Fluids and Electrolytes

Importance of fluid + electrolytes

• maintenance of homeostasis

• constantly move between intracellular and extracellular compartments

Causes of changes in movement of fluids

disease processes cause the changes in movements of fluids

Compartment Syndrome

Fluid Compartments

• Intracellular compartments

• Extracellular compartments (ECF further divided into interstitial fluid (around cells = 80%) and intravascular fluid (in blood vessels = around 20%)

Volumes of

Composition of body fluids

• 60% water

Role of water in body fluids

• lubricant

• transport to cells

• removes waste products from cells

• regulates temperature

• optimum medium for cell function and chemical reactions

• breaks down food particles in the GI tract

• contains dissolved substances (electrolytes, gases, nutrients, enzymes, hormones)

Fluid intake

• drinking (60%) - 1400-1800ml

• food (30%) - 700-1000ml

• oxidation (10%) - 300-400ml

Total balance: 2400-3200ml

Fluid output

• urine (60%) - 1400-1800ml

• faeces (2%) - 100ml

• expiration of lungs (28%) - 600-800ml

• skin (10%) - 300-600ml

Total balance: 2400-3200 ml

Osmosis

water moves from area of high volume to low volume through a selectively permeable membrane. (movement dependent on concentrations)

• membrane impermeable to certain solutes

Diffusion

solutes move from high conc to low conc

• small molecules: via concentration gradient

• large molecules: via facilitated diffusion

Active transport

solutes moved across membrane AGAINST concentration gradient, process requires energy (e.g. Sodium Potassium pump)

Measuring concentration

Osmolarity vs Osmolality

• Osmolality: weight based

• Osmolarity: volume based

Osmolality

amount of solutes per kg of solvent (water) = weight based

• precise

• unaffected by temperature

Osmolarity

amount of solutes per liter of solvent (water) = volume based

• calculated

• measured in a lab

• affected by temperature

Tonicity

• Isotonic: same osmolality as body fluids (e.g. 0.9% sodium chloride)

• Hypotonic: lower conc of solutes (e.g. 0.45% Sodium Chloride)

• Hypertonic: higher conc of solutes (e.g. 5% Dextrose)

Electrolytes

Chemical compounds which dissociate in water to form charge particles called ions (either + or - charged)

Cations

Positively charged

• Potassium (intracellular) -

• Sodium (extracellular)

• Calcium (intracellular)

• Magnesium (intracellular)

Anions

Negatively charged

• Chloride (extracellular)

• Phosphate (intracellular)

• Bicarbonate (extracellular)

Sodium (Na+)

• generation of action potential

• important in fluid/ electrolyte balance

Potassium (K+)

• establish resting membrane potential

• regulates pH balance

• Maintains intracellular fluid volume

Calcium (Ca++)

• important clotting factor

• neurotransmitter release in neurons

• maintains muscle tone

• excitability of nervous and muscle tissue

Magnesium (Mg++)

• maintains normal nerve and muscle function

• maintains regular heart rate

• regulates blood glucose and blood pressure

• essential for protein synthesis

Chloride (Cl-)

• maintains balance of anions in different fluid compartments

Bicarbonate (HCO3-)

• main buffer of hydrogen ions in plasma

• maintains balance of cations and anions in ICF and ECF

Phosphate (HPO4-)

• essential for digestion of proteins, carbs, fats and absorption of Ca++

• essential for bone formation

Sulphate (

Hormonal regulation of electrolytes

• ADH by hypothalamus by osmoreceptors

• Aldosterone by cortex of adrenal glands - facilitates water and sodium retention)

ADH

• fluid intake regulated by thirst

• fluid is lost = serum osmolarity increases

• osmoreceptors in hypothalamus sense the increase = release ADH = trigger kidneys to retain water = produces thirst sensation

RAAS

Renin- Angiotensin – Aldosterone system

• Trigger = LOW BLOOD PRESSURE

• Juxtaglomerular apparatus in the kidney senses low flow and produces renin

• Renin stimulates the liver to produce angiotensinogen which converts to angiotensin 1 .

