Nephrology

• Excretion of metabolic waste and many drugs/toxins.
• Regulation of extracellular volume, osmolality and electrolyte composition.
• Acid–base homeostasis through bicarbonate handling and net acid excretion.
• Blood-pressure regulation through sodium balance, RAAS and neurohumoral mechanisms.
• Endocrine functions: renin, erythropoietin and activation of vitamin D.
• Metabolic functions including gluconeogenesis and hormone metabolism.

Core concept: The kidney is not simply a filter. It continuously filters plasma and then selectively reabsorbs, secretes and generates substances to preserve internal homeostasis.

The filtration barrier comprises fenestrated endothelium, glomerular basement membrane and podocyte slit diaphragm. Size and charge characteristics restrict passage of cells and most large proteins.

• Podocyte injury commonly produces proteinuria.
• Capillary-wall inflammation can produce hematuria and nephritic manifestations.
• Loss of filtration surface or severe hemodynamic disturbance reduces GFR.

GFR is the volume of plasma ultrafiltrate formed by all functioning glomeruli per unit time and is a central measure of kidney filtration function.

• It depends on net filtration pressure, filtration coefficient and functioning nephron mass.
• Serum creatinine is an imperfect surrogate affected by muscle mass, diet, tubular secretion and non-steady-state conditions.
• Estimated GFR is most useful when interpreted with clinical context and albuminuria.

High yield: A “normal” creatinine can coexist with substantially impaired kidney function in a patient with low muscle mass.

Kidneys receive a large fraction of cardiac output. Autoregulation helps stabilize renal blood flow and GFR across a physiological pressure range.

• Myogenic response alters afferent arteriolar tone.
• Tubuloglomerular feedback links distal sodium chloride delivery to afferent tone and renin release.
• Sepsis, shock, advanced vascular disease and certain drugs can disrupt effective autoregulation.

• Afferent constriction tends to reduce renal blood flow and glomerular pressure.
• Moderate efferent constriction can support glomerular capillary pressure when renal perfusion falls.
• Excessive efferent constriction can ultimately reduce renal blood flow and impair filtration.
• Prostaglandins help preserve afferent vasodilation in vulnerable states; NSAIDs can therefore precipitate hemodynamic AKI.
• Angiotensin II preferentially supports efferent tone; RAAS blockade can lower intraglomerular pressure.

Filtration fraction = GFR / renal plasma flow. It describes the fraction of renal plasma flow that becomes glomerular ultrafiltrate.

Changes in afferent/efferent resistance can alter GFR, renal plasma flow and filtration fraction differently—one reason serum creatinine cannot be interpreted purely as a structural kidney marker.

• The macula densa senses distal tubular sodium chloride delivery.
• High delivery promotes signals that reduce afferent filtration pressure.
• Low delivery promotes renin release and mechanisms supporting filtration/volume conservation.
• This mechanism links tubular workload with glomerular filtration.

Reduced renal perfusion, reduced macula-densa sodium chloride delivery and sympathetic stimulation promote renin release.

• Renin initiates angiotensin II generation.
• Angiotensin II supports vascular tone, efferent arteriolar resistance and aldosterone secretion.
• Aldosterone promotes distal sodium retention and potassium/hydrogen secretion.
• Persistent RAAS activation contributes to hypertension, glomerular hypertension and cardiorenal disease.

• Reabsorbs most filtered sodium and water in near-isosmotic fashion.
• Reabsorbs most filtered bicarbonate.
• Normally reabsorbs essentially all filtered glucose below transport saturation.
• Major site for phosphate and amino-acid reabsorption.
• Secretes organic acids/bases and participates in ammoniagenesis.

Generalized proximal tubular dysfunction produces a recognizable pattern: bicarbonate, phosphate, glucose and amino-acid wasting.

• Descending limb is relatively water permeable.
• Thick ascending limb reabsorbs Na-K-2Cl and is relatively impermeable to water.
• Separation of salt and water handling generates the medullary osmotic gradient.
• This gradient allows ADH to produce concentrated urine downstream.

