March 10 - Fluids and Electrolytes

Question 1

Describe the total body water distribution of the human body

Answer 1

60% of the body is water
Of that 60%, 66% is intracellular fluid (ICF) and 33% is extracellular fluid (ECF)
Of the 33% of ECF, 20% is plasma and 80% is interstitial fluid

Question 2

How does the total body water vary?

Answer 2

Its percentage of body weight varies from 45-75%. It is variable depending on sex, body fate, skeletal muscle, age. In males, 60% of body weight is body water; in females its only 50% of body weight. The more body fat a person has, the less body water. As a person ages, their total body water %of body weight decreases

Question 3

Describe the movement across capillary membrane

Answer 3

There is easy movement across this membrane (very leaky) for water and electrolytes

Question 4

Describe the movement across the plasma (cell) membrane

Answer 4

There is limited movement of electrolytes. It is tightly regulated, and there are lots of pumps and channels. Water moves freely

Question 5

What is the force driving calcium between the plasma, interstitial fluid and the ICF

Answer 5

The concentration of calcium in the plasma is 5 mmol/L. In the interstitial fluid, it’s 3 mmol/L. In the ICF, it’s 10^-7 mmol/L

Question 6

What is the driving force of magnesium between the plasma, the interstitial fluid and the ICF

Answer 6

In the plasma, the concentration of magnesium in the plasma is 2 mmol/L. In the interstitial fluid, it’s 1 mmol/L. In the ICF, it’s 7 mmol/L

Question 7

What it the driving force of sulfate between the plasma, interstitial fluid and the ICF

Answer 7

In the plasma, the concentration of sulfate is 1 mmol/L
In the interstitial fluid, the concentration of sulfate is 1 mmol/L
In the ICF, the concentration of sulfate is 116 mmol/L

Question 8

What is the driving force of hydrogen phosphate between the plasma, the interstitial fluid and the ICF?

Answer 8

The concentration of hydrogen phosphate in the plasma is 2 mmol/L
In the interstitial fluid, it’s 2 mmol/L
In the ICF, it’s 116 mmol/L

Question 9

How are cations and anions distributed?

Answer 9

Once an ion gradient is established, there is little movement of ions between the ECF to the ICF. Water moves easily across the plasma membranes is based on osmotic forces. If sodium is added to the ECF, the amount of sodium in the ECF increase, and the amount of the sodium in the ICF is unchanged

Question 10

Describe the intracellular fluid

Answer 10

The volume is contained within the cells
It is very tightly controlledPlasma (cell) membrane limits the movement of electrolytes (water moves easily)
It accounts for approximately 40% of the body weight
Most of it is found in the skeletal muscle (the larger the muscle mass, the larger percentage of water of total body weight)

Question 11

Describe the extracellular fluid

Answer 11

The volume is not within the cells (plasma is confined to the vascular space and the interstitial fluid confined to between cells)
It is not as tightly controlled (but it is still important)
Capillary wall allows easier movement of fluid and electrolytesIt accounts for approximately 20% of the body weight

Question 12

What are the Starling forces?

Answer 12

The forces movement the water and solute between the plasma and interstitium. The forces moving fluid out of the capillary are P sub C (hydrostatic pressure in the capillary) and pi sub i (oncotic (colloid) pressure in the interstitium). The forces moving fluid into the capillary are P sub i (hydrostatic pressure in the interstitium) and pi sub C (oncotic (colloid) pressure in the plasma early in the capillary)

Question 13

Describe the oncotic pressure

Answer 13

The oncotic pressure is produced by proteins
The oncotic pressure in the interstitum is more or less negligible (approximately 0 mmHg - there are not proteins in the interstitium).
The oncotic pressure in the plasma early in the capillary is 25 mmHg and rises - the proteins in the blood pulls the water. It changes from 25 to 30 because things leave near the arterioles (fluids) but proteins stay so there is a higher concentration of proteins downstream towards the venule

Question 14

Describe the hydrostatic pressure

Answer 14

The hydrostatic pressure in the capillary changes along the capillary from 37 mmHg to 20 mmHg. If it increases, the venous pressure increases (oedema).The hydrostatic pressure in the interstitum is about 1 mmHg

Question 15

What happens in left-sided heart failure?

