LEC 21 Fluid, Electrolyte, and Acid-Base Balance

Fluid Balance

• Amount of water gained

• Amount of water lost to environment

Electrolyte balance

• absorption during digestion

• losses at kidneys (and sweat)

Acid/Base balance

• production and loss of H+ ions (Ph)

• We generate acids during metabolism - kidney’s and lungs counteract

Fluid importance

• fluids are vital to sustain life

  • Temperate reg, cell shape, transport of gases, nutrients, wastes

Gain = Losses

• measurable losses: kidneys digestive system

• Immeasurable losses: skin, lungs

Fluid distribution %

• Fluid makes up a Large % of our body mass

Two main fluid compartments

• Intracellular fluid: ICF

• Extracellular fluid: ECF

ECF categorized into:

1. IF - fluid between cells

2. Plasma - Fluid in blood

Intracellular

• fluid portion of cytosol

Extracellular

• IF

• Plasma (intravascular)

Transcellular

• CSF, Pleural cavity, lymph, joint, eyes

Arteries »»» Arterioles

• branch progressively smaller and smaller into capillaries

• many of which have pores in them

Continuous Capillaries

• Deliver gasses, water, glucose, hormones

Fenestrated Capillaries

• deliver nutrients

Sinusoidal capillaries

• in liver, spleen, bone marrow

Pores

• allow for certain fluid and solutes to be delivered to (and brought back from) the IF

The starling principle

• fluid movement between blood and tissues are determined by differences in the hydrostatic and blood colloid osmotic pressures between the plasma inside capillaries and the fluid outside of them

Extracellular fluid is redistributed:

• from blood plasma »»» IF »»» lymphoid system and back

• interactions between opposing forces results in continuous filtration of fluid

ECF volume is made up of:

• 80% IF

• 20% plasma

Isotonic

• equally concentrated with other solutions

Hypotonic

• less concentrated than other solutions

Hypertonic

• more concentrated than other solutions

Isotonic fluids

• on either side of the membrane, solutions are equally concentrated

• no net fluid shift occurs between isotonic solutions

Hypotonic Fluids

• when a less concentrated (hypotonic) solution is placed next to a more concentrated solution

• fluids will shift to equalized concentrations

Hypertonic fluids

• when a solution has more solutes, and less, fluid, relative to an adjacent solution

• fluid will shift from low »»» high solution until equal concentrations exist

Osmosis

• fluids move passively from area with relatively more fluid (and fewer solutes) to areas with relatively less fluid (and more solutes)

Diffusion

• solutes more from high »»» Low until their concentration is equal in both areas

Active Transport

• energy from ATP molecules move solutes from Low »»» High

• ATP pushes against concentration gradient

Pitting Edema

• excess fluids build, swelling leaves an indentation (pit) when pressed that remains after pressure is applied

Pulmonary Edema

• pulmonary congestion

• excess fluid buildup in the lungs

• impairs gas exchange

• difficulty breathing

• hypoxemia

• respriotry failure

• CHF

Kwashiorkor

• serve protein deficiency

• usually affects infants and children

• prevalent in extreme cases of starvation and poverty-stricken regions worldwide

euvolemic hyponatremia

• total amount of water in body is increased, but NA+ levels remain normal

• Causes:

• Endurances sports

• drinking too much water, too quickly

• excess alcohol or use of ecstasy

Membrane functions

• a cells plasma membrane is selectively permeable

• ions enter or leave via specific channels

• carrier mechanisms move specific ion into or out of cell

Fluid Compartments

• water helps to keep the balance

Osmotic conecntration of ICF and ECF

• is nearly identical

• osmosis eliminates minor differences in concentration because plasma membranes are permeable to H2O

Blood vessels

• pore is BV allow for exchange between plasma and interstitum, so similarities exist between IF and Plasma

Blood serum

• tests measures electrolytes

• NA+, CA2+, Cl-, HCO3-

• Ions are not exchangeable across cellular membrane, however…

• water can pass through (via osmosis) based on concentration gradients

Symptoms of low electrolyte levels

• muscle cramps and weakness

• fatigue and lethargy

• nausea and vomiting

• headaches

• irritability and confusion

Severe electrolyte disruption

• severe muscle cramps

• seizures

• changes in blood pressure

Symptoms of high electrolyte levels

• numbness and tingling

• muscle twitching or spasms

• swelling and fluid retention

• high blood pressure

• confusion and agitation

Significant elevation in electrolytes

• muscle paralysis

• seizures

• breathing difficulties heart arrhythmias

Two rules of electrolyte balance

• most common problems with electrolyte balance are caused by imbalances between gains and losses of sodium NA+ ion

• Problems with potassium K+ balances are less common, but more dangerous than sodium imbalance

Solutes in electrolyte balance

• the extracellular solute levels may remain the same, but because there is now less water, the extracellular solute concentrations increase

