The master class begins with an introduction to acid-base balance, focusing on the concept of pH. The normal body pH ranges from 7.35 to 7.45, with an average of 7.4. A pH less than 7.4 indicates acidity, while greater than 7.4 indicates alkalinity. The Henderson-Hasselbalch equation (pH = pKa + log([conjugate base]/[weak acid])) is introduced as a key formula for calculating pH.

The discussion moves to fluid compartments, establishing that total body water is approximately 60% of body weight (42 liters). This is divided into 20% extracellular fluid (ECF - 14 liters) and 40% intracellular fluid (ICF - 28 liters). Males have a higher total body water (60%) than females (50%), while infants have the highest (70-75%). Sodium is the most abundant cation in the ECF, and potassium is most abundant in the ICF, with a 3:2 sodium-potassium ratio. Chloride, bicarbonate, and calcium are also found in the ECF, while phosphate and magnesium are in the ICF.

Specific indicators are used to measure fluid compartments: antipyrine for total body water, inulin for ECF, and Evans Blue Dye for plasma volume. The Darrow-Yannet diagram is introduced to illustrate fluid shifts, emphasizing homeostasis as the equilibrium state of the internal environment.

Six scenarios of fluid imbalance are discussed: isotonic fluid contraction (diarrhea, burns), isotonic fluid expansion (infusion of normal saline or lactated Ringer's), hypertonic fluid contraction (dehydration, fever, sweating, diabetes insipidus), hypertonic fluid expansion (high sodium chloride intake, rapid NSS infusion), hypotonic fluid contraction (adrenal insufficiency/Addison's disease, cerebral salt wasting), and hypotonic fluid expansion (SIADH). Clinical examples and their physiological implications are provided for each scenario.

Normal arterial blood gas levels are presented: pCO2 between 35-45 mmHg and bicarbonate between 20-26 mEq/L. Metabolic acidosis, characterized by decreased pH and bicarbonate, is linked to causes like DKA, severe diarrhea, and various toxins. The anion gap formula (Na+ + K+ - Cl- + HCO3-) is critical, with a normal range of 12-14. High anion gap acidosis causes include lactic acidosis and ketoacidosis. Metabolic alkalosis (increased pH and bicarbonate), respiratory acidosis (decreased pH and increased pCO2 due to hypoventilation), and respiratory alkalosis (increased pH and decreased pCO2 due to hyperventilation/hysteria) are also covered, along with their compensatory mechanisms.

Normal kidney size (10x5x5.5 cm) and weight (150g) are discussed. Exceptions to decreased kidney size in CKD (normal size in HIV, diabetic nephropathy, amyloidosis) are noted. The functional units of the kidney, including the glomerulus, Bowman's capsule, proximal and distal convoluted tubules, loop of Henle (thin descending, thin ascending, thick ascending), connecting tubules, and collecting ducts, are outlined. The brush border in the proximal tubules, crucial for absorption, and the juxtaglomerular apparatus (JG cells secreting renin, macula densa sensing sodium, mesangial cells) are highlighted.

The sites of action for various diuretics are presented: acetazolamide and canagliflozin (SGLT2 inhibitors) in the proximal convoluted tubules; osmotic diuretics throughout the tubules; loop diuretics in the thick ascending limb of the loop of Henle; thiazide diuretics and potassium-sparing diuretics (e.g., spironolactone, triamterene) in the distal tubules and collecting ducts. Potassium shifts are discussed in the context of hyperkalemia and hypokalemia. Drugs for hyperkalemia (calcium gluconate, insulin, beta-adrenergic agonists like salbutamol) are mentioned, with calcium gluconate being the first-line for cardiac stabilization.

Important vascular concepts include renal infarction (coagulative necrosis), abdominal aortic aneurysm (most common location is infrarenal), and renal Nutcracker syndrome (compression of the left renal vein).

The three basic renal processes – glomerular filtration, tubular reabsorption, and tubular secretion – are explained. Filtration occurs in the glomerular capillaries, and reabsorption in the peritubular capillaries. The majority of water reabsorption happens in the proximal convoluted tubules. The structure of the glomerular filtration barrier, including fenestrated endothelium, glomerular basement membrane (GBM with negative charge and Type IV collagen), and podocytes (foot processes), is detailed. Effacement of podocytes is characteristic of minimal change disease (lipoid nephrosis), treated effectively with steroids. Alport syndrome, a genetic disorder of Type IV collagen, presents with nephritis and deafness.

Inulin is used to measure glomerular filtration rate (GFR) as it is freely filtered and neither reabsorbed nor secreted. Para-aminohippuric acid (PAH) measures renal plasma flow because it is freely filtered and 100% secreted. The micturition reflex is described, with the first urge to void at 150 ml and marked fullness at 400-500 ml. Bethanechol is used for post-op urinary retention, but bradycardia is a dreaded side effect due to its parasympathetic effects.

Renin, secreted by JG cells, converts angiotensinogen (from the liver) to angiotensin I. Erythropoietin (EPO), produced in the peritubular capillaries of the kidney, is stimulated by hypoxia and promotes RBC production. EPO is used to treat normochromic normocytic anemia in CKD patients. Vitamin D (1,25-dihydroxycholecalciferol or calcitriol), is activated in the kidney by 1-alpha hydroxylase and is crucial for calcium and phosphate metabolism. Vitamin D deficiency causes rickets in children and osteomalacia in adults.

The ascending limb of the loop of Henle is impermeable to water, while the descending limb is highly permeable due to aquaporins. Neuromyelitis Optica (NMO or Devic's disease) involves antibodies against aquaporins. The countercurrent multiplier (loop of Henle) establishes the medullary osmotic gradient, and the countercurrent exchanger (vasa recta) maintains it. Aldosterone is produced in the zona glomerulosa of the adrenal cortex.

Starling's forces (capillary hydrostatic, capillary oncotic, Bowman's hydrostatic pressure) determine net filtration pressure, which is typically 10 mmHg. Renal autoregulation mechanisms include myogenic response and tubuloglomerular feedback (TGF). The kidneys receive 20-25% of cardiac output. The renal threshold for glucose is 180-200 mg/dL.

The three urinary buffers are bicarbonate (strongest), phosphate, and ammonia (NH4). The critical reaction of CO2 + H2O forming carbonic acid, catalyzed by carbonic anhydrase (requiring zinc) in RBCs, is reviewed. This leads to bicarbonate leaving the RBC and chloride entering (chloride shift). Hemoglobin's oxygen-binding capacity is 1.34 mL O2 per gram. Hemoglobin exists in relaxed (better binding) and tense/taut forms.