Chapter 21

Water and electrolytes are interdependent in the body; changes in one affect the other.
Electrolytes: Ions dissolved in water; crucial for various bodily functions.
Most important electrolytes: Na+, K+, Ca+2, H+, OH-, Cl-, and Mg+
Homeostasis: Requires that the amount of water and electrolytes entering the body equals the amount exiting.
Balance is maintained through mechanisms that replace lost water/electrolytes and excrete excesses.

Body fluids are not uniformly distributed; they occupy compartments of varying volumes and compositions.
Major Fluid Compartments:
- Intracellular Fluid (ICF): Fluid inside cells (63% of body water).
- Extracellular Fluid (ECF): Fluid outside cells (37% of body water), including:
  - Interstitial Fluid: In tissue spaces.
  - Blood Plasma: In blood vessels, containing proteins.
  - Lymph: In lymphatic vessels.
  - Transcellular Fluid: Separate by epithelial layers (e.g., cerebrospinal fluid, synovial fluid in joints, serous fluid).
Females have more adipose tissue (low in water) and males have more muscle (high in water)

Body fluids are solutions of electrolytes in water with distinct compositions:
- Extracellular fluids: high concentrations of sodium (Na+).
- Blood plasma contains more proteins than interstitial fluid or lymph.
- Intracellular fluid has high concentrations of potassium (K+).
Factors Regulating Fluid Movement:
- Hydrostatic Pressure: Stabilizes fluid levels in cells and interstitial fluids.
- Osmotic Pressure: Exerted by impermeant solutes; primarily affects fluid movement.

Water Balance Definition: When water intake equals water output.
Control of Water Intake: Governed by thirst centers in the brain; influenced by changes in extracellular fluid volume or osmotic pressure.
Control of Water Output: Regulated by the kidneys through mechanisms such as urine production, which is influenced by hormones like aldosterone and antidiuretic hormone (ADH).

Thirst Mechanism: Activated by loss of water, increased osmotic pressure, and reduced blood volume
-Thirst sensation is derived from change in volume or osmotic pressure of
extracellular fluids (ECF)
-Osmotic pressure is due to presence of impermeant solutes (which
cannot cross cell membrane)
-Osmoreceptors in hypothalamus detect changes in osmotic pressure of
body fluids

Response to loss of 1% of body water:
• Osmoreceptors detect increase in osmotic pressure, and stimulate
thirst mechanism
• Stretch receptors in blood vessels detect water loss due to
hemorrhage, and stimulate thirst center
• Decrease in blood pressure activates renin-angiotensin system;
angiotensin II stimulates thirst center
• Drinking fluids results in stomach distension, which inhibits thirst
center

Distal tubules and collecting ducts of Kidneys manage water excretion, influenced by anti-diuretic hormone (ADH):
- During dehydration, ADH increases water reabsorption in kidneys.
- Excess water loss causes osmoreceptors to lose water and shrink, which leads to the stimulation of thirst and the release of ADH to promote water retention in the body.
- Excess water intake inhibits ADH, leading to increased urine volume.

Electrolyte Balance: Occurs when intake equals output; crucial for cellular function.
Sources of Electrolytes: Primarily from dietary intake, drinks, and metabolism.
Methods of Electrolyte Output:
- Primarily through urine, with some loss in sweat and feces.

Positively charged ions are vital for physiological functions (e.g., nerve conduction, muscle contractions).
Kidneys adjust electrolyte losses in urine
Aldosterone: Hormone regulating sodium levels; increases absorption in renal tubules.

Acids and Bases:
- Acids release H+ ions in solution; bases combine with H+ to reduce acidity.
Normal pH range of internal environment: 7.35 to 7.45.
Slight pH changes can significantly impact metabolic reactions.

Strong Acids: Ionize completely (e.g., HCl); weak acids ionize partially (e.g., carbonic acid).
Strong Bases: Ionize entirely; weak bases ionize less completely (e.g., bicarbonate).

Maintenance of acid-base balance involves chemical buffers, respiratory regulation, and renal excretion.

Buffers: Stabilize pH in body fluids by combining with acids or bases (e.g., bicarbonate, phosphate buffers).
Bicarbonate Buffer System: Converts strong acids to weak acids, maintaining pH levels.
Phosphate Buffer System: Plays a crucial role in intracellular fluid and urine, helping to regulate pH by neutralizing excess acids or bases.
Protein Buffer System: Utilizes proteins, such as hemoglobin, which can bind to hydrogen ions, thereby minimizing changes in pH and providing a critical mechanism for maintaining acid-base balance in the body.

Respiratory Center:

Respiratory Excretion: CO2 regulation through breathing patterns.
Renal Excretion: Nephrons regulate H+ concentrations in urine through tubular secretion.

Acidemia: pH below 7.35; occurs from acid accumulation or base loss (e.g., metabolic acidosis).
Alkalemia: pH above 7.45; occurs from base accumulation or acid loss (e.g., respiratory alkalosis).
Types of Acidosis:
- Respiratory Acidosis:
  - Caused by increased CO2 levels in the blood.
  - Symptoms may include labored breathing and cyanosis.
  - Causes: Brainstem injury, airway obstruction, or diseases affecting gas exchange.
- Metabolic Acidosis:
  - Caused by the accumulation of acids or loss of bases.
  - Causes: Kidney disease, prolonged diarrhea, or diabetes mellitus (which produces acidic ketones).
Types of Alkalosis:
- Respiratory Alkalosis:
  - Caused by excessive loss of CO2.
  - Results from hyperventilation due to anxiety, fever, poisoning, high altitude, or playing a musical instrument.
- Metabolic Alkalosis:
  - Caused by loss of ions or accumulation of bases.
  - Causes: Gastric drainage, vomiting, diuretics, or excessive antacid use.

Compensation: Physiological adjustments made to counteract pH shifts during imbalances (chemical buffers, respiratory changes, renal adaptations).