Anatomy Ch. 24: Fluid, electrolyte, and acid-base balance

Types of Homeostatic Balance

1. Fluid balance

2. Electrolyte balance

3. Acid-base balance

Fluid Balance

Achieved when daily water gains equal losses

Electrolyte Balance

Maintained when the electrolytes ingested match those excreted

Acid-base Balance

Maintained when hydrogen ions (H+) are excreted at the same rate they are produced

Total Body Water (TBW)

• Percentage of body water depends on:

  • Biological gender

  • Age

  • Body composition

Intracellular Fluid (ICF)

• 65%

• Inside cells

Extracellular Fluid (ECF)

• 35%

• Outside cells:

  • Tissue (interstitial) fluid

  • 25%

  • Between cells

  • Blood plasma & Lymph

  • 8%

  • Circulating fluid

  • Transcellular fluid

  • 2%

  • CSF, synovial, pleural, peritoneal

Water Movement Across Capillaries

Water moves via capillary filtration into tissue fluid

Water Movement Across Cell Membranes

Water moves via osmosis

Driving Force:

• Osmotic movement is determined by solute concentration in ICF vs. ECF

(Na+) - mostly in ECF

(K+) - mostly in ICF

Water Gains (Input)

• Preformed water- from food and drink

• Metabolic water- produced during cellular metabolism

Water Losses (Output)

• Sensible losses- detectable, urine, sweat

• Insensible losses- not easily noticed

  • Cutaneous transpiration (through skin, evaporates)

  • Respiratory loss (increases in cold air)

• Losses vary with physical activity and environmental conditions

Water Intake Regulation (Thirst Mechanism)

Main control: Thirst (regulated by hypothalamus)

When you are thirsty:

Saliva production decreases → dry mouth

• Hypothalamus inhibiting salivary glands

• Low blood pressure or high osmolarity

After Drinking Water

Short-term Control:

• Prevents over drinking

• Satiety lasts 30-45 minutes

Long-term Control:

• Blood osmolarity decreases

• Hypothalamus stops thirst signals

Water Output Regulation Mechanisms

Water output is mainly controlled by adjusting urine volume in two ways:

1. Sodium (Na+) Reabsorption

2. Antidiuretic Hormone (ADH)

Sodium (Na+) Reabsorption

• Water follows sodium (osmosis)

• If Na+ is reabsorbed → water is reabsorbed → less urine

• If Na+ is excreted → water is lost → more urine

Antidiuretic Hormone (ADH)

• ADH acts on the kidney collecting ducts

• Causes cells to insert aquaporins (water channels)

• This allows more water to be reabsorbed into the blood

• Less urine volume

• More urine concentration

Negative Feedback Loop (Dehydration Example)

1. Dehydration → increase blood osmolarity

2. Hypothalamus detects this (osmoreceptors)

3. Posterior pituitary releases ADH

4. Kidneys reabsorb more water

5. Blood osmolarity decreases → system shuts off

Hypovolemia (Volume Depletion)

• Loss of water + sodium together

• Causes:

  • Blood loss

  • Burns

  • Vomiting/diarrhea

Dehydration (Negative Water Balance)

• Water loss > sodium loss

• Causes:

  • Not drinking enough

  • Extreme temperatures

  • Heavy sweating

• Affects all fluid compartments

• Infants are more vulnerable

Fluid Volume Excess

• Too much water + sodium retained

Example: Kidney (renal) failure

Water Intoxication

• Too much water relative to sodium

Example: Sweating a lot and only replacing with plain water

• Dangerous → dilutes electrolytes

Electrolyte Balance

Balance between electrolytes absorbed and lost from the body

Functions of Electrolytes

• Participate in metabolism, chemically reactive

• Help create electrical signals (potential) across cell membranes

• Control osmolarity

• Regulate water distribution in the body

Cations (+)

• Na+ (sodium)

• K+ (potassium)

• Ca2+ (calcium)

• Mg2+ (magnesium)

• H+ (hydrogen)

Anions (-)

• Cl- (choloride)

• HCO3-(bicarbonate)

