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Chapter 15
Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Balance Concept
• Internal pool – the quantity of any particular
substance in the ECF
• If quantity is to remain stable within the body
– Input must be balanced with output
•Ingestion
•Metabolic
consumption
•Excretion
•Metabolic
consumption
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Balance Concept
• Input must equal output to maintain a stable balance
in ECT.
– Positive balance exists when input exceeds
output
– Negative balance exists when output exceeds
input
– Input
• Input of substances into plasma is poorly controlled or
not controlled
• Eating habits are variable
– Output
• Compensatory adjustments usually occur on output
side by urinary excretion
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Balance Concept
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Fluid Balance
• Water
– Most abundant substance in body
– Amount varies in different kinds of tissues
– Content remains fairly constant within an individual
• Minor ECF components
– Lymph
– Transcellular fluid
•
•
•
•
•
Cerebrospinal fluid
Intraocular fluid
Synovial fluid
Pericardial, intrapleural, and peritoneal fluids
Digestive juices
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Classification of Body Fluids
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Barriers Separating Body-Fluid Compartments
• Barrier between plasma and interstitial fluid
– Blood vessel walls
• Barrier between ECF and ICF
– Cellular plasma membranes
– Major differences between ECF and ICF
• Presence of cell proteins in ICF that cannot permeate
the cell membrane to leave the cells
• Unequal distribution of Na+ and K+ and their attendant
ions as a result of the action of the membrane-bound
Na+ - K+ ATPase pump present in all cells
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Ionic Composition of the Major Body-Fluid
Compartments
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Fluid Balance
• ECF serves as an intermediary between the cells
and external environment
• Two factors are regulated to maintain fluid balance
in the body
– ECF volume must be closely regulated to help
maintain blood pressure
• Maintaining salt balance is very important in long-term
regulation of ECF volume
– ECF osmolarity must be closely regulated to
prevent swelling or shrinking of cells
• Maintaining water balance is very important in
regulating ECF osmolarity
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Salt Balance
• Very important in regulating ECF volume
• Salt input occurs by ingestion
– Often not well controlled
• Salt balance maintained by outputs in urine
– Salt also lost in perspiration and in feces
• Kidneys keep salt constant in ECF
– Glomerular filtration rate (GFR)
– Tubular reabsorption of sodium
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Daily Salt Balance
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Osmolarity
• Measure of the concentration of individual solute
particles dissolved in a fluid
• Circumstances that result in a loss or gain of free
H2O lead to changes in ECF osmolarity
– Deficit of free water in ECF
• Osmolarity becomes hypertonic
• Often associated with dehydration
– Excess of free water in ECF
• Osmolarity becomes hypotonic
• Usually associated with overhydration
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Osmolarity
• Hypertonicity
– Cells tend to shrink
– Causes
• Insufficient water intake
• Excessive water loss
• Diabetes insipidus
– Symptoms and effects
• Shrinking of brain neurons
– Confusion, irritability, delirium, convulsions, coma
• Circulatory disturbances
– Reduction in plasma volume, lowering of blood pressure,
circulatory shock
• Dry skin, sunken eyeballs, dry tongue
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Osmolarity
• Hypotonicity
– Cells tend to swell
– Causes
• Patients with renal failure who cannot excrete a dilute urine
become hypotonic when they consume more water than
solutes
• Can occur in healthy people when water is rapidly ingested
and kidney’s do not respond quickly enough
• When excess water is retained in body due to inappropriate
secretion of vasopressin
– Symptoms and effects
• Swelling of brain cells
– Confusion, irritability, lethargy, headache, dizziness, vomiting,
drowsiness, convulsions, coma, death
• Weakness (due to swelling of muscle cells)
• Circulatory disturbances (hypertension and edema)
• Water intoxication
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
H2O Input and Output
• In order to maintain stable
water balance, water input
must equal water output.
