Chapter 27 fluid electrolyte and acid base balance
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Chapter 27: Fluid, Electrolyte and Acid-base Balance. BIO 211 Lecture Instructor: Dr. Gollwitzer. 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

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Chapter 27: Fluid, Electrolyte and Acid-base Balance

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Chapter 27 fluid electrolyte and acid base balance

Chapter 27: Fluid, Electrolyte and Acid-base Balance

BIO 211 Lecture

Instructor: Dr. Gollwitzer

Chapter 27 fluid electrolyte and acid base balance

  • 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



  • 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

Body fluid compartments

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

Body fluid compartments1

Body Fluid Compartments

Figure 27–1a-2

Body fluid balance

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

Fluid water balance

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

Fluid gains and losses

Fluid Gains and Losses

Figure 27–3

Electrolyte ion balance

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

Acid base ph balance

Acid-base (pH) Balance

  • Balances production and loss of H+

  • Gains

    • GI (from food and liquid)

    • Metabolism

  • Losses

    • Kidneys (secrete H+)

    • Lungs (eliminate CO2)

Fluid components

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

Cations in body fluids

Cations in Body Fluids

Figure 27–2 (1 of 2)

Anions in body fluids

Anions in Body Fluids

Figure 27–2 (2 of 2)

Cations and anions in body fluids

Cations and Anions in Body Fluids

  • In ECF

    • Na+

    • Cl-

    • HCO3-

  • In ICF

    • K+ (98% of body content)

    • Mg2+

    • HPO42-

    • Negatively charge proteins

Principles of fluid and electrolyte regulation

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

Principles of fluid and electrolyte regulation1

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

Principles of fluid and electrolyte regulation2

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

Primary hormones for fluid and electrolyte balance

Primary Hormones for Fluid and Electrolyte Balance

  • ADH

  • Aldosterone

  • Natriuretic peptides (e.g., ANP)

Chapter 27 fluid electrolyte and acid base balance


  • 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

Chapter 27 fluid electrolyte and acid base balance


  •  Water conservation

    • Increases water absorption  decreased osmotic concentration (by diluting Na+)

    • Stimulates thirst center in hypothalamus  increased fluid intake

Chapter 27 fluid electrolyte and acid base balance

Figure 27–4



  • Mineralocorticoid secreted by adrenal cortex

  • Produced in response to:

    • Decreased Na+ or increased K+

    • In blood arriving at:

      • Adrenal cortex

      • Kidney (renin-angiotensin system

Renin angiotensin system

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)



  • 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

Natriuretic peptides

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

Chapter 27 fluid electrolyte and acid base balance

Figure 27–5, 7th edition

Fluid and electrolyte balance

Fluid and Electrolyte Balance

  • When body loses water:

    • Plasma volume decreases

    • Electrolyte concentrations increase

  • When body loses electrolytes:

    • Electrolyte concentrations decrease

    • Water also lost

Disorders of fluid and electrolyte balance

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

Disorders of fluid and electrolyte balance1

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

Disorders of fluid and electrolyte balance2

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

Summary disorders of electrolyte balance

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

Chapter 27 fluid electrolyte and acid base balance

  • 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 acid-base 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

Acid base balance

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

Acid base balance1

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

Acid base balance2

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

Acid base balance3

Acid-Base Balance

  • Carbonic acid (H2CO3)

    • Most important factor affecting pH of ECF

  • CO2 + H2O  H2CO3  H+ + HCO3-

Relationship between pco 2 and ph

Relationship between PCO2 and pH

  • PCO2 inversely related to pH

Figure 27–9

Acid base balance4

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



  • Compounds dissolved in body fluids

  • Stabilize pH

  • Can provide or remove H+

Buffer systems in body fluids

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

Chapter 27 fluid electrolyte and acid base balance

Figure 27–7

Carbonic acid bicarbonate buffer system

Carbonic Acid–Bicarbonate Buffer System

Figure 27–9

Maintenance of acid base balance

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

Conditions affecting acid base balance

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

Disturbances of acid base balance

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

Respiratory compensation

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 acid-bicarbonate buffer system

Respiratory compensation1

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

Carbonic acid bicarbonate buffer system1

Carbonic Acid–Bicarbonate Buffer System

Figure 27–9

Renal compensation

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

The carbonic acid bicarbonate buffer system and regulation of plasma ph

The Carbonic Acid–Bicarbonate Buffer System and Regulation of Plasma pH

Figure 27–11a

The carbonic acid bicarbonate buffer system and regulation of plasma ph1

The Carbonic Acid–Bicarbonate Buffer System and Regulation of Plasma pH

Figure 27–11b

Disturbances of acid base balance1

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

Respiratory acid base disorders

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

Respiratory acidosis

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

Respiratory acidosis1

Respiratory Acidosis

Figure 27–12a, 7th edition

Respiratory alkalosis

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

Respiratory alkalosis1

Respiratory Alkalosis

Figure 27–12b, 7th edition

Metabolic acid base disorders

Metabolic Acid-Base Disorders

  • Result from:

    • Production of acids during metabolic processes

    • Conditions that affect concentration of HCO3- in ECF

  •  Metabolic

    • Acidosis

    • Alkalosis

Metabolic acidosis

Metabolic Acidosis

  • Results from:

    • Production of large numbers of acids

      • H+ overloads buffer systems

    • Inability to excrete H+ at kidneys

    • Severe HCO3- loss

Metabolic acidosis1

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

Metabolic acidosis2

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+

Responses to metabolic acidosis

Responses to Metabolic Acidosis

Figure 27–13

Metabolic alkalosis

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

Metabolic alkalosis1

Metabolic Alkalosis

Figure 27–14

Detection of acidosis and alkalosis

Detection ofAcidosis and Alkalosis

  • Includes blood tests for:

    • pH

    • PCO2

    • HCO3-

  • Recognition of acidosis or alkalosis

  • Classification as respiratory or metabolic

Diagnostic chart for acid base disorders

Diagnostic Chart for Acid-Base Disorders

Figure 27–18

Blood chemistry and acid base disorders

Blood Chemistry and Acid–Base Disorders

Table 27–4

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