Principles OF Homeostasis PDF

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PRINCIPLES OF HOMEOSTASIS ● Define the terms: ○ Homeostasis ○ Compensation ○ Decompensation ○ Positive and negative feedback ● Explain that homeostatic mechanisms are responsible for maintaining optimal tissue function ● Explain that loss of homeostasis occurring as a result of external insults can lead to disease Homeostasis ● Tendency to resist change in order to maintain a stable, relatively constant internal environment. ● It is how an organism regulates its internal environment to maintain stability ● Relative constancy of the body to maintain an equilibrium in body’s environment ● Compensation = the return to homeostasis after a system is challenged ● Decompensation = the failure to compensate, adapt, heal to challenges Circular pathway to allow the body to return to its homeostatic state 1. Stimulus produces a change in variable 2. Change is detected by receptor 3. Information is sent along an afferent [to CNS] pathway to the control centre 4. Information is sent along an efferent [from CNS] pathway to an effector 5. The response of the effector feeds back to influence the magnitude of the stimulus and returns the variable to homeostasis Autonomic Nervous System ● Primary function is to maintain homeostasis via involuntary control ● Homeostasis is a collaboration between the ANS and endocrine system ● Therefore body can survive lack of food, water, extremes of temperature etc ● It is not subject to voluntary control ● Innervates smooth muscle, cardiac muscle and glands ● Two nerve fibres are connected by a synapse ○ One from receptor to CNS, and one from CNS to effector [reflex arc] ● Stimulates or inhibits a target cell once the receptor has received the impulse ● Regulates the internal environment, and mobilizes body resources during times of stress Negative Feedback ● Body’s ability to counteract a change in the environment ● These loops act to oppose the stimulus, or cue, that triggers them. ● Sensory nerve fibres communicate the change to the CNS ● Reference centre = region in hypothalamus ○ Notes any differences compared the normal

● Motor neurons affect the cells from CNS via altered impulse frequency ○ For example, if your body temperature is too high, a negative feedback loop will act to bring it back down towards the set point. Positive Feedback ● These loops amplify the starting signal. ● Positive feedback loops are usually found in processes that need to be pushed to completion ○ E.g. oxytocin during birth to increase speed and intensity of contractions; pepsin can convert pepsinogen to create more pepsin [autocatalytic reaction] Homeostatic mechanisms are responsible for maintaining optimal tissue function Osmoreceptors ○ Most water comes from the diet, small amount from aerobic respiration ○ Water leaves body by sweat, urine, exhalation ○ Water levels in the cells in the body must be maintained ● A stimulus disrupts homeostasis by increasing osmolarity [measured compared to other ions, high osmotic value = high concentration of ions] of plasma and interstitial fluid ● Osmoreceptors in the hypothalamus input nerve impulses to the hypothalamus and posterior pituitary gland ● This causes the increased release of ADH ○ Therefore target cells [aquaporins] in the kidney become more permeable to water which increases water reabsorption into the blood ○ This causes a decrease in plasma osmolarity [lower concentration of ions] ● Decreased blood volume is detected by baroreceptors in the aorta [blood pressure] so the heart can increase rate of contraction to compensate for decreased blood volume. Normal Blood Calcium Level ● Rising blood calcium level [hypercalcaemia] ○ Stimulus causes thyroid gland to release calcitonin ○ This stimulates calcium deposition in the bones and reduces calcium uptake in the kidneys/more calcium excreted from body ● Falling blood calcium level [hypocalcaemia] ○ Stimulus causes parathyroid gland to release parathyroid hormone [PTH] ○ This stimulates calcium release from bones and increases calcium uptake in kidneys and intestines ○ When normal levels are reached PTH levels fall Temperature ● If you get either too hot or too cold, sensors in the periphery and the brain tell the temperature regulation center of your brain [hypothalamus] that your temperature has strayed from its set point. ● Rising body temperature [hyperthermia/pyrexia [high temperature caused by inflammation and fever]] ○ Nervous system signals dermal blood vessels to dilate and sweat glands to secrete ○ Body heat is lost to its surroundings ● Falling body temperature [hypothermia] ○ Nervous system signals dermal blood vessels to constrict and sweat glands remain inactive

○ Body temperature is conserved ○ If body temperature continues to drop, nervous system signals muscles to contract involuntarily [shivering] ■ The muscle activity generates body heat Loss of homeostasis occurring as a result of external insults can lead to disease Cell Death ● Apoptosis - programmed cell death ● Cell shrinks [condensation] and starts to fragment so parts of the cell can break away to form apoptotic bodies ● ATP is involved in apoptosis ● The cell will have normal morphology ○ Removes cells during development, eliminates potentially cancerous and virus-infected cells, and maintains balance in the body. ○ They shrink and develop bubble-like protrusions (technical name: “blebs”) on their surface. ○ The DNA in the nucleus gets chopped up into small pieces, and some organelles of the cell, such as the endoplasmic reticulum, break down into fragments. ○ In the end, the entire cell splits up into small chunks, each neatly enclosed in a package of membrane. ● Cells at the top of villi will undergo apoptosis ● Too much cell death = not able to absorb nutrients properly - example of homeostasis ● Necrosis - cells die due to external insult [spontaneous] ● Cell swelling that is reversible up to a point ● Past that point the swelling is irreversible and the membrane disintegrates [enzymes digest cell membrane] ● ATP is not involved in necrosis ● Caspases are not involved in necrosis ● The cell will have random morphology ○ The damaged cell’s plasma membrane can no longer control the passage of ions and water so the cell swells up, and its contents leak out through holes in the plasma membrane. ○ This often causes inflammation in the tissue surrounding the dead cell. Protein Misfolding ● Misfolding of disease-causing proteins results in the disruption of protein homeostasis when misfolded monomers accumulate and begin to form intermediate soluble oligomers or fibrils, and eventually form insoluble aggregates. [original protein has mutated] ○ Chaperone proteins assist in the correct folding of proteins and prevent the formation of toxic/misfolded proteins ○ Increasing the expression of chaperone proteins enhances the ability of cells to maintain protein homeostasis even in the presence of aggregation-prone proteins ● E.g. huntington's, parkinson's, BSE [mad cow disease]

Aging ● Aged cells have accumulated damage over time and no longer respond as quickly as they should, which makes it harder for the body to restore homeostasis. ● The longer an organism lives, the more damage occurs to their DNA and to the materials outside of their cells. ● The changes inside and outside of cells prevents organs from responding as robustly as they used to. ● These effects of aging can be dramatically observed in the function of the heart and the circulatory system. ● The dysfunctional stem cells of an older person also contribute to the inability to restore homeostasis. Hormones ● Homeostasis often works by the release of hormones from one organ, which communicates with distant organs. ● However, as a person ages the target organs become less responsive to the signals that are sent from other organs. ● Furthermore, hormone levels may not be at their proper levels in the blood....