Module 6 - Fluid & Electrolyte Disturbances Flashcards
What system controls fluid
Mostly renal system
How much of adult body weight is water
60% of adult weight is total body water (42L in a 70kg person) –> higher in infants
Do people who are fatter have more or less total body water
Less
As fat is water repelling (hydrophobic) and very little water found in adipose cells
Describe intracellular fluid
- Comprises all the fluid within cells – 2/3 of total body water
- Where metabolic processes occur
- High conc. of potassium, magnesium, phosphate, glucose and oxygen
Describe extracellular fluid
• All fluid outside the cells which constitutes the remaining fluid volume and is divided into interstitial and intravascular fluid
Additional smaller components include lymph, cerebrospinal fluid, synovial fluid, sweat, urine, pleural, peritoneal, etc.
• Where transport of molecules occurs
• Higher conc. of sodium, chloride and bicarbonate
What is interstitial fluid
Space between cells and outside the blood vessels (where most of extracellular fluid found)
What is intravascular fluid
The plasma
Where does most water intake and output occur
- Intake body water mostly through drinking, or water from food and metabolism of food
- Most lost through urine, small amounts through sweat, lungs and faeces
- Around 2.5L enters and exits body per day which maintains homeostasis
How does water move between plasma and interstitial fluid
- Occurs as a result of changes in hydrostatic pressure (pushing) and osmotic/oncotic pressure (pulling) at arterial and venous ends of capillaries
- Water, sodium and glucose readily cross membrane, but larger proteins do not
- Larger proteins help maintain osmolarity (particularly albumin) as it generates effective oncotic pressure
What are the four main forces present for movement between plasma and interstitial fluid
- Capillary hydrostatic pressure (blood pressure) – tends to force water out into interstitial fluid
- Capillary oncotic pressure – occurs as proteins attract fluid, high levels of protein in blood so reasonable pressure generated which draws water into capillary
- Interstitial hydrostatic pressure – facilitates movement from interstitial into capillary or lymph vessels
- Interstitial oncotic pressure – due to proteins, attracts water from capillary into interstitial space
Last two have minimal effect
What law does movement between plasma and interstitial fluid follow
STARLING’S LAW OF THE CAPILLARY:
Net filtration = forces favouring filtration – forces opposing filtration
Favouring- capillary hydrostatic pressure and interstitial oncotic pressure
Opposing- capillary oncotic pressure and interstitial hydrostatic pressure
What forces are most present at venous and venous and arterial ends
- At arterial end- capillary hydrostatic exceeds capillary oncotic so fluid exits capillary into the space
- At venous end- capillary hydrostatic less due to rapid exit of water from arterial end, oncotic is the same at both ends as amount of proteins doesn’t change, so oncotic exceeds hydrostatic and fluid enters capillary
Explain movement of water between interstitial fluid and intracellular fluid
- Result of osmotic forces
- Oncotic force proteins which is relatively constant within cells, draws water into intracellular fluid
- Moves freely via diffusion through lipid of membrane and aquaporins
- Sodium responsible for ECF osmotic balance, K+ for ICF
- Osmotic force of proteins in ICF controlled through active transport with ions
- Osmolarity of total body water needs to be at equilibrium (balanced
What is oedema
Accumulation of fluid in interstitial space (swelling)
• Conditions that cause increased sodium and water retention or venous obstruction, lead to increased blood volume which increases blood hydrostatic pressure contributing to oedema e.g. hypertension, heart failure and renal failure
• Also affected by low plasma oncotic pressure such as decrease in albumin or malnutrition, increased capillary membrane permeability i.e. inflammation or lymphatic obstruction (maybe a tumour)
Explain first, second and third spacing
When distribution between ICF and ECF are normal it is called first spacing
When people have oedema second spacing occurs due to excess fluid in interstitial space - can be treated by elevating limb or using diuretic agents
Third spacing occurs in areas not considered to be normal fluid compartments and not able to easily drained e.g. burns site or damaged tissue - main concern is hypovolaemia (low blood volume as fluid is in third space and not in normal compartments, can lead to dehydration)
Manifestations of swelling
Weight gain, limited movements, impaired blood flow, slow wound healing, increased risk infection and increase risk pressure sores and injuries
Treatment for swelling
Elevate limb, fluid restriction, administer diuretics, treat underlying cause
Describe antidiuretic hormone
Controls water balance
