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Chapter 27
Fluid, Electrolyte, and Acid-Base Balance: An Overview
▪Extracellular fluid- ECF▪Intracellular fluid- ICF▪Normal volume and composition of these fluids is necessary for survival.
▪Stabilizing the volumes, solute concentrations, and pH of the ECF and the ICF
involves three interrelated processes:
1. Fluid balance2. Electrolyte balance3. Acid-Base BalanceAn Introduction to Fluid and Electrolyte Balance
▪The ECF and the ICF- exchange between these two fluids occur across cell
membranes by osmosis, diffusion, and carrier-mediated transport.
▫ECF is subdivided into interstitial fluid and plasma.
▫Fluid compartments▫The principle ions in the ECF are:
▫The ICF contains an abundance of:
▫Despite the differences in the concentration of specific substances, the
ICF and ECF osmotic concentrations are identical.
▪Basic Concepts in the Regulation of Fluids and Electrolytes- four basic principles
necessary for the understanding of fluid and electrolyte balance.
1)
2)
3)
4)
▪An Overview of the Primary Regulatory Hormones- major physiological
adjustments affecting fluid balance and electrolyte balance are mediated by 3
hormones: ADH, aldosterone, natriuretic peptides (ANP and BNP).
▫ADH-
-Increased release of ADH has two important effects:
1)
2)
▫Aldosterone-The higher the plasma concentration of aldosterone, the more
efficiently the kidneys conserve Na+.
-Aldosterone also increases the sensitivity of salt receptors on the
tongue, increase interest in and consumption of salty foods.
-Aldosterone is secreted in response to rising K+ or falling Na+
levels in the blood that reaches the adrenal cortex, or in response
to the renin-angiotensin system.
▫Natriuretic Peptides (ANP and BNP):
-Reduce thirst, and block the release of ADH and aldosterone,
results in diuresis which lowers both blood pressure and plasma
volume.
▪The Interplay Between Fluid Balance and Electrolyte Balance- must be seen as
separate entities, even though they are very closely related.
Fluid Balance
▪Water moves freely within the ECF compartment, allowing for important
circulation among minor compartments.
▪Fluid Movement within the ECF
▫The relationship between the net hydrostatic pressure (NHP) and the
blood colloid osmotic pressure (BCOP) determine the exchange between
the plasma and the interstitial fluid.
▫This results in continuous filtration of fluid from the capillaries into the
interstitial fluid.
▫Any factor that affects the NHP or BCOP will alter the distribution of fluid
within the ECF.
▪Fluid Gains and Losses- common routes of fluid exchange with the environment
▫Water losses-Temperature rise accompanying a fever▫Water gains-
▪Fluid Shifts- water movement between the ECF and the ICF
▫Fluid shifts occur rapidly in response to changes in the osmotic
concentration of the ECF and reach equilibrium within minutes to hours.
▫If the osmotic concentration of your ECF increases:
▫If the osmotic concentration of your ECF decreases:
▫Allocation of water loss-Dehydration (water depletion)
-Hypernatremia-
▫Distribution of water gains-Hyperhydration-Hyponatremia results when:
1)
2)
3)
Electrolyte Balance
▪Electrolyte balance is important because:
▫Total electrolyte concentrations directly affect water balance
▫The concentrations of individual electrolytes can affect cell functions
▪Na+ and K+ are important because they are major contributors to the osmotic
concentrations of the ECF and ICF, respectively. They directly effect normal
functioning of cells.
-More than 90% of the osmotic concentration of the ECF results
from the presence of sodium salts (sodium chloride and sodium
bicarbonate)
-Normal concentrations of Na+ in body fluids averages 140 mEq/L
while inside cells, it is 10 mEq/L
-Potassium concentrations within cells reach 160 mEq/L, while only
around 3.8 to 5 mEq/L in body fluids.
▫Two general rules about sodium and potassium balance:
1)
2)
▪Sodium Balance- the total amount of sodium in the ECF represents a balance
between two factors:
1)
2)
▫A change in sodium content of the ECF does not produce a change in the
sodium concentration.
-A corresponding gain or loss of water tends to keep the sodium
concentration constant.
▫When sodium gains exceed sodium losses:
▫When sodium losses exceed sodium gains:
▫ECF volume changes are related to blood plasma volume, thus blood
volume changes. Blood volume changes are monitored by looking at
blood pressure.
*see figure 27-5 and 27-6 to familiarize yourself with fluid volume regulation
and sodium ion concentrations in body fluids.*
▪Potassium Balance- roughly 98% of the K+ content of the human body is in the
ICF.
▫The K+ concentration in the EFC is a balance between K+ absorbed at
the digestive epithelium and the loss at the kidney.
