Fluid, Electrolyte & ph Balance

(1)

Fluid, Electrolyte

& pH Balance

Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids Body Fluids:

1) Water: (universal solvent)

Fluid / Electrolyte / Acid-Base Balance

Body water varies based on of age, sex, mass, and body composition

H2O ~ 73% body weight Low body fat

Low bone mass H2O (♂) ~ 60% body weight H2O (♀) ~ 50% body weight

♀ =  body fat /  muscle mass H₂O ~ 45% body weight

Intracellular Fluid (ICF) Volume = 25 L

(40% body weight)

Extracellular Fluid (ECF) Total Body Water

Interstitial Fluid

Plasma

Volume = 12 L

Volume = 3 L

Volume = 15 L (20% body weight) Volume = 40 L

(60% body weight) Fluid / Electrolyte / Acid-Base Balance

Body Fluids:

Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids

1) Water: (universal solvent)

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Clinical Application:

The volumes of the body fluid compartments are measured by the dilution method

Step 1:

Identify appropriate marker substance

A marker is placed in the system that is distributed

wherever water is found Marker: D2O

Extracellular Fluid Volume:

A marker is placed in the system that can not cross

cell membranes Marker: Mannitol Total Body Water:

Step 2:

Inject known volume of marker into individual

Step 3:

Let marker equilibrate and measure marker

• Plasma concentration

• Urine concentration

Step 4:

Calculate volume of body fluid compartment

Volume = Amount Concentration (L)

Amount:

Amount of marker injected (mg) – Amount excreted (mg)

Concentration:

Concentration in plasma (mg / L)

Note:

ICF Volume = TBW – ECF Volume (mg)

A) Non-electrolytes (do not dissociate in solution – neutral)

Although individual [solute] are different between compartments, the osmotic

concentrations of the ICF and ECF are usually identical…

B) Electrolytes (dissociate into ions in solution – charged)

• Mostly organic molecules (e.g., glucose, lipids, urea)

• Inorganic salts

• Inorganic / organic acids

• Proteins

Marieb & Hoehn (Human Anatomy and Physiology, 8th ed.) – Figures 26.2 Fluid / Electrolyte / Acid-Base Balance

Body Fluids:

Cell function depends not only on continuous nutrient supply / waste removal, but also on the physical / chemical homeostasis of surrounding fluids

2) Solutes:

Electrolytes have great osmotic power:

MgCl2  Mg2+ + Cl^- + Cl^-

Fluid / Electrolyte / Acid-Base Balance Fluid Movement Among Compartments:

The continuous exchange and mixing of body fluids are regulated by osmotic and hydrostatic pressures

Intracellular Fluid Interstitial Fluid Plasma

Hydrostatic Pressure

Osmotic Pressure

The volume of a particular compartment depends on amount of solute present

The shift of fluids between compartments depends on osmolarity of solute present In a steady state,

intracellular osmolarity is equal to extracellular osmolarity

300 mosm

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Ingested liquids (60%) Solid foods (30%) Metabolism(10%) Water Intake

2500 ml/day

= 0

Urine (60%) Skin / lungs (30%)

Sweat (8%) Water Output

2500 ml/day Feces (2%)

> 0

< 0

Osmolarity rises:

Osmolarity lowers:

ICF functions as a reservoir

• Thirst

• ADH release Fluid / Electrolyte / Acid-Base Balance

Water Balance:

For proper hydration: Waterintake = Wateroutput

• Thirst

• ADH release

 volume /

 osmolarity of extracellular fluid

 saliva

secretion Dry mouth

Osmoreceptors stimulated (Hypothalamus)

Sensation of thirst

 osmolarity /

 volume of extra. fluid

Drink (-)

Fluid / Electrolyte / Acid-Base Balance Water Balance:

Thirst Mechanism:

(-)

Intracellular Fluid Extracellular

Fluid

300 mOsm

Pathophysiology:

Disturbances that alter solute or water balance in the body can cause a shift of water between fluid compartments

Volume Contraction: A decrease in ECF volume

Isomotic Contraction Diarrhea 300 mOsm

Fluid

300 mOsm

Fluid

< 300 mOsm

300 mOsm

Fluid

> 300 mOsm

ECF osmolarity = No change

Hyperosmotic Contraction ECF volume = Decrease

Water deficiency

< 300 mOsm

ECF volume = Decrease Aldosterone insufficiency NaCl not reabsorbed

from kidney filtrate

Hyposmotic Contraction

< 300 mOsm

> 300 mOsm Fluid / Electrolyte / Acid-Base Balance

ICF volume = No change ICF osmolarity = No change

ECF volume = Decrease

ICF osmolarity = Increase ICF volume = Decrease

ECF osmolarity = Increase ECF osmolarity = Decrease ICF volume = Increase ICF osmolarity = Decrease

