Integrative Physiology: Fluid and Electrolyte Balance (copy)

Rutgers University, School of Environmental and Biological Sciences, Integrative Physiology Course (11:067:300)

Total Body Water Composition:
  - In a normal weight adult human, body mass in fluids comprises 55–60%.
  - Body fluids are made up of two compartments:
    - Intracellular Fluid (ICF): Fluid within the cells (approximately 2/3 of total body fluid).
    - Extracellular Fluid (ECF): Fluid surrounding the cells, including plasma and interstitial fluid (approximately 1/3 of total body fluid).
Body Fluids Breakdown:
  - For females: 45% solids, 55% fluids
  - For males: 40% solids, 60% fluids
  - ECF distribution: 80% interstitial fluid, 20% plasma

Water Gain:
- Ingestion: A primary source of water intake.
- Metabolic synthesis: Water produced as a byproduct of metabolic processes.
Water Loss:
  - Water loss must equal water gain to maintain homeostasis.
  - Loss pathways include:
    - Urine
    - Perspiration
    - Lung exhalation
    - Feces

Stimuli for thirst include:
  - Increased blood osmolarity
  - Decreased blood volume: Triggers thirst response pathways.
    - Stimulates osmoreceptors located in the hypothalamus.
    - Decreases activity of atrial volume receptors and baroreceptors in blood vessels, which also lowers blood pressure.
    - Increases the release of renin from the kidneys, stimulating thirst center in the hypothalamus and subsequently increasing angiotensin II formation, leading to increased thirst and water intake.
Physiological response: Increased water intake decreases blood osmolarity, increases blood volume and blood pressure, alleviating mouth dryness.

Integrated Responses to Changes in Blood Pressure and Volume:
  - Blood volume and pressure are monitored by:
    - Volume receptors in atria
    - Carotid and aortic baroreceptors
  - Receptors send signals to:
    - Cardiovascular system (affects cardiac output and causes vasoconstriction)
    - Kidneys (conserve water to minimize further volume loss)
    - Thirst (causing increased water intake)
Kidney Function:
- Conserve H2O to regulate body fluid volumes.

The kidneys help conserve volume but cannot fully replace lost volumes.
Glomerular Filtration Rate (GFR) can be adjusted; if volume falls too low, GFR stops.
Kidneys regulate the reabsorption of water to manage volume loss and maintain body fluid balance.

Osmolarity of Urine:
  - Urine osmolarity can range from 50–1200 mOsM.
  - Changes in osmolarity occur as filtrate flows through the nephron:
    - Descending Loop of Henle: Water is reabsorbed, increasing concentration (becomes more concentrated in descending limb).
    - Ascending Loop of Henle: Salt reabsorption occurs but is impermeable to water, resulting in dilute fluid exiting.
Hormonal Control:
- Hormones regulate kidney permeability to water and solutes, dictating urine osmolarity dependent on distal nephron reabsorption.

Antidiuretic Hormone (ADH):
- Increases collecting duct permeability to water, facilitating water reabsorption into the bloodstream.
Aldosterone:
- Enhances Na+ reabsorption, favoring water reabsorption.
Atrial Natriuretic Peptide (ANP):
- Acts to prevent Na+ reabsorption and consequently reduces water retention, leading to increased urine output.

Stimuli for ADH Release:
  - Increased blood osmolarity
  - Decreased blood volume and pressure
  - Pain, nausea, and stress responses also trigger ADH release.
ADH Pathway:
- Increased synthesis and release of ADH occurs in response to stimuli.
- ADH acts on the late distal tubules and collecting ducts, increasing their permeability to water, which boosts water reabsorption and ultimately increases blood volume and decreases osmolarity.

Water Reabsorption Features:
- Vasopressin increases permeability of the collecting duct, increasing reabsorption rates and influencing urine concentration.
- The process is characterized by the insertion of Aquaporin-2 (AQP2) water channels into cell membranes, enhancing osmosis into the bloodstream.

Electrolyte Functions:
  - Sodium (Na⁺): Main cation in extracellular fluid; contributes significantly to osmolarity. Controlled by ADH, aldosterone, and ANP.
  - Chloride (Cl⁻): Predominantly balances anions across compartments; readily crosses membranes via Cl⁻ channels.
  - Potassium (K⁺): Vital for establishing resting membrane potential; regulated by aldosterone.
  - Bicarbonate (HCO₃⁻): Central in pH regulation and CO₂ homeostasis.
  - Calcium (Ca²⁺): Predominantly stored in bones; acts as a signaling molecule for various cellular processes.
  - Phosphate (HPO₄²⁻): Functions prominently as a buffer.
  - Magnesium (Mg²⁺): Acts as a cofactor for numerous enzymatic reactions.

Body’s Response to Increased Sodium:
- Ingestion of sodium (e.g., salt) stimulates ADH secretion, causing renal water reabsorption and an increase in blood volume.
- Aldosterone enhances sodium reabsorption in distal nephron, which collectively contributes to blood volume recovery and maintenance.

pH Regulation:
  - Cosntant homeostasis of blood pH (~7.4) is multifaceted, influenced by:
    - Buffer systems (e.g., bicarbonate)
    - Ventilation rates (exhalation of CO₂)
    - Renal mechanisms (excretion and reabsorption of H⁺)
Disturbances in Acid-Base Balance:
- Acidosis: Blood pH < 7.35 due to excess H⁺; can arise from respiratory failure ( high PCO₂) or metabolic dysfunctions (HCO₃ loss). - Alkalosis: Blood pH > 7.45; results from improper ventilation (low PCO₂) or excess HCO₃ intake.

Detailed overview of various stimuli and the responses triggered in cardiovascular systems, kidneys, and hypothalamus indicating shifts in blood pressure and osmolarity.
Tables refer to stimuli (increased/decreased blood pressure) affecting renal functions, cardiovascular responses, and thirst mechanisms to maintain fluid and electrolyte homeostasis.