Transcript Chapter 21: Blood Vessels and Circulation

Chapter 27: Fluid, Electrolyte and
Acid-base Balance
BIO 211 Lecture
Instructor: Dr. Gollwitzer
1
• Today in class we will discuss:
– The importance of water and its significance to fluid
balance in the body
– Definitions and the importance of:
• Fluid Balance
• Electrolyte balance
• Acid-base balance
– Extracellular fluid (ECF) and intracellular fluid (ICF)
and compare their composition
– Fluid and electrolyte balance
• Hormones that regulate them
• Importance of key electrolytes
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Introduction
• Water critical to survival
– 50-60% total body weight
– 99% of extracellular fluid (ECF)
– Essential component of cytosol (intracellular
fluid, ICF)
• All cellular operations rely on water
– Diffusion medium for gases, nutrients, waste
products
3
Body Fluid Compartments
• Body must maintain normal volume and
composition of:
– ICF
– ECF = all other body fluids
• Major - IF, plasma
• Minor - lymph, CSF, serous and synovial fluids
• ICF > total body water than ECF
– Acts as water reserve
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Body Fluid Compartments
Figure 27–1a-2
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Body Fluid Balance
• Must maintain body fluid:
– Volume (fluid balance)
– Ionic concentration (electrolyte balance)
– pH (acid-base balance)
• Gains (input) must equal loss (output)
• Balancing efforts involve/affect almost all
body systems
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Fluid (Water) Balance
• = amount of H20 gained each day equal to
amount of H2O lost
• Regulates content and exchange of body
water between ECF and ICF
• Gains
– GI (from food, liquid)*
– Catabolism
• Losses
– Urine*
– Evaporation (from skin, lungs)
– Feces
* Primary route
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Fluid Gains and Losses
Figure 27–3
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Electrolyte (Ion) Balance
• Balances gains and losses of all electrolytes
(ions that can conduct electrical current in solution)
• Gains
– GI (from food, liquid)
• Losses
– Urine
– Sweat
– Feces
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Acid-base (pH) Balance
• Balances production and loss of H+
• Gains
– GI (from food and liquid)
– Metabolism
• Losses
– Kidneys (secrete H+)
– Lungs (eliminate CO2)
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Fluid Components
• ECF components (plasma and IF) very similar
• Major differences between ECF and ICF
• ICF very different because of cell membrane
– Selectively permeable
– Specific channels for ions
– Active transport into/out of cell
• Water exchange between ECF and ICF occurs across
cell membranes by:
– Diffusion
– Osmosis
– Carrier-mediated transport
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Cations in Body Fluids
Figure 27–2 (1 of 2)
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Anions in Body Fluids
Figure 27–2 (2 of 2)
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Cations and Anions in Body
Fluids
• In ECF
– Na+
– Cl– HCO3-
• In ICF
– K+ (98% of body content)
– Mg2+
– HPO42– Negatively charge proteins
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Principles of Fluid and Electrolyte
Regulation
• All homeostatic mechanisms that monitor and
adjust body fluid composition respond to
changes in ECF, not ICF
– Because:
• A change in ECF spreads throughout body and affects
many or all cells
• A change in ICF in one cell does not affect distant cells
15
Principles of Fluid and Electrolyte
Regulation
• No receptors directly monitor fluid or
electrolyte balance
– Electrolyte balance = electrolytes gained equals
the electrolytes lost
• Monitor secondary indicators
– Baroreceptors – for plasma volume/pressure
– Osmoreceptors – for osmotic (solute)
concentration
• Solutes = ions, nutrients, hormones, all other materials
dissolved in body fluids
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Principles of Fluid and Electrolyte
Regulation
• Cells cannot move water by active transport
– Passive in response to osmotic gradients
• Fluid balance and electrolyte balance are
interrelated
• Body’s content of water and electrolytes:
– Increases if gains exceed losses
– Decreases if losses exceed gains
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Primary Hormones for Fluid and
Electrolyte Balance
• ADH
• Aldosterone
• Natriuretic peptides (e.g., ANP)
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ADH
• Produced by osmoreceptor neurons in
supraoptic nuclei in hypothalamus (and released
by posterior pituitary)
– Osmoreceptors monitor osmotic concentrations in
ECF
– Osmotic concentration increases/decreases when:
