Chapter008

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins


Chapter 8

Disorders of Fluid, Electrolyte, and Acid-Base Balance

Chapter 8

Disorders of Fluid, Electrolyte, and Acid-Base Balance


Fluid DistributionFluid Distribution

• Intracellular compartment

• Extracellular compartment

– Interstitial spaces

– Plasma (vascular) compartment

– Transcellular compartment


Distribution of WaterDistribution of Water


ScenarioScenario• An athlete ran a marathon even though he felt ill…• After the race he collapsed. He was pale with a low blood

pressure and sunken eyes. One knee and ankle were badly swollen, and his abdomen was distended with fluid. The doctor diagnosed appendicitis and dehydration.

Question:• What has happened to his:

– Blood osmolarity?– Cell size? – Transcellular fluid volume? – Vascular compartment volume?


Forces Moving Fluid In and Out of CapillariesForces Moving Fluid In and Out of Capillaries


Question Question

What forces work to keep blood in the capillary?

a. Capillary colloid osmotic pressure (COP) & tissue COP

b. Capillary hydrostatic pressure & tissue COP

c. Capillary hydrostatic pressure & tissue hydrostatic pressure

d. Capillary COP & tissue hydrostatic pressure


Answer Answer

d. Capillary COP & tissue hydrostatic pressure

Hydrostatic pressure can be thought of as “pushing pressure,” and osmotic pressure can be thought of as “pulling” pressure. Pressure in the capillary that pulled/kept fluid in (capillary COP) and pressure pushing fluid out of the tissue (tissue hydrostatic pressure) would result in more fluid in the capillary.


SodiumSodium

• Normal level is 135–145 mEq/L

• Regulates extracellular fluid volume and osmolarity

Question:

• Why would “retaining sodium” cause high blood pressure?


ScenarioScenarioIt's a very hot day and you fall down the stairs on the way to

see the doctor about your hepatitis and renal disease.

• Explain why you have edema in your sprained ankle and foot.


Controlling Blood OsmolarityControlling Blood Osmolarity

• High osmolarity causes:

– Thirst increased water intake

– ADH release water reabsorbed from urine

• Low osmolarity causes:

– Lack of thirst decreased water intake

– Decreased ADH release water lost in urine


Question Question

Tell whether the following statement is true or false:

Increased levels of ADH decrease urine output.


Answer Answer

True

ADH prevents diuresis by causing more water to be absorbed in the kidney tubules. If more water is absorbed, there is less water left to eliminate as waste, decreasing urine output.


Dehydration Due to HypodipsiaDehydration Due to Hypodipsia

• A common problem in elderly people

Scenario:

• Dr. Bob thinks it could be treated with ADH given in a nasal spray

• Dr. Bill thinks renin injections would be better

Question:

• What is your evaluation of these two theories?


ADH ImbalancesADH Imbalances

• Diabetes insipidus (DI)

– Neurogenic

– Nephrogenic

• Syndrome of inappropriate ADH (SIADH)

• Which will cause hyponatremia?


Sodium ImbalancesSodium Imbalances

• Hyponatremia (<135 mEq/L)

– Hypertonic

– Hypotonic (dilutional)

• Hypernatremia (>145 mEq/L)

– Water deficit

– Na+ administration


ScenarioScenario

• A man with hypernatremia was severely confused.

Question:

• The doctor said this was due to a change in the size of his brain cells. Why would this happen?

• A medical student suggested giving him a hypotonic IV. Why?

• The doctor said that might worsen the change in his brain cell size, and that his blood osmolarity should be corrected very slowly. Why?


PotassiumPotassium

• Normal level is 3.5–5.0 mEq/L

• Maintains intracellular osmolarity

• Controls cell resting potential

• Needed for Na+/K+ pump

• Exchanged for H+ to buffer changes in blood pH


What Will Happen to Blood K+ Levels When the Client Has:What Will Happen to Blood K+ Levels When the Client Has:

• Hyperaldosteronism?

• Alkalosis?

• An injection of epinephrine?

• Convulsions?

• Loop diuretics?


The Basics of Cell FiringThe Basics of Cell Firing

• Cells begin with a negative charge— resting membrane potential

• Stimulus causes some Na+ channels to open

• Na+ diffuses in, making the cell more positive

Threshold potential

Resting membrane potential stimulus


The Basics of Cell Firing (cont.)The Basics of Cell Firing (cont.)

• At threshold potential, more Na+ channels open

• Na+ rushes in, making the cell very positive: depolarization

• Action potential: the cell responds (e.g., by contracting)

Threshold potential


Action potential


The Basics of Cell Firing (cont.)The Basics of Cell Firing (cont.)

• K+ channels open

• K+ diffuses out, making the cell negative again: repolarization

• Na+/K+ ATPase removes the Na+

from the cell and pumps the K+ back in

Threshold potential


Action potential


Blood K+ Levels Control Resting PotentialBlood K+ Levels Control Resting Potential

• Hyperkalemia raises resting potential toward threshold

– Cells fire more easily

– When resting potential reaches threshold, Na+ gates open and won’t close

Threshold potential

Normal resting membrane potential

Hyperkalemia


Blood K+ Levels Control Resting Potential (cont.)Blood K+ Levels Control Resting Potential (cont.)

