Kidney and Interstitium: Renal Physiology Q&A

Interstitium is the fluid space between cells.

Nephrons are the functional units of the kidney.

The nephron components are Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct.

Nephrons cannot be regenerated.

The two main functions of the kidney are to make urine and maintain blood pressure.

Adaptive changes in remaining nephrons compensate for loss.

Fluid becomes concentrated going down the nephron and becomes dilute going up the nephron, creating a gradient from top to bottom.

The hyperconcentrated environment at the bottom of the nephron/interstitium allows water retention.

ADH increases water reabsorption by opening water channels in the collecting duct.

Glomerular filtrate is plasma devoid of proteins, and concentrations of most non-protein solutes (salts, organic molecules) are the same as plasma.

(1) The balance of hydrostatic and colloid osmotic forces acting across the capillary membrane and (2) the capillary filtration coefficient (Kf).

The capillary filtration coefficient (Kf) is the product of permeability and filtering surface area.

They have high glomerular hydrostatic pressure and a large Kf.

It refers to the structure and properties of the filtration barrier in the glomerulus.

(1) Capillary endothelium, (2) Basement membrane, and (3) Layer of epithelial cells (podocytes).

It forms the first filtration layer of the glomerulus.

It acts as a negatively charged filtration barrier between the glomerulus and Bowman's capsule.

Podocytes are epithelial cells of Bowman's capsule that attach to the basement membrane.

The basement membrane is negatively charged, which allows permeability to most substances but restricts proteins.

Basement membrane problems decrease glomerular osmotic colloid pressure, which increases GFR.

The basement membrane is located between the glomerulus and Bowman's capsule.

High permeability occurs because of fenestrae in the endothelium and slit pores in the podocytes.

All components have a strong negative charge that hinders the passage of plasma proteins.

Negatively charged large molecules are not easily filtered due to charge repulsion and size restriction.

The afferent arteriole carries blood into the glomerulus.

Increased afferent arteriolar resistance reduces blood flow into the glomerulus, decreasing GFR.

The efferent arteriole carries blood out of the glomerulus.

Mild efferent arteriolar constriction increases glomerular hydrostatic pressure, which increases GFR.

Severe efferent arteriolar constriction reduces renal blood flow and decreases hydrostatic pressure and GFR.

GFR is determined by (1) net filtration pressure and (2) the glomerular filtration co-efficient (Kf).

Net filtration pressure is the sum of hydrostatic and colloid osmotic forces across the glomerular membrane.

Kf is the filtration coefficient of the glomerulus that contributes to GFR.

Increased Kf increases GFR.

Decreased Kf decreases GFR.

Disease states increase thickness of the basement membrane, lowering Kf.

Increased Bowman's capsule pressure decreases GFR.

Urinary tract obstruction from stones.

Increased glomerular colloid osmotic pressure decreases GFR.

GFR decreases.

Approximately 60 mm Hg.

(1) Arterial pressure, (2) afferent arteriolar resistance, and (3) efferent arteriolar resistance.

It reduces glomerular capillary hydrostatic pressure, decreases GFR, and reduces renal blood flow.

It increases glomerular capillary hydrostatic pressure, generally increases GFR, but also reduces renal blood flow.

Moderate constriction increases hydrostatic pressure causing a mild increase in GFR, while severe constriction decreases renal blood flow causing a decrease in GFR.

The Sympathetic Nervous System and Angiotensin II.

Strong activation causes renal vasoconstriction, decreased renal blood flow, and decreased GFR.

GFR decreases.

Epinephrine and norepinephrine have similar effects, causing vasoconstriction and decreasing GFR.

It primarily constricts efferent arterioles.

It helps maintain GFR via the renin/angiotensin system.

It refers to intrinsic kidney mechanisms that regulate GFR through the macula densa and juxtaglomerular cells.

They sense changes in NaCl concentration.

They are located in the walls of efferent and afferent arterioles, release renin, and cause vasoconstriction of efferent arterioles.

It maintains glomerular hydrostatic pressure for GFR and leads to fluid and water reabsorption to maintain fluid volume.

Decreased arterial pressure decreases glomerular hydrostatic pressure and GFR; macula densa senses NaCl change; afferent resistance decreases; juxtaglomerular cells release renin; angiotensin II is formed, increasing efferent arteriolar resistance.

It turns on when decreased arterial pressure or decreased glomerular hydrostatic pressure occurs.

