Chapter 26 - Fluid, Electrolyte, and Acid-Base Balance

Intracellular Fluid (ICF)

• ~66% of total body water found here

• K+, proteins, hydrogen phosphate 

Extracellular fluid (ECF) 

• ~33% of reming body water found here

• Two subcompartments 

  • Plasma 

  • Interstitial fluid (IF) 

    • Lymph, cerebrospinal fluid, humors. serous fluid, synovial fluid 

• Na+, Cl-, bicarbonate 

Electrolytes 

• anything that dissociate into ions in water

•  (+)  or (-) charge

• Most abundant solutes 

• More responsible for fluid shifts/movement of water 

  • Ex: inorganic salts, acids & bases, some proteins 

Non-electrolytes 

• do not dissociate in water

• No charge

• Make up the bulk of the body fluids 

• Ex: glucose, urea, lipids, etc

Optimal body water content depends on 

• Age

• Sex

• Body fat %

Sources of water intake 

• Ingested food and liquid 

• Metabolic water 

Sources of water output 

• Insensible water loss: lungs, skin 

• Sensible water loss: sweat, urine, feces 

"When properly hydrated, water intake = […]"

water output 

"Hydration allows body to properly maintain osmolarity of […]"

~300 mOsm

Hypothalmic thirst center controls the thirst mechanism, is activated by: 

1. Osmoreceptors -> detect changing ECF osmolarity 

2. Dry mouth -> salovary glands cannot draw water from blood to produce saliva 

3. Decreasing blood volume/pressure -> ~5-10% drop initiates thirst mechanism 

Feelings of thirst stop almost as soon as we drink water. Why is this important? 

Increase blood volume and increase blood pressure. Push it upwards 

Obligary water loss

The body will always lose water, even if we never drink water 

Why do we have obligary water loss? 

Insensible water loss, kidneys never stop functioning 

"Urine output depends on […], […], […]. Excess water is eliminated in urine. "

fluid intake, diet, other sources of water loss

"ADH causes aquaporins to be inserted in […]"

collecting ducts 

Release of ADH dependent on 

1. Osmoreceptors monitoring osmolarity of ECF

2. Baroreceptors monitoring blood pressure 

Deficiencies in ADH release 

"

• Central diabetes insipidus -> decrease in ADH produced by hypothalamus or released by posterior pituitary 
• Symptoms: polyuria (followed by polydipsia), very dilute urine, fatigue, eventual dehydration 
• Nephrogenic diabetes insipidus -> ADH is produced and released in normal amounts, but the kidneys are unresponsive to it 

"

Importance of Electrolyte Balance 

Influence water movement in body, excitability of neurons, membrane permeability, etc

Salt intake comes mostly from…

diet, with small amount coming from metabolic processes 

Salt loss

• Urine and feces, sweat, vomit 

• Renal processes help body retain what is needed 

"NaHCO₃ and NaCL account for […] of total ECF solute "

~280 mOsm

Why is sodium a key player in maintaining ECF volume? 

1. Most important in establishing osmotic gradient -> water moves with Na+

2. Plasma membranes are impermeable to Na+ -> almost always kept out of cells and in the ECF 

"Most […] reabsorbed in PCT and nephron loop ([…]) "

• Na+

• (~85%) 
  

Hormonal regulation - Aldosterone 

• Release causes increased reabsorption of Na+ in DCT and collecting ducts 

  • Side effect of aldosterone release: increase in ECF volume 

Hormonal regulation - Atrial Natriuretic peptide (ANP)

•  Release causes decreased reabsorption of Na+

  • Is diuretic and natriuretic 

Hormonal regulation - Sex hormones 

• Estrogen exterts similar effect as aldosterone

• Progestrone is slightly diuretic 

Hormonal regulation - Glucocorticoids 

• In high plasma levels, exterts very strong aldosterone-like effects 

  • Can contribute substantially to edema 

What is the importance of potassium balance? 

