Essentials of Pathophysiology

CHAPTER 8 DISORDERS OF FLUID, ELECTROLYTE, AND ACID-BASE BALANCE

PRE LECTURE QUIZ

(TRUE/FALSE) F F F T F  The extracellular compartment contains approximately two thirds of the body water in healthy adults, and is the larger of the two compartments.

 Potassium is the most abundant cation in the body.

 Edema of the brain, larynx, or lungs is not an acute, life-threatening condition.



Polydipsia

is a term that means “excessive thirst.”  Decreased vascular volume would yield manifestations such as full and bounding pulses, venous distention, and signs and symptoms of pulmonary edema, such as shortness of breath, crackles, dyspnea, and cough.

PRE LECTURE QUIZ

ADH Calcium Cardiac Metabolic pH

     Diabetes insipidus is a condition that is caused by a deficiency or a decreased response to __________, also known as vasopressin.

The most serious side effect of hyperkalemia is on ______________ function.

Serum _______________ is directly or indirectly regulated by parathyroid hormone (PTH) and vitamin D.

Respiratory acidosis is characterized by a decrease in __________, reflecting a decrease in ventilation and an increase in PCO 2 .

Vomiting, removal of gastric secretions through the use of nasogastric suction, and low potassium levels resulting from diuretic therapy are the most common causes of _________________ alkalosis in hospitalized patients.

FLUID DISTRIBUTION

 Intracellular compartment  Extracellular compartment  Interstitial spaces  Plasma (vascular) compartment  Transcellular compartment

DISTRIBUTION OF WATER

•Intracellular •Extracellular •Interstitial •Plasma •3 rd Space

SCENARIO

  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 Capillaries and Cells OSMOSIS: WHICH WAY WILL WATER MOVE?

Blood: (ECF) Few solutes Lots of water Cell: (ICF) Many solutes Less water Water Follows Solutes

FORCES MOVING FLUID IN AND OUT OF CAPILLARIES

 Hydrostatic pressure  Pressure created by the Heart  i.e. Blood Presssure  Forces water out of capillaries

FORCES MOVING FLUID IN AND OUT OF CAPILLARIES

Hydrostatic pressure generated by the heart Greatly reduced Hydrostatic pressure due to capillary resistance

QUESTION

What forces work to keep blood in the capillary?

a.

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

c.

d.

Capillary hydrostatic pressure & tissue COP Capillary hydrostatic pressure & tissue hydrostatic pressure Capillary COP & tissue hydrostatic pressure

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.

SODIUM

  Normal level is 135–145 mEq/L Regulates extracellular fluid volume and osmolarity Question:  Why would “retaining sodium” cause high blood pressure?

SCENARIO

It’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 OSMOLARITY

  High osmolarity causes: (Hypothalamic detection)  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 the urine

QUESTION

True or False: Increased levels of ADH decrease urine output.

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 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 IMBALANCES

   Diabetes insipidus (DI)  Neurogenic  Nephrogenic Syndrome of inappropriate ADH (SIADH) Which will cause hyponatremia?

SODIUM IMBALANCES

  Hyponatremia (<135 mEq/L)  Hypertonic  Hypotonic (dilutional) Hypernatremia (>145 mEq/L)  Water deficit  Na + administration

SCENARIO

 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?

POTASSIUM

     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

+

WHEN THE CLIENT HAS: LEVELS

     Hyperaldosteronism?

Alkalosis?

An injection of epinephrine?

Convulsions?

Loop diuretics?

THE BASICS OF CELL FIRING

   Cells begin with a negative charge— resting membrane potential Stimulus Na + causes some channels to open Na + diffuses in, making the cell less negative, i.e. more positive Threshold potential Resting membrane potential stimulus

THE BASICS OF CELL FIRING (CONT.)

   At threshold potential, more Na + channels open Na + rushes in, making the cell positive: depolarization Action potential: the cell responds (e.g., by contracting) Action potential Threshold potential Resting membrane potential stimulus

THE BASICS OF CELL FIRING (CONT.)