• Na+ increases/decreases or
• H2O decreases/increases
• Increased osmotic concentration  increased
ADH
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ADH
•  Water conservation
– Increases water absorption  decreased osmotic
concentration (by diluting Na+)
– Stimulates thirst center in hypothalamus 
increased fluid intake
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Figure 27–4
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Aldosterone
• Mineralocorticoid secreted by adrenal cortex
• Produced in response to:
– Decreased Na+ or increased K+
– In blood arriving at:
• Adrenal cortex
• Kidney (renin-angiotensin system
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Renin-Angiotensin System
• Renin released in response to:
– Decreased Na+ or increased K+ in renal circulation
– Decreased plasma volume or BP at JGA
– Decreased osmotic concentration at DCT
• Renin   angiotensin II activation in lung
capillaries
• Angiotensin II 
– Adrenal cortex  increased aldosterone
– Posterior pituitary  ADH
– Increased BP (hence it’s name)
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Aldosterone
• In DCT and collecting system of kidneys 
– Increased Na+ absorption (and associated Cl- and
H2O absorption)
– Increased K+ loss
•  Increased sensitivity of salt receptors on
tongue  crave salty foods
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Natriuretic Peptides
• Released by cardiac muscle cells stretched by:
– Increased BP or blood volume
• Oppose angiotensin II and cause diuresis
–  Decreased ADH  increased H2O loss at kidneys
–  Decreased aldosterone  increased Na+ and
H2O loss at kidneys
– Decreases thirst  decreased H2O intake
• Net result = decreased stretching of cells
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Figure 27–5, 7th edition
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Fluid and Electrolyte Balance
• When body loses water:
– Plasma volume decreases
– Electrolyte concentrations increase
• When body loses electrolytes:
– Electrolyte concentrations decrease
– Water also lost
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Disorders of Fluid and Electrolyte
Balance
• Dehydration = water depletion
– Due to:
• Inadequate water intake
• Fluid loss, e.g., vomiting, diarrhea
• Inadequate ADH (hypothalamic/pituitary malfunction)
– Leads to:
•
•
•
•
Too high Na+ = hypernatremia
Thirst, wrinkled skin
Decreased blood volume and BP
Fatal circulatory shock
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Disorders of Fluid and Electrolyte
Balance
• Overhydration = water excess
– Due to:
• Excess water intake (>6-8 L/24 hours)
– Seen in hazing rituals (water torture)
– Marathon runners/paddlers
– Ravers on ecstasy who overcompensate for thirst
• Chronic renal failure
• Excess ADH
– Leads to
• Too low Na+ = hyponatremia
• Increased blood volume and BP
• CNS symptoms (water intoxication); can proceed to
convulsions, coma, death
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Disorders of Fluid and Electrolyte
Balance
• Hypokalemia
– Too low K+
– Caused by diuretics, diet, chronic alkalosis (plasma
pH >7.45)
– Results in muscle weakness and paralysis
• Hyperkalemia
– Too high K+
– Caused by diuretics (that block Na+ reabsorption)
– Renal failure, chronic acidosis (plasma pH<7.35)
– Results in severe cardiac arrhythmias
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Summary: Disorders of Electrolyte
Balance
• Most common problems with electrolyte
balance
– Caused by imbalance between gains/losses of Na+
• Uptake across digestive epithelium
• Excretion in urine and perspiration
• Problems with potassium balance
– Less common, but more dangerous
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• Today in class we will discuss:
– Acid-base balance and
• Three major buffer systems that balance pH of ECF and ICF
• Compensatory mechanisms involved in maintaining acidbase balance
– Respiratory compensation
– Renal compensation
• Causes, effects, and the body’s response to acid-base
disturbances that occur when pH varies
– Respiratory acid-base disorders
» Respiratory acidosis
» Respiratory alkalosis
– Metabolic acid-base disorders
» Metabolic acidosis
» Metabolic alkalosis
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Acid-Base Balance
• Control of pH
– Acid-base balance = = production of H+ is precisely
offset by H+ loss
• Body generates acids (H+) during metabolic
processes
– Decrease pH
• Normal pH of ECF = 7.35 – 7.45
– <7.35 = acidosis (more common than alkalosis)
– >7.45 = alkalosis
• <6.8 or >7.7 = lethal
33
Acid-Base Balance
• Deviations outside normal range extremely
dangerous
– Disrupt cell membranes
– Alter protein structure (remember hemoglobin?)