• PO43-(phosphate)

Sodium (Na+) Functions

• Essential for nerve and muscle signaling

• Hydration in cartilages

• Major factor determining total body water

• Provides energy for transport of other substances (glucose, potassium, calcium)

Sodium Regulation Mechanisms (Homeostasis)

1. Aldosterone (“Salt-Retaining Hormone”)

2. ADH

3. Natriuretic Peptides

Aldosterone (“Salt-Retaining Hormone”)

• Low Na+ levels in the blood (hyponatremia) → the body wants to conserve salt

• High K+ levels (hyperkalemia) → aldosterone also helps get rid of extra potassium

• Low blood pressure or low blood volume → detected by the kidneys, which activate the RAAS System (Renin-Angiotensin-Aldosterone System) to raise BP

Sodium Regulation - ADH

Mainly controls water reabsorption, but indirectly affects sodium concentration

• High Na+ (hypernatremia) in the blood triggers ADH release from the posterior pituitary

• ADH makes the kidney’s reabsorb more water → blood becomes more diluted

ADH Affect on Sodium

More water reabsorbed → lowers sodium concentration in blood

→ ADH controls water, not sodium directly

Sodium Regulation - Natriuretic Peptides

Removes excess sodium and water

Triggered by: High blood volume/pressure → released from heart

Natriuretic Peptides Effects

• Decrease sodium reabsorption → more sodium lost in urine

• Water follows sodium → increases urine volume

• Lowers blood volume and pressure

Functions of Potassium

• Most abundant cation inside cells (ICF)

• Main role in intracellular osmolarity → helps control cell volume

Works with Sodium to:

• Create resting membrane potential

• Generate action potentials (nerve & muscle activity)

• Maintain the Na+/K+ pump

Also important for:

• Protein synthesis

• Enzyme function

Regulation (Homeostasis) of K+

• Controlled mainly by aldosterone

• Relationship with sodium

  • Increase Na+ reabsorption, increase K+ excretion

  • Decrease Na+ reabsorption, decrease K+ excretion

More sodium in urine = less potassium in urine (And other way around)

Functions of Calcium

• Provides strength to bones

• Needed for muscle contraction (sliding filament mechanism)

• Acts as a second messenger for some hormones and neurotransmitters

• Triggers neurotransmitter release (exocytosis)

• Essential for blood clotting

Calcitriol (Vitamin D)

Increase blood Ca2+ by increasing intestinal absorption

Parathyroid Hormone (PTH)

Increase blood Ca2+ by:

• Increasing bone breakdown

• Increasing kidney reabsorption

Calcitonin

Decreases blood Ca2+

• Promotes calcium storage in bone

Chloride

• Most abundant anion in extracellular fluid

• Helps maintain osmolarity (fluid balance)

• Needed to make stomach acid (HCl)

• Important for pH regulation

• Closely follows sodium → they move together

Magnesium

• Acts as a cofactor (helper) for enzymes, transporters, and nucleic acids

• Absorbed in intestines → regulated by vitamin D

• Lost through feces and urine

Phosphate

• Needed for ATP

• Helps buffer and stabilize pH

• Constantly filtered by kidneys

• If levels are low → kidneys reabsorb more phosphate

Acid-Base Balance

Normal blood pH: 7.35 - 7.45

• Enzymes depend on proper pH → even small changes can:

  • Stop metabolic pathways

  • Change protein structure/function

• Acid-base balance = maintaining stable pH in body fluids

Acids

Release H+

• Strong (ex: HCl) → big pH drop

• Weak (ex: carbonic acid) → small effect

Bases

Accept H+

• Strong → big pH increase

• Weak → small effect

Buffers

• Prevent big pH changes

• Convert strong acids/bases → weak ones

• Keep pH stable

Physiological Buffers (Whole-body system)

A system that controls output of acids, bases, or CO2

• Respiratory System

  • Fast (minutes)

  • Controls CO2

• Urinary (Kidneys)

  • Slow (hours-days)

  • Stronger effect on pH

Chemical Buffers

Act within seconds to prevent sudden pH changes

• Work by either:

