Homeostasis and Negative Feedback Mechanisms

Introduction to Homeostasis

• Etymology and Definition: Homeostasis translates to "staying the same." It is defined as the process of maintaining a constant, internal environment within narrow limits, despite changes that take place internally and externally.
• Internal Environment and Tissue Fluid:
  • The internal environment of the body is the tissue fluid.
  • Cells are surrounded by tissue fluid, and their health depends on its conditions.
  • Functions of Tissue Fluid:
    • Supplies essential substances to cells.
    • Removes metabolic waste.
    • Maintains the correct $pH$.
    • Provides an optimal temperature for cellular processes.
    • Allows for maximum enzyme and metabolic functioning.
• Critical Factors to be Maintained: Factors that must be kept constant include glucose, water, salts, carbon dioxide concentration, temperature, and $pH$.
• Negative Feedback Mechanisms: These are used to control:
  • Thyroxin levels.
  • Blood glucose levels.
  • Blood carbon dioxide levels.
  • Water balance (osmoregulation).
  • Salt levels.

The Mechanism of Negative Feedback

• Definition of Negative Feedback: The process where a change from the norm is detected and corrected, usually in the opposite direction.
• Components of a Negative Feedback System:
  1. Receptor: Sensory neurons and organs that detect changes in the internal environment.
  2. Control Centre: The brain and nervous system (e.g., the hypothalamus or medulla oblongata) that process information and activate responses.
  3. Effector: Endocrine glands, hormones, or body organs that respond to correct the change back to the set point.
• General Sequence of Events (MTG Framework):
  • Step 1: An imbalance is detected.
  • Step 2: A control centre is stimulated.
  • Step 3: The control centre responds.
  • Step 4: A message is sent to target organs (effectors).
  • Step 5: The target organ responds.
  • Step 6: The response opposes or reverses the imbalance.
  • Step 7: Balance (homeostasis) is restored.

Regulation of Blood Glucose Concentration

• Concentration Thresholds:
  • Ideal Level: $70-110\,mg$ per $100\,ml$ of blood (or $3.5-5.5\,mmol/100\,ml$).
  • Hyperglycaemia: Blood glucose levels above $180\,mg$.
  • Hypoglycaemia: Blood glucose levels below $70\,mg$.
• Regulatory Organ: The pancreas (specifically insulin-secreting and glucagon-releasing cells).
• When Blood Glucose is Too High (Excessive):
  • Step 1: Glucose level increases above normal levels.
  • Step 2: The pancreas is stimulated.
  • Step 3: Pancreas secretes insulin into the blood.
  • Step 4: Insulin travels to the liver (and muscles).
  • Step 5: Insulin stimulates the conversion of excess glucose to glycogen, which is then stored in the liver.
  • Step 6: Body cells take up more glucose.
  • Step 7: Glucose level decreases back to normal.
• When Blood Glucose is Too Low (Deficient):
  • Step 1: Glucose levels decrease below normal levels.
  • Step 2: The pancreas is stimulated.
  • Step 3: Pancreas secretes glucagon into the blood.
  • Step 4: Glucagon travels to the liver.
  • Step 5: Glucagon stimulates the conversion of stored glycogen back into glucose.
  • Step 6: Glucose is released into the blood.
  • Step 7: Glucose level increases and returns to normal.
• Disorder: Diabetes mellitus is caused by an imbalance in blood glucose levels.

Regulation of Carbon Dioxide Concentration

• Biological Context: Carbon dioxide ($CO_2$) is a waste product of cellular respiration. Increased exercise requires more energy, leading to higher respiration rates and increased $CO_2$ production.
• Mechanism when $CO_2$ is Too High:
  • Step 1: $CO_2$ levels in the blood increase above normal.
  • Step 2: Blood becomes more acidic (decrease in $pH$) due to the formation of carbonic acid.
  • Step 3: Receptor cells in the carotid artery in the neck and chemoreceptors in the medulla oblongata detect high $CO_2$ (or low $pH$).
  • Step 4: Impulses are sent to the medulla oblongata (control centre).
  • Step 5: The medulla oblongata stimulates breathing muscles (intercostal muscles and diaphragm) and the heart.
  • Step 6: Breathing muscles contract more actively, increasing the rate and depth of breathing (faster and deeper).
  • Step 7: The heart beats faster, bringing blood with $CO_2$ to the lungs more rapidly.
  • Step 8: Excess $CO_2$ is exhaled from the lungs.
  • Step 9: $CO_2$ level in the blood decreases back to normal.