Clinical link: Loop diuretics inhibit transport in the thick ascending limb and impair the kidney's concentrating machinery.

• Distal nephron performs fine control rather than bulk reclamation.
• Principal cells participate in sodium reabsorption and potassium secretion.
• Intercalated cells are central to hydrogen and bicarbonate handling.
• Aldosterone and distal sodium delivery strongly influence potassium secretion.
• ADH controls collecting-duct water permeability through aquaporin regulation.

Total-body sodium primarily determines extracellular volume; serum sodium primarily reflects water balance relative to exchangeable body solute.

• A patient may be sodium overloaded yet hyponatremic.
• A patient may have sodium depletion with normal serum sodium.
• Volume status and tonicity must therefore be assessed separately.

• Plasma osmolality reflects the concentration of dissolved particles.
• Tonicity describes the effective osmotic force across cell membranes.
• ADH and thirst are the principal regulators of water balance.
• Water excess relative to solute tends to lower serum sodium; water deficit tends to raise it.

Think of sodium disorders first as disorders of water balance, then determine the underlying volume and hormonal physiology.

• ADH release rises with increased effective osmolality and with sufficiently strong non-osmotic stimuli such as reduced effective circulating volume.
• ADH increases collecting-duct water permeability.
• Concentrated urine requires both ADH action and an intact medullary gradient.
• Suppression of ADH permits excretion of dilute urine when renal diluting capacity is intact.

The kidney responds to perceived effective arterial filling, not simply total-body fluid volume.

• Heart failure and cirrhosis can produce total-body sodium/water excess while the kidney senses reduced effective arterial volume.
• This activates RAAS, sympathetic pathways and ADH, worsening sodium/water retention.

• Most body potassium is intracellular.
• Serum potassium is influenced by total-body stores and transcellular shifts.
• Insulin and beta-adrenergic activity shift potassium into cells.
• Acid–base state, cell breakdown and osmolality can alter distribution.
• Renal excretion depends heavily on distal sodium delivery, tubular flow and aldosterone activity.

A rapid change in serum potassium can occur without a comparable change in total-body potassium.

• Kidneys regulate phosphate excretion and activate vitamin D.
• PTH modifies renal calcium/phosphate handling and vitamin-D activation.
• CKD disrupts phosphate balance, vitamin-D metabolism and parathyroid regulation, contributing to CKD-mineral bone disorder.
• Magnesium is filtered and reabsorbed at multiple nephron sites; renal failure and tubular/diuretic effects can alter levels.

• Filtered bicarbonate is largely reclaimed rather than simply excreted.
• Kidneys generate new bicarbonate while excreting net acid.
• Ammonium production is a major adaptive mechanism during chronic acid load.
• Distal hydrogen secretion permits urinary acidification.

The lungs regulate CO₂ rapidly; kidneys regulate bicarbonate and net acid over a slower timescale.

Detailed diagnosis and management will be developed in the electrolyte/acid–base phase.

• Creatinine generation depends substantially on muscle mass.
• Creatinine rises late relative to an abrupt fall in GFR because a new steady state takes time.
• Urea is influenced by protein intake, catabolism, gastrointestinal bleeding, volume state and tubular reabsorption.
• Cystatin C can add information in selected settings but also has non-GFR determinants.

• Creatinine clearance approximates filtration but can overestimate true GFR because creatinine is also secreted by tubules.
• Timed urine collection introduces collection error.
• eGFR equations estimate filtration from serum markers and demographic/biological variables but are less reliable in rapidly changing kidney function.

Clearance is the virtual volume of plasma completely cleared of a substance per unit time.

• If a freely filtered substance is neither reabsorbed nor secreted, its clearance reflects GFR.
• Clearance below GFR suggests net reabsorption.
• Clearance above GFR suggests net secretion.

