Chapter 27 - Fluid, Electrolyte & Acid-base Homeostasis Flashcards
4 Types of Homeostasis
- Fluid Homeostasis
- Electrolyte Homeostasis
- Acid-base Homeostasis
- Nitrogen Homeostasis
Interstitial Fluid (6 Types)
- CSF
- Lymph
- Synovial Fluid
- Aqueous & Vitreous humours
- Pleural, Pericardial & Peritoneal Fluids
- Endolymph & Perilymph
Extracellular Fluid/ECF (2 Subdivisions)
- Interstitial Fluid: 80%
- Plasma: 20%
*Contains 15 L of the 40 L of total body H2O
Intracellular Fluid/ICF
- Contains 25 L of the 40 L of total body H2O
- ICF & ECF compartments maintain distinctive compositions of Na+ & Cl- as well as K+, proteins, & PO4(-3)
4 Rules for Fluid/Electrolyte Balance
- Homeostatic mechanisms respond to changes in ECF
- ECF receptors respond to changes in plasma volume (via baroreceptors) & osmolarity (via osmoreceptors)
- All H2O movements occur passively in response to osmotic gradients (H2O follows solute)
- Body content of H2O salts will accumulate if intake > outflow & vice-versa
4 Hormones Involved in Fluid/Electrolyte Balance
- Antidiuretic Hormone
- Angiotensin 2
- Aldosterone
- Atrial Natriuretic Peptide
Antidiuretic Hormone
- Causes H2O reabsorption in renal CT & CD via aquaporin-2
- ADH release triggered by osmoreceptors or by a large decrease in blood volume
Angiotensin 2
- Stimulates PCT Na+/H+ antiporters -> Increased NaCl & H2O reabsorption -> Increased BP
- Also causes vasoconstriction and secretion of aldosterone
Aldosterone
-Increases Na+, Cl- & H2O reabsorption in CT & CD
2 Triggers of Aldosterone
- JGA cells activate renin-angiotensin-aldosterone mechanism
- Direct release in response to hyperkalemia
Atrial Natriuretic Peptide
- Inhibits ADH & aldosterone
- Promotes fluid/electrolyte losses in urine (natriuresis) by causing relaxation of renal mesangial cells
- Inhibits Na+ & H2O reabsorption by the PCT & CD
Methods of H2O Loss
- Urine, feces, evaporation from skin & lung: 2300 mL
- Sweating: 200 mL
Methods of H2O Gain
- Eating & drinking
- Production of H2O primarily during oxidative phosphorylation by mitochondria (Metabolic Generation)
- Also from dehydration synthesis reactions
H2O Loss > H2O Gains
Results in dehydration & hypotension
H2O Gains > H2O Loss
Results in overhydration, hypertension & hemodilution
Restoration of Osmotic Equilibrium
- If ECF tonicity > ICF tonicity: H2O travels from ICF -> ECF
- If ECF tonicity < ICF tonicity: H2O travels from ECF -> ICF
2 Types of Fluid Imbalances
- Excessive ECF -> Overhydration & H2O intoxication
2. Depleted ECF -> Circulatory shock & hypotension
Urinary NaCl Losses
Main determinant of total body H2O fluid volume
2 Rules for Na+ and K+ balance
- Most salt homeostasis problems due to imbalances between Na+ gains & Na+ losses
- K+ homeostasis problems are uncommon, but dangerous
Sodium (Na+) Balance
- Na+ is the most abundant extracellular ion in the ECF
- Inputs come from dietary intake (salt)
- Outputs done through urine & sweat
*Normal range of blood [Na+] = 136 -148 mEq/L of plasma H2O
4 Hormones Controlling Na+ Homeostasis
- Aldosterone
- Angiotensin 2
- Antidiuretic Hormone
- Atrial Natriuretic Peptide
- All 4 hormones also control ECF volume to promote fluid homeostasis
- Decreased ECFV -> 1,2, and 3 are secreted
- Increased ECFV -> 4 is secreted
Hypernatremia
=Excessive Na+ levels
- If due to dehydration -> thirst, dry skin & decreased blood volume & BP
- If due to inadequate renal excretion of Na+ or increased dietary Na+ -> increased blood volume, BP & edema
Hyponatremia
=Inadequate Na+ levels
- Leads to muscle weakness, hypotension, dizziness & disturbed CNS function
- “H2O Intoxication” -> dilutional hyponatremia & possible cytotoxic brain edema
- Aldosterone deficiency in adrenal insufficiency -> hyponatremia, hypovolemia & decreased BP
Potassium (K+) Balance
- K+ is the most abundant intracellular cation
- Concentration in ECF is controlled by aldosterone
3 Functions of K+
- Maintain cell fluid volume
- Action potential conduction
- Helps regulate PH (w/ K+/H+ antiporters)
- In acidosis, K+/H+ antiporters -> H+ influx & K+ efflux
- Alkalosis has reverse effect
2 Reasons for Increased K+ Excretion
- ECF [K+] increases above normal levels
2. Renin-angiotensin-aldosterone pathway activated
Hyperkalemia
=Excessive K+ levels
