Fluid, Electrolyte, and Acid-Base Balance Lecture Flashcards

Composition of Extracellular Fluid (Blood)

• Sodium ($Na^+$) is the #1 positive ion in the blood.

• Chloride ($Cl^-$) is the #1 negative ion in the blood.

• Bicarbonate ($HCO_3^-$) levels are high in the blood.

• Kidney Function: As evidenced by kidney diagrams, sodium, chloride, and bicarbonate are constantly reabsorbed by the kidneys.

Overhydration (Hypotonic Hydration)

• Definition: Occurs when the blood becomes hypotonic to the cells.

• Mechanism of Action: The Extracellular Fluid (ECF/blood) becomes hypotonic in relation to the cells.

• Osmotic Movement: Water flows into the cell (Intracellular Fluid or ICF) via osmosis.

• Cellular Effect: Cells fill with fluid and can burst or lyse.

• Severity: Overhydration can be fatal.

• Primary Causes: 1. Overtreating dehydration too quickly after strenuous exercise. 2. Kidney failure, resulting in the retention of too much water. 3. Excessive Antidiuretic Hormone ($ADH$) secretion. 4. Giving water to babies younger than $6 ext{ months}$ of age.

• Symptoms of Overhydration: Nausea and Vomiting ($N/V$), headache, confusion, seizures, and death.

• Physiological Consideration: Neurons are surrounded by bone, making tissue swelling particularly dangerous.

• Blood State: The blood is considered to be too dilute.

Dehydration

• Definition: Occurs when water loss is greater than water gain.

• Electrolyte Impact: Fluid loss serves to concentrate electrolytes within the blood.

• Fluid Shift: Fluid is lost from the Extracellular Fluid (ECF/blood), which subsequently draws water from the Intracellular Fluid (ICF/cells).

• Cellular Impact: Water leaves the cells, causing crenation (shriveling), in order to enter the blood (ECF).

• Causes of Dehydration: 1. Excessive sweating. 2. Water deprivation. 3. Hyposecretion of aldosterone. 4. Hyposecretion of Antidiuretic Hormone ($ADH$). 5. Eating disorders and episodes of vomiting or diarrhea.

• Blood State: The blood becomes hypertonic to the cells and is highly concentrated.

Composition of Intracellular Fluid (ICF) and Potassium Dynamics

• Primary Ion: Potassium ($K^+$) is the ion found in the highest concentration inside the cells.

• Clinical Characteristic: Potassium ($K^+$) has a low therapeutic index.

• Focused Inquiry: What specific factors can bring about changes in potassium levels within the bloodstream?

Factors Influencing Potassium ($K^+$) Levels

• Role of Aldosterone: The basic function of this hormone is to save sodium ($Na^+$) to the blood and eliminate excess potassium ($K^+$) to the urine.

• Hyperaldosteronism (Excessive Aldosterone): Too much of this hormone will move too much $K^+$ into the urine, leading to a decrease in blood $K^+$ levels, a condition known as hypokalemia.

• Hypoaldosteronism (Insufficient Aldosterone): Too little of this hormone allows $K^+$ to elevate in the blood because the rate of $K^+$ elimination is decreased, a condition known as hyperkalemia.

• Metabolic Acidosis: In this state, the kidney prioritizes getting rid of hydrogen ions ($H^+$) more than potassium ($K^+$). As a result, $K^+$ will increase in the blood, causing hyperkalemia.

Respiratory Acidosis

• Hypercapnia: Defined as elevated carbon dioxide ($CO_2$) levels resulting from low ventilation.

• pH Correlation: Hypercapnia leads to a low $pH$, where an increase in $CO_2$ corresponds to a decrease in $pH$.

Homeostatic Maintenance and Respiratory Acidosis

• Disturbance: Homeostasis is disturbed by increasing Partial Pressure of Carbon Dioxide ($P_{CO_2}$).

• Cause: Hypoventilation.

• Receptor Activation: Arterial and Cerebrospinal Fluid ($CSF$) chemoreceptors are stimulated by the rising $P_{CO_2}$.

• Immediate Effect: Increased $P_{CO_2}$ results in a decreased blood $pH$.

• Systemic Responses to Acidosis: 1. Increased respiratory rate to clear $CO_2$. 2. Kidneys secrete $H^+$ and generate bicarbonate ($HCO_3^-$). 3. Buffer systems other than the carbonic acid-bicarbonate system accept $H^+$ ions.

• Outcome: Decreased $P_{CO_2}$, decreased $H^+$, and increased $HCO_3^-$ return the blood $pH$ to normal, restoring homeostasis.

Metabolic Acidosis: Characteristics and Causes

• Definition: Acidosis not caused by $CO_2$.

• Major Causes: 1. Cells producing large numbers of metabolic acids. 2. Diabetes mellitus, particularly in patients who are noncompliant with treatment. 3. Strenuous exercise, such as lactic acid accumulation in a marathon runner. 4. Chronic renal (kidney) failure, resulting in the inability to excrete hydrogen ions ($H^+$). 5. Depletion of bicarbonate, often due to severe diarrhea.

Compensatory Mechanisms for Metabolic Acidosis

• Lactic Acid Scenarios: In cases like a marathon runner, blood $pH$ drops due to increased $H^+$ concentration.

• Renal Compensation: The kidney eliminates $H^+$ ions into the urine and reabsorbs $HCO_3^-$ into the blood to build a good reserve.

• Respiratory Compensation: Exhaling $CO_2$ improves acidosis and brings $pH$ levels back up.

• Chemical Mechanism: Carbonic Acid ($H_2CO_3$) is a weak acid that dissociates into water ($H_2O$) and $CO_2$, the latter of which is exhaled.

Integrated Responses to Acidosis (Respiratory and Metabolic)

• Neurological Action: The brain (specifically the Dorsal Respiratory Group or DRG) increases the respiratory rate.

• Renal Action: The kidneys eliminate hydrogen ions and conserve bicarbonate ions.

• Protein Buffering: The amino group on protein buffers functions as a weak base, absorbing hydrogen ions ($H^+$) from the blood.

• Carbonic Acid Buffer Limitations: This system cannot buffer against acidosis that is specifically caused by $CO_2$; therefore, it is only efficient in helping to offset metabolic acidosis.