroreceptors?
- Baroreceptors are found on the aortic arch and the carotid sinus, and are used control blood pressure by sending signals to the cardiovascular control centre in the medulla
- The medulla then uses the sympathetic and parasympathetic nervous systems to either increase or decrease blood pressure by changing cardiac output and total peripheral resistance levels
- In conditions such as hypertension, the baroreceptors ‘reset’ which alters blood pressure
The Hypothalamic-pituitary axis – Control of Plasma osmolarity
Where are hormone synthesised and where are they released?
Where do other neurosecretory cells get released into and via what?
What deetcs rise in osmolarity and how? what is released and where does it act?
- Neurones in the hypothalamus synthesise and release hormones from the posterior pituitary gland, the two are directly linked
- Other neurosecretory cells in the hypothalamus release their hormones into the portal capillaries, which are directly linked to the endocrine cells of the anterior pituitary gland
- Control of extracellular fluid – the osmoreceptor-ADH feedback system:
o When osmolarity increases (more Na+), osmorececptors in the hypothalamus shrink
o AP’s are generated signalling to the posterior pituitary to release stored ADH
o ADH is transported to kidneys to increase water permeability of collecting tubules & DCT
o This leads to increased water reabsorption and more concentrated urine
Vasopressin (ADH) also has a role in the control of blood pressure
After a haemorrhage, blood volume (and hence blood pressure) are reduced, to restore blood pressure several homeostatic mechanisms are used:
o The baroreceptor reflex to increase CO and TPR
o ADH secretion to increase blood volume
• Feedback loops are integrated to control sodium balance, blood pressure & fluid volume:
(LOOK AT DIAGRAM)
The hypothalamic pituitary axis also releases CRH (corticotrophin releasing hormone)
What doe CRH stimulate and from where? what does this secretion stimulate and what does it release?
Effects of this last secretion
What are diseases from a lack and excess of it?
How can the set clock be influenced?
o CRH stimulates ACTH secretion from the anterior pituitary gland
o ACTH stimulates the adrenal cortex to release corticosteroids (cortisol)
o Effects of cortisol in the blood:
- Gluconeogenesis
- Protein mobilisation
- Fat mobilisation
- Anti-inflammatory effects
o Lack of cortisol results in Addison’s disease
o Excess cortisol results in Cushing’s disease
o The set point can be influenced by your ‘body clock’ where levels are high in the morning and low at night, and by stress which keeps cortisol levels high
The Principles of negative and positive feedback
Why is it less common?
Name two scenarios where it takes place
Describe one of the processes
- Negative feedback is where a change in a controlled variable is reversed
- Positive feedback is where a change in a controlled variable is increased
o Less common as it is a “runaway train”
o Response of the effector output reinforces the stimulus (e.g. blood clotting, childbirth)
- In labour, oxytocin stimulates the contraction of uterine muscles
- Cervix dilates and activates stretch receptors
- AP’s then sent to hypothalamus to release more oxytocin
The major endocrine glands of the body are the
• Hypothalamus
• Pituitary gland
(both of these form the hypothalamic-pituitary axis)
• Thyroid, adrenal cortex/medulla, gonads (these are regulated by hypothalamic-pituitary axis)
• Pancreas
• Parathyroid glands
(these are not regulated by hypothalamic-pituitary axis, instead regulated by other controlled variables e.g. insulin in pancreas and glucagon, which are depdendent on glucose levels, parathyroid regulate calcium levels)
What do the major endocrine glands secrete?
The hypothalamus is responsible for releasing and inhibiting hormones
Pituitary gland anterior lobe releases trophic (growth) hormones, while the posterior lobe releases oxytocin and vasopressin (ADH)
These act on various organs such as thyroid gland, adrenal gland and gonads.
Thyroid gland releases thyroxine and triidothyronine
The adrenal gland produces different hormones depending on the part, the cortex produces cortisol and aldosterone. The medulla releases adrenaline/noradrenaline.
The gonads release oestrogens, androgens and progestagens (these are the family names)
Pancreas releases insulin and glucagon.