- Leads to increased neuromuscular excitability
- Causes cardiac arrhythmias, cardiac arrest & possible death (Heart)
- Causes muscle twitching & weakness (Skeletal Muscle)
- Causes CNS irritability (CNS)
Hypokalemia
=Inadequate K+ levels
- Leads to decreased neuromuscular excitability
- Causes cardiac arrhythmias & possible cardiac arrest (Heart)
- Causes flaccid paralysis & weakness (Skeletal Muscle)
- Causes hypoventilation (Lungs)
- Causes mental confusion (CNS)
Calcium (Ca+2) Balance
- Ca+2 is the most abundant ion in the body
- Functions in blood clotting, exocytosis & muscle contractions
- PTH & calcitriol increase Ca+2 levels
- Calcitonin decreases Ca+2 levels
Hypercalcemia
=Excessive Ca+2 levels
- Occurs in primary hyperparathyroidism, vitamin D3 toxicity, some cancers & chronic Ca+2 overconsumption
- Leads to decreased neuromuscular excitability; causes excessive membrane splinting, especially affects excitable cells
- Also leads to confusion, CNS depression, “metastatic” calcification of soft tissues
Hypocalcemia
=Inadequate Ca+2 levels
- Occurs in hypoparathyroidism & hypomangesemia
- Leads to increased neuromuscular excitability; causes insufficient membrane splinting, especially affects excitable cells
- Also leads to hyper-reflexia, muscle spasms, convulsions, laryngospasm & paresthesias
2 Mechanisms for Hypomagnesemia -> Hypocalcemia
- Mg+2 for Ca+2 “exchange” in bone
2. PTH secretion inhibited
Magnesium (Mg+2) Ion
- Primarily an intracellular electrolyte
- Is an important activator of protein kinase A & C
- Is a cofactor for ATPases
- Is required for PTH secretion
Hypermagnesemia
=Excessive Mg+2 levels
- Occurs in overdosing w/ magnesium-containing supplements & renal failure
- Leads to decreased neuromuscular excitability, lethargy, confusion & respiratory depression
Hypomagnesemia
=Inadequate Mg+2 levels
- Occurs in poor diet, alcoholism & severe diarrhea
- Leads to increased neuromuscular excitability, which causes muscle weakness, cramps & cardiac arrhythmias
Phosphate (PO4-3)
- Primarily an intracellular ion
- Required for synthesis of nucleic acids
- Is important in buffer systems
3 Different Forms of Phosphate Ion
- H2PO4-
- HPO4-2
- PO4-3
4 Hormones that Regulate Phosphate
- Parathyroid Hormone: Inhibits phosphate reabsorption by kidneys
- Calcitriol: Promotes absorption of dietary phosphate
- Calcitonin: Inhibits osteoclastic activity
- Fibroblast Growth Factor-23: Decreases GI absorption & increases renal excretion
Hyperphosphatemia
=Excessive PO4-3 levels
- Occurs in high phosphate intake, renal failure, cancer chemotherapy & hypoparathyroidism
- Leads to muscular weakness, vomiting, hyperactive reflexes & tetany
Hypophosphatemia
=Inadequate PO4-3 levels
- Occurs in poor diet, malabsorption syndrome, some kidney diseases, hyperparathyroidism, vitamin D3 deficiency & overuse of Al+3 containing drugs
- Leads to dizziness & memory loss
Chloride (Cl-) Ion
- Is a major extracellular anion
- Regulates osmotic pressure
- Part of stomach acid
- Important in RBC chloride shift
- Blood level is indirectly controlled by aldosterone
Hyperchloremia
=Excessive Cl- levels
- Occurs in dietary chloride excess, dehydration, aldosteronism & renal failure
- Caused by hyperkalemia, muscle weakness & metabolic acidosis
Hypochloremia
=Inadequate Cl- levels
- Occurs in excessive vomiting, hypokalemia, primary Addison’s disease & diuretic overuse
- Caused by metabolic alkalosis, anorexia, muscle cramps, tetany & slow/ shallow ventilation
Normal pH Range
=7.35 -7.45
pH > 7.45 = alkalosis
pH < 7.35 = acidosis
3 Types of Acids in the Body
- Volatile Acids
- Fixed Acids
- Organic Acids
Volatile Acids
=Acids able to leave solution & enter the atmosphere
-Example: Carbonic acid (HCO3-)
Fixed Acids
=Acids that are non-volatile; remain in the body until excreted
-Examples: Sulfuric acid & phosphoric acid
Organic Acids
=By-products of cellular metabolism
-Examples: Lactic acid & ketone bodies
Rapid Regulation of pH
- Acid-base balance is maintained by controlling H+ concentration of body fluids
- Buffer systems = weak acid & the salt of a weak acid
- Example: H2CO3/NaHCO3
3 Main Buffer Systems
- Protein Buffer Systems
- Carbonic Acid-bicarbonate Buffer System
- Phosphate Buffer System
Protein Buffer Systems
- Some amino acids can accept or release H+ ions