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Fluid

300 mOsm

Pathophysiology:

Disturbances that alter solute or water balance in the body can cause a shift of water between fluid compartments

Volume Expansion: An increase in ECF volume

Isomotic Expansion Osmotic IV 300 mOsm

Fluid

300 mOsm

Fluid

< 300 mOsm

Fluid

> 300 mOsm

Hyperosmotic Expansion

Yang’s lunch SIADH

Greater than normal water reabsorption

Hyposmotic Expansion IV bag

300 mOsm

> 300 mOsm NaCl

Water intoxication

IV bags of varying solutes allow for manipulation of ECF / ICF levels…

ECF osmolarity = No change

ECF volume = Increase ECF volume = Increase

ICF volume = No change ICF osmolarity = No change

ECF volume = Increase

ICF osmolarity = Increase ICF volume = Decrease

ECF osmolarity = Increase ECF osmolarity = Decrease ICF volume = Increase ICF osmolarity = Decrease

300 mOsm

Acid-Base Balance:

Acid-base balance is concerned with maintaining a normal hydrogen ion concentration in the body fluids

Costanzo (Physiology, 4th ed.) – Figure 7.1

[H⁺] = 40 x 10^-9Eq / L

pH = -log

10

[H

⁺

]

Normal [H⁺]

pH = -log

10

[40 x 10

^-9

] pH = 7.4

Note:

Normal range of arterial pH = 7.37 – 7.42 Compatible range for life

Problems Encountered:

1) [H⁺] and pH are inversely related 2) Relationship is logarithmic, not linear

1) Disruption of cell membrane stability 2) Alteration of protein structure

3) Enzymatic activity change

⁺

2) Fixed Acids: Acids that do not leave solution (~ 50 mmol / day) Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance:

Acid production in the human body has two forms:

volatile acids and fixed acids

End product of aerobic metabolism (13,000 – 20,000 mmol / day)

Products of normal catabolism:

Products of extreme catabolism:

Ingested (harmful) products:

Eliminated via the lungs

• Sulfuric acid (amino acids)

• Phosphoric acid (phospholipids)

• Lactic acid (anaerobic respiration)

• Acetoacetic acid

(ketobodies) Diabetes

mellitus

• B-hydroxybutyric acid

• Salicylic acid (e.g., aspirin)

• Formic acid (e.g., methanol)

• Oxalic acids (e.g., ethylene glycol) Eliminated via the kidneys

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Chemical Buffer:

A mixture of a weak acid and its conjugate base or a weak base and its conjugate acid that resist a change in pH

Fluid / Electrolyte / Acid-Base Balance Acid-Base Balance:

H⁺ Gain:

• Across digestive epithelium

• Cell metabolic activities

H⁺ Loss:

• Release at lungs

• Secretion into urine Distant from

one another

Brønsted-Lowry Nomenclature:

Weak acid:

Acid form = HA = H⁺ donor Base form = A^- = H⁺ acceptor

Weak base:

Acid form = BH⁺ = H⁺ donor Base form = B = H⁺ acceptor 11.4 L

11.4 L 150 mEq H⁺ 150 mEq H⁺

7.44  7.14 7.00  1.84 Robert Pitt:

The Henderson-Hasselbalch equation is used to calculate the pH of a buffered solution

Henderson-Hasselbalch Equation:

pH = pK + log [A^-] [HA]

pH = -log10 [H⁺] pK = -log10 K (equilibrium constant) [A^-] = Concentration of base form of buffer (mEq / L) [HA] = Concentration of acid form of buffer (mEq / L) pK is a characteristic value for a buffer pair

(strong acid =  pK; weak acid =  pK)

Costanzo (Physiology, 4th ed.) – Figure 7.2 Most effective

buffering -1 +1 For the human body, the most effective physiologic buffers

will have a pK at 7.4  1.0

1) HCO₃^- / CO₂ Buffer:

Utilized as first line of defense when H⁺ enters / lost from system:

The major buffers of the ECF are bicarbonate and phosphate

H

2

CO

3

HCO

3-

+ H

⁺

CO

2

+ H

2

O

(HA) (A^-)

(1) Concentration of HCO3- normally high at 24 mEq / L (2) The pK of HCO3- / CO2 buffer is 6.1 (near pH of ECF) (3) CO2 is volatile; it can be expired by the lungs

