Water and Acid-Base Balance

Fluid Compartments

The human body maintains water and solute balance through distinct fluid compartments. Understanding these compartments is essential for grasping fluid and electrolyte regulation.

• Extracellular Fluid (ECF): Includes all fluid outside cells, such as plasma and interstitial fluid.

• Intracellular Fluid (ICF): Refers to fluid within cells, making up the majority of total body water.

• Fluid Shift: Movement of water between ECF and ICF, often driven by osmotic gradients.

Example: Dehydration causes water to move from ICF to ECF to maintain blood volume.

Fluid Balance

Fluid balance is achieved when water intake equals water loss. This balance is vital for cellular function and overall homeostasis.

• Fluid Balance: The equilibrium between water gained (via ingestion and metabolism) and water lost (via urine, sweat, feces, and respiration).

• Electrolyte Balance: Maintenance of appropriate concentrations of ions (e.g., Na+, K+, Cl-) in body fluids.

• Acid-Base Balance: Regulation of hydrogen ion concentration to keep pH within a narrow range.

Example: Excessive sweating can disrupt both fluid and electrolyte balance, leading to dehydration and hyponatremia.

Regulation of Water and Solute Loss

Homeostatic Regulation

Homeostasis monitors the ECF to regulate water and solute losses and gains. Receptors detect changes indirectly, and active transport mechanisms cannot move water directly.

• Losses vs. Gains: The body must balance water and solute losses (e.g., urine, sweat) with gains (e.g., drinking, eating).

• Receptors: Monitor fluid and electrolyte status indirectly, such as by sensing blood pressure or osmolarity.

• Active Transport: Moves solutes but not water; water movement occurs via osmosis.

Example: Osmoreceptors in the hypothalamus detect changes in plasma osmolarity and trigger thirst or ADH release.

Regulation of Water/Solute Loss

Water and solute loss are regulated by hormonal mechanisms and can result in disorders if imbalanced.

• NaCl Loss: Loss of sodium chloride can lead to hyponatremia (low sodium) or hypernatremia (high sodium).

• Hyponatremia: Occurs when sodium concentration in ECF falls below normal.

• Hypernatremia: Occurs when sodium concentration in ECF rises above normal.

Hormonal Control

• Angiotensin II: Stimulates thirst and release of aldosterone, increasing sodium retention.

• Aldosterone: Promotes sodium reabsorption in kidneys, increasing water retention.

• Antidiuretic Hormone (ADH): Increases water reabsorption in kidneys, reducing urine output.

• Natriuretic Peptides: Promote sodium and water excretion, lowering blood volume and pressure.

Example: High blood pressure triggers natriuretic peptide release, leading to increased urine production.

Acids and Bases

Definitions and Properties

Acids and bases are fundamental to understanding pH and homeostasis in the body.

• Acid: Substance that increases the concentration of hydrogen ions ([H+]) in a solution.

• Base: Substance that decreases [H+] by absorbing H+ or forming OH- ions.

• Strong Acid/Base: Completely dissociates in solution, significantly affecting pH.

• Weak Acid/Base: Partially dissociates, providing a buffering effect.

Example: Hydrochloric acid (HCl) is a strong acid; acetic acid (CH3COOH) is a weak acid.

pH Balance

Normal Range and Regulation

The body maintains pH within a narrow range for optimal enzyme function and cellular processes.

• Normal pH Range: 7.35 – 7.45

• Minimum/Maximum pH: 6.8 (minimum) and 7.7 (maximum) for physiological survival.

• Homeostasis: Maintains pH through buffer systems and compensatory mechanisms.

• Buffer Systems: Chemical systems that resist changes in pH.

• Compensation: Adjustments by respiratory and renal systems to restore pH.

Example: The kidneys excrete excess H+ to compensate for acidosis.

Buffer Systems

Types of Buffer Systems

Buffer systems stabilize pH by neutralizing excess acids or bases.

• Protein Buffer System: Proteins in plasma and cells act as buffers by binding or releasing H+.

• Phosphate Buffer System: Important in intracellular fluid and urine; uses phosphate ions to buffer pH.

• Carbonic Acid-Bicarbonate Buffer System: Major buffer in ECF; involves reversible reactions between CO2, H2CO3, and HCO3-.

Example: The carbonic acid-bicarbonate system buffers blood pH during exercise.

Bicarbonate Buffer System Equation

The carbonic acid-bicarbonate buffer system is represented by the following equation:

Acid-Base Disorders

Definitions and Types

Acid-base disorders occur when pH regulation fails, leading to abnormal blood pH.

• Acidemia: Blood pH below 7.35; "acidosis" refers to the process causing acidemia.

• Alkalemia: Blood pH above 7.45; "alkalosis" refers to the process causing alkalemia.

• Compensation: The body's response to restore pH, which can be complete or partial, and involve respiratory or renal mechanisms.

Respiratory Acid-Base Disorders

• Respiratory Acidosis: Caused by hypoventilation or impaired gas exchange, leading to hypercapnia (excess CO2).

• Acute: Sudden onset, such as airway obstruction.

• Chronic: Long-term, such as in chronic obstructive pulmonary disease (COPD).

• Respiratory Alkalosis: Caused by hyperventilation, leading to hypocapnia (reduced CO2).

Example: Panic attacks can cause respiratory alkalosis due to rapid breathing.

Metabolic Acid-Base Disorders

• Metabolic Acidosis: Results from increased acid production or loss of bicarbonate.

• Lactic Acidosis: Accumulation of lactic acid, often due to hypoxia.

• Ketoacidosis: Accumulation of ketone bodies, common in uncontrolled diabetes.

• Kidney Damage: Impaired excretion of acids.

• Bicarbonate Loss: Loss through diarrhea or renal dysfunction.

• Metabolic Alkalosis: Excessive loss of acids or gain of bases; can occur during the "alkaline tide" after meals.

Example: Prolonged vomiting can cause metabolic alkalosis due to loss of gastric acid.

Summary Table: Acid-Base Disorders

Additional info: The "alkaline tide" refers to a temporary increase in blood bicarbonate after eating, due to gastric acid secretion.