• Hypokalemia lowers resting potential away from threshold

– Cells fire less easily

Threshold potential

Normal resting membrane potential Hypokalemia


Question Question

What effect does a potassium level of 7.5 mEq/L have on resting membrane potential (RMP)?

a. RMP becomes less negative, and it takes a greater stimulus in order for cells to fire.

b. RMP becomes less negative, and it takes less of a stimulus in order for cells to fire.

c. RMP becomes more negative, and it takes a greater stimulus in order for cells to fire.

d. RMP becomes more negative, and it takes less of a stimulus in order for cells to fire.


Answer Answer

b. RMP becomes less negative, and it takes less of a stimulus in order for cells to fire.

A potassium level of 7.5 mEq/L is considered hyperkalemic. In hyperkalemia, RMP is moved closer to the threshold (it becomes less negative). Because RMP is nearer to the threshold, a weaker stimulus will cause the cell to fire (a lesser distance must be overcome).


CalciumCalcium

• Normal level is 8.5–10.5 mg/dL

• Extracellular: blocks Na+ gates in nerve and muscle cells

• Clotting

• Leaks into cardiac muscle, causing it to fire

• Intracellular: needed for all muscle contraction

• Acts as second messenger in many hormone and neurotransmitter pathways



Scenario:Scenario:

• A man with metastatic cancer complains of bone pain and sudden weakness.

Question:

• Why did the doctor measure:

– PTH?

– Calcium levels?

– Vitamin D levels?


MagnesiumMagnesium

• Normal level is 1.8–2.7 mg/dL

• Cofactor in enzymatic reactions

– Involving ATP

– DNA replication

– mRNA production

• Binds to Ca2+ receptors

• Can block Ca2+ channels


Extracellular Calcium Controls Nerve FiringExtracellular Calcium Controls Nerve Firing

• Hypercalcemia

– Blocks more Na+ gates

– Nerves are less able to fire

• Hypocalcemia

– Blocks fewer Na+ gates

– Nerves fire more easily

• Which would cause Trousseau sign?


Question Question


Both hyperkalemia and hypercalcemia cause cells to fire more easily.


Answer Answer

False

Recall that hyperkalemia cause cells to fire more easily by moving RMP closer to the threshold. Hypercalcemia, on the other hand, blocks more sodium gates. If less sodium enters the cell, it cannot depolarize as quickly (it is less likely to fire). Hypocalcemia blocks fewer sodium gates–cells depolarize more quickly (they are more likely to fire).



Acid (H+)Acid (H+)

• Normal value: pH = 7.35–7.45

• Blocks Na+ gates

• Controls respiratory rate

• Individual acids have different functions:

– Byproducts of energy metabolism (carbonic acid, lactic acid)

– Digestion (hydrochloric acid)

– “Food” for brain (ketoacids)


Respiratory or Volatile AcidRespiratory or Volatile Acid

• CO2 + H2O H2CO3 (carbonic acid)

• H2CO3 H+ + HCO3- (bicarbonate ion)

• An increase in CO2 will cause

– Increases in CO2 (increased PCO2)

– Increases in H+ (lower pH)

– Increases in bicarbonate ion


Respiratory Acidosis and AlkalosisRespiratory Acidosis and Alkalosis

• CO2 + H2O H2CO3 H+ + HCO3- (bicarbonate ion)

Respiratory acidosis Respiratory alkalosis

Increased PCO2

Increased carbonic acid

Increased H+ = low pH (<7.35)

Increased bicarbonate

Decreased PCO2

Decreased carbonic acid

Decreased H+ = high pH (>7.45)

Decreased bicarbonate


Question Question


Serum levels of pH and CO2 levels are directly proportional.


Answer Answer

False

As blood levels of CO2 increase, pH becomes more acidic (decreases).


Respiration and Buffers Adjust Blood pHRespiration and Buffers Adjust Blood pH

Scenario:• A woman was given an acidic IV. Soon she began to

breathe more heavily. Why?• When her blood was tested, it had:

– Slightly lowered pH– Low bicarbonate– Low PCO2

– Slightly increased K+

• Her urine pH was slightly lowered• Why?


Buffer SystemsBuffer Systems


Metabolic Acid ImbalancesMetabolic Acid Imbalances

• Metabolic acidosis

– Increased levels of ketoacids, lactic acid, etc.

– Decreased bicarbonate levels

• Metabolic alkalosis

– Decreased H+ levels

– Increased bicarbonate levels


Metabolic Acidosis and AlkalosisMetabolic Acidosis and Alkalosis

• Increased metabolic acids raise H+ levels

• Some H+ combines with bicarbonate, decreasing it

• Breathing adjusts CO2 levels to bring pH back to normal

Metabolic acidosis Metabolic alkalosis

Increased H+ = low pH (<7.35)

Decreased bicarbonate

Heavier breathing causes decreased PCO2

Decreased H+ = high pH (>7.45)

Increased bicarbonate

Lighter breathing causes increased PCO2

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