  • Binding H+ (removing acid → raises pH)

  • Releasing H+ (adding acid → lowers pH)

Main Buffer Systems

• Bicarbonate (most important in ECF)

• Phosphate (ICF + Kidneys)

• Protein (Most abundant & powerful overall)

Bicarbonate Buffer System

A solution of carbonic acid and bicarbonate ions

CO₂ + H₂O <→ H₂CO₃ ←> HCO₃^- + H⁺

Balance between:

• CO2 (Controlled by lungs)

• HCO₃^- (Controlled by kidneys)

• H⁺ (determines pH)

If the Reaction Shifts RIGHT (→)

CO₂ + H₂O → H₂CO₃ → HCO₃^- + H⁺

• More H⁺ produced

• pH drops → more acidic (acidosis)

• Happens when:

  • CO₂ builds up (ex: slow breathing)

If the Reaction Shifts LEFT (←)

CO₂ + H₂O ← H₂CO₃ ← HCO₃^- + H⁺

• H⁺ is used up (bound)

• pH rises → more basic (alkalosis)

• Happens when:

  • CO₂ is removed (ex: rapid breathing)

How the lungs Control the Bicarbonate Buffer System

• Controls CO₂ levels

  • Exhale more CO₂ → decrease H⁺ , increase pH

  • Retain CO₂ → increase H⁺ , decrease pH

How the kidneys Control the Bicarbonate Buffer System

• Control H⁺ and HCO^-₃

  • Excrete H⁺ → increase pH

  • Reabsorb HCO^-₃ → increase pH

  • Excrete HCO^-₃ → decrease pH

Phosphate Buffer System Components

• H₂ PO^-₄ (dihydrogen phosphate) → weak acid

• HPO₄^2- (monohydrogen phosphate) → weak base

H₂ PO^-₄ ←> HPO₄^2- + H⁺

Phosphate Buffer System Shift (RIGHT →)

• Release H⁺

• Decrease pH (more acidic)

Phosphate Buffer System Shift (LEFT →)

• Binds H⁺

• Increases pH (more basic)

Where Phosphate Buffer System Function

• Intracellular fluid (ICF)

• Renal (Kidney) tubules

Why Phosphate Buffer System is Effective in those areas

• Optimal pH = 6.8

• ICF pH= ~7.0 → close to optimal → works efficiently

• Renal tubules have lower (more acidic) pH than blood → ideal conditions for this

Phosphate Buffer System Physiological Role

• Helps neutralize metabolic acids produced inside cells

• In kidneys:

  • H⁺ binds to HPO₄^2- → forms H₂PO₄^-

  • Allows safe excretion of H⁺ in urine

Protein Buffer System

• Proteins = ~75% of buffering capacity

• Work because amino acids can act as acid OR base

Carboxyl group (-COOH)

• Released H⁺ → lowers pH

Amino group (-NH₂)

• Binds H⁺ → raises pH

Breathing & pH (Respiratory Control)

• CO2 increases, H+ increases → pH decreases (acidic)

• CO2 decreases, H+ decreases → pH increases (basic)

Body Response Low pH (acidic)

• Brain senses this

• Increases pulmonary ventilation

• CO2 removed → pH rises

Physical activity

Body Response High pH (basic)

• Reduced pulmonary ventilation

• CO2 retained → pH drops

Relaxation

Acidosis (pH < 7.35)

• Too much H⁺ in body

• Causes hyper polarization (cells harder to excite)

Symptoms:

• Muscle weakness

• Fatigue

• Confusion → coma

Alkalosis (pH > 7.45)

• Too little H⁺

• Causes overexcitable cells

Symptoms:

• Muscle spasms

• Tetany (sustained contraction)

• Seizures

Acidosis - Why symptoms happen

• H⁺ moves into cells

• K⁺ moves out of cells

• Inside becomes more negative → hyper polarized

• Harder to reach threshold → decrease nerve activity

Alkalosis - Why symptoms happen

• H⁺ moves out of cells

• K⁺ moves into cells

• Membrane closer to threshold

• Nerve fire too easily → spasms + tetany