• Input
– Drinking liquids
– Eating solid foods
– Metabolically produced
water
• Output
– Insensible loss
• Lungs
• Nonsweating skin
– Sensible loss
• Sweating
• Feces
• Urine excretion
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Vasopressin
• Produced by hypothalamus
• Stored in posterior pituitary gland
• Released on command from hypothalamus
– Also location of thirst center
• Hypothalamic osmoreceptors
– Located near vasopressin-secreting cells and
thirst center
– Osmolarity increase → vasopressin secretion and
thirst stimulated
– Osmolarity decrease → vasopressin secretion
decreased and thirst suppressed
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Vasopressin
• Left atrial receptors
– Monitor pressure of blood flowing through
(reflects ECF volume)
– Upon detection of major reduction in arterial
pressure, receptors stimulate vasopressin
secretion and thirst
– Upon detection of elevated arterial pressure,
vasopressin and thirst are both inhibited
• Angiotensin II
– Stimulates vasopressin secretion and thirst when
renin-angiotensin-aldosterone mechanism is
activated to conserve Na+
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Nonregulatory Factors Not Linked to Vasopressin
and Thirst
• Regulatory factors that do not link vasopressin and
thirst
– Dryness of mouth stimulates thirst but not
vasopressin
• Oral metering
– Some animals will rapidly drink only enough H2O
to satisfy its H2O deficit
– Mechanism is less effective in humans
• Nonphysiologic influences on fluid intake
– Fluid intake often influenced by habit and
sociological factors
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Acid-Base Balance
• Refers to precise regulation of free H+ concentration in body
fluids
• Acids
– Group of H+ containing substances that dissociate in
solution to release free H+ and anions
• Bases
– Substance that can combine with free H+ and remove it
from solution
• pH
– Designation used to express the concentration of H+
– pH 7 – neutral
– pH less than 7 → acidic
– pH greater than 7 → basic
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
pH
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Comparison of pH Values of
Common Substances
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Acid-Base Balance
• Arterial pH less than 6.8 or greater than 8.0 is not
compatible with life
• Acidosis
– Exists when blood pH falls below 7.35
• Alkalosis
– Occurs when blood pH is above 7.45
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Acid-Base Balance
• Consequences of fluctuations in pH
– Changes in excitability of nerve and muscle cells
– Marked influence on enzyme activity
– Changes influence K+ levels in body
• Sources of H+ in the body
– Carbonic acid formation
– Inorganic nutrients produced during breakdown of
nutrients
– Organic acids resulting from intermediary
metabolism
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Lines of Defense Against pH Changes
• Chemical buffer systems
• Respiratory system
• Kidneys
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Chemical Buffer Systems
• Minimize changes in pH by binding with or yielding
free H+
• First line of defense
• Body has four buffer systems
– H2CO3-, HCO3- buffer system
• Primary ECF buffer for noncarbonic acids
– Protein buffer system
• Primary ICF buffer; also buffers ECF
– Hemoglobin buffer system
• Primary buffer against carbonic acid changes
– Phosphate buffer system
• Important urinary buffer; also buffers ICF
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Respiratory System
• Second line of defense again changes in pH
• Acts at a moderate speed
• Regulates pH by controlling rate of CO2 removal
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Kidneys
• Third line of defense against change in hydrogen ion
concentration
• Kidneys require hours to days to compensate for
changes in body-fluid pH
• Control pH of body fluids by adjusting
– H+ excretion
– HCO3- excretion
– Ammonia secretion
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Acid-Base Imbalances
• Can arise from either respiratory dysfunction or
metabolic disturbances
• Deviations divided into four general categories
– Respiratory acidosis
– Respiratory alkalosis
– Metabolic acidosis
– Metabolic alkalosis
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Respiratory Acidosis
• Result of abnormal CO2 retention arising from hypoventilation
• Possible causes
– Lung disease
– Depression of respiratory center by drugs or disease
– Nerve or muscle disorders that reduce respiratory muscle
activity
– Holding breath
• Compensations
– Chemical buffers immediately take up additional H+
– Kidneys are most important in compensating for
respiratory acidosis
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Respiratory Alkalosis
• Primarily due to excessive loss of CO2 from body as result of
hyperventilation
• Possible causes
– Fever
– Anxiety
– Aspirin poisoning
– Physiologic mechanisms at high altitude
• Compensations
– Chemical buffer systems liberate H+
– If situation continues a few days, kidneys compensate by
conserving H+ and excreting more HCO3-
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Metabolic Acidosis
• Includes all types of acidosis other than those caused by
excess CO2 in body fluids
• Causes
– Severe diarrhea
– Diabetes mellitus
– Strenuous exercise
– Uremic acidosis
• Compensations
– Buffers take up extra H+
– Lungs blow off additional H+ generating CO2
– Kidneys excrete more H+ and conserve more HCO3-
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Metabolic Alkalosis
• Reduction in plasma pH caused by relative
deficiency of noncarbonic acids
• Causes
– Vomiting
– Ingestion of alkaline drugs
• Compensations
– Chemical buffer systems immediately liberate H+
– Ventilation is reduced
– If condition persists for several days, kidneys
conserve H+ and excrete excess HCO3- in the
urine
Chapter 15 Fluid and Acid-Base Balance
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning