Hydrogen Ion homeostasis and blood gases

ECF osmolality contributor

• Sodium #1 cation in extracellular fluid, 

• Anions Chloride Cl- and bicarbonate HCO3 follows sodium

• Glucose

Urea

 has no effect on osmolality and water distribution

Osmolality equation

1.86 x Na + (glucose/18) + (BUN/2.8) + 9

Normal sodium levels

136 - 145 mmol/L

Sodium renal threshold

110 - 130 mmol/L

ANP

• atrial natriuretic peptide inhibits Na reabsorption

• Inhibits release of renin

• suppresses effects of norepinephrine and angiotensin II

ANP release

Is released in response to cardiac atria expansion due to fluid expansion / fluid overload

BNP

• Beta type natriuretic peptide 

• secreted by cardiac ventricles upon expansion

• Marker for congestive heart failure

CNP

C-type natriuretic peptide is mostly present in vascular endothelium; works as vasodilator

Potassium Distribution

• Main cation of intracellular fluid

• 90% free in cell cytoplasm

• 8% bound inside red cells

• Major function is the contraction of skeletal and cardiac muscles

Normal potassium levels

3.5 - 5.0 mmol/L, no renal threshold

Potassium and clotting

• during clotting platelets release potassium, 

• 0.2 - 0.3 mmol/L higher than plasma potassium

• Serum = may have higher potassium due to clotting

Normal osmolality

275 - 295 mOsm/kg in serum

Normal osmolality in 24hr urine collection

 301 - 1090 mOsm/kg

Osmometry 

• used to measure osmolality

• Depression freezing point

• vapor pressure

Depression freezing point

• Direct relation

• More particles the lower the freezing point

• Molal freezing point depression of pure water is -1.86*C

• rapid and inexpensive, more accurate

Depression freezing point disadvantage

• Samples must be low viscosity

• May not be suitable for high molality or colloidal solutions

Vapor pressure

• evaporation will decrease with the higher number of particles present in solution

• indirect relation

• less accurate

• cannot use volatile solutes

Osmolal gap

difference between measure and calculated osmolality, OG = MO - CO

Normal osmolal gap

<10 - 20 mOsm/kg, indicative of the presence of unmeasured anions

Beta-hydroxybutyrate

ketone formed in diabetic ketoacidosis ingestion of ethylene glycol or methanol

Increases Osmolal gap

• S - salicylate intoxication (aspirin)