It maintains glomerular hydrostatic pressure, causes angiotensin II to release aldosterone, and directly acts on the nephron to cause fluid retention.

when the primary pump is pushing sodium out of cell so secondary takes advantage by bringing in sodium and glucose or amino acids Secondary active transport uses the sodium gradient created by the primary pump to bring sodium and glucose or amino acids into the cell.

They include primary active transport and secondary active transport.

Primary active transport directly uses or hydrolyzes ATP to move solutes against their electrochemical gradient and sets up the primary gradient for secondary transport.

Secondary active transport uses the electrochemical gradient of one substance (often Na+) to move another substance.

Glucose enters tubular cells from the lumen coupled with Na+ entry (down its concentration gradient), and glucose then exits the cell on the basolateral side down its own concentration gradient.

Downhill movement of Na+ ions is coupled with uphill movement of protons in the opposite direction.

It describes how different parts of the nephron reabsorb water and solutes.

Concentrated fluid moving down and dilute fluid moving up maintains a hyperosmotic interstitium, allowing water to be reabsorbed from the collecting duct when needed.

The proximal tubule reabsorbs bulk amounts of filtered sodium and water while maintaining osmolarity, and a little filtered chloride, with no change in concentration.

It is an important site for secretion of organic acids and bases (e.g., bile salts, oxalate, urate, catecholamines).

Thin descending segment, thin ascending segment, and thick ascending segment.

It is highly permeable to water and reabsorbs water but not solutes, and it is not very permeable to sodium chloride and urea.

It is impermeable to water, reabsorbs some sodium chloride, and makes tubular fluid dilute.

It is impermeable to water, reabsorbs large amounts of solutes due to active transport, and makes urine very dilute by the time it reaches the distal tubule, which is crucial for regulating urine concentration.

They inhibit the 1-sodium, 2-chloride, 1-potassium cotransporter in the thick ascending loop, causing ions and water to stay in the lumen and increasing urine formation (diuresis).

They inhibit the sodium chloride cotransporter, causing ions and water to stay in the lumen and increasing urine formation.

It describes how different nephron segments reabsorb water and solutes.

Concentrated fluid moving down and dilute fluid moving up maintains a hyperosmotic interstitium, allowing water to be reabsorbed from the collecting duct when needed.

It refers to reabsorbing large amounts of filtered sodium and water.

The proximal tubule reabsorbs large amounts of sodium and water while maintaining osmolarity, reabsorbs a small amount of chloride, and does not change overall concentration.

It is an important site for secretion of organic acids and bases (e.g., bile salts, oxalate, urate, catecholamines).

Thin descending segment, thin ascending segment, and thick ascending segment.

It is highly permeable to water and reabsorbs water but not solutes, and it is not very permeable to sodium chloride and urea.

It is impermeable with no reabsorption of water, reabsorbs some sodium chloride, and makes tubular fluid dilute.

It is impermeable to water, reabsorbs large amounts of solutes due to active transport, and makes urine very dilute by the time it reaches the distal tubule, which is crucial for regulating urine concentration.

They inhibit the cotransporter so ions and water stay in the lumen, increasing urine formation (diuresis).

They inhibit the sodium chloride cotransporter so ions and water remain in the lumen, increasing urine formation.

They are similar to the thick ascending segment of the loop of Henle and further dilute urine.

It reabsorbs small amounts of sodium chloride.

Thiazide diuretics inhibit the sodium chloride cotransporter, reducing sodium and chloride reabsorption.

Principal cells and intercalated cells.

They reabsorb sodium and water and secrete potassium.

Permeability to water depends on levels of antidiuretic hormone (ADH).

Na⁺/K⁺ ATPase on the basolateral membrane pumps Na⁺ out; luminal Na⁺ enters cells via specialized channels; ATPase activity increases intracellular K⁺, which is secreted into the lumen through specialized channels.

They inhibit aldosterone or sodium channels to reduce sodium reabsorption and potassium secretion.

Aldosterone activates the sodium potassium ATPase.

Aldosterone increases sodium reabsorption.

Sodium remains in the cell, so more water stays in the cell and potassium remains in the cell.

Sodium channel blockers prevent sodium from entering the cell and potassium from leaving, causing sodium and water to remain in the lumen and increasing urine output.

They maintain acidase balance.

It reabsorbs small amounts of water and solutes, which are critical for final urine processing and water/solute output.

