Homeostasis

• Homeostasis


  • Homeostasis is the maintenance of the internal environment of an organism within preset limits, despite fluctuations in the external environment.
  • Examples of homeostatic variables in humans include body temperature, blood pH, blood glucose concentration, and blood osmotic concentration.
• Positive Feedback Loops


  • Positive feedback loops are mechanisms where a change in a variable causes an additional change in the same direction, amplifying the original response.
  • In a positive feedback loop, the output of the system is used to further increase the input, leading to a rapid and self-reinforcing change.
  • Positive feedback loops are less common in homeostasis, but they can play important roles in certain physiological processes.
• Homeostasis comes from the Greek words "homeo" (similar) and "stasis" (standing still). It's the process by which biological systems maintain stability while adjusting to changing external conditions.
• Body Temperature

  • Normal range: 36.5°C to 37.5°C (97.7°F to 99.5°F)
  • Regulated by the hypothalamus in the brain

  Example: When you exercise and your body temperature rises, you start sweating to cool down. Conversely, when you're cold, you shiver to generate heat.
• Blood pH

  • Normal range: 7.35 to 7.45
  • Maintained by buffer systems, respiratory system, and kidneys

  Even small deviations from this range can be life-threatening!
• Blood Glucose Concentration

  • Normal fasting range: 3.9 to 5.5 mmol/L (70 to 100 mg/dL)
  • Regulated primarily by insulin and glucagon hormones
• Blood Osmotic Concentration

  • Normal range: 275 to 295 mOsm/kg
  • Controlled mainly by antidiuretic hormone (ADH) and the kidneys
• The Homeostatic Mechanism

  Homeostasis typically involves three components:

  1. Receptor: Detects changes in the internal environment
  2. Control center: Processes information and initiates a response
  3. Effector: Carries out the necessary actions to restore balance
• What are feedback loops in homeostasis?

  Feedback loops are biological mechanisms whereby homeostasis is maintained. This occurs when the product or output of an event or reaction changes the organism's response to that reaction
• Why Negative Feedback preferred:

  1. Self-Correcting: Negative feedback counteracts changes, bringing the system back to its set point.
  2. Stability: It prevents extreme fluctuations, maintaining balance.
  3. Efficiency: It responds proportionally to the deviation, conserving energy.
• Negative Feedback

  Negative feedback is a self-regulating mechanism in biological systems that responds to changes by initiating actions that counteract those changes, thereby maintaining stability
• Negative vs. Positive Feedback


  While negative feedback promotes stability, positive feedback amplifies changes, often leading to instability.
• Example of Negative feedback: 


  Rising blood sugar triggers insulin release, lowering blood sugar. Positive feedback: During childbirth, contractions stimulate oxytocin release, which increases contractions further.
• Positive Feedback Loop


  A mechanism where the response to a stimulus amplifies the stimulus itself, leading to an exponential increase in the response, often seen in biological systems.
• Positive feedback is rarely used in homeostasis, but it's crucial in certain biological processes like blood clotting and childbirth.
• How Negative Feedback Works

  Negative feedback operates in a cyclical manner:

  1. Stimulus: A change occurs in the internal environment.
  2. Receptor: Detects the change.
  3. Control Center: Processes the information and determines the response.
  4. Effector: Initiates actions to counteract the change.
  5. Return to Set Point: The variable returns to its normal range.
• Returning to Set Point from Above and Below

  Negative feedback is bidirectional, meaning it can correct deviations above or below the set point.

  For Example: Body Temperature Regulation;

  • Too High: Sweating increases, blood vessels dilate
  • Too Low: Shivering begins, blood vessels constrict
• Overview of Blood Glucose Regulation

  The body aims to maintain blood glucose levels between 3.9 to 5.5 mmol/L (70 to 100 mg/dL). This is primarily achieved through the antagonistic actions of two pancreatic hormones: insulin and glucagon.
• Pancreatic Endocrine Cells


  The pancreas contains specialized endocrine cells in regions called islets of Langerhans:

  1. Beta (β) cells: Produce insulin
  2. Alpha (α) cells: Produce glucagon
• Insulin:

  The Blood Glucose Lowering Hormone

  1. Secretion Control:

  • Triggered by high blood glucose levels
  • Ca2+Ca2+ influx in β-cells leads to insulin release

  1. Transport:

  • Travels in blood plasma (not bound to carrier proteins)
• Insulin Effects on Target Cells


  • Increases glucose uptake by cells (especially muscle and fat cells)
  • Stimulates conversion of glucose to glycogen in liver and muscles
  • Promotes conversion of glucose to fat for storage
  • Inhibits gluconeogenesis and glycogenolysis
• Glucagon:

  The Blood Glucose Raising Hormone

  Glucagon primarily acts on liver cells, while insulin has more widespread effects.