Secreted when total plasma concentration (osmolarity) increases or circulating blood volume or pressure decreases
Increased plasma concentration occurs with water deficit or sodium excess which stimulates hypothalamic osmoreceptors
In addition to causing thirst, osmoreceptors signal posterior pituitary to release ADH
ADH increases reabsorption of water by kidneys so water retained and not lost as urine (urine concentration increases and plasma volume increases)
Plasma concentration, therefore, decreases returning it to normal
ADH also released by signals sent for baroreceptors and receptors in heart muscle
What 3 factors affect sodium balance
Increased glomerular filtration rate, aldosterone and natriuretic peptides
What role does sodium play in the body
It is the main ion in extracellular fluid that regulates osmotic forces and water balance
Also works with other ions for transport, regulating acid-base balance and neuron signalling
What is the normal plasma sodium range
135-145mmol/L
Mostly controlled by tubular absorption in kidneys
How is sodium hormonally controlled
By ALDOSTERONE secreted from adrenal cortex as part of RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM
Aldosterone secretion influenced by blood volume and plasma concentrations of sodium and potassium (secreted when sodium low and potassium high)
What does aldosterone do
Increases reabsorption of sodium and secretion of potassium in the kidneys - sodium concentration in extracellular fluid increases
What are natriuretic peptides
Released by heart (atrial NP and brain NP) when increase in BV and BP - decreases release of renin to increase sodium and water excretion, also cause vasodilation
Explain the renin-angiotension-aldosterone system
Blood volume or pressure low, decreased sodium or increased potassium causes juxtaglomerular cells of kidneys to release renin
Renin stimulates release of angiotensin I from angiotensinogen
Angiotensin I is converted to Angiotensin II by angiotensin converting enzymes in pulmonary vessels
Angiotensin II causes release aldosterone and vasoconstriction - sodium and water retention is promoted to increase blood volume
Vasoconstriction increases BP to stop release of renin
What is tonicity
Concentration of solutes in relation to water
Describe isotonic alterations
Total body water changes may be accompanied by changes in electrolytes so osmolarity remains within normal range of 280-300 mOsm/kg
For example, if individual loses pure plasma or extracellular fluid, fluid volume depleted but conc. and type electrolytes remains the same
• Isotonic- same conc. of solutes as normal plasma concentration
• Example: Hypovolaemia or Hypervolemia
• Example of a solution is 0.9% sodium chloride
Explain hypovolaemia
Low extracellular volume (low blood volume) caused by insufficient fluid intake, haemorrhage (bleeding), wound drainage, excessive sweating, vomiting, diarrhoea or high doses diuretic hormones
Both fluid and extracellular sodium are lost together (compared to dehydration which is just the loss of water)
Indicators- rapid HR, flattened neck veins, decreased BP
Treatment- blood transfusion, fluid replacement, fluid challenge
Explain hypervolaemia
- High ECF (high blood volume and high BP)
- Can be caused by fluid overload, impaired mechanism that maintain homeostasis
- Often results in bradycardia
- Effects include distended neck veins, cough, respiratory crackles, pink, frothy sputum, increased urine output, peripheral oedema
- Treatment- diuretics, fluid restriction and low sodium diet
Describe hypertonic alterations
- When solute conc. is higher than in the blood (solution has less water than in the blood)
- Develop when conc. extracellular is elevated above normal osmolarity
- Most common causes- increased conc. sodium (hypernatremia) or deficit in ECF water
- ECF hypertonicity seen in plasma attracts water from ICF causes ICF dehydration
- Example of solution is 3% sodium chloride, 50% dextrose
Explain hypernatraemia
Serum sodium levels above 145mmol/L
• Caused by acute gain in sodium or net loss of water
• Although sodium is mainly in ECF, increased levels cause ICF dehydration as fluid moves out of cell into ECF
• As chloride follows sodium, hypernatremia is accompanied by hyperchloremia (no symptoms associated with excess chloride)
• Commonly occurs when oversecretion of aldosterone or adrenocorticotropic hormone or inappropriate administration of hypertonic saline solutions e.g. sodium bicarbonate as treatment for acidosis during cardiac arrest
Describe hypotonic alterations
- Lower solute conc. (more dilute) than in the blood
- Occur when ECF conc. is less than normal (less then 280mOsm/kg)
- Common cause is sodium hyponatremia (sodium deficit) or water excess
- Leads to intracellular overhydration or cell swelling/oedema
Explain hyponatraemia
- When serum sodium conc. falls below 135mmol/L
- Frequent among hospitalised elderly patients
