Homeostasis and Feedback Loops

Homeostasis

• Process of maintaining a stable internal environment
• Body fluid compartments:
  • Intracellular fluid (ICF): inside cells
  • Extracellular fluid (ECF): outside cells, composed of interstitial fluid and blood plasma
• Key controlled physiological variables:
  • Temperature
  • Heart Rate
  • Blood pressure
  • Body Fat
  • pH
  • Gas levels (O2, CO2)
  • Osmolality
  • Electrolytes
  • Calcium
  • Glucose
  • Hormones

Feedback Loops

• Control Centres:
  • Integrate signals from receptors and effectors and determine the appropriate response.
• Receptors:
  • Detect changes in the internal environment.
• Effectors:
  • Carry out the response to restore homeostasis.
• Stimulus:
  • The change in the internal environment.
• Effects:
  • The response of the effectors, which should negate the stimulus and restore homeostasis.
• Negative Feedback:
  • The primary mechanism of homeostasis
  • The response of the effectors negates the stimulus, pushing the value back toward the set point.
  • Example: Increase in body temperature triggers sweating, which cools the body, bringing the temperature back to the set point.

Types of Receptors

• Baroreceptors:
  • Monitor pressure, for example blood pressure in carotid arteries.
• Chemoreceptors:
  • Monitor levels of chemical substances, for example oxygen, carbon dioxide, and pH in carotid arteries.
• Thermoreceptors:
  • Monitor temperature, for example in skin and internal organs.
• Osmoreceptors:
  • Monitor osmolarity (solute concentration).
• Glucoreceptors:
  • Monitor levels of glucose, for example beta-cells in the pancreas.

Homeostasis Mechanisms

• Local (Intrinsic) Control:
  • Regulation within a tissue, direct effect, for example autoregulation (cells responding to their own local environment).
• Reflex/Extrinsic Control:
  • Control system outside the tissue being controlled, for example negative feedback.
• Intracellular Control:
  • Control of intracellular processes, for example enzymes, mediating biochemical reactions.

Homeostatic Models

• Model of Homeostasis:
  • 1. Sensor: Measures the regulated variable.
  • 2. Set Point: Established ‘normal range’ for the regulated variable.
  • 3. Error Detector: Compares sensor signal with the set point, generating an error signal.
  • 4. Controller: Interprets the error signal and determines effector outputs.
  • 5. Effectors: Determine the value of the regulated variable.

Other Feedback Loop Concepts

• Sensitization:
  • The effects of multiple stimuli can be greater than the sum of their individual effects.
  • Example: Low oxygen and high carbon dioxide both increase breathing rate.
• Feed Forward:
  • The response to a stimulus is initiated before the threshold is reached.
  • Similar to anticipation but the response is reduced before the threshold is reached.
  • Example: Thirst reflex. When you drink, you may stop drinking even before osmoreceptors detect a return to normal plasma osmolality.

Transcellular Fluid

• Transcellular fluid is a part of the extracellular fluid (ECF) and encompasses fluids secreted by epithelial tissue, including cerebrospinal fluid and extraocular fluid.
• ECF contains high sodium ion concentration and low potassium ion concentration, whereas the opposite is true for intracellular fluid (ICF).

Homeostasis

• Definition: Maintenance of a relatively constant internal environment despite changes in the external environment.

Components of the Internal Environment Under Homeostatic Control

• pH
• Temperature
• Blood glucose
• Blood gases (oxygen and carbon dioxide)
• Electrolyte concentration (sodium, potassium, calcium)

Homeostatic Control Mechanisms

• Set point: The ideal value for a regulated variable.
• Threshold: The point at which a change in a variable triggers a response from the control system.
• Receptors: Detect changes in the internal environment.
• Integrating center: Processes information from receptors and determines the appropriate response.
• Effectors: Carry out the response to restore the variable to its set point.
• Oscillation: The value of a regulated variable fluctuates around the set point due to the action of the control system.

Positive Feedback

• The effect of a change on a variable is amplified, leading to a further change in the same direction.
• Requires an external influence to break the cycle.
• Rarely involved in maintaining homeostasis.
• Example: During labor, the stretching of the uterus triggers the release of oxytocin, which further stimulates uterine contractions.

Temperature Set Point

• Varies among individuals.
• The range of "normal" body temperature can be wide.
• Average rectal temperature in a group of healthy students: 36.8°C.

Circadian Cycles

• The body's internal clock influences the set points for several variables, leading to daily high-low rhythms.
• Example: Body temperature is typically lower during sleep.

Blood Glucose Homeostasis

• The blood glucose concentration is maintained within a relatively narrow range (20-100mg/dl).
• Irregular feeding can lead to greater fluctuations in blood glucose levels.

Calcium Homeostasis

• The range of normal calcium levels in blood is narrower (9-11mg/100ml) compared to blood glucose.
• Fluctuations in calcium levels can lead to hypocalcemic tetany, characterized by spontaneous twitching of hands and feet and increased reflexes.

Fever

• Elevated body temperature caused by an increase in the hypothalamic set point.
• Triggers: Endogenous pyrogens released from bone marrow macrophages in response to bacterial infection.
• Benefits: May help defend against microbial invasion by enhancing white blood cell activity, increasing antibody production, and directly killing or inhibiting the growth of some bacteria and viruses.

Effects of Fever at the Extremes of Age

• Infants: May not develop fever during the first few days of life. Young infants (less than 60-90 days old) often present with fever and no other symptoms, making diagnosis difficult. Children develop higher temperatures than adults for minor infections. Febrile seizures before the age of 5 are more common.
• Older Adults: May have subtle or atypical responses to infection, including dehydration, alternating hypothermia and high fever.