• L - Lactic acidosis

• U - Unmeasured ions

• M - Methanol

• P - Poisoning

• E - Ethanol

• D - Diabetic ketoacidosis

Water loss 

osmolality of ECF will increase

Hypothalamus 

release antidiuretic hormone (ADH) / vasopressin

ADH 

• increase water reabsorption in kidney’s collecting tubules 

• Increase urine concentration

• Decrease serum osmolality

• Increase blood pressure

Aldosterone 

• Mineralocorticoid steroid hormone secreted from adrenal gland

• Controls Na/Cl and H2O retention

• Excretes K+ and H+

• Activated by renin-angiotensin system to increase blood pressure

Maintaining blood pressure increase

• ADH stimulation

• Constricted renal arterioles

• Systemic vasoconstriction

• Activation of renin-angiotensin-aldosterone system

Water/Sodium depletion

• sodium accompanied by water loss 

• water volume decreases

• Increase in Osmolality

• increase in aldosterone

• Significant increase in ADH

Pure water loss

no change in total volume loss, no change in osmolality

Water and sodium excess

• Excessive intake

• Cerebral overhydration, edema

• ICF water into ECF = dehydrate cells

Secondary aldosterism

• Hyperaldosterism; renin angiotensin disorder,

• increased sodium, low potassium

• High osmolality

Hyponatremia

 true loss of total body Na+

• Overuse of diuretics

• Low aldosterone

• diarrhea/vomiting

• Severe burns/ trauma

• sickle cell syndrome

Addison’s disease

• Low aldosterone

• Decreased cortisol

• Increased ACTH

• Increased beta MSH, hyperpigmentation

• Decreased blood glucose

• Decreased chloride/ sodium

• Increased potassium

SIADH

• Overhydrating

• Overproduction of ADH

• low sodium, increased water sodium

Hyponatremia dilutional 

• Hyperglycemia, increase water

• Congestive heart failure, BNP fluid build up

• liver cirrhosis

• Nephrotic syndrome

Hypernatremia

• Diarrhea at onset

• Diabetes insipidus

• Hyperaldosteronism

• Cushings syndrome

Hyperaldosteronism

• Primary : Conn syndrome, adrenal

• Secondary : Renin-angiotensin disorder, not at organ

• Overproduce aldosterone

Hypokalemia

• Potassium depletion 

• High aldosterone ; Conn and Cushings

• Insulin overdose

• Excreted in urine no renal threshold

Cushings syndrome

Overproduction of cortisol leading to over production of Aldosterone

Hyperalkemia 

• Risk of cardiac arrest K+ concentrations > 6.5 mmol/L

• Leakage of potassium from red cells

Falsely increased potassium

• Hemolysis (released from RBC)

• Delayed centrifugation (platelets/clotting)

• EDTA contamination

• abnormal blood

Ion selective electrode 

• Reference electrode is silver-silver chloride

• voltage change measuring ions

Direct ISE

• Requires no sample dilution

• Lipids and proteins no effect

• Whole blood can be used

• Not suitable for urine

Indirect ISE

• Requires sample dilution

• Cannot use whole blood

• Suitable for urine sample

• Elevated lipids or proteins may falsely decrease

Sodium selectivity electrode

glass ion exchange membrane

Potassium selectivity electrode

 valinomycin membrane

Reference range pH

7.35 - 7.45

Bicarbonate renal threshold

 >26-30 mmol/L

Bicarbonate decrease

• Diuretic abuse

• Diarrhea

• enteric fistula

• Addison’s disease

• Renal dysfunction

Henderson Hasselbach equation

pH = 6.1 + log (HCO3 / (0.0307 x CO2))

Primary Metabolic Acidosis

• diabetic ketoacidosis or starvation

• Reduced renal excretion of acids

• excessive loss of bicarbonate from diarrhea or drainage from GI tubes

• Beta hydroxybuterate

Primary Respiratory acidosis

• Most lungs diseases, emyphysema, bronchopneumonia

• Drugs cause hypoventilation

• Aspiration

• Congestive heart failure - less blood to the lungs

Primary metabolic alkalosis

• Excess bicarb

• ingestion of bicarbonate producing salts like sodium lactate, citrate or acetate

• Excess loss of acid, vomiting

• Prolonged use of diuretics 

Primary respiratory alkalosis

• Salicylates (aspirin), respiratory stimulation

• Increased temp

• Fever

• hysteria

• pulmonary emboli/ fibrosis

Uncompensated

• either pCO2 or HCO3- will be within range while pH is out of range, 

• ONE VALUE IS STABLE, pH and other out of range

Partially compensated

• all values out of range, 

• reverse respiratory, 

• same metabolic

Fully compensated

• pH is normal, but partial acid/basic

• other values out of range

Mixed respiratory and metabolic disorder

• pCO2 and HCO3 are abnormal in opposite directions

• base off pH

• Metabolic + Respiratory acidosis = pCO2 elevated

• M+R alkalosis = HCO3 elevated

Aspirin overdose

• pH fluctuates, other values decreased

2,3-diphosphoglycerate

(2,3-DPG) reduces affinity between O2 and hemoglobin

Left shift

• Decreased

  • temp

  • 2,3 - DPG

  • H+

  • pCO2

• Increased

  • pO2

  • pH

• Abnormal hemoglobin disorders

  • Carboxyhemoglobin

  • Methemoglobin

• High affinity to oxygen

Right shift 

• Increased

  • temp

  • 2,3 - DPG

  • H+

  • pCO2

  • Lactate

  • EPO

• Decreased

  • pO2

  • pH

• Reduced affinity to oxygen