– Change activities of enzymes
• Affects all body systems
– Especially CNS and CVS
34
Acid-Base Balance
• CNS and CVS especially sensitive to pH
fluctuations
– Acidosis more lethal than alkalosis
– CNS deteriorates  coma  death
– Cardiac contractions grow weak and irregular 
heart failure
– Peripheral vasodilation  decreased BP and
circulatory collapse
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Acid-Base Balance
• Carbonic acid (H2CO3)
– Most important factor affecting pH of ECF
• CO2 + H2O  H2CO3  H+ + HCO3-
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Relationship between PCO2 and pH
• PCO2 inversely related to pH
Figure 27–9
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Acid-Base Balance
• H+
– Gained
• At digestive tract
• Through cellular metabolic activities
– Eliminated
• At kidneys by secretion of H+ into urine
• At lungs by forming H2O and CO2 from H+ and HCO3-
– Sites of elimination far from sites of production
– As H+ travels through body, must be neutralized to
avoid tissue damage
– Accomplished through buffer systems
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Buffers
• Compounds dissolved in body fluids
• Stabilize pH
• Can provide or remove H+
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Buffer Systems in Body Fluids
• Phosphate buffer system (H2PO4-)
– In ICF and urine
• Protein buffer systems
– In ICF and ECF
– Includes:
• Hb buffer system (RBCs only)
• Amino acid buffers (in proteins)
• Plasma protein buffers (albumins, globulins…)
• Carbonic acid-bicarbonate buffer system
– Most important in ECF
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Figure 27–7
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Carbonic Acid–Bicarbonate
Buffer System
Figure 27–9
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Maintenance of Acid-Base Balance
• For homeostasis to be preserved:
– H+ gains and losses must be balanced
• Excess H+ must be:
– Tied up by buffers
• Temporary; H+ not eliminated, just not harmful
– Permanently tied up in H2O molecules
• Associated with CO2 removal at lungs
– Removed from body fluids
• Through secretion at kidneys
• Accomplished by:
– Respiratory mechanisms
– Renal mechanisms
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Conditions Affecting Acid-Base Balance
• Disorders affecting buffers, respiratory or
renal function
– Emphysema, renal failure
• Cardiovascular conditions
– Heart failure or hypotension
– Can affect pH, change glomerular filtration rates,
respiratory efficiency
• Conditions affecting CNS
– Neural damage/disease that affects respiratory
and cardiovascular reflexes that regulate pH
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Disturbances of Acid-Base Balance
• Serious abnormalities have an:
– Acute (initial) phase
• pH moves rapidly out of normal range
– Compensated phase
• If condition persists
• Physiological adjustments move pH back into normal
range
• Cannot be completed unless underlying problem
corrected
• Types of compensation
– Respiratory
– Renal
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Respiratory Compensation
• Changes respiratory rate
– Increasing/decreasing respiratory rate changes pH
by lowering/raising PCO2
– Helps stabilize pH of ECF
• Occurs whenever pH moves outside normal
limits
• Has a direct effect on carbonic acidbicarbonate buffer system
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Respiratory Compensation
• Increased PCO2 
– Increased H2CO3  increased H+  decreased pH
(acidosis)
– Increased respiratory rate  more CO2 lost at lungs
 CO2 decreases to normal levels
• Decreased PCO2 
– Decreased H2CO3  decreased H+  increased pH
(alkalosis)
– Decreased respiratory rate  less CO2 lost at lungs
 CO2 increases to normal levels
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Carbonic Acid–Bicarbonate
Buffer System
Figure 27–9
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Renal Compensation
• Changes renal rates of H+ and HCO3– Secretion
– Reabsorption
• In response to changes in plasma pH
– Increased H+ or decreased HCO3- 
• Decreased pH (acidosis)  more H+ secreted and/or less
HCO3- reabsorbed
– Decreased H+ or increased HCO3- 