• Normal glomeruli restrict large proteins and proximal tubules reclaim much of the small filtered protein load.
• Albuminuria commonly reflects glomerular barrier dysfunction.
• Low-molecular-weight proteinuria may indicate tubular reabsorptive dysfunction.
• Albuminuria is both a kidney-disease marker and an important cardiovascular/renal prognostic marker.

• Erythropoietin: renal interstitial cells respond to tissue oxygen signaling and stimulate erythropoiesis.
• Renin: central to RAAS regulation.
• Vitamin D: renal activation supports calcium/phosphate homeostasis.

CKD anemia and mineral-bone disease are direct consequences of losing endocrine/metabolic kidney functions—not merely filtration failure.

Long-term blood-pressure regulation is tightly linked to the kidney's ability to excrete sodium at a given arterial pressure. A shift toward requiring higher pressure to achieve sodium balance contributes to sustained hypertension.

Understanding nephron physiology predicts electrolyte and acid–base adverse effects of each class.

SGLT2 in the proximal tubule reabsorbs filtered glucose together with sodium. SGLT2 inhibition increases glycosuria/natriuresis and alters tubuloglomerular feedback, reducing intraglomerular pressure.

Clinical connection: The kidney benefits of SGLT2 inhibition extend beyond glucose lowering and are now central to modern cardiorenal therapy in appropriate patients.

• Remaining nephrons can increase single-nephron filtration after nephron loss.
• Initially adaptive hyperfiltration may become maladaptive when sustained.
• Intraglomerular hypertension contributes to progressive albuminuria and nephron injury.

• GFR and renal reserve tend to decline with ageing.
• Older adults are more vulnerable to volume depletion, nephrotoxins and electrolyte disturbances.
• Low muscle mass can conceal impaired filtration when creatinine is interpreted without context.

• Filtration fraction: GFR / renal plasma flow.
• Clearance: urine concentration × urine flow / plasma concentration.
• Filtered load: GFR × plasma concentration of a freely filterable solute.
• Excretion rate: urine concentration × urine flow.
• Fractional excretion: fraction of filtered solute ultimately excreted.

Formulas are useful only when their physiological assumptions and limitations are understood.

• The kidney filters first and then extensively modifies the filtrate.
• Proximal tubule performs bulk reabsorption; distal nephron performs fine regulation.
• GFR depends on both perfusion/hemodynamics and functioning filtration surface.
• NSAIDs reduce protective prostaglandin-mediated afferent vasodilation in vulnerable patients.
• RAAS blockade reduces intraglomerular pressure by altering efferent tone.
• Serum sodium is primarily a water-balance measurement, not a direct measure of total-body sodium.
• ADH determines collecting-duct water permeability.
• Distal sodium delivery and aldosterone are major determinants of potassium secretion.
• Kidneys regenerate bicarbonate and excrete net acid.
• A normal creatinine does not guarantee normal renal function.
• eGFR is unreliable as a simple static number during rapidly changing AKI.
• Albuminuria provides diagnostic and prognostic information beyond GFR.
• CKD affects endocrine and metabolic kidney functions as well as filtration.
• Understanding nephron transport explains how diuretics work and why they cause predictable electrolyte disturbances.
• Sustained glomerular hyperfiltration can become maladaptive.

• Serum creatinine is influenced by generation, distribution and renal excretion; it is not a direct measurement of GFR.
• Early AKI may be present before creatinine reaches its eventual peak, so urine output and trajectory matter.
• Low muscle mass can produce a deceptively low creatinine despite clinically important kidney dysfunction.
• A creatinine rise after a hemodynamic change should be interpreted with volume status, blood pressure, medications and urine findings rather than labelled automatically as intrinsic renal injury.

Clinical pearl: Never interpret creatinine without asking: compared with what baseline, over what time, and in what physiological context?

• Serum sodium primarily reflects the relationship between body water and exchangeable sodium/potassium, not total-body sodium alone.
• A patient can be edematous yet have reduced effective arterial blood volume.
• Hyponatremia is approached by tonicity, urine dilution and volume physiology rather than by sodium replacement reflexively.