Homeostasis

•  What is homeostasis?

  The maintenance of a relatively stable internal environment in the body, despite external changes. It involves physiological processes that keep variables such as temperature, pH, and glucose levels within a narrow range.
•  What is a "controlled variable" in homeostasis?

  A specific physiological parameter that the body regulates, such as body temperature, blood glucose concentration, or blood pressure.
• What is the "set point" in homeostasis?

  The set point is the ideal or target value of a controlled variable that the body aims to maintain. For example, the set point for core body temperature is around 37°C.
• What is the role of a "sensor" in homeostasis?

  Detects changes in a controlled variable and sends information (usually as electrical or chemical signals) to a control centre. Example: thermoreceptors in the skin detect temperature changes.
• What is a "comparator" in a homeostatic system?

  Part of the control centre that compares the current value of the controlled variable (from the sensor) to the set point and determines if an adjustment is needed.
• What is an "error signal" in homeostasis?

  The difference between the current value of a controlled variable and the set point. It is used to determine the strength and direction of the response needed to restore homeostasis.
•  What are "controlling elements" in a homeostatic system?

  Controlling elements are effectors that respond to signals from the control center to correct deviations. These may include muscles (e.g., shivering to produce heat) or glands (e.g., insulin secretion).
•  Give an example of homeostasis using all six key terms.

  • Controlled variable: body temperature
  • Set point: ~37°C
  • Sensor: thermoreceptors detect temperature drop
  • Comparator: hypothalamus compares actual temp to set point
  • Error signal: temperature below 37°C triggers a response
  • Controlling elements: muscles shiver to generate heat
•  What part of the brain detects changes in body temperature?

  The hypothalamus, specifically the preoptic area of the anterior hypothalamus.
• What are the key effectors involved in body temperature regulation?

  • Sweat glands (increase sweating)
  • Blood vessels (vasodilation or vasoconstriction)
  • Skeletal muscles (shivering)
  • Thyroid gland (alters metabolism via thyroxine release)
• Which receptors detect changes in blood pressure? Where are they located?

  Baroreceptors in the carotid sinus and aortic arch.
•  Describe the baroreceptor reflex in response to high blood pressure

  • Baroreceptors increase firing.
  • Signals sent to cardiovascular center in the medulla.
  • Parasympathetic activity increases; sympathetic decreases.
  • Heart rate and cardiac output decrease.
  • Blood vessels dilate → BP decreases.
• What happens when blood pressure drops suddenly?

  • Baroreceptors decrease firing.
  • Sympathetic nervous system activated.
  • Heart rate and force increase.
  • Vasoconstriction occurs.
  • BP increases.
• Draw a diagram illustrating the baroreceptor reflex for blood pressure regulation.

  Diagram should include:

  • Baroreceptors (carotid/aortic)
  • Afferent nerves → medulla
  • Efferent signals to heart and vessels
  • Changes in HR, CO, and TPR
  • Negative feedback loop
• What is the axis involved in cortisol regulation?

  Hypothalamic-pituitary-adrenal (HPA) axis
• Describe the regulation of plasma cortisol levels via the HPA axis.

  • Hypothalamus releases CRH
  • Pituitary releases ACTH
  • Adrenal cortex releases cortisol
  • Cortisol exerts negative feedback on both hypothalamus and pituitary
• What triggers an increase in plasma cortisol levels?

  • Stress (physical or emotional)
  • Low blood glucose
  • Diurnal rhythm (early morning peak)
• Draw a diagram of the HPA axis regulating cortisol.

  • Hypothalamus (CRH) →
  • Anterior pituitary (ACTH) →
  • Adrenal cortex (cortisol)
  • Arrows showing negative feedback to hypothalamus and pituitary
• What is a physiological example of a change in set point in homeostasis?

  Fever (pyrexia): During infection, pyrogens (like IL-1, IL-6, and TNF-α) stimulate the hypothalamus to raise the body's temperature set point via prostaglandin E2 (PGE2) production. This helps the immune system fight pathogens by creating an environment less favorable for microbial growth.
• How does the hypothalamus contribute to fever as a set point change?

  The hypothalamus detects pyrogens and increases PGE2 production, which shifts the set point for core body temperature upward. The body responds by generating and conserving heat (e.g., shivering, vasoconstriction) until the new set point is reached.
• What is a pathophysiological example of a change in homeostatic set point?

  Hypertension (chronic high blood pressure): In some individuals, the cardiovascular system adapts to maintain a higher arterial pressure as the new "normal." The baroreceptors reset their sensitivity and tolerate elevated pressure, which contributes to sustained hypertension.
• Why is a fever considered a beneficial physiological set point?

  change?

  A fever helps enhance immune cell activity (e.g., increased T-cell response), inhibits bacterial growth, and speeds up tissue repair. It is a temporary, reversible adjustment designed to protect the body during infection
• Why is hypertension considered a harmful pathophysiological set point change?

  Chronic hypertension leads to increased cardiac workload, endothelial damage, and risk of stroke, heart failure, and renal disease. Unlike fever, this change is maladaptive and often irreversible without treatment.
• What is positive feedback in physiological control systems?

  A control mechanism that amplifies a change or deviation from the normal state, rather than reversing it. It leads to an exaggerated response in the same direction as the initial stimulus and typically requires an external event to stop it. Unlike negative feedback, it does not promote stability.
• Name a physiological example of positive feedback involved in childbirth

  Oxytocin release during labor is a positive feedback loop.

  • Uterine contractions push the baby against the cervix.
  • This stimulates stretch receptors in the cervix.
  • Nerve impulses are sent to the hypothalamus, triggering oxytocin release from the posterior pituitary.
  • Oxytocin increases uterine contractions, which further stimulate the cervix.
  • The cycle continues until the baby is delivered.
• How does oxytocin contribute to positive feedback in childbirth?

  Oxytocin enhances uterine muscle contractions, which increase cervical stretch. This further stimulates the release of more oxytocin. The loop continues and intensifies until parturition (birth) occurs, which removes the stimulus and ends the feedback cycle.
• What is a physiological example of positive feedback in blood clotting?

  Blood clotting (coagulation cascade) is a positive feedback process:

  • When a blood vessel is damaged, platelets adhere to the site and release chemicals.
  • These chemicals attract more platelets to the area.
  • The accumulation of platelets continues until a clot is formed and the bleeding stops.
  • This cascade amplifies the clotting response.
• Explain how the blood clotting cascade represents positive feedback.

  Platelets release pro-coagulant factors that activate more platelets, amplifying the clotting process. The cascade accelerates until a stable fibrin clot is formed. Once the vessel is sealed, the stimulus (bleeding) ends, stopping the feedback loop.