Parathyroid gland releases parathyroid hormone.
Other tissues also release hormones, such as the kidneys (vitamin D, EPO), CVS (ANP, uendothelins), pineal gland (melatonin), thymus gland (thymic hormones), bone (phosphate) and adipose tissue (leptin).
Signalling mechanisms in the Endocrine System
There are four types of signalling. NAme ands how they work
Endocrine: Hormones are released by an endocrine cell into the general circulation and act on distant target sites
Paracrine: Hormones released by endocrine cell which act locally on adjacent cells
Autocrine: Hormones released by a cell which act back on the same cell
Intracrine: Conversion of an inactive hormone to an active hormone that acts within the cell
General Functions of Hormones
Hormones play a key role in the body, regulating many things including
Reproduction, growth and development - sex steroids, thyroid hormones, prolactin, growth hormone
Maintenance of internal environment (homeostasis) – aldosterone, parathyroid hormone, vitamin D
Energy production, utliisation and storage – Insulin, glucagon, thyroid hormones, cortisol, growth hormone
Chemical Classification of Hormones
How can hormones be classified into different groups? What are the 4 main groups?
GIve examples for each of the 4 groups
Hormones can be classified into different groups depending on what they are made from (chemical structure), there are four main groups:
Protein/peptide hormones (long chain amino acids), steroid hormones (derivatives of cholesterol), amino acid derivatives (one or two amino acids, mostly deriving from tyrosine, very small) and fatty acid derivatives
- Protein/peptide hormones include: Hypothalmic hormones, pituitary hormones, insulin, PTH and calcitonin (these tend to act in classical endocrine signalling)
- Steroid hormones include: Cortisol, aldosterone, oestrogens, androgens, progestagens, vitamin D
- Amino acid derivatives: Adrenaline, Noradrenaline, thyroid hormones (all from tyrosine), melatonin (from tryptophan)
- Fatty acid derivatives: Prostaglandins, thromboxanes, prostacyclin
Chemical nature or hormones
How do protein and peptide hormones intially start?
What is the precursor for all steroid hormones?
What is the nature of transport and half life ofr the different hormones?
Protein and peptide hormones will initially start as a prohormone and then through additions of extra polypeptides and enzymes and new bonds forming the hormone will be made.
Cholesterol is the precursor of all steroid hormones, so all steroid hormones are made from a steroid nucleus which is arrived at from the cleaving of certain groups of cholesterol. Major steroids include: cortisol, aldosterone, oestrogens, progestogens, androgens and vitamin D.
Hormones have different natures of transport in the body, they will spend different lengths of time in circulation and also the mode of transport will vary. These given below.
Proteins and peptide hormones have a half life in circulation of minutes and they are most circulating freely (unbound).
Tyrosine derivatives (called catacholamines) have half lives in the seconds range, while thyroid hormones have half lives in the hours. Thyroid hormones are also bound to plasma proteins in their transport in the circulation.
Cholesterol derivatives (i.e. all steroids) have half lives of hours –days, they are also bound to plasma proteins.
Anatomy and functional connections of the hypothalamic-pituitary axis and the hormones secreted by the anterior and posterior lobes of the pituitary gland
What is neuroendocrine integration?
What is the magnocellular neuron and where does it project? what is this projected area supplied by?
Whta is the parvoceullar neuron and what capillaries supply this area and projected area?
The nervous system and endocrine system are linked and interact, this is called neuroendocrine integration.
The magnocellular neuron projects to the posterior lobe of the pituitary gland and regulates this area. The posterior lobe is supplied by the inferior hypophyseal artery which then capillarises and then becomes the inferior hypophyseal vein. The posterior lobe will release oxytocin and ADH.
The parvocellular neuron project down to the capillaries in the median eminence. Running into this is the superior hypophyseal artery. These capillaries become the hypophyseal portal vein which travels into the anterior lobe to become a secondary capillary network and finally becomes the hypophyseal portal vein again. Hormones such as GH, prolactin, TSH, ACTH, LH, FSH are released from the anterior lobe of the pituitary gland