▫Urinary K+ losses are usually limited to the amount gained by absorption.
(50-150 mEq/L per day)
▫The K+ concentration in the ECF is controlled by adjustments in the rate
of active secretion along the DCT and collecting system, rate varies in
response to:
1)
2)
3)
▫Hypokalemia▫Hyperkalemia▪Other Electrolytes
▫Calcium balance- calcium is the most abundant mineral in the body, 99%
deposited in the bones.
▫Magnesium balance- more Mg2+ is found in the ICF, about 26 mEq/L,
1.5-2.5 mEq/L is found in the ECF.
▫Phosphate balance- phosphate ions are bound up in the mineral salts of
the skeleton, and used for formation of high energy compounds,
activation of enzymes, and synthesis of nucleic acids.
▫Chloride balance- most abundant anions in the ECF.
Acid-Base Balance
▪The Importance of pH Control
▫Any deviation outside the normal range (7.35-7.45) is extremely
dangerous, because changes in H+ concentrations disrupt the stability of
cell membranes, alter the structure of proteins, and change the activities
of important enzymes.
▫The nervous and cardiovascular systems are particularly sensitive to
fluctuations in pH, for example, severe acidosis (under 7.0) can be deadly
because:
1)
2)
3)
▫Acidosis is more common, due to the fact that acids are produced by
normal cellular activities.
▪Types of Acids in the Body- there are three categories of acids in the body;
volatile acids, fixed acids, and organic acids.
▫Volatile acids-
▫Fixed acids▫Organic acids▪Mechanisms of pH Control
▫To maintain acid-base balance, your body must balance gains and losses
of hydrogen ions.
▫Buffer system▫Phosphate buffer system▫Protein buffer system- depends on the ability of amino acids to respond
to changes in pH by accepting or releasing H+
▫Hemoglobin buffer system- hemoglobin molecules buffer the H+
concentrations as carbonic acid dissociates.
▫Carbonic Acid-Bicarbonate Buffer System-
-The primary role of the carbonic acid-bicarbonate buffer system is
to prevent changes in pH caused by organic acids and fixed acids in
the ECF. There are three important limitations:
1)
2)
3)
▪Maintenance of Acid-Base Balance- buffer systems cannot work for an extended
period of time, they temporarily tie up H+, rendering them harmless.
▫The maintenance of acid-base balance includes balances H+ gains and
losses, involves coordinating the buffer systems with the renal and
respiratory systems. These mechanisms support buffer systems by:
1)
2)
3)
▫Respiratory compensation- change in the respiratory rate that helps
stabilize the pH of the ECF.
*review figure 23-26 for a reminder of this mechanism*
▫Renal compensation- change in the renal rates of H+ and HCO3secretion or reabsorption in response to changes in plasma pH.
*see figure 27-10 for illustrations of these buffer systems*
-When body fluid pH drops (acidosis), the kidney’s respond by:
-When body fluid pH increases (alkalosis), the kidney’s respond by:
Disturbances of Acid-Base Balance
▪Serious and prolonged disturbances of acid-base balance can result from any
factor that affects one of the principal regulatory mechanisms:
▪Acute phase▪Compensated phase▪Respiratory Acidosis▫The primary symptom is low plasma pH due to hypercapnia (elevated
plasma pCO2)
▫Acute respiratory acidosis▫Chronic respiratory acidosis-CNS injuries, desensitized respiratory centers, emphysema,
congestive heart failure, pneumonia are causes of chronic
respiratory acidosis.
-Kidney’s will increase the rate of H+ secretion into tubular fluid,
but cannot alone return the pH to normal until the underlying
respiratory or circulatory problems are corrected.
-Artificial respiration, mechanical ventilator, or induced
bronchodilation can be used to increase pulmonary exchange.
▪Respiratory Alkalosis- relatively uncommon, develops when respiratory activity
lowers plasma pCO2 levels to below normal levels, hypocapnia.
▫Hyperventilation can cause temporary hypocapnia.
▫If continued, individual will become unconscious and the breathing rate
returns to normal.
▫Other problems with respiratory alkalosis are rare and involve:
▪Metabolic Acidosis- second most common type of acid-base imbalance, it has
three major causes:
1)
2)
3)
▫Respiratory and metabolic acidosis are typically linked, because oxygenstarved tissues generate large quantities of lactic acid and because
sustained hypoventilation leads to decreased PO2.
▪Metabolic Alkalosis- occurs when HCO3- concentrations become elevated, taking
H+ out of solution and generating H2CO3
▪The Detection of Acidosis and Alkalosis
*use figure 27-18 and table 27-4 to help you identify acid-base disorders*