Renal Physiology

Understanding renal physiology is fundamental for finals, as it underpins common conditions like acute kidney injury (AKI), chronic kidney disease (CKD), electrolyte imbalances, and hypertension. The kidney's role extends beyond waste excretion to crucial endocrine functions and acid-base balance. You'll need to grasp how the nephron, the functional unit, filters blood and selectively reabsorbs/secretes substances to maintain homeostasis. This knowledge is vital for interpreting U&Es, managing fluid status, and understanding diuretic mechanisms.

Glomerular filtration is a passive process driven by Starling forces across the filtration barrier: fenestrated endothelium, negatively charged glomerular basement membrane, and podocyte slit diaphragms. This barrier restricts large proteins and cells. GFR is tightly regulated by afferent and efferent arteriolar tone, which is autoregulated via the myogenic mechanism and tubuloglomerular feedback (macula densa sensing NaCl delivery). The proximal convoluted tubule (PCT) is where bulk reabsorption occurs (e.g., 65% Na+, 100% glucose via SGLT2, 85% HCO3- via carbonic anhydrase). The Loop of Henle, particularly the thick ascending limb's active Na+/K+/2Cl- transport, is critical for establishing the medullary osmotic gradient (countercurrent multiplier), which the vasa recta preserves. The distal convoluted tubule (DCT) and collecting duct (CD) fine-tune electrolyte and water balance under hormonal control: ADH increases water permeability (aquaporins) and aldosterone increases Na+ reabsorption/K+ excretion.

Renal physiology is central to understanding AKI (pre-renal, intrinsic, post-renal causes), CKD progression and its complications (anaemia, bone disease, metabolic acidosis), and electrolyte disorders (e.g., hyperkalaemia, hyponatraemia). Diuretics target specific nephron segments (e.g., loop diuretics inhibit the Na+/K+/2Cl- cotransporter in the thick ascending limb, causing significant diuresis). Conditions like nephrotic syndrome (massive proteinuria, oedema) result from damage to the glomerular filtration barrier. Understanding the RAAS is key to managing hypertension and heart failure. Recognising signs of fluid overload or depletion is a core clinical skill.

Interpreting U&Es (urea, creatinine, Na+, K+) is paramount. Elevated creatinine and urea indicate reduced GFR. eGFR provides an estimate of kidney function but can be inaccurate in AKI or extremes of body habitus. Urinalysis (dipstick) screens for proteinuria, haematuria, and glycosuria. Urine albumin:creatinine ratio (ACR) is a sensitive marker for early kidney damage, particularly in diabetes. Imaging (renal ultrasound) is crucial for identifying obstruction (hydronephrosis) or structural abnormalities. Blood gas analysis helps assess acid-base status, often deranged in significant renal impairment.

Management of renal conditions often involves addressing the underlying cause, optimising fluid balance, and correcting electrolyte disturbances. For AKI, this means identifying and removing the insult (e.g., stopping nephrotoxic drugs like NSAIDs/ACEi in hypovolaemia), ensuring adequate perfusion, and managing complications (e.g., IV calcium gluconate for severe hyperkalaemia). CKD management focuses on blood pressure control (often with ACEi/ARBs), managing anaemia (EPO), bone disease (phosphate binders, activated Vitamin D), and preparing for renal replacement therapy if needed. Diuretics are used to manage fluid overload.