It is controlled by ADH; high ADH levels increase water reabsorption and reduce urine volume.

It can reabsorb urea and secrete H⁺.

(1) High ADH levels and (2) high osmolarity of the renal medullary interstitium.

They increase water reabsorption in distal tubules and collecting ducts.

It provides the osmotic gradient necessary for water reabsorption with high ADH.

It remains isosmotic with plasma (~300 mOsm/L) because solutes and water are reabsorbed in equal proportion.

Water is reabsorbed due to the hypertonic renal interstitium, causing the fluid to become very concentrated.

Sodium and other solutes are reabsorbed, but water is not, causing the fluid to become very dilute.

Water is not reabsorbed, and fluid can become as dilute as 50 mOsm/L.

More water is reabsorbed, and urine becomes concentrated.

They have short loops of Henle and extensive peritubular capillaries.

They make up 20-30% of nephrons, have glomeruli at the cortex-medulla junction, and long loops of Henle that penetrate deep into the medulla.

The vasa recta maintains the hyperosmotic renal interstitium through a parallel balance mechanism.

It is the mechanism that produces a hyperosmotic renal medullary interstitium.

It depends on the special anatomical arrangement of the loops of Henle and the vasa recta.

The loop of Henle initially fills with isosmotic fluid.

An active ion pump in the thick ascending limb reduces tubular concentration and raises interstitial concentration, establishing a gradient.

Tubular fluid in the descending limb and interstitial fluid reach osmotic equilibrium by water osmosis out of the descending limb, increasing interstitial osmolarity.

Additional fluid flow pushes hyperosmotic fluid into the ascending limb, generating a further gradient and raising interstitial osmolarity.

Repeated steps lead to rising interstitial fluid osmolarity.

It adds more solute than water to the medulla.

The fluid leaving the loop of Henle is dilute.

High ADH levels lead to avid water reabsorption in distal and collecting tubules, concentrating urine.

Further reabsorption occurs in the medullary collecting ducts.

It preserves the high solute concentrations and hyperosmolarity of the renal medulla.

(1) Low medullary blood flow and (2) vasa recta act as countercurrent exchangers, minimizing washout of solutes.

Low medullary blood flow.

The vasa recta act as countercurrent exchangers, minimizing washout of solutes.

It becomes progressively more concentrated due to solute entry from the interstitium and water loss into it.

The U-shape allows large exchange of solute and fluids with little net change in concentration, preventing dissipation of medullary hyperosmolarity.

They prevent dissipation of medullary hyperosmolarity but do not create it.

It refers to mechanisms that adjust tubular reabsorption based on physiological needs.

It is a mechanism where tubules increase reabsorption rates in response to increased tubular load.

If GFR increases, proximal tubular reabsorption increases.

It refers to hormonal mechanisms that regulate tubular reabsorption.

Aldosterone is a hormone involved in sodium reabsorption and potassium/proton secretion.

Angiotensin 2 stimulates secretion of aldosterone.

It is an important regulator of sodium reabsorption and potassium/proton secretion.

Principal cells of the cortical collecting tubule.

Aldosterone stimulates activity of the sodium-potassium ATPase pump on the basolateral membrane of cortical collecting tubule cells.

(1) Increased extracellular potassium concentration and (2) increased angiotensin II levels.

Increased angiotensin II levels increase renal sodium and water retention, raising extracellular fluid volume and restoring blood pressure.

It refers to disorders caused by absence or excess of aldosterone.

absence/deficiency in relation to aldosterone Marked loss of sodium and accumulation of potassium.

excess aldosterone Sodium retention and decreased plasma potassium due to excessive renal potassium secretion.

It is crucial for regulating renal potassium excretion and potassium concentration in body fluids, more than for sodium concentration.

It is a hormone involved in vasoconstriction and regulation of sodium and water balance.

Angiotensin II causes vasoconstriction of the efferent arteriole, increases fluid and salt reabsorption, and stimulates release of aldosterone.

ADH promotes water reabsorption.

It is the body's most powerful sodium-retaining hormone.

Low blood pressure and/or low extracellular fluid volume stimulate formation of Angiotensin II.

(1) Stimulates aldosterone secretion, (2) constricts efferent arterioles, and (3) directly increases sodium and water reabsorption.

In the proximal tubules, loops of Henle, distal tubules, and collecting tubules.

It stimulates sodium reabsorption in multiple nephron segments.

