Fluid and Electrolyte Balance Part 1

What is the main component of all body fluids?

Water

What makes 2/3rds and 1/3rd of the body’s fluid?

• 2/3 located in cells called intracellular fluid (ICF)

• 1/3 is extracellular fluid (ECF)

What are the components of ECF?

80% interstitial fluid and 20% blood plasma

Where is ICF localized?

Lymph, cerebrospinal fluid, GI tract, synovial fluid, fluids of eyes and ears, pleural, pericardial, peritoneal, glomerular filtrate in kidneys

What do selectively permeable membranes separate? Plasma membranes? Blood vessel walls?

1. Separate body fluids into distinct compartments

2. Plasma membrane separate intracellular fluid from interstitial fluid

3. Walls divide interstitial fluid from blood plasma

What is fluid balance?

• Means various body compartments contain the required amount of water according to their needs

• Osmosis is the primary way in which water moves in and out of a compartment

• Most solutes in body fluids are electrolytes that dissociate into ions

What is the source of body water gain and loss?

• Water makes up a lot of the weight, depending on age and fat

• Fluid intake = fluid output so that the body maintains a constant volume

What does metabolic water volume depend on?

Depends on level of aerobic cellular respiration, which reflects the demand of ATP in body cells

What is the main way to regulate water balance in the body?

• By adjusting the volume of water intake

• Water loss = dehydration

What is the stimulus for water intake?

• Dehydration, resulting in thirst sensation

• Stimulate thirst in the hypothalamus by renin-angiotensin II pathway, which responds to decreased blood volume and decreased blood pressure

What hormones under normal conditions stimulate fluid output?

Antiduretic hormone (ADH), atrial naturietic peptide (ANP), Angiotensin II, and aldosterone

(controls urine production)

What does a change in osmolarity in body fluids mean?

• Means a imbalance in intracellular fluid and interstitial fluid

• Often caused by a change in Na+ concentration

• When water is consumed too fast than kidneys can excrete, it could lead to water intoxication

What are the four general functions electrolytes in the body serve for?

1. Control the osmosis of water between body compartments

2. Maintains acid-base balance

3. Carry electrical current, which allow for AP and GP and controls secretions of some hormones and neurotransmitters; important for development

4. Ions are cofactors needed for optimal activity of enzymes

How do we compare the charge carried by ions in different solutions?

• Concentration is expressed in milliequivalent/Liters (mEQ/L)

• This gives concentration of cations or anions in a soln

What are the differences between plasma and interstitial fluid?

1. Plasma contains protein anions, while interstitial fluid can’t since its plasma proteins can’t move out of blood vessel walls

2. Plasma contains more sodium ions but fewer chloride ions than interstitial fluid

Why is sodium the most abundant extracellular ion?

• It is involved in impulse transmission, muscle contraction, and fluid and electrolyte balance by creating most of the osmotic pressure in ECF

What do the kidneys do with sodium?

Excrete excess sodium and conserve it during sodium restrictions

What hormones control sodium in the blood?

Aldosterone, ADH, ANP

What does excess sodium in the body result in? Excess loss?

1. Edema

2. Excess loss causes excessive loss of water, which results in hypovolemia (low blood volume)

Why is chloride a major extracellular anion?

• Cl- regulates osmotic pressure between compartments and forming HCl in the stomach

What hormones regulates Cl- balance?

Indirectly controlled by aldosterone

• Aldosterone controls sodium reabsorption, negative Cl- follows positive sodium Na+ by electrical attraction

Why is potassium (K+) the most abundant cation?

Maintains fluid volume, impulse conduction, muscle contraction, and regulating pH

What hormone controls K+?

Mineralcorticoids like aldosterone

Where is bicarbonate prominent in?

