Chapter 27: Fluid, Electrolyte, and Acid-Base Balance

fluid

water

extracellular fluids (ECF): blood plasma, interstitial fluids, other

intracellular fluids (ICF): cytosol (within cells)

electrolyte

inorganic compounds that dissociate in solution to form ions

most IMPORTANT electrolyte:

Na+ = sodium

• most abundant ion in the ECF

• main contributor to osmolarity of blood

factors that must be controlled

1. fluid balance:

fluid in = fluid out

normal blood volume: ~5L

2. electrolyte balance:

electrolytes in = electrolytes out

normal blood osmolarity: 300 mOsmn/L

gains:

• food

• drink

• metabolism

losses

• urine

• feces

• sweat

• breathing

application: suppose you eat an extra-large jumbo tub of movie popcorn with extra salt and butter

blood osmolarity will: increase

blood volume will: not change

how does the body detect these changes

osmoreceptors in the hypothalamus detect on increase in blood osmolarity

• sense an increase in Na+ of blood

how does the body respond to these changes

increase the secretion of antidiuretic hormone (ADH)

affect will be:

1. increase thirst - consume liquids

2. decrease water loss at kidneys

decrease the secretion of aldosterone

affect will be:

1. decrease Na+ reabsorption in kidney

overall affect will be:

1. restore osmolarity of blood by increase retention of H2O and decrease reabsorption of sodium

2. kidneys produce small volume of concentrated urine

application: suppose you drink a very large volume of water

blood osmolarity will: decrease

blood volume will: increase

how does the body detect these changes

osmoreceptors in the hypothalamus detect a decrease in blood osmolarity

how does the body respond to these changes

decrease the secretion of antidiuretic hormone

affect will be:

1. water reabsorption in DCT and CD

2. thirst

increase the secretion of aldosterone

affect will be:

1. increase sodium reabsorption

overall affect will be:

• kidneys produce large volume of dilute urine

• blood volume will decrease

• blood osmolarity will increase

application: suppose you consume a large volume of an isotonic solution

blood osmolarity will: not change

blood volume will: increase

how does the body detect these changes

baroreceptors will detect an increase in blood pressure due to an increased blood volume

how does the body respond to these changes

increase the secretion of atrial naturetic peptide

affect will be:

1. ADH secretion will decrease

2. aldosterone secretion will decrease

the overall affect will be:

• kidneys produce increased volume of urine with increase Na+ content

application: suppose you are in a very severe automobile accident and are suffering from excessive blood loss

blood osmolarity will: not change

blood volume will: decrease

how does the body detect these changes

as blood volume decreases, blood pressure declines

how does the body respond to these changes

increase the secretion of renin will result in an increase in angiotension II

the affect will be:

1. increase aldosterone - increase Na+ retention in the kidneys because water follows salt, water will also be retained

2. causes vasoconstriction = increase BP

3. promotes release of ADH

   1. increase thirst - consume liquids

   2. decrease water loss at kidneys

the overall affect will be:

• both work to return blood volume and pressure to normal

what is pH

concentration of H+ ions in solution

[H+] = 1×10^-7 → pH=7

pH of body fluids

ECF = pH 7.35-7.45

acidosis: pH below 7.35, very serious problem

alkalosis: pH above 7.45, also serious, but not as common

both primarily effects CNS and cardiovascular functions

why is the maintenance of pH a problem

H+ gains = H+ loses

• constantly producing H+ through metabolic activities

  • CO2 + H2O →← H+ + HCO3-

• H+ must be eliminated or tied up to maintain pH

three mechanisms

1. buffer system

2. respiratory system

3. urinary system

buffer system

buffer: a compound that stabilizes the pH of a solution by providing or removing H+ ions

1. protein buffer system: amino acids accept or release H+ ions

   1. hemoglobin: maintains pH as pCO2 changes

2. carbonic acid-bicarbonate buffer system

   1. CO2 + H2O ←→ (H2CO3) ←→ H+ + HCO3-

   2. can prevent changes in pH after addition of H+ ions

   3. → equation shifts to the left; extra CO2 is released in lungs

3. phosphate buffer system

   1. H2PO4- ←→ H+ + HPO4-2

   2. plays a supporting role in maintaining pH

respiratory system

• changes in the respiratory rate stabilize the pH of the ECF

• direct effect on the carbonic acid/bicarbonate buffer system

  • CO2 + H2O ←→ (H2CO3) ←→ H+ + HCO3-

• decreased pCO2 = increase pH

• to return pCO2 to normal levels the respiratory rate must decrease

• the affect: less CO2 loss at lungs = pCO2 and pH return to normal

• increased pCO2 = decrease pH

• to return the pCO2 to normal levels the respiratory rate must increase

• the affect: more CO2 loss at lunhs = pCO2 and pH return to normal

urinary system

changes in the rate of H+ and HCO3- secretion or reabsorption by the kidneys regulates the pH of the ECF

occurs along the PCT, DCT and CD

principle buffering system: carbonic acid/bicarbonate system

• CO2 + H2O ←→ (H2CO3) ←→ H+ + HCO3-

• CO2 - from tubular fluid, blood and tubular cells

• relies on carbonic anhydrase in tubular cells

decreased pH of the ECF leads to:

• H+ secretion into tubular fluid and HCO3- released into blood

• the affect: increase pH of ECF

increased pH of the ECF leads to:

• HCO3- secretion into tubular fluid and the rate of H+ secretion declines

• the affect: decrease pH of ECF