--- module: 017-02 language: en chapter: 17 title: "Glomerular Filtration, Tubular Transport, Concentration, and Clearance" module_title: "Glomerular haemodynamics, tubular energetics, and renal diagnostic patterns" source_sha256: b9e77b98c8bc7464d595944cfc3cf9977d51fc52800e335e821634c841e7397b --- # Glomerular filtration, tubular transport, concentration, and clearance ## Filtration is pressure constrained ### Needs plasma flow, pressure gradient, intact surface ### Afferent tone sets inflow, efferent sets outflow ### Moderate efferent constriction preserves glomerular pressure #### Renal blood flow falls, filtration fraction rises ### Angiotensin two supports efferent tone in low volume #### Blockade risky with renal-artery disease or depletion #### Long term it protects proteinuric kidneys ### Prostaglandins maintain afferent dilation under stress #### Afferent loss, efferent loss, diuretics combine ### Below the autoregulatory range filtration follows pressure ### Above it, transmitted pressure injures capillaries ## Barrier selectivity and protein patterns ### Endothelium, basement membrane, podocyte slits differ ### The injured layer sets the finding combination ### Nephritic: red cells, low filtration, hypertension, oedema ### Nephrotic: hypoalbuminaemia, oedema, infection, thrombosis ### Syndromes describe physiology, not one histology ### Protein may be glomerular, tubular, overflow, post-renal #### Albumin predominance supports glomerular leakage #### Light chains exceed reabsorptive capacity ## Tubular transport and its oxygen cost ### Filtered sodium load creates most transport work ### Demand is highest where reclamation is greatest ### The medulla transports much on little oxygen ### Proximal failure loses bicarbonate, phosphate, glucose, amino acids #### Generalised dysfunction gives a Fanconi pattern ### Thick limb dilutes fluid and builds the gradient ### Potassium loss also follows distal delivery and flow ## Concentrating and diluting urine ### Gradient, water permeability, and delivery all needed ### Antidiuretic hormone supplies aquaporin permeability ### Water diuresis gives dilute high-volume urine ### Solute diuresis raises excreted osmoles ### Volume and osmolality separate the mechanisms ### Concentrating fails early in tubulointerstitial disease ## Clearance and non-steady states ### Clearance links excretion to plasma concentration ### Reabsorption lowers clearance, secretion raises it ### Creatinine rises slowly after abrupt filtration loss #### Early values underestimate severity #### Dilution and low muscle mass lower it further ### Estimating equations are population tools ## Urine sediment in miniature ### Red-cell casts support glomerular inflammation ### White-cell casts point to interstitial inflammation ### Granular casts and tubular cells: acute tubular injury ### Crystals may be incidental or reveal supersaturation ### Dipstick blood detects haem pigment, not cells ### Dipstick protein misses light chains ## Integrating the renal diagnosis ### State filtration, output, sediment, protein, obstruction ### Ultrasound assesses size, structure, and drainage #### Normal imaging does not exclude disease ### Ask which mechanism is actually operating ### Review every medicine against current function ### Recovery is more than creatinine normalisation ### Residual albuminuria and reduced reserve persist ## Endocrine failure over time ### Reduced erythropoietin contributes to anaemia #### Assess iron, bleeding, inflammation, haemolysis, vitamins ### Phosphate retention and low active vitamin D ### Altered calcium and rising parathyroid hormone #### Bone and vessels remodel progressively ### Treatment follows stage, diet, symptoms, dialysis ### Over-treatment creates vascular harm ## Dialysis decisions ### No single creatinine value starts dialysis ### Refractory hyperkalaemia and acid-base disturbance ### Pulmonary oedema, poisons, uraemic complications ### Judged by clinical context and rate of change ### Modality matched to haemodynamics and access ### Mechanical clearance is not kidney recovery ## Acute kidney injury timeline ### Baseline value and last normal value ### Urine output, pressure, contrast, surgery, infection ### Record when each medicine started ### Several causes commonly coincide on one day ### Avoid applying a single label prematurely ### Oliguria resistant to volume needs reassessment ### Drug accumulation feeds a further injury cycle