- Can result from sodium loss from body, inadequate sodium intake or dilution of body’s sodium level by water excess
- Often caused by vomiting, diarrhoea, GI drainage or burns, or renal loses of sodium for use of diuretics, inadequate sodium intake is rare, insufficient aldosterone secretion
- Common adverse effect of diuretics such as Frusemide
- Hypochloraemia may also occur with hyponatremia
Describe alterations in potassium balance
• K maintained by sodium-potassium pump • Needed for resting membrane potential and AP transmission • Disturbances are life-threatening • Regulated by kidneys Normal - 3.5-5.0mmol/L
Explain hypokalaemia
- Potassium deficiency – serum potassium level below 3.5mmol/L
- Potassium loss from ECF, conc. gradient change will favour movement of potassium from cells to ECF – the ICF/ECF conc. ratio is maintained but total body potassium is depleted
Describe hyperkalameia
- Elevation of K levels above 5.5mmol/L
- Relatively rare due to renal control
- Causes- increased intake, shift from cells to ECF, decreased renal excretion, drugs that limit renal excretion such as angiotensin receptor blockers
Describe calcium
• In ECF
• Controlled by parathyroid hormone
• If low levels, stores in bones are broken down and released
Normal 2.15-2.6mmol/L
Explain hypercalcaemia
- Result of hyperparathyroidism
- Elevated PTH causes increased of calcium to be released from bones
- Can cause muscle weakness, fatigue, constipation, polyuria, thirst, arrythmias and possible hypertension
Explain hypocalcaemia
- Low levels ECF Ca with normal amounts stored in bones or decrease in total body stores
- Can occur due to hypoparathyroidism or renal failure
- Can cause tetany (lead to bronchospasm and respiratory arrest), convulsions, bradycardia, ventricular arrythmias
Describe phosphate
• ICF
• Stores found within bone
• Needed for ATP production and acid-base balance
• Vital for RBC function and oxygen delivery
• Energy required for neurons of heart to function
Normal 0.8-1.5mmol/L
Describe hyperphosphataemia
- Caused by acute or chronic renal failure
- ICF to ECF movement can occur as result of chemotherapy, sepsis or hypothermia
- Muscle cramps and pain, tingling around mouth, tetany (due to low Ca which develops secondary to high phosphate)
Explain hypophosphataemia
Due to malabsorption
• Can result from re-feeding syndrome – increase in glucose stimulates insulin release stimulating glucose and phosphate into cells = decreasing ECF levels
• Symptoms only occur if levels very low – muscle weakness, confusion, seizures possible acute respiratory failure
Describe magnesium
• ICF • Large stores found in bones • Excreted by the kidneys • Deficiency will affect whole body Normal 0.8-1.0mmol/L
Explain hypermagnesaemia
- Often in people with renal failure
- Interferes with neuromuscular transmission and depresses the CNS
- Muscle weakness, hypotension, bradycardia, cardiac arrest, respiratory depression and coma
Explain hypomagnesaemia
- Links with cardiovascular disease
- Caused by insufficient ingestion or absorption or renal loss to medication and kidney dysfunction
- Chronic alcoholism can contribute
- Tetany, confusion/hallucinations, muscle cramps, convulsions, hypotension, fatal cardiac arrythmias,
What are the normal sodium and bicarbonate levels
Sodium 135-145mmol/L
Bicarbonate 22-32mmol/L
Paediatrics and distribution of fluids
- Birth, total body weight is 75-80% water, decreases to 67% in first year of life
- Infants susceptible to changes in body fluids due to high metabolic rate and greater body surface area compared with adults
- Greater fluid intake and output in relation to their size also
- Immaturity can mean that renal mechanisms can’t account as well for losses related to vomiting, diarrhoea - dehydration
- Symptoms include: thirst, decreased skin elasticity, decreased urine output, decreased body weight, sunken fontanels, absent tears, dry mucous membranes, increased HR and irritability
When does total water level reach adult levels
• Water level in body reaches adult level in adolescence and gender differences appear (males larger % due to increased muscle mass, females increased body fat due to oestrogen so have less water)
Aging and distribution of body fluids
- Aging reduces renal mass – someone aged 30 has 20-25% more kidney mass than someone aged 85
- Disease can decrease body fluid content and dehydration can become life threatening
- Kidneys less effective at filtering urine
- Not due to changes in ADH release but because of sodium-conserving responses reduced, thirst perception and cognitive function can also decline
- In aged, total body water is around 50% of body mass for men and 45% for women
- Increased risk ECF overload due to increase response of atrial natriuretic peptide, renin release also reduced so aldosterone response weakened leading to leakage of sodium in the urine
- K excretion limited due to reduction in kidney mass – increased risk of hyperkalaemia
