Fluid, Electrolyte & pH Balancelemastm/Teaching/BI336/Unit... · 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

2

Intracellular Fluid (ICF)

Volume = 25 L (40% body weight)

Extracellular Fluid (ECF)

Total Body Water

Interstitial

Fluid

Pla

sm

a

Volume = 12 L

Vo

lum

e =

3 L




Body Fluids:



1) Water: (universal solvent)

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)

3

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


Body Fluids:



2) Solutes:

Electrolytes have great

osmotic power:

MgCl2 Mg2+ + Cl- + Cl-


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

Osmotic

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

300 mosm

300 mosm

4

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


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

(-)


Water Balance:

Thirst Mechanism:

(-)

5

Intracellular

Fluid

Extracellular

Fluid

300 mOsm

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

Intracellular

Fluid

Extracellular

Fluid

300 mOsm

300 mOsm

Intracellular

Fluid

Extracellular

Fluid

< 300 mOsm

300 mOsm

Intracellular

Fluid

Extracellular

Fluid

> 300 mOsm

ECF osmolarity = No change

Hyperosmotic Contraction

ECF volume = Decrease

Water

deficiency

< 300 mOsm


Aldosterone

insufficiency

NaCl not reabsorbed

from kidney filtrate

Hyposmotic Contraction

< 300 mOsm > 300 mOsm


ICF volume = No change

ICF osmolarity = No change


ICF osmolarity = Increase

ICF volume = Decrease

ECF osmolarity = Increase ECF osmolarity = Decrease

ICF volume = Increase

ICF osmolarity = Decrease

Intracellular

Fluid

Extracellular

Fluid

300 mOsm

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

Intracellular

Fluid

Extracellular

Fluid

300 mOsm

300 mOsm

Intracellular

Fluid

Extracellular

Fluid

< 300 mOsm

< 300 mOsm

Intracellular

Fluid

Extracellular

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

6


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 10-9 Eq / L

pH = -log10 [H+]

Normal

[H+]

pH = -log10 [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

1) Volatile Acids: Acids that leave solution and enter the atmosphere

H2CO3

carbonic

acid

CO2 + H2O HCO3- + H+

2) Fixed Acids: Acids that do not leave solution (~ 50 mmol / day)


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

7

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


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


Acid-Base Balance:

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)


Most effective

buffering -1 +1

For the human body, the

most effective physiologic buffers

will have a pK at 7.4 1.0

http://www.easyvectors.com/assets/images/vectors/afbig/70f38a6bec1218ec7468c2af949e109b-dog-simple-drawing-clip-art.jpg

8

1) HCO3- / CO2 Buffer:

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


Acid-Base Balance:

The major buffers of the ECF are bicarbonate and phosphate

H2CO3 HCO3- + H+ CO2 + H2O

(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 PCO2

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

1) HCO3- / CO2 Buffer:


Acid-Base Balance:



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)

9

2) H2PO4- / HPO4

2- Buffer:


Acid-Base Balance:


(HA) (A-)

H2PO4- HPO4

2- + 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:


Acid-Base Balance:

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 – NH2 + H+ R – NH3

+ 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++ Ca++

Ca++

H+ H+

Normal pH

Normal circulating Ca++

A Ca++

Ca++

Ca++ Ca++

H+ H+

Acidemia Alkalemia

H+

Hypercalcemia Hypocalcemia

Ca++

R – COOH R – COO- + H+ If pH rises

10

1) Respiratory Regulation:

If H+ begins to rise in

system, respiratory

centers excited

H2CO3

carbonic

acid


CO2 leaves

system

H2CO3

carbonic

acid


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…


Maintenance of Acid-Base Balance:

2) Renal Regulation:

A) HCO3- reabsorption:

Process continues

until 99.9% of

HCO3- reabsorbed





Kidneys are ultimate acid-base

regulatory organs

• Reabsorb HCO3-

• Secrete fixed H+


(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.

11


B) Secretion of fixed H+:






regulatory organs

• Reabsorb HCO3-


(1)

Excretion of H+

as titratable acid


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)


B) Secretion of fixed H+:






regulatory organs

• 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

12

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…


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