• Angiotensin 1 is converted to angiotensin 2 in the lungs

• Angiotensin 2 triggers the adrenal glands to produce aldosterone and causes vasoconstriction

• Aldosterone causes Na+ retention, K+ excretion and water reabsorption

• BLOOD PRESSURE INCREASES

Causes of electrolyte imbalance

• environmental conditions

• disease

Fluid imbalances

• Hypovolaemia (dehydration) = too little

• Hypervolaemia = too much

• Oedema = abnormal distribution (In the wrong places)

Hypovolaemia

Loss of blood volume and extracellular fluid

• loss of water caused by vomiting/ sweating/ climate/ hormonal dysfunction (lack of ADH)

• loss of blood (internal or external haemorrhage)

Hypervolaemia

Overload of fluid in blood vessels and extracellular fluid

• too much water (over infusion of crystalloid fluid or polydipsia (excessive thirst))

• too much intravascular fluid (over infusion of colloid fluid (e.g. blood products)

Abnormal distribution of fluids

• fluid in the wrong places: cardiac failure (pulmonary/ peripheral odema), obesity (increased fluid pressure and Na+ retention), liver failure (hypoabluminaemia causing ascites), inflammation (effusions)

• electrolytes in the wrong places (hyponatraemia (SIADH = damage to the hypothalamus/ side effect of some drugs)

Oedema

• abnormal accumulation of fluid (mainly water) in the interstitial space

• may be local (around an injury)

• generally temporary and resolves without intervention, but can also be generalised (e.g. cardiac failure leads to swollen ankles because weak heart cannot pump blood up from vessels so fluid leaks into surrounding tissues)

Result of too much ADH

SIADH = body retains too much water

Result of too little ADH

body does not retain enough water

Dehydration

A consequence of insufficient intake or exessive losses, including insensible losses

Consequences of dehydration

• constipation

• UTI

• Increased blood viscosity = increases risk of Coronary Heart Disease

• Poor blood glucose control in diabetics

• falls in the elderly

• acute kidney injury

Hyponatraemia

• a serum sodium level >145 mmol/L

• a deficit of total body water relative to sodium, causing cellular dehydration

• can cause headache, confusion, dizziness, brain swelling and death

Hypernatraemia

• serum sodium concentration below 135 mmol/L

• an excess water relative to sodium, leading to cell swelling

• thirst, weakness, neuromuscular excitability, hyperreflexia, confusion, seizures, coma.

• brain shrinkage caused by osmotic imbalances

Electrolyte disturbances

Treating electrolyte imbalances

• easier to treat deficiencies than excess (like adding salt to cooking) by using supplements

Managing excess water

• acetazolomide

• loop diuretics (bumetanide, furosemide, torsemide)

• Thiazides

• Spironolactone (aldosterone antagonist), amiloride, triamterine

Managing excess electrolytes

1. duiretics: loss of electrolytes with water via kidneys

2. insulin activates Na+/K+ pump and shifts potassium in to the cells. need to give glucose at the same time to prevent associated hypoglycaemia – take about 10-20 minutes to work.

Types of fluid replacement and their advantages/ disadvantages

Crystalloid

• isotonic (maintain electrolyte balance)

• hypotonic (create conc gradient to move electrolytes in to the intravenous space)

• Hypertonic (create a conc gradient to move water in to the intravascular space)

Big shifts in fluids

Burns: huge losses of fluid into interstitial space

• use of specific formulas - calculations of volume over time

Diabetic Ketoacidosis: extremely high Blood glucose levels

Renal failure

• Peritoneal dialysis
• Uses the peritoneum as the semi-permeable membrane to utilise
osmosis and diffusion of solutes . Dialysate fluid is instilled in to
the peritoneal cavity, and left for a period of time for equilibrium to
occur, then drained off.
• Diffusion occurs both ways, so the constitution of the dialysate
fluid determines amount of fluid and electrolyte shift.