Answer 15

Blood backs up into the lungs (fluid in the lungs aka pulmonary oedema). It is life threatening. Sitting up helps (more blood in the veins)

Question 16

What happens in right-sided heart failure?

Answer 16

Blood backs up into the periphery (systemic oedema)

Question 17

What maintains solute/electrolyte levels?

Answer 17

Na/K ATPase establishes and maintains ICF solute/electrolyte levels. Although Na/K pumps maintain the gradient, water moves freely between the two compartments (ICF:ECF). Water movement is driven by differences is osmolality

Question 18

What is osmolality?

Answer 18

It is the number of particles in the water that want to hold onto water

Question 19

What is the osmolality of the ICF and ECF?

Answer 19

Osmolalities of the ICF and the ECF in steady state are the same. This is due to the movement of water (there is not solute movement)

Question 20

What is the importance of sodium?

Answer 20

Sodium is the major “osmotically active” cation outside the cell (it can change a fair amount)

Question 21

What is the importance of potassium?

Answer 21

Potassium is the major “osmotically active” cation inside the cell. It does not change very much. Potassium is important in keeping resting membrane potential. Changes in the ECF will change the resting membrane potential

Question 22

What drives movement between the plasma the interstitial space?

Answer 22

Changes in the pi sub C and the P sub C (moves fluid and electrolytes)

Question 23

What drives the movement of fluid between plasma and intracellular fluid?

Answer 23

Sodium (think osmolality changes and water movement)

Potassium (think resting membrane potential changes)

Question 24

What is the importance of plasma potassium (P sub K)

Answer 24

Potassium is important in determining intracellular osmolality - cells tightly regulate their internal milieu (changes little). Changes occur int he extracellular potassium relative to the intracellular potassium - this has major effects on the resting membrane potential. This can be a problem in the electrical conducting tissues (nerves, electrical conduction pathways of the heart). Small changes in the P sub K are potentially lethal

Question 25

Describe potassium homeostasis?

Answer 25

Extracellular potassium is important in the regulation of the resting membrane potential. Small changes have major effects on RMP. 98% of the total body potassium is intracellular. 2% of potassium is extracellular. Intracellular concentration is 140-150 mmol/L; serum concentration is 3.5-5.0 mmol/L. Even small changes in the serum levels is life threatening

Question 26

How does the body response to changes in potassium?

Answer 26

Acute - drive potassium into or out of the cells (hide or release potassium)
Chronic - the kidney becomes important; the distal nephron sodium delivery and renal actions of aldosterone

Question 27

How can calcium help in the case of hyper/hypokalemia?

Answer 27

Plasma potassium alters the resting membrane potential. Plasma calcium alters the threshold level. Giving calcium is a short term fix to ensure that the RMP does not get close to threshold potential

Question 28

How is hypokalemia treated?

Answer 28

Administration of potassium, slowly (diet, intravenous)

Must be aware of producing hyperkalemia

Question 29

How is hyperkalemia treated?

Answer 29

Driving potassium into the cells (short term solution)
-in emergencies insulin stimulates Na/K pumps and drive K into the cell (has to be given with glucose to avoid hypoglycemia)
-beta-adrenergic receptor stimulation
-alkalosis (H leaves the cell in exchange for K entering)
Aldosterone (kidney - long term) - sodium reabsorption and potassium secretion
Calcium to depolarize the threshold potential (short term solution)

Question 30

Define osmolality

Answer 30

Instrument measurement of solutes in a solution and reported as mOsmol/Kg of solvent. Osmolarity relates to the calculated solute concentration using sodium, glucose and plasma measurements which is based on volume (therefore value is mOsmol/L)

Question 31

Define osmotic pressure

Answer 31

A pressure that needs to be applied to a solution to prevent the movement of water across a semipermeable membrane

Question 32

Define tonicity

Answer 32

Osmotic pressure gradient of two solutions separated by a semipermeable membrane (cell wall) which must be selective in blocking movement of solutes. Usually in reference to plasma

Question 33

What determines water movement between the intracellular and extracellular environment?

Answer 33

The difference in osmolality

Question 34

What is the importance of tonicity?

