Transcript Fluid, Electrolyte, and Acid-Base Balance - Websupport1

Anatomy & Physiology
SIXTH EDITION
Lecture 27: Fluid, Electrolyte,
and Acid-Base Balance
Lecturer: Dr. Barjis
Room: P313
Phone: (718) 260-5285
E-Mail: [email protected]
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Frederic H. Martini
Fundamentals of
Learning Objectives
• Explain what is meant by “fluid balance,”
“electrolyte balance,” and “acid-base balance”
• Compare the compositions of intracellular and
extracellular fluids
• Identify the hormones that play important roles
in regulating fluid and electrolyte balance
• Describe the movement of fluid that takes place
within the ECF, between the ECF and the ICF,
and between the ECF and the environment
Learning Objectives
• Discuss how sodium, potassium, calcium and
chloride ions are regulated to maintain
electrolyte balance
• Explain the buffering systems that balance the
pH of the intracellular and extracellular fluids
• Describe the compensatory mechanisms
involved in acid-base balance
Maintenance of normal fluid volume and
composition is vital
• Extracellular fluid (ECF) includes:
• Interstitial fluid, plasma, lymph and other
body fluids
• Intracellular fluid (ICF) includes:
• The cytosol (fluid inside the cell)
• Makes up about two-thirds of the total body
water
Fluid and electrolyte balance
• Fluid balance
• Fluid is in a balance when the amount of water gained
(e.g. through the digestive system) each day equals the
amount of fluid lost (e.g. through urinary system, sweat
glands) each day
• Electrolyte balance
• The ion gain each day equals the ion loss
• Electrolytes are ions released through the dissociation of
inorganic compounds
• Acid-base balance
• H+ gain is offset by their loss
• When acid–base balance exists, the pH of body fluids
remains within normal limits (7.35-7.45).
The Composition of the Human Body
Regulation of fluids and electrolytes
• Homeostatic mechanisms respond to changes in
ECF
• No receptors directly monitor fluid or electrolyte
balance.
• However fluid and elctrolyte can be monitored by
responding to changes in plasma volume or
osmotic concentrations
• All water moves passively in response to
osmotic gradients
• Body content of water or electrolytes rises if
intake exceeds outflow
Cations in Body Fluids
• Major cations inside the cell (ICF)
include
•
Sodium (Na)
•
Potassium (K)
•
Magnesium (Mg)
• Major cations outside the cell (ECF)
include
•
Sodium (Na)
•
Potassium (K)
•
Calcisum (Ca)
• The concentration of cations inside the
cell and outside the cell differs as shown
in figure on the right e.g. there is much
higher concentration of potassium in the
ICF than in the ECF, there is much
higher concentration of sodium in the
ECF than in the ICF.
Anions in Body Fluids
•
•
•
Major Anions inside the cell (ICF) include
•
Chloride (Cl)
•
Proteins
•
Phosphates (HPO4)
•
Bicarbonate (HCO3)
•
SO4
Major Anions outside the cell (ECF) include
•
Chloride (Cl)
•
Proteins
•
Phosphates (HPO4)
•
Bicarbonate (HCO3)
•
SO4
The concentration of anions inside the cell and
outside the cell differs as shown in figure on the
right e.g. there is much higher concentration of
proteins and Phosphate in the ICF than in the
ECF, there is much higher concentration of
chloride in the ECF than in the ICF.