• not necessarily a good thing

If extracellular NA+ fluid rises

• intracellular fluid travels out of cell to balance ion levels

ICF & ECF

• ICF should roughly equal ICF based on the movement of water

• water follows salt

If extracellular NA+ falls…

• extracellular fluid travels into cell to balance ion levels

Concentration gradients

• water will travel to wherever is a higher concentration of solutes

If someone eats salty food, and does not ingest water…

• the plasma NA_ increases, followed by a shift in fluid

• Fluid will travel ICF»»»ECF (plasma) to help decrease the NA+ in the plasma

• Consequently, that cell will shrink (crenation)

• also, ADH secretion will restrict H2) loss and stimulate thirst

ADH

• stimulates water conservation at kidneys

  • reducing urinary water loss

  • aquaporin channels open

  • helps to concentrate urine

• promotes fluid intake

  • Stimulates thirst center

Aldosterone

• adrenal cortex secretes in response to:

  • increases K+ or decreased NA+ levels

  • Activation of renin-angiotensin system

  • Determines rate of NA+ absorption and K+ loss along DCT and CD

High aldosterone plasma concentration

• causes kidneys to conserve salt

• this conservation of NA+ will stimulate water retention

• water follows salt

Atrial naturistic peptide

• released by cardiac muscle cells in response to abnormal stretching of heart walls

• reduces thirst

• blocks release of ADH

• blocks release of aldosterone

• causes diuresis

  • increased formation of urine by kidneys

• lowers plasma volume and blood pressure

Large changes in ECF volume

• are connected by homeostatic mechanisms that regulate blood volume and BP

IF ECF volume rises

• blood volume goes up, which will elevate BP

If ECF volume drops

• blood volume foes down, which will decrease BP

If plasma ECF volume is too large?

• Venous return increases

• Salt and H2) loss at kidneys increase

• ECF volume declines

If plasma ECF volume is inadequate

• here, blood volume and blood pressure will decline

• so, renin - angiotensin system is activated

• so, NA+ and H2O losses are reduced

• this will help ECF volume increase

Potassium K+

• 98% of potassium in the human body is in ICF

  • Cells expend energy to recover potassium ions diffused from ICF (cytoplasm) into ECF

Calcium CA2+

• calcium is most abundant mineral in the body

• a typical individual has 2.2-4.4lbs of this element

• 99% of which is deposited in skeleton

Magnesium Mg

• a cofactor for important enzymatic reactions

Phosphate ion (PO4 3-)

• in ICF, it is required for formation of high-energy compounds, activation of enzymes, and synthesis of nucleic acids

Chloride Ions

• absorbed across digestive tract with NA+

Acidosis

• lower than 7.35

Alkalosis

• higher than 7.45

CO2 + H20 »»» H2CO3 »»» HCO3- + H+

• When we breathe, we produce CO2

• Eventually, this CO@ enters the blood and mixes with the water H20 of the blood

• Here the CO2 and H20 combines and produces carbonic acid H2CO3

• Carbonic acid splits and produces bicarbonate ion HCO3- and hydrogen ion H+

• Therefore, the measurement of pH is the amount or concentration of hydrogen

If CO2 level rise, more H2CO3 forms

• this leads to additional release of HCO3- and H+

• The increases H+ outweighs the rest of the equation, thus producing more acid - meaning pH decreases

• The more acidic, the lower we go below 7.35… acidosis

If CO2 levels fall, H2CO3 dissociates into H2O and CO2

• This removes H+ ion from solution

• This produces less acid - meaning pH increases

• The less acidic, the higher we go above 7.45… Alkalosis

Respiratory acidosis

• CO2 levels rise and combine with H20 to form H2CO3

• H+ levels increase and outweigh everything else

• therefore, pH goes down…Acidosis

• Develops when the respiratory system cannot eliminate all CO2 generated by peripheral tissues

  • Primary cause: hypoventilation

Respiratory Alkalosis

• When CO2 levels fall

• there is not enough CO2 to combine with H20

• Therefore, they do not form H2CO3, and do not produce a high concentration of H+ ions

• So, it develops because there is an abundance of HCO3- ion, and pH goes up… Alkalosis

  • Primary cause: hyperventilation - rapid, uncontrolled breathing

Metabolic Acidosis

• When there is an increase in the amount of H+ ions

• when there is a decrease in the amount of HCO3-

• HCO3- binds H+ ions and soaks up the excess H+ ions to eliminate them

  • If HCO3- is gone, there remains an excess of free H+ ions

  • Bloodwork will show a drop in dicorbonate and drop in Ph… Acidosis

Metabolic Alkalosis

• Production of:

• Too many HCO3- ions

• or not enough H+ ions

• If uncompensated HCO3- ion levels, go up and H+ ion goes down

  • H+ is not outweighing anything…Alkalosis

  • patient must slow their breathing (hypoventilation)

Most diagnostic blood tests screen for

• pH and buffer function:

• blood Ph

• PCO2 (partial pressure of CO2)

• HCO3- (bicarbonate)