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

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

      • 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 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 Compartments

Figure 27–1a-2

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

  • = 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

Figure 27–3

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

  • Balances production and loss of H+

  • Gains

    • GI (from food and liquid)

    • Metabolism

  • Losses

    • Kidneys (secrete H+)

    • Lungs (eliminate CO2)

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

Figure 27–2 (1 of 2)

Anions in Body Fluids

Figure 27–2 (2 of 2)

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

  • 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 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 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

  • ADH

  • Aldosterone

  • Natriuretic peptides (e.g., ANP)


  • 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


  •  Water conservation

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

    • Stimulates thirst center in hypothalamus  increased fluid intake

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 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

  • 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

Figure 27–5, 7th edition

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

  • 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 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 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

  • 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

  • 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

  • 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 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 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 Balance

  • Carbonic acid (H2CO3)

    • Most important factor affecting pH of ECF

  • CO2 + H2O  H2CO3  H+ + HCO3-

Relationship between PCO2 and pH

  • PCO2 inversely related to pH

Figure 27–9

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

  • 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

Figure 27–7

Carbonic Acid–Bicarbonate Buffer System

Figure 27–9

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

  • 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

  • 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

  • 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 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 System

Figure 27–9

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

Figure 27–11a

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

Figure 27–11b

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

  • 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

  • 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 Acidosis

Figure 27–12a, 7th edition

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 Alkalosis

Figure 27–12b, 7th edition

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

  • Results from:

    • Production of large numbers of acids

      • H+ overloads buffer systems

    • Inability to excrete H+ at kidneys

    • Severe HCO3- loss

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 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

Figure 27–13

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 Alkalosis

Figure 27–14

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

Figure 27–18

Blood Chemistry and Acid–Base Disorders

Table 27–4

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