Cations and Anions in Body Fluids
• Despite the differences in the concentration of
specific substances, the ICF and ECF osmotic
concentrations are identical
• If the cell membrane were freely permeable,
diffusion would continue until these ions were
evenly distributed across the membrane
Primary regulatory hormones
• Fluid balance and electrolyte balance are
mediated by three hormones:
• Antidiuretic hormone (ADH)
• Stimulates water conservation and the thirst
center
• Aldosterone
• Controls Na+ absorption and K+ loss along
the DCT
• Natriuretic peptides (ANP and BNP)
• Reduce thirst and block the release of ADH
and aldosterone
Fluid movement within the ECF
• Fluid moves freely within ECF compartment
• Water, electrolytes, proteins, fats and other
nutrients and molceules are normally balanced
by gains via
• Eating
• Drinking
• Metabolic generation
• Losses of water and other molcules are by:
• Urine
• Respiratory losses
• Perspiration
• Fecal Loss
Fluid Exchanges
Water excess and depletion
• Hyponatremia
• Na+ concentration in the ECF is reduced
(overhydration)
• Hypernatremia
• Na+ in the ECF is abnormally high
• Dehydration
• Develops when water loss outpaces water gains
Fluid shifts
• Water movement between ECF and ICF
• If ECF becomes hypertonic relative to ICF,
water moves from ICF to ECF
• If ECF becomes hypotonic relative to ICF,
mater moves from ECF into cells
Problems with Electrolyte Balance
• Usually result from sodium ion imbalances
• Potassium imbalances are less common, but
more dangerous
Sodium balance
• Rate of sodium uptake across digestive tract
directly proportional to dietary intake
• Sodium losses occur through urine and
perspiration
• Shifts in sodium balance result in expansion or
contraction of ECF
• Large variations corrected by homeostatic
mechanisms
• Too low, ADH / aldosterone secreted
• Too high, ANP secreted
The Homeostatic Regulation of Normal Sodium
Ion Concentrations in Body Fluids
The Integration of Fluid Volume Regulation and
Sodium Ion Concentrations in Body Fluids
Potassium balance
• Potassium ion concentrations in ECF are low
• Not as closely regulated as sodium
• Potassium ion excretion increases as
• ECF concentrations rise
• Aldosterone secreted
• pH rises
• Potassium retention occurs when pH falls
ECF Concentrations of other electrolytes
• Calcium balance
• Bone reserves, absorption in the digestive
tract, and loss at kidneys
• Magnesium balance
• Absorbed by the PCT to keep pace with urinary
losses
Acid-base Balance
The importance of pH control
• The pH of the ECF remains between 7.35 and
7.45
• If plasma levels fall below 7.35 (acidemia),
acidosis results
• If plasma levels rise above 7.45 (alkalemia),
alkalosis results
• Alteration outside these boundaries affects all
body systems e.g. can result in coma, cardiac
failure, and circulatory collapse
Common Acids
• Carbonic acid is most important factor affecting
pH of ECF
• CO2 reacts with water to form carbonic acid
• Inverse relationship between pH and
concentration of CO2
Mechanisms of pH control
• Buffer system consists of a weak acid and its
anion
• Three major buffering systems:
• Protein buffer system
• Amino acid
• Hemoglobin buffer system
• H+ are buffered by hemoglobin
• Carbonic acid-bicarbonate
• Buffers changes caused by organic and fixed
acids
Protein buffer system
• If pH climbs, the carboxyl group of amino acid acts as a
weak acid
• If the pH drops, the amino group acts as a weak base
• Hemoglobin buffer system
• Prevents pH changes when PCO2 is rising or falling
Carbonic Acid-Bicarbonate Buffering System
• Carbonic acid-bicarbonate buffer system
• CO2 + H2O  H2CO3  H+ + CO3–
• Has the following limitations:
• Cannot protect the ECF from pH changes due
to increased or depressed CO2 levels
• Only functions when respiratory system and
control centers are working normally
• It is limited by availability of bicarbonate ions
(bicarbonate reserve)
The Carbonic Acid-Bicarbonate Buffer System
Maintenance of acid-base balance
• Lungs help regulate pH through carbonic acid bicarbonate buffer system
• Changing respiratory rates changes PCO2
• Respiratory compensation
• Kidneys help regulate pH through renal
compensation
The Central Role of the Carbonic AcidBicarbonate Buffer System in the Regulation of
Plasma pH
The Central Role of the Carbonic AcidBicarbonate Buffer System in the Regulation of
Plasma pH
Changes with age include
• Reduced total body water content
• Impaired ability to perform renal compensation
• Increased water demands
• Reduced ability to concentrate urine
• Reduced sensitivity to ADH/ aldosterone
• Net loss of minerals
• Inability to perform respiratory compensation
• Secondary conditions that affect fluid, electrolyte, acidbase balance
You should now be familiar with:
• What is meant by “fluid balance,” “electrolyte
balance,” and “acid-base balance”
• The compositions of intracellular and
extracellular fluids
• The hormones that play important roles in
regulating fluid and electrolyte balance
• The movement of fluid that takes place within the
ECF, between the ECF and the ICF, and between
the ECF and the environment
You should now be familiar with:
• How sodium, potassium, calcium and chloride
ions are regulated to maintain electrolyte balance
• The buffering systems that balance the pH of the
intracellular and extracellular fluids
• The compensatory mechanisms involved in acidbase balance