• It is a significant anion in electrolyte balance

• Major component in acid-base buffer system secreted by the kidneys

What is calcium’s importance in EC fluid? (Ca²+)

• Structural component of teeth and bone

• Functions in blood coagulation, neurotransmitter release, maintenance of muscle tone, and excitability of nervous and muscle tissue

What hormones regulate calcium levels?

Parathyroid hormone and calcitonin

Phosphate is present as…

Calcium phosphate salts and contributes to 100 mEQ/L of anions in ICF, and a buffer

What are the hormones that regulate phosphate?

• PTH and calcitriol

• PTH increases urinary excretion of phosphate and lowers blood phosphate levels

• Calcitriol increases absorption of phosphate and calcium from GI tract

Why is magnesium abundant in ICF?

• Activates enzymes in metabolism of carbohydrates and proteins and is needed for operation of sodium pump

• Also functions in neuromuscular activity, neural transmission within the CNS, and myocardial functioning

What factors regulate Mg+ ion concentration in plasma?

1. Hypo/hyper calcemia

2. Hypo/hyper magnesemia

3. Inc or dec in ECF volume

4. Inc or dec of PTH

5. Acidosis or alkalosis

What maintains acid-base balance of the body?

• H+ concentration, especially in ECF

• Normal pH is 7.35-7.45

• Homeostasis of pH is maintained by buffer systems, exhalation of CO2, and kidneys excretion

What does the buffer system consist of and do?

• Consists of a weak acid and the salt of that acid (weak base)

• Functions in preventing rapid, drastic changes in pH of a body fluid by changing strong acids and bases into weak ones

Give some examples of a buffer system

• Protein system - most abundant in body cells and plasma; hemoglobin is a good buffer for carbonic acid

• Carbonate acid-bicarbonate system - regulator of blood pH

• Phosphate system - regulator of pH in RBCs and kidney tubular fluids

Describe how the exhalation of carbon dioxide adjusts pH of body fluids?

1. An increase in the rate and depth of breathing causes more CO2 exhalation, thereby increasing pH

2. A decrease in respiration rate and depth means less CO2 exhalation, thereby decreasing pH

How do the kidneys help maintain the body's pH balance through the actions of cells in the proximal convoluted tubule (PCT) and collecting ducts

• PCT and collecting ducts secrete H+ into the tubular fluid

• In the PCT, Na+/H+ antiporters secrete H+ and reabsorb Na+

• Apical surfaces include proton pumps that push H+ into tubular fluid while HCO3 antiporters in the basolateral membrane reabsorb HCO3

What two cells maintain body fluid pH?

1. PCT

2. Intercalated cells that have proton pumps

3. They excrete H+ when pH is too low and excrete HCO3 when pH is too high

Describe acidosis

• When below 7.35

• Principal effect is depression of the CNS thru depression of synaptic transmission

Describe alkalosis

• When pH above 7.45

• Principal effect is overexcitability of CNS thru facilitation of synaptic transmission

What are primary disorders of PCO2? What about bicarbonate concentration?

1. Respiratory acidosis or respiratory alkalosis

2. Metabolic acidosis or metabolic alkalosis

What is respiratory acidosis characterized by?

• Elevated PCO2 and decreased pH

• Caused by hypoventiliation or reduced gas exchange

What is respiratory alkalosis characterized by?

• Decreased PCO2 and increased pH

• Caused by hyperventilation

What is metabolical acidosis characterized by?

• Decreased bicarbonate levels and decreased pH levels

• Results from an abnormal increase in acid metabolic products, loss of bicarbonate, or failure of kidneys to secrete H+

What is metabolic alkalosis characterized by?

• Increased bicarbonate levels and increased pH levels

• Results from non-respiratory loss of acid (VOMIT) or excess intake of alkaline drugs

What are the 4 general step processes of acid-base imbalances?

1. Note whether the pH is high or low

2. Decide which value of PCO2 or HCO2 could cause the abnormality

3. Specific whether it is respiratory or metabolic

4. Look at the non-causative value and determine if it is compensating for the problem