The pH of arterial blood can be calculated with the Henderson-Hasselbalch equation

pH = pK + log HCO3-

0.03 x P_CO2

pH = 6.1 + log 24 0.03 x 40 pK = 6.1

[HCO3-] = 24 mmol / L Solubility = 0.03 mmol / L / mm Hg PCO2 = 40 mm Hg

pH = 7.4

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1) HCO3- / CO2 Buffer:

Acid-base map:

Shows relationship between the acid and base forms of a buffer and the pH of the solution

Isohydric line (‘same pH’) Ellipse:

Normal values for arterial blood

Note:

Abnormal combinations of PCO2 and HCO3- concentration can yield normal values of pH (compensatory mechanisms)

2) H₂PO₄^- / HPO₄^2- Buffer:

(HA) (A^-)

H

2

PO

4-

HPO

42-

+ H

⁺

Why isn’t inorganic phosphate the primary ECF buffer?

(1) Lower concentration than bicarbonate (~ 1.5 mmol / L) (2) Is not volatile; can not be cleared by lung

1) Organic phosphates:

The major buffers of the ICF are organic phosphates and proteins H⁺ enters cells via a build up of CO2 in ECF, which then enters cells,

or to maintain electroneutrality

• ATP / ADP / AMP

• Glucose-1-phosphate

• 2,3-diphosphoglycerate

pKs range from 6.0 to 7.5

2) Proteins:

R – NH

2

+ H

⁺

R – NH

3+ If pH falls The most significant ICF protein

buffer is hemoglobin

Proteins also buffer H⁺ in the ECF

A relationship exists between Ca⁺⁺, H⁺,

and plasma proteins

A

^Ca

++

Ca⁺⁺

H⁺ H⁺ Normal pH

Normal circulating Ca⁺⁺

A

^Ca

++

Ca⁺⁺

3-

+ H

⁺ If H⁺ begins to fall in

system, respiratory centers depressed

 CO₂ leaves system

Doubling / halving of areolar ventilation can raise / lower blood pH by 0.2 pH units

( H; homeostasis restored)

( H; homeostasis restored) Buffers are a short-term fix to the problem; in the long term,

H⁺ must be removed from the system…

Fluid / Electrolyte / Acid-Base Balance Maintenance of Acid-Base Balance:

2) Renal Regulation:

A) HCO3- reabsorption:

Process continues until 99.9% of HCO3- reabsorbed

Buffers are a short-term fix to the problem; in the long term, H⁺ must be removed from the system…

Kidneys are ultimate acid-base regulatory organs

• Reabsorb HCO3-

• Secrete fixed H⁺

Costanzo (Physiology, 4th ed.) – Figure 7.5 (brush

border) (intracellular) Net secretion of

H⁺ does not occur

Net reabsorption of HCO3- does occurs THUS

pH of filtrate does not significantly

change

If HCO3- loads reach 40 mEq / L, transport maximized and

HCO3- excreted.

B) Secretion of fixed H⁺:

• Reabsorb HCO3-

(1) Excretion of H⁺ as titratable acid

Costanzo (Physiology, 4^th ed.) – Figure 7.6 Titratable Acid:

H⁺ excreted with buffer

Recall:

Only 85% of phosphate reabsorbed from filtrate

CO2

New HCO3- replaces HCO3- that is lost when

CO2 exits blood Minimum urinary

pH is 4.4 (Maximum H⁺ gradient

H⁺ ATPase can work against)

(Removes 40% of fixed acids – 20 mEq / day)

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B) Secretion of fixed H⁺:

• Reabsorb HCO3-

(2) Excretion of H⁺

as NH4+

(Removes 60% of fixed acids – 30 mEq / day)

Diffusional trapping:

Lipid soluble NH3 is able to diffuse into tubule but once combined with

NH4+ it is unable to leave

Disturbances of Acid-Base Balance:

1) Respiratory Acid / Base Disorders:

Respiratory Acidosis:

 CO₂ retained in body

(Most common acid / base disorder)

B) Respiratory Alkalosis:

 CO₂ retained in body Cause:

Hypoventilation (e.g., emphysema)

Cause:

Hyperventilation (e.g., stress) (Rarely persists long

enough to cause clinical emergency)

2) Metabolic Acid / Base Disorders:

A) Metabolic Acidosis:

 fixed acids generated in body Causes:

Starvation ( ketone bodies)

Excessive HCO3- loss (e.g., chronic diarrhea)

 Alcohol consumption ( acetic acid)

A) Metabolic alkalosis:

 HCO₃^- generated in body Causes:

Repeated vomiting (alkaline tide ‘amped’) Antacid overdose (Rare in body) In the short term, respiratory / urinary system

will compensate for disorders…