Transcript Ch 27 Severely Shortened Student Revised

Chapter 27
Fluids,
Electrolytes, and
Acid Base
Balance
Fluids -- Electrolytes -- Acid-base
Three types of homeostatic balance
1. water balance - Body fluids homeostasis is essential to life
 55%- 75% of body weight based on gender, body mass, age
• newborn baby’s body weight 75% water
• obese and elderly people as little as 45% by weight
2. electrolyte balance -conduct electrical current in solution
• 2/3 of body fluids are inside cells:
• 1/3 of body fluids is outside cells: extracellular fluid (ECF)
2. acid-base balance -production and loss of hydrogen ions (pH)
Water intake: preformed water (2,500 mL/day)
– ingested in food (1000 mL/day) and drink (1200 mL/day)
– metabolically generated (300 mL/day) in cells
Water eliminated
– Sensible: urination and heavy perspiration, feces
– Insensible: cutaneous transpiration, respiration.
Fluid Movements
• OSMOSIS responsible for water movement between body cells,
interstitium, and blood. Osmosis determined by the RELATIVE
CONCENTRATIONS OF SOLUTES in each compartment
– Fluid will MOVE TO higher concentration of SOLUTES
– Electrolytes – play the principal role in governing the
body’s water distribution and total water content
 Homeostatic mechanisms that monitor and adjust body fluid
composition respond to changes in the EXTRACELLULAR
FLUID (ECF), not in the INTRACELLUALR FLUID (ICF)
• Cells cannot move water molecules by active transport
• ECF osmotic concentration INCREASES - more solutes to
fluid
• ECF becomes HYPERTONIC to ICF - H2O from Cells to ECF
• ECF osmotic concentration DECREASES -less solutes to
fluid
• ECF becomes HYPOTONIC to ICF - H2O from ECF to Cells
HYPERTONIC
solids
CELL
water
solids
solids
solids
solids
water
solids
solids
Cat = water
Mouse = MORE solid substances
HYPOTONIC
water
water
CELL
water
water
water
solids
solids
water
water
Cat = water
Mouse = LESS solid substances
Disorders of Water Balance
FLUID DEFICIENCY fluid output exceeds intake over long time
• volume depletion (hypovolemia) H20 & Na+ lost w/o replacement
– hemorrhage, severe burns, chronic vomiting, or diarrhea
• dehydration (negative water balance) more H20 than (Na+)
– Infants vulnerable to dehydration- high metabolic rate; high urine
excretion, immature kidneys cannot concentrate urine, diarrhea
– Elderly dehydration = depend on others to provide fluid intake
FLUID EXCESSES
– volume excess = both Na+ and water retained
– (water intoxication) more H20 than (Na+) ingested- water drinking
contests or from long bouts of exercise- high water consumption
FLUID SEQUESTRATION –EXCESS fluid accumulates in a
PARTICULAR LOCATIONS. Total body water may be normal
• edema - abnormal accumulation of fluid in the INTERSTIUM
• lymphedema -Edema caused by blockage of LYMPH drainage
• hemorrhage - BLOOD pools in the tissues is lost to circulation
• pleural effusion –fluid accumulates in the pleural cavity
– caused by some LUNG infections
Electrolytes
Electrolytes are any substance containing free ions that make the
substance electrically conductive.
– strongly affect osmolarity of body fluids (they are solutes)
– determine electrical potential (charge difference) across cell
membranes
• Cations are positively charged: Na+, Ca2+, K+, Mg2+ , H+.
• Anions are negatively charged: HCO3- , Cl-, PO43- (phosphate)
• Electrolyte depletion can occur through excessive perspiring,
vomiting, dehydration
• Very important to maintain electrolyte balance. EX: if you lose
500 mg of Na+ in the urine you need to replenish the loss by food
and drink intake to restore balance.
Sodium / Chloride / Potassium Ions
SODIUM (Na+) ions are the dominant ECF cations.
– DeterminesTOTAL H20 volume and water distribution body wide;
promotes osmotic pressure; electrical / resting membrane potentials
– Accounts for 90%- 95% of fluid osmolarity
• Regulation of Na+ ions:ANP, Aldosterone, ADH
CHLORIDE (Cl-) ions are the most abundant anions in the ECF.
• help maintain the resting membrane potential of the cell.
• Component of bicarbonate ions during the chloride shift.
• Gastric glands utilize to make hydrochloric acid in the stomach.
POTASSIUM (K+) ions - 98% found inside cells
• Resting membrane potentials; repolarization; hyperpolarization
• Hyperkalemia: Lowers action potential threshold in nerve and
muscle cells ABNORMALLY EXCITABLE
• Hypokalemia: Causes hyperpolarization; nerve and muscle cells
LESS EXCITABLE
• Both conditions may affect cardiac function, blood pressure, and
neuromuscular interaction.
• KCl used as lethal injection in high doses 1st induces sleep, 2nd
respiratory failure, finally fibrillation and cardiac arrest
Bicarbonate / Phosphate Ions
BICARBONATE (HCO3-) ions are the main buffer in the ECF.
• HCO3- is the main form of transport of CO2 in blood plasma:
• CO2 + H2O  H2CO3  HCO3- + H+
• CHLORIDE SHIFT- exchange of bicarbonate (HCO3−) and chloride
(Cl−) across the membrane of red blood cells (RBCs)
PHOSPHATE (PO43-) Ions
• PO43- ions are found in bone, DNA, phospholipids, ATP associated
to Ca2+ ions.
