Homeostasis and Negative Feedback Loops — Study Notes Flashcards

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Homeostasis

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A dynamic equilibrium in which biological systems actively regulate internal variables to stay near a stable set point. It enables cells and organisms to maintain conditions necessary for proper biochemical function and survival, often keeping the internal environment different from the external one. Homeostasis involves sensors, responses, and transport across barriers to correct deviations.

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Set point

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The target value or narrow range that a physiological variable is maintained near by homeostatic mechanisms. Set points (e.g., core temperature or blood glucose) guide sensors and responses that correct deviations. They represent a regulated equilibrium rather than a fixed absolute value.

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Cell membrane

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A selective barrier that separates the internal cellular environment from the external space and controls movement of substances. Its selective permeability allows cells to regulate ion concentrations, nutrients, and $H_2O$ balance. Membrane transport mechanisms are central to maintaining intracellular conditions.

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Feedback loop

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A regulatory circuit that detects deviations from a set point and generates responses to correct or amplify those deviations. A typical loop includes a stimulus, a sensor, and a response, and it can be negative (stabilizing) or positive (amplifying). Feedback loops coordinate many physiological processes to maintain homeostasis.

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Stimulus

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The change in an internal or external condition that moves a variable away from its set point. Examples include rising or falling body temperature or changes in blood glucose. The stimulus initiates detection by sensors, which then trigger appropriate responses.

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Sensor

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A structure or mechanism that detects changes in a specific variable relative to its set point. An example is the hypothalamus sensing core temperature shifts in humans. Sensors transmit information to effectors that generate corrective responses.

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Response

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The actions taken by effectors to counteract or amplify a change detected by sensors. Responses can include physiological adjustments like vasodilation, sweating, shivering, or activation of ion pumps. Effective responses restore variables toward their set points or drive processes toward a particular outcome.

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Negative feedback

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A feedback mechanism that reduces the effect of the original stimulus and returns the system toward its set point. It is stabilizing and is the primary method organisms use to maintain homeostasis. Examples include thermoregulatory responses that reverse temperature deviations.

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Positive feedback

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A feedback mechanism that amplifies a change, driving the system further from its set point and often producing a rapid outcome. Positive feedback is less common because it moves systems away from equilibrium; examples include blood clotting cascades and escalating uterine contractions during labor. Such loops are typically self-limiting by external factors or end conditions.

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Vasodilation

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The widening of blood vessels that increases blood flow to the skin, promoting heat loss from the body. It is a physiological response often triggered by the hypothalamus during heat stress. Vasodilation helps lower core temperature by transferring heat to the environment.

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Sweating

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The secretion of watery fluid onto the skin surface that increases evaporative heat loss. Sweating is activated during heat stress as part of thermoregulatory negative feedback. It helps cool the body but requires adequate hydration to be effective.

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Shivering

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Involuntary, rapid muscle contractions that generate heat and raise core body temperature. Shivering is triggered in response to cold stress as part of a negative feedback loop. It increases metabolic heat production to help return temperature toward the set point.

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Blood clotting

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A cascade of biochemical reactions in which activated clotting factors activate additional factors, amplifying the response to vascular injury. This positive feedback mechanism rapidly forms a clot to stop bleeding. The cascade is regulated by checks that limit excessive clot formation.

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Passive transport

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Movement of substances down their concentration or electrochemical gradients without direct ATP use. Examples include diffusion and osmosis, which help equalize concentrations across spaces and contribute to short-term homeostatic adjustments. Passive transport relies on permeability properties of membranes and existing gradients.

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Diffusion

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The net movement of particles from regions of higher concentration to regions of lower concentration due to random molecular motion. Diffusion requires no ATP and helps distribute solutes across compartments. It is a fundamental passive transport process underlying many homeostatic exchanges.

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Osmosis

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The passive movement of $H_2O$ across a semipermeable membrane from areas of lower solute concentration to areas of higher solute concentration. Osmosis balances water distribution between compartments and influences cell volume and pressure. It plays a key role in fluid homeostasis and is driven by solute gradients.

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Active transport

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The movement of substances against their concentration or electrochemical gradients using energy, typically from $ATP$. Active transporters and ion pumps maintain essential gradients (e.g., Na\/K pumps) that passive processes cannot uphold. These gradients are vital for cellular functions like electrical signaling and nutrient uptake.

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ATP

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Adenosine triphosphate ($ATP$) is the primary cellular energy currency used to power active transport and many other energy-requiring processes. Hydrolysis of $ATP$ provides the work needed to move molecules against gradients, synthesize macromolecules, and drive mechanical actions. Maintaining adequate $ATP$ supply is essential for homeostatic responses.

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Blood glucose

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The concentration of glucose in the blood, normally around $90\text{--}120\ \mathrm{mg/dL}$ in humans. It must be regulated to supply cells with energy while avoiding toxic effects of hyper- or hypoglycemia. Hormonal feedback (e.g., insulin and glucagon) and transport processes maintain glucose homeostasis.

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Body temperature

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Core body temperature is typically about $98.6^\circ F\ (\approx\ 37^\circ C)$ in humans and is tightly regulated for optimal enzyme function. Thermoregulatory systems use sensors, negative feedback, and effectors like vasodilation, sweating, and shivering to keep temperature near the set point. Large or prolonged deviations can impair metabolism and damage tissues.

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Failed homeostasis

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When regulatory systems cannot restore variables to their set points, short-term symptoms (fatigue, irritability) or severe outcomes (organ failure, coma, death) can occur. Chronic or extreme deviations—such as severe hypothermia, hyperthermia, or hypoglycemia—overwhelm compensatory mechanisms. Preventing and correcting failures of homeostasis is central to health and clinical care.