1. Heavy regulation due to effect on resting membrane potential 

2. Buffer - K+ moves in the opposite direction of H+ to balance pH

Primary mechanism of Potassium balance 

• Renal

• Principal cells secrete K+ in the DCT and colecting ducts 

  • Can alter how much based on what needs to be excreted 

• Type A inertcalated cells can reabsorb K+ when levels are exceptionally low 
  

  • The Kidneys are VERY limited in reabsorption capabilities K+ 

Potassium secretion depends on - Plasma concentration 

• High ECF K+ concentrations drive excess K+ into principal cells -> increased secretion and excretion of K+ 

• Low ECF K+ concentrations promotes reabsorptions 

Potassium secretion depends on - Aldosterone 

• Stimulates K+ secretion 

• Adrenal cortex secretes aldosterone when K+ ECF concentrations are high 

• This has a limited effect -> large shifts in Na+ and volume concentrations do not affect K+ concentrations overall 

  • Renal mechanisms will preserve desirable K+ concentration 

"Optimal pH of arterial blood is […]"

7.35 to 7.45 

"pH 7.45 or higher -> […] "

alkalosis 

"pH of 7.35 or lower -> […] "

physiological acidosis 

Sources of H+ in the body 

1. Ingested food

2. Metabolic processes -> lactic acid, loading of CO2, phosphoric acid, etc 

Chemical Buffer Systems 

• One or more compounds that resist changes in pH when strong acids and bases are intorudced 

• Release H+ when pH rises, binds H+ when pH drops 

Buffer system - Bicarbonate buffer system 

• Important for ECF 

• Mixture of carbonic acid (weak acid) and bicarbonate salt (weak base) 

  • Bicarbonate salt ties up free H+ from a strong acid -> converted to carbonic acid 

    • Conversion of strong acid to weak acid lowers the pH only slightly 

• Cabronic acid ties up free OH- from a strong base -> converted to bicarbonate salt 

  • Converstion of strong base to weak base raises the pH only slightly 

Phosphate buffer system 

"Similar to bicarbonate buffer system, but utilizes different weak acids and bases 
Salts of dihydrogen phosphate (weak acid) and monohydrogen phosphate (weak base) 
The end result is the same -> prevents drastic pH changes "

Protein buffer system 

"important in ICF and blood plasma 

• Carboxyl group (-COOH) can release H+ when pH rises 
• Amine group (NH2) group can bind free H+ when pH decreases 
• Amphoteric molecule: a single protein can react as either an acid or base 
• Depends on the pH of the enviornment 

"

Respiratory Regulation of B+ 

• CO2 accumulation lowers pH of blood

• Rising PCO2 activates respiratory centers 

  • Respiratory rate + depth increases 

  • pH rises as more CO2 is blown off 

• Decreasing PCO2 depresses respiatroy centers 

  • Respiratory rate + depth decrease 

  • pH decreases as CO2 accumulates 

Renal Regulation 

• Important for long-term acid base balance

• Primary mechanism of acid-base balance: adjusting amount of bicarbonate in blood 

Generating new bicarbonate 

"

• PCT and Type A intercalated cells of colelctimg ducts can generate new bicarbonate ions to be reabsorbed 
• H+ must be secreted into filtrate at the same time 

"

Secretion of bicarbonate 

• Type B intercalated cells in collecting ducts can be reabsorb H+ while secreting bicarbonate ions from filtrate 

• Secretion of bicarbonate is not efficient -> even in alkalosis, more bicarbonate will be reabsorbed than secreted 

Respiatory Acidosis 

• PCO2 > 45 mm Hg 

• Respiration is shallow/slow (hyperventilation) 

• Caused by: many respiratory diseases/conditions 

Respiratory alkalosis 

• P_CO2 < 35 mm Hg

• Respiration is deep/fast (hyperventialtion) 

• Caused by: stress/anxiety, pain 

Metabolic acidosis and alkalosis 

• Any acid- base imbalance that does not involve CO2

• Especially bicarbonate ion imbalances 

Metabolic acidosis 

• Low bicarbonate levels

• Common causes: excessive alchol intake, long-term diarrhea 

Metabolic alkalosis 

• High bicarbonate levels 

• Common causes: excessive vomiting, excessive base intake 

Blood pH limits are 6.8 and 7.8. Below and above is

• Below 6.8 - CNS depression -> coma and death 

• Above 7.8 - overstimulated CNS 

  • Muscle tetany (the bad kind), restlessness/nervousness, convulsions, death 

"Kidneys or lungs can act to restore […]"

pH when other organ fails 

Respiatory compensation 

• Changes in respiratory rate and depth evident when lungs must compensate for metabolic imbalances 

  • Metabolic acidosos -> respiratory rate + depth increase 

    • This blows off excess CO2 to increase blood pH again

  • Metabolic alkalosos -> respitaory rate + depth decrease 

    • Conserves CO2 to decrease blood pH to desirable level 

Renal compensation 

• Kidneys can compensate for acid-base imbalances of respiratory origins

• Respiratory acidosis -> kidneys conserve more bicarbonate ions 

• Respiratory alkalosis -> kidneys either secrete more bicarbonate ions or simply do not reabsorb it