   K + channels open K + diffuses out, making the cell negative again: repolarization Na + /K + ATPase removes the Na + the cell and pumps the K + back in from Threshold potential Action potential Resting membrane potential stimulus

BLOOD K+ LEVELS CONTROL RESTING POTENTIAL

 Hyperkalemia raises resting potential toward threshold  Cells fire more easily  Increased K + resting potential to threshold, Na can move + gates open and won’t close Threshold potential Hyperkalemia Normal resting membrane potential

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

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.

d.

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

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

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).

CALCIUM

      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

Calcium Regulation 1.

2.

Blood Ca 2+ can be increased by: Increased intestinal absorption Release from bones

CALCIUM REGULATION

Blood Ca 2+ can be increased by: 1.

Increased intestinal absorption 2.

Release from bones

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?

MAGNESIUM

 Normal level is 1.8–2.7 mg/dL  Cofactor in enzymatic reactions  Involving ATP  DNA replication  mRNA production  Binds to Ca 2+ receptors  Can block Ca 2+ channels

EXTRACELLULAR 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’s & Chvostek’s sign?

Hypocalcemia because they indicate easy nerve firing

CHVOSTEK’S SIGN &TROUSSEAU’S SIGN

              CHVOSTEK’S SIGN Elicitation: Tapping on the face at a point just anterior to the ear and just below the zygomatic bone Postitive response: Twitching of the ipsilateral facial muscles, suggestive of neuromuscular excitability caused by hypocalcemia TROUSSEAU’S SIGN Elicitation: Inflating a sphygmomanometer cuff above systolic blood pressure for several minutes Postitive response: Muscular contraction including flexion of the wrist and metacarpophalangeal joints, hyperextension of the fingers, and flexion of the thumb on the palm, suggestive of neuromuscular excitability caused by hypocalcemia

QUESTION

True or False: Both hyperkalemia and hypercalcemia cause cells to fire more easily.

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).

Balance between blood pH, HCO 3 = and H 2 CO 3 Normal Balance Metabolic Acidosis Respiratory Alkalosis Important Relationships Insert fig. 6-16

H 2 CO 3 = .03 * PaCO 2

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 ACID

 CO 2 + H 2 O   H 2 CO 3 (carbonic acid)  H 2 CO 3   H + + HCO 3 (bicarbonate ion)  An increase in CO 2 will cause  Increases in CO 2 (increased PCO 2 )  Increases in H + (lower pH)  Increases in bicarbonate ion

RESPIRATORY ACIDOSIS AND ALKALOSIS

 CO 2 + H 2 O   H 2 CO 3   H + + HCO 3  (carbonic acid)   (bicarbonate ion)

Respiratory acidosis

Increased PCO 2 Increased carbonic acid Increased H + (<7.35) = low pH Increased bicarbonate

Respiratory alkalosis

Decreased PCO 2 Decreased carbonic acid Decreased H + (>7.45) = high pH Decreased bicarbonate  Respiratory distress may be an attempt to compensate for low serum pH

QUESTION

Tell whether the following statement is true or false: Serum levels of pH and CO 2 proportional.

levels are directly

ANSWER

False As blood levels of CO 2 increase, pH becomes more acidic (decreases).

RESPIRATION 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 PCO 2 Slightly increased K + Her urine pH was slightly lowered Why?

BUFFER SYSTEMS

METABOLIC 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 ALKALOSIS

 Increased metabolic acids raise H + levels  Some H+ combines with bicarbonate, decreasing it  Breathing adjusts CO 2 to normal levels to bring pH back

Metabolic acidosis Metabolic alkalosis

Increased H + (<7.35) = low pH Decreased H + (>7.45) = high pH Decreased bicarbonate Heavier breathing causes decreased PCO 2 Increased bicarbonate Lighter breathing causes increased PCO 2