- If pH increases, free carboxyl (-COOH) groups of amino acids can dissociate, releasing H+ & becoming -COO-
- If pH decreases, free amine groups (-NH2) of amino acids can accept an additional H+ -> NH3+
- Examples: Hemoglobin of RBCs & albumin of plasma
Carbonic Acid-bicarbonate Buffer System
- Uses H2CO3/HCO3-
- Exchange reactions occur (AB +CD -> AD + BC)
- If plasma becomes too alkaline, H2CO3 becomes HCO3- by releasing an H+ ion
- If plasma becomes too acidic, HCO3- becomes H2CO3 by binding to an H+ ion
Phosphate Buffer System
- Seen in ICF & urine
- Uses H2PO4-/HPO4-2
- If plasma becomes too alkaline, OH- is buffered by H+ from H2PO4-
- If plasma becomes too acidic, excess H+ is bound by HPO4-2
- Buffering of H+ is “quick-fix”
Respiratory System & Acid-base Homeostasis
- Exhalation of CO2 & H2O removes excess H+ from blood
- If ECF pH decreases, chemoreceptors signal medullary DRG to increase ventilation rate -> decreased PCO2 -> pH increases (vice-versa during pH increase)
- Change in ventilation rate = respiratory compensation
- Anxiety can double ventilation rate -> alkalosis
- Intentional slowing of respiration -> acidosis
Urinary System & Acid-base Homeostasis
- Kidneys can vary their rates of H+ secretion, HCO3- secretion and HCO3- reabsorption
- H+ secretion done by Na+/H+ anitporters in PCT and H+ ATPases of intercalated cells in CT/CD
- HCO3- secretion done by Cl-/HCO3- antiporter of intercalated cells in CT/CD
- HCO3- reabsorption done by PCT
Acidosis
=When blood pH < 7.35
-Principal effect: Decreased neuromuscular excitability, can lead to possible coma & death
Alkalosis
=When blood pH > 7.45
-Principal effect: Increased neuromuscular excitability, can lead to nervousness, muscle spasms, convulsions & possible death
Respiratory Acidosis
=Excessive CO2 levels in body fluids
- Symptoms: Fatigue, confusion, dyspnea & somnolence
- Seen in hypoventilation, emphysema, airway obstruction & pulmonary edema
- Renal compensation: Increasing H+ secretion & HCO3- reabsorption, while decreasing HCO3- secretion
- Renal compensation may be partial or complete
Respiratory Alkalosis
=Inadequate CO2 levels in body fluids
- Symptoms: vertigo, numbness &/or muscle spasms in hands & feet
- Seen in high altitude sickness, stroke, anxiety states
- Renal compensation: Decreasing H+ secretion & HCO3- reabsorption, while increasing HCO3- secretion
- Renal compensation may be partial or complete
Metabolic Acidosis
=Depletion of HCO3- reserve
- Symptoms: Rapid/shallow breathing, confusion, somnolence, anorexia
- Respiratory Compensation: Increased ventilation rate to blow off more CO2 & H2O
- Respiratory compensation may be partial or complete
5 Causes of HCO3- Depletion
- Deficient renal H+ secretion
- Excessive production of fixed acids
- Excessive production of organic acids
- Chronic diarrhea
- Nephrotic syndrome
Metabolic Alkalosis
=Excessive levels of HCO3- in blood
- Symptoms: Myalgia, polyuria & cardiac arrhythmias
- Caused by prolonged vomitting or excessive consumption of alkaline substances
- Respiratory Compensation: Decreased ventilation rate to blow off less CO2 & H2O
- Respiratory compensation may be partial or complete
Diagnosis of Acidosis & Alkalosis (3 Tests)
- Systemic Arterial Blood pH
- Blood PCO2 levels
- Blood HCO3- levels
Age-related Changes in Homeostasis
- 75% of total bodyweight of newborn = H20 (60% for adults)
- ICF:ECF volume ratio of premature infants = 1:2 (2:1 for adults)
- Infants have a 2x faster metabolic rate than adults due to immature kidneys, making acidosis common
3 Other Infantile Problems w/ Homeostasis
- More H2O loss via skin due to having greater ratio of body surface area/volume
- More H2O loss via lungs due to high respiration rates
- Difficult tubular H+ secretion due to higher K+ and Cl- levels in blood
- Hyperkalemia: less H+ losses via intercalated cells
- Hyperchloremia: H+ harder to secrete
- Leads to higher risk of metabolic acidosis
Elderly Problems w/ Homeostasis
- Impaired fluid/electrolyte, acid-base homeostasis
- Respiratory or renal compensations often inadequate
- Pharmaceuticals may contribute to fluid/electrolyte imbalances
4 Elderly Susceptibilities for Homeostatic Imbalances
- Dehydration & hypernatremia
- Hyponatremia
- Hypokalemia
- Acidosis
