A&P Kai

Osmolarity Maintenance: ECF/ICF osmolarity is $\approx 300 \text{ mOsm}$ , maintained by sodium ( $\text{Na}^+$ ).
Kidney Function: Nephrons filter blood, reabsorb useful substances (e.g., >99\% of filtered sodium), and secrete unwanted substances.
$\text{Na}^+$ & Water Reabsorption: When $\text{Na}^+$ is reabsorbed, water follows by osmosis, increasing blood volume and pressure but not osmolarity.
Hormonal Regulation:
- Aldosterone: Increases $\text{Na}^+$ and water reabsorption; primarily controls blood volume/pressure in response to low BP/volume.
- Antidiuretic Hormone (ADH): Increases water reabsorption independently; controls blood osmolarity (high osmolarity triggers release).
- Atrial Natriuretic Peptide (ANP): Decreases $\text{Na}^+$ and water reabsorption, reducing blood volume.

Capillaries & Interstitial Fluid: Capillaries have gaps, allowing fluid and nutrients to move into interstitial fluid.
Normal Exchange (Starling Forces):
- Filtration: Fluid moves out of capillaries due to capillary hydrostatic pressure (arterial end).
- Reabsorption: Fluid moves into capillaries due to capillary colloidal osmotic pressure (proteins in blood).
Lymphatic System: Returns excess interstitial fluid and proteins to circulation, preventing fluid loss.

Edema: Accumulation of interstitial fluid due to:
- Increased capillary hydrostatic pressure (e.g., heart failure).
- Decreased capillary colloidal osmotic pressure (e.g., liver/kidney disease).
- Increased capillary permeability (e.g., inflammation).
- Lymphatic obstruction.
- Pulmonary edema is life-threatening as it impairs gas exchange.
Isotonic Hypovolemia (Fluid Volume Deficit): Proportionate loss of $\text{Na}^+$ and water, decreasing ECF volume (osmolarity unchanged).
- Causes: Inadequate intake, excessive GI/renal/skin losses, third-space losses.
- Manifestations: Thirst, weight loss, decreased urine, postural hypotension, tachycardia, dry membranes, decreased tissue turgor.
Isotonic Hypervolemia (Fluid Volume Excess): Proportionate increase in ECF $\text{Na}^+$ and water.
- Etiology: Increased total body sodium, decreased renal elimination, excessive intake.
- Manifestations: Weight gain, edema, distended neck veins, full pulse, pulmonary edema.

A. Sodium Balance

Overview: Main ECF cation; vital for osmolarity, ECF volume, acid-base, nerve function.

1. Hyponatremia (plasma \text{Na}^+ < 135 \text{ mEq/L} )

Types: Hypertonic (water shift from ICF), Hypotonic (dilutional due to water retention - hypovolemic, euvolemic, hypervolemic).
Manifestations: Cell swelling (especially brain), muscle cramps, GI issues, CNS symptoms (lethargy, confusion, seizures, coma).

2. Hypernatremia (plasma \text{Na}^+ > 145 \text{ mEq/L} )

Characterized by: ECF hypertonicity and cellular dehydration.
Causes: Net water loss (DI), sodium gain, impaired thirst.
Manifestations: Thirst, decreased urine, dry skin/mucous membranes, ECF loss signs (tachycardia, low BP), CNS symptoms (agitation, seizures, coma) due to brain cell shrinkage.

B. Potassium Balance

Overview: Major ICF cation ( $140 \text{-} 150 \text{ mEq/L}$ ), ECF ( $3.5 \text{-} 5.0 \text{ mEq/L}$ ); critical for cell integrity, acid-base, nerve/muscle excitability.
Regulation: Renal elimination, transcellular shifts (insulin, acid-base).

1. Hypokalemia (plasma \text{K}^+ < 3.5 \text{ mEq/L} )

Etiology: Inadequate intake, excessive losses (renal, GI), transcellular shifts.
Manifestations: Renal (impaired concentration), GI (constipation, ileus), CV (dysrhythmias, digitalis toxicity), Neuromuscular (weakness, paralysis).

2. Hyperkalemia (plasma \text{K}^+ > 5 \text{ mEq/L} )

Etiology: Decreased renal elimination (renal failure), rapid administration, movement from ICF to ECF (tissue trauma).
Manifestations: Decreased neuromuscular excitability (paresthesias, muscle weakness), serious cardiac conduction issues (peaked T waves, wide QRS, arrest).

C. Calcium Balance

Overview: $99\%$ in bone; ionized ( $\text{Ca}^{2+}$ ) is vital for neuromuscular/cardiac excitability, muscle contraction, clotting.
Regulation: Vitamin D (absorption), Parathyroid Hormone (PTH - raises $\text{Ca}^{2+}$ from bone, kidney, activates Vit D), Calcitonin (lowers $\text{Ca}^{2+}$ ).

1. Hypocalcemia (plasma \text{Ca}^{2+} < 8.5 \text{ mg/dL} )

Etiology: Impaired bone mobilization (hypoparathyroidism, Vit D deficiency), abnormal renal losses, increased protein binding.
Manifestations: Increased neuromuscular excitability (paresthesias, cramps, tetany, positive Chvostek/Trousseau signs, prolonged QT interval), cardiac insufficiency.

2. Hypercalcemia (plasma \text{Ca}^{2+} > 10.5 \text{ mg/dL} )

Etiology: Increased bone resorption (neoplasms, hyperparathyroidism), increased intestinal absorption, decreased elimination.
Manifestations: Decreased neural excitability (dulling, weakness, coma), GI (constipation, nausea), Kidney (polyuria, stones), CV (hypertension, shortened QT).