• Increased pH (alkalosis)  less H+ secreted and/or more
HCO3- reabsorbed
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The Carbonic Acid–Bicarbonate
Buffer System and
Regulation of Plasma pH
Figure 27–11a
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The Carbonic Acid–Bicarbonate
Buffer System and
Regulation of Plasma pH
Figure 27–11b
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Disturbances of Acid-Base Balance
• Conditions named for:
– Uncompensated or
• Compensated
– Primary source of problem
• Respiratory or metabolic
• Mixed (both)
– Primary effect
• Acidosis or alkalosis
• e.g.,
– Compensated or uncompensated
• Respiratory acidosis or alkalosis
• Metabolis acidosis or alkalosis
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Respiratory Acid-Base Disorders
• Result from imbalance between:
– CO2 generated in peripheral tissues (ECF)
– CO2 excreted at lungs
• Cause abnormal CO2 levels in ECF
•  Respiratory
– Acidosis
– Alkalosis
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Respiratory Acidosis
• Most common challenge to acid-base equilibrium
• Primary sign is hypercapnia (increased PCO2)
• Develops when respiratory system cannot eliminate
all CO2 generated by peripheral tissues
• Usual cause is hypoventilation
• Acute situation may be immediate, life-threatening
condition
– Requires bronchodilation or mechanical breathing
assistance (ventilator)
• pH can get as low as 7.0
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Respiratory Acidosis
Figure 27–12a, 7th edition
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Respiratory Alkalosis
• Relatively uncommon
• Primary sign is high pH
• Develops when increased respiratory activity
(hyperventilation) lowers plasma PCO2 to
below normal levels (hypocapnia)
• Seldom of clinical significance
• pH can get as high as 7.8 – 8.0
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Respiratory Alkalosis
Figure 27–12b, 7th edition
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Metabolic Acid-Base Disorders
• Result from:
– Production of acids during metabolic processes
– Conditions that affect concentration of HCO3- in
ECF
•  Metabolic
– Acidosis
– Alkalosis
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Metabolic Acidosis
• Results from:
– Production of large numbers of acids
• H+ overloads buffer systems
– Inability to excrete H+ at kidneys
– Severe HCO3- loss
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Metabolic Acidosis
• Production of large number of acids
– Lactic acidosis from anaerobic respiration
• After strenuous exercise
• From prolonged tissue hypoxia (O2 starvation)
– Ketoacidosis from generation of ketone bodies
during metabolism
• When peripheral tissues cannot obtain adequate
glucose from bloodstream and begin metabolizing
lipids and ketone bodies), e.g.,
– Starvation
– Complication of poorly controlled diabetes mellitus
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Metabolic Acidosis
• Inability to excrete H+ at kidneys
– With severe kidney damage (glomerulonephritis)
– Caused by diuretics that interfere with H+
secretion into urine
• Severe HCO3- loss
– From chronic diarrhea
– Loss interferes with buffer system ability to
remove H+
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Responses to Metabolic Acidosis
Figure 27–13
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Metabolic Alkalosis
• Relatively rare
• Occurs after repeated vomiting
– Stomach continues to generate HCl to replace lost
acids
– Is associated with increased HCO3- in ECF
– HC03- + H+  H2CO3
• Reduces H+  alkalosis
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Metabolic Alkalosis
Figure 27–14
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Detection of
Acidosis and Alkalosis
• Includes blood tests for:
– pH
– PCO2
– HCO3-
• Recognition of acidosis or alkalosis
• Classification as respiratory or metabolic
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Diagnostic Chart for Acid-Base Disorders
Figure 27–18
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Blood Chemistry and Acid–Base Disorders
Table 27–4
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