Osmoregulation: Water and Salt Balance

• Importance: The human body is more than $66\%$ water. Proper water and solute concentrations are necessary for cell metabolism and blood pressure regulation.
• Water Balance via ADH (Anti-Diuretic Hormone):
  • In Case of Excessive Water Loss (Dehydration):
    1. Osmoreceptors in the hypothalamus detect dehydration.
    2. Hypothalamus activates the pituitary gland to release more ADH into the blood.
    3. ADH is transported to the kidneys (effector organ).
    4. ADH causes the pores of the collecting ducts in the nephrons to become more permeable to water.
    5. More water is reabsorbed from the renal tubules back into the blood.
    6. The result is a smaller volume of concentrated urine.
    7. Water levels in the blood rise back to the set point.
  • In Case of Excessive Water Intake (Overhydration):
    1. Hypothalamus detects high water levels.
    2. Pituitary gland releases less ADH.
    3. The collecting ducts of the nephron become less permeable.
    4. Less water is reabsorbed; more water is excreted in the urine.
    5. Water balance is restored.
• Salt Regulation via Aldosterone:
  • Biological Importance: Sodium ($Na^+$) and potassium ($K^+$) ion concentrations determine osmolarity/osmotic pressure, which is critical for cell metabolism.
  • Mechanism for Low Salt/High Potassium:
    1. Low $Na^+$ or high $K^+$ levels are detected by receptor cells in the afferent arterioles of the kidneys.
    2. Hormones (renin) signal the adrenal cortex (control centre).
    3. The adrenal cortex releases aldosterone.
    4. Aldosterone travels to the distal tubules in the nephrons of the kidneys.
    5. Aldosterone causes more sodium ions to be reabsorbed into the blood and excess potassium ions to be excreted.
    6. Ion levels return to the set point.
  • Mechanism for High Salt:
    1. High salt is detected by receptors in the arterioles of the kidneys.
    2. The adrenal glands release less aldosterone.
    3. The kidneys reabsorb less salt (less $Na^+$).
    4. Salt balance is restored.

Thermoregulation and the Role of the Skin

• Definition: Thermoregulation is the maintenance of a constant internal body temperature ($37^\circ\text{C}$) by balancing heat production and heat loss. Optimal enzyme functioning depends on this; enzymes denature above $42^\circ\text{C}$.
• Biological Types:
  • Endothermic animals: (e.g., mammals and birds) Regulate their own temperature via respiration; temperature remains constant.
  • Ectothermic animals: (e.g., reptiles and fish) Obtain heat from the environment; temperature varies.
• Methods of Heat Loss:
  • Radiation: Loss of heat from a warm body to a colder environment.
  • Conduction: Loss of heat through physical contact with colder objects.
  • Convection: Warm air rises from a hot object and is replaced by cold air.
  • Evaporation: The main cooling method on hot days; body heat is used to change sweat into water vapor.
• Major Structures of the Skin:
  • Epidermis: Outer layer.
  • Dermis: Middle layer containing glands and vessels.
  • Hypodermis/Subcutaneous fat: For insulation.
  • Sebaceous glands: Produce sebum (oily/waxy) to prevent dehydration.
  • Blood vessels: Involved in vasodilation and vasoconstriction.
  • Sweat glands: Produce sweat for evaporative cooling.
  • Sensory/Thermo-receptors: Detect temperature changes.

Negative Feedback in Temperature Control

• When the Body is Too Hot (Vasodilation):
  • Hypothalamus is stimulated by increased blood temperature or impulses from skin thermoreceptors.
  • Vasodilation: Circular muscles of dermal arterioles relax, and capillaries dilate (widen).
  • Increased blood flow near the skin surface leads to heat loss via radiation, conduction, and convection.
  • Sweat glands become more active; increased evaporation of sweat cools the blood.
  • Metabolic rate decreases to produce less heat.
• When the Body is Too Cold (Vasoconstriction):
  • Hypothalamus is stimulated by decreased blood temperature or cold detection by skin receptors.
  • Vasoconstriction: Circular muscles in dermal arterioles constrict, making capillaries narrower.
  • Blood is diverted from the skin surface to deeper tissues to reduce heat loss.
  • Sweat glands become inactive, reducing evaporation.
  • Metabolic rate increases to generate more heat.
  • Skeletal muscles are activated to cause shivering, generating additional heat.

Disorders and Imbalances

• Thyroxin Imbalance: Leads to a Goitre.
• Blood Glucose Imbalance: Causes Diabetes mellitus.
• Hypothermia: Occurs when body temperature remains below $37^\circ\text{C}$ for a long period; the body loses more heat than it can produce.
• Hyperthermia: Occurs when body temperature remains above $37^\circ\text{C}$ for a long period; the body absorbs more heat than it can lose.

Comparative Summary Table