  
  

  Secretion Control:

  • Triggered by low blood glucose levels
  • α-cells release glucagon when glucose levels fall

  1. Transport:

  • Circulates in blood plasma
• Glucagon Effects on Target Cells

  • Stimulates glycogenolysis (breakdown of glycogen to glucose) in liver
  • Promotes gluconeogenesis (synthesis of glucose from non-carbohydrate sources)
  • Increases lipolysis (breakdown of fats)
• This system operates as a negative feedback loop:


  1. Blood glucose rises (e.g., after a meal)
  2. β-cells detect the increase and release insulin
  3. Insulin lowers blood glucose
  4. As glucose falls, insulin secretion decreases
• Conversely of the Feedback Loop:

  1. Blood glucose falls (e.g., during fasting)
  2. α-cells detect the decrease and release glucagon
  3. Glucagon raises blood glucose
  4. As glucose rises, glucagon secretion decreases
• Molecular Mechanisms

  1. Insulin Signaling:

  • Binds to insulin receptors (tyrosine kinase receptors)
  • Activates a signaling cascade leading to GLUT4 translocation

  1. Glucagon Signaling:

  • Binds to G-protein coupled receptors
  • Activates adenylyl cyclase, increasing cAMP levels
  • cAMP activates protein kinase A, leading to glycogen breakdown
• Physiological Changes of Type 1 Diabetes:

  1. Autoimmune destruction of β-cells: The immune system attacks and destroys insulin-producing β-cells in the pancreas.
  2. Insulin deficiency: Due to β-cell destruction, little or no insulin is produced.
  3. Hyperglycemia: Blood glucose levels remain consistently high due to lack of insulin.
  4. Ketoacidosis: Body breaks down fats for energy, producing ketones that can lead to a life-threatening condition.
• Risk factors of Type 1 Diabetes:

  • Genetic predisposition
  • Environmental triggers (e.g., viral infections)
  • Family history of autoimmune diseases
• Treatment of Type 1 Diabetes:

  1. Insulin therapy: Regular insulin injections or insulin pump
  2. Blood glucose monitoring: Regular testing to adjust insulin doses
  3. Dietary management: Carbohydrate counting and meal planning
• Physiological Changes of Type 2 Diabetes: 


  1. Insulin resistance: Cells become less responsive to insulin.
  2. Compensatory insulin overproduction: Initially, β-cells produce more insulin to overcome resistance.
  3. β-cell exhaustion: Over time, β-cells may become dysfunctional or die due to overwork.
  4. Relative insulin deficiency: Insulin production becomes insufficient to maintain normal blood glucose levels.
  5. Hyperglycemia: Blood glucose levels remain consistently high.
• Risk factors of Type 2 Diabetes:

  • Obesity
  • Sedentary lifestyle
  • Age (risk increases with age)
  • Family history
  • Ethnicity (higher risk in certain populations)
  • Gestational diabetes history
• Prevention of Type 2 Diabetes:

  1. Maintain a healthy weight
  2. Regular physical activity
  3. Balanced diet rich in fiber, low in processed foods
  4. Regular health check-ups
• Treatment of Type 2 Diabetes: 


  1. Lifestyle modifications: Diet and exercise
  2. Oral medications: e.g., Metformin (reduces glucose production)
  3. Injectable medications: e.g., GLP-1 receptor agonists
  4. Insulin therapy: In advanced cases or when other treatments are insufficient
• Overview of Thermoregulation

  The human body aims to maintain a core temperature of approximately 37°C (98.6°F). This process involves several key components:

  1. Sensors (Thermoreceptors)
  2. Control Center (Hypothalamus)
  3. Effectors (Muscles, Blood Vessels, Sweat Glands)
• Thermoregulation:

  A Model of Negative Feedback Control

  Thermoregulation is an excellent example of how the body maintains homeostasis through negative feedback.
• Components of the Thermoregulatory System

  1. Peripheral Thermoreceptors
  2. Hypothalamus
  3. Pituitary Gland
  4. Thyroid Gland
  5. Effectors
• Peripheral Thermoreceptors


  • Located in the skin
  • Detect changes in external temperature
  • Two types: cold receptors and warm receptors
• Hypothalamus

  • Acts as the body's thermostat
  • Contains central thermoreceptors that monitor blood temperature
  • Integrates signals from peripheral and central thermoreceptors
  • Initiates appropriate responses to temperature changes
• Pituitary Gland


  • Releases hormones that influence metabolism and heat production
  • Particularly important: Thyroid Stimulating Hormone (TSH)
• Thyroid Gland

  • Produces thyroxine in response to TSH
  • Thyroxine increases metabolic rate and heat production