• (HPO42−) and phosphoric acid H3PO4) act as buffers to stabilize
the pH of body fluids
• renal control – phosphate continually lost by glomerular filtration
• parathyroid hormone
– increases excretion of phosphate in urine occurs by reducing
reabsorption in kidney tubule
– Calcium and phosphate have an inverse relationship
Calcium (Ca2+) ions
Calcium - Ca2+most abundant ion in body; 99% of Ca2+ stored in
bones as calcium phosphate; 50% bound to plasma proteins albumin
• Inverse relationship between Ca and P ratio of is 2-to-1; careful
balance.; as Ca2+ rises PO43- declines.
• Excess amounts inside cells can results in crystallization
• Structural role in bones; blood clotting, release of neurotransmitter,
muscle contraction, nerve and muscle function.
• Controlled by PTH and Calcitonin
• Hypocalcemia: Chronic renal failure; vitamin D deficiency
– Lowers threshold potential for depolarization. Action potential more
easily obtained resulting in OVER EXCITEMENT of nervous and
muscular systems. SIMILAR TO POTASSIUM (K+)
• Hypercalcemia: Due to over secretion of PTH
– increased calcium RAISES the threshold (hyperpolarization)
causing muscle weakness by SLOWING THE STIMULUS to
muscles. A higher threshold takes longer to reach depolarization.
– High levels of Ca2+ can increase secretions of gastrin. This would
stimulate HCl production possibly leading to peptic ulcers
Acid Base Balance
• One of the most important aspects of homeostasis
• Acid base balance achieved by controlling H+ in body fluids.
• H+ homeostasis is absolutely necessary for protein structure, and
blood pH (Blood pH range: 7.35 to 7.45)
– pH 6.8-7.0 CNS DEPRESSED –person goes into coma
– pH 7.8-8.0 CNS OVERSTIMULATION of respiratory muscles
impairs breathing–death results from respiratory arrest.
• Acids release H+ into solution; Bases remove H+ from solution
Challenges to acid-base balance:
• metabolism constantly produces acid
– lactic acids from anaerobic fermentation
– phosphoric acid from nucleic acid catabolism
– fatty acids and ketones from fat catabolism
– carbonic acid from carbon dioxide
The 3 types of pH regulation:
• Buffer regulation; Respiratory regulation; Renal regulation
Buffer Systems
• Buffer = Mechanism that resists changes in pH
– When H+ is added, buffer removes it
– When H+ is removed, buffer replaces it
• Types of buffer systems
– Physiological buffer- body control
• urinary system- MOST EFFECTIVE but VERY SLOW
respiratory system –FAST buffers within minutes but not
as effective as urinary system; release in expiration or
chloride shift
– Chemical buffer- bind or release of H+
• Protein buffer
• Carbonic acid / bicarbonate ion
• Phosphate buffer
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Protein Buffer Systems
Intracellular and plasma proteins bind or release H+
Provide about 75% of the buffer capacity of the body.
Hemoglobin act as a buffer in erythrocytes.
Albumin act as a buffer in blood plasma.
Amino acids of individual proteins produce buffering capacity
– pH rises [too alkaline]- the carboxyl group of amino acid
releases a H+
– pH drops [too acidic]- Carboxylate ion (RCOO-) and
amino group accepts H+ (NH3 ammonia)
• The amino group binds H+ when pH gets too low (acidic)
Carbonic acid / bicarbonate ion / Phosphate
• Carbonic acid (H2CO3) is a weak acid: slight release of H+
• Bicarbonate ion (HCO3−) is a weak base: BINDS LESS H+
forming carbonic acid, resulting in a smaller net increase in
acidity ( H2CO3 = H+ + HCO3- )
• Not as powerful as the protein buffer, but important for the
maintenance of blood pH
• Phosphate buffer system consists of anion H2PO4– (a weak
acid); slight release of H+
• Important buffer in renal tubules (urine).
• Consists of di-hydrogen phosphate ions - (H2PO4-) as H+ donor
(acid) and hydrogen phosphate ions (HPO42-) as H+ acceptor
(base)
– H2PO4H+ + HPO42• Provide only temporary solution to acid–base imbalance
– Does not eliminate H+ ions
Respiratory / Renal Regulation
• Neutralizes 2x to 3x as much
acid as chemical buffers
• RESPIRATORY regulation of
pH is achieved through carbonic
acid/bicarbonate buffer system
– CO2 levels increase, pH
decreases (acidic)
– CO2 levels decrease,
pH increases (basic)
• Increased CO2 and decreased pH stimulate pulmonary
ventilation- Hypoventilation
• Increased pH inhibits pulmonary ventilation- Hyperventilation
• KIDNEYS neutralize MORE ACID OR BASE THAN EITHER the
respiratory system or chemical buffers by changing the rate of H+
secretion in the renal tubules
Acidosis Alkalosis
 Respiratory acidosis pH below 7.35 to 7.45 caused by inadequate
ventilation
• CO2 accumulates in the ECF and lowers its pH
 Respiratory alkalosis - plasma pH above 7.45
– hyperventilation or CO2 eliminated faster than it is produced
 Metabolic acidosis results of all conditions other than respiratory
that DECREASE pH; build up of acid compounds
→ increased lactic acid, ketone bodies (ketoacidosis) seen in
alcoholism, and diabetes mellitus
→ loss of base due to chronic diarrhea, laxative overuse
→ chronic renal insufficiency/failure- inability to remove H+
 Metabolic alkalosis is the results of all conditions other than
respiratory that increase pH= Rarely occurs
• overuse of bicarbonates (antacids and IV bicarbonate
solutions)
• loss of stomach acid (chronic vomiting)