Homeostasis

• Definition: Maintenance of a functional internal environment despite changes in internal and external conditions.
  • Involves keeping physiological parameters within tightly regulated tolerance limits, rather than static conditions.

Homeostatic Mechanisms

• React to large external fluctuations affecting the internal environment:
  • Example: Temperature changes, dehydration, or over-hydration.
• Maintain small internal fluctuations at the cellular level.

Hormonal Control Systems

• Key Systems with Hormonal Control:
  • Sleep
  • Fluid balance
  • Blood pressure
  • Blood glucose level
  • Energy level

Fluid Regulation

• Fluid Compartments:
  • Intracellular fluid: 67%
  • Extracellular fluid: 33% (7% blood plasma, 26% interstitial fluid)

Osmosis

• Definition: Movement of water across a semipermeable membrane from areas of lower solute concentration to those of higher.

Osmoregulation

• Definition: Maintenance of constant osmotic pressure in bodily fluids by regulating water and salt concentrations.
  • Normal condition: Isotonic at 0.9% saline.
  • Hypertonic: More concentrated extracellular fluid.
  • Hypotonic: More dilute extracellular fluid.

Types of Thirst

1. Osmotic Thirst:
   • Triggered by eating salt/sugar, increasing plasma osmolality and drawing water out from cells.
   • Adding water dilutes extracellular fluid, decreasing osmolality.
2. Hypovolemic Thirst:
   • Results from loss of bodily fluids (hemorrhage, vomiting).
   • Fluid replacement must be isotonic to rehydrate effectively.

Regulation of Fluid Balance

• Osmotic Dehydration:

  • Cerebral osmoreceptors release ADH (antidiuretic hormone) to conserve water via kidneys.
  • Thirst response activated if dehydration persists.

• Hypovolemic Dehydration:

  • Baroreceptors respond to low blood volume, increasing heart rate, releasing ADH, renin, and aldosterone.
  • Retains water and salts, triggering thirst as well.

Hormones in Fluid and Electrolyte Balance

• ADH:

  • Produced in hypothalamus, released by the posterior pituitary.
  • Increases water retention in kidneys, enhances collecting duct's water permeability.

• Aldosterone:

  • Secreted from adrenal cortex in response to angiotensin II.
  • Stimulates sodium reabsorption in the kidneys.

Renin-Angiotensin-Aldosterone System (RAAS)

• Baroreceptors activate RAAS in response to low blood pressure:
  • Renin from kidneys → converts angiotensinogen to angiotensin I.
  • ACE in lungs converts angiotensin I to angiotensin II.
  • Angiotensin II promotes vasoconstriction and stimulates ADH and aldosterone release, aiding in blood pressure regulation.

Blood Pressure Regulation

• Blood pressure influenced by:
  • Blood volume
  • Resistance in blood vessels
  • Blood viscosity

Blood Glucose Regulation

• Glucose: Primary energy source for body's cells; must be kept within narrow range.
  • Hyperglycemia: Too much glucose; can lead to diabetes.
  • Hypoglycemia: Too little glucose.
  • Hormonal regulation with insulin (lowers blood glucose) and glucagon (raises blood glucose).

Insulin Action

• Produced by beta cells of pancreas, promotes glucose uptake post-meal, facilitating energy production or storage (glycogenesis).

Glucagon Action

• Produced by alpha cells in response to low glucose, stimulates liver to produce glucose from glycogen (glycogenolysis) and amino acids (gluconeogenesis).

Balancing Insulin and Glucagon

• Insulin decreases blood glucose while glucagon increases it, maintaining balance.

Diabetes Overview

• Type I Diabetes: Insulin deficiency due to autoimmune destruction of pancreatic beta cells.
• Type II Diabetes: Insulin resistance; often related to diet and lifestyle.

Hormonal Control of Energy Balance

• Hormones involved: Insulin, glucagon, ghrelin, leptin, CKK.
  • Ghrelin: Hunger hormone, increases appetite.
  • Leptin: Satiety hormone, reduces appetite when energy stores are sufficient.
  • CCK: Also promotes satiety after food intake.

Guiding Questions for Study

• What is homeostasis, and why is it not a static process?
• Identify five systems under hormonal control for homeostatic processes.
• What are the two types of thirst, and how are they remedied?
• Detail how blood pressure and blood glucose are maintained in the body.
• Explain energy balance and the hormones involved in its regulation.