Answer 34

Tonicity of the surrounding fluid (plasma) determines water movement into and out of cells

Question 35

What is the importance of sodium in determining osmolality

Answer 35

Sodium is important to determine plasma osmolality and total body water. Sodium and associated isons represent the majority of osmotically active solutes in the plasma (other solutes are much smaller amounts and play a lesser role)

Question 36

What is normal osmolality?

Answer 36

280 to 300 mOsm/L. If it’s higher than 290 mOsm/L, that means there is something else in the blood

Question 37

How does plasma osmolality affect fluid distribution?

Answer 37

Changes in plasma osmolality alter water distribution (e.g., ingestion of a large volume of water over a period of days?): ingestions of water -> water absorbed from gut into plasma - hyponatremia and decreased osmolality -> osmotic gradient now favours movement of fluid into cells -> water moves (is drawn) into cells to balance osmotic gradient -> cell size increases (normally not a problem except in the brain)

Question 38

What is hypovolemia? What is hypervolemia?

Answer 38

Hypovolemia: low volume of plasmaHypervolemia: high volume of plasma

Question 39

Why is sodium important in determining total body water?

Answer 39

Changes in plasma sodium concentration change plasma osmolality.
Plasma osmolality changes:
-water distribution (intracellular and extracellular) - osmolal (acute correction normally)
-hormone levels (main ones for water and sodium regulation; vasopressin and aldosterone; required for chronic correction)

Question 40

What does vasopressin do?

Answer 40

Vasopressin (aka ADH or antidiuretic hormone) is released from the posterior pituitary and it increases water reabsorption in collecting duct of kidney

Question 41

What does aldosterone do?

Answer 41

Aldosterone is released from the adrenal medulla and it increases sodium reabsorption in distal tubule of the kidney

Question 42

Total body sodium determines ECF volume. Explain

Answer 42

Increased sodium in the plasma increases vascular volume and total body water.Ingestion/infusion of sodium and little/no water (increased total body sodium) -> plasma sodium concentration rises, increasing plasma osmolality (this increases vasopressin release) -> water moves (is pulled) out of cells to balance osmolality -> plasma osmolality still slightly elevated when balanced -> vasopressin retains water in the kidney and helps dilute plasma back to normal osmolality

Question 43

Total boyd sodium determines plasma volume.

Answer 43

Decreased sodium in the plasma - decreased vascular volume and total body water.Loss of sodium and lesser/little loss of water -> plasma sodium concentration drops, decreasing plasma osmolality (this turns off vasopressin but increases aldosterone release) -> water moves (is pulled) into the cells to balance osmotically -> fluid shifts from ECF to ICF: decreases plasma volume -> plasma osmolality still slightly decreased aldosterone retains sodium in the kidney helps correct low sodium

Question 44

So why do we calculate osmolality when it is normally measured anyway?

Answer 44

1. The equation nicely shows the major importance of sodium? Yes but who cares
2. More importantly: -what if measured osmolality&raquo_space; calculated osmolarity-something is increasing osmolality that is normally not present (foreign; a solute other than sodium, glucose and BUN and it usually points to a foreign substance in the blood)

Question 45

What is osmolal gap?

Answer 45

Osmlality can be measured directly (freezing point dpression), or calculated as osmolarity (based on the concentration of the sodium glucose and BUN)
The apparent difference between the measured and calculated values in plasma is known as the osmolal gap. Normally there is a gap of 10 mOsmol/L

Question 46

What happens if the osmolal gap is bigger than 10 mOsmol/L?

Answer 46

This means an unknown “osmotically active” substance must be present. A high osmolal gap is indicative of poisoning with alcoholic beverages, wood alcohol, rubbing alcohol or antifreeze

Question 47

Plasma volume reflects total body sodium. Explain.

Answer 47

Extracellular fluid volume and total body sodium are closely linked. Hypovolemia (low plasma volume) is a reflection of low total body sodium. Hypervolemia is a reflection of high total body sodium

Question 48

What is the anion gap?

Answer 48

The plasma is electroneutral (cations=anions; Na - (HCO3 + Cl = 11 mEg/L). So if the anion gap increases when calculated, this means sodium is combining with something other than bicarbonate and chloride