Constriction of efferent arterioles helps maintain GFR, ensuring normal excretion of metabolic waste products while Angiotensin II is retaining water.

Both increase extracellular fluid sodium and volume by increasing water reabsorption, having little effect on sodium concentration except under extreme conditions.

ADH is a hormone that regulates water reabsorption in the kidneys.

ADH is effective only when the renal interstitium is hyperosmotic, allowing water reabsorption to occur.

Decreased ADH results in very dilute urine.

It increases water permeability of the distal tubule, collecting tubule, and collecting duct epithelia.

(1) Increased ECF osmolarity and (2) decreased blood pressurelood volume.

Increased ECF osmolarity causes osmoreceptor cells in the anterior hypothalamus to shrink, signaling the posterior pituitary to release ADH.

Signals from the carotid sinus and aortic arches travel via vagus and glossopharyngeal nerves to stimulate ADH release.

(1) Inappropriate secretion of ADH, (2) inability of the nephron to respond to ADH, and (3) impairment of the countercurrent mechanism.

A disorder characterized by inability to produce ADH.

Inability to produce ADH.

Distal tubular segments cannot reabsorb water.

Large volume of dilute urine.

Administration of desmopressin (synthetic ADH analog).

A disorder where renal tubules cannot respond properly to ADH.

Normal or elevated ADH levels are present, but renal tubules cannot respond to ADH because the renal interstitium is not hyperosmotic.

Failure of the countercurrent mechanism or failure of distal/collecting tubules/ducts to respond to ADH.

Many renal diseases, diuretics that inhibit electrolyte reabsorption in the loop of Henle, and certain drugs such as lithium or antibiotics.

Correcting the underlying renal disorder; it does not respond to desmopressin.

It refers to methods used to quantify kidney excretory function.

It quantifies the effectiveness of kidney excretion.

The volume of plasma completely cleared of a substance by the kidneys per unit time.

A method to estimate GFR using inulin clearance.

The substance must be freely filtered by the glomerulus and not reabsorbed or secreted by renal tubules.

A polysaccharide that meets GFR criteria and must be administered intravenously.

A by-product of muscle metabolism cleared almost entirely by glomerular filtration.

Creatinine clearance assesses GFR, and a two-fold increase in plasma creatinine suggests a 50% decrease in GFR.

A method to estimate renal plasma flow using PAH clearance.

If a substance is completely cleared from plasma, its clearance rate equals total RPF.

Complete clearance requires tubular secretion in addition to filtration.

It is approximately 90% cleared from plasma, making its clearance a good approximation of RPF.

A calculation representing the fraction of plasma filtered through the glomerular membrane.

The fraction of plasma filtered through the glomerular membrane.

FF = GFR / RPF.

Azotemia is elevation of blood urea nitrogen and creatinine levels, typically indicating decreased GFR.

Prerenal azotemia is caused by hypoperfusion of the kidneys, reducing GFR without parenchymal damage.

Postrenal azotemia is caused by obstruction of urine flow below the kidney level and is reversible upon relief of obstruction.

Uremia is azotemia with clinical manifestations and systemic biochemical abnormalities.

Failure of renal excretory function and metabolic and endocrine alterations due to renal damage.

Many kidney diseases are immune mediated.

Gastrointestinal (uremic gastroenteritis), neuromuscular (peripheral neuropathy), and cardiovascular (uremic fibrinous pericarditis).

Nephritic syndrome results from glomerular injury.

Acute gross hematuria, oliguria, azotemia, and hypertension.

Inflammation, hematuria, reduced GFR, and hypertension.

Nephrotic syndrome is a glomerular syndrome.

Increased urinary protein loss leads to hypoalbuminemia, decreasing osmotic pressure and activating the renin-angiotensin system, causing salt and water retention and edema.

Increased lipid synthesis due to hypoproteinemia and increased permeability to lipoproteins.

Heavy proteinuria, hypoalbuminemia, severe edema, hyperlipidemia, and lipiduria.

Nephritic presents with blood in urine and less proteinuria, while Nephrotic presents with more proteinuria, severe edema, hyperlipidemia, and lipiduria.

Most primary glomerular diseases are caused by immune mechanisms.

Most primary glomerular diseases result from immune mechanisms.

Antigens may be planted in the basement membrane or may be endogenous glomerular proteins.

Immune complexes formed elsewhere can deposit in the glomerulus.

A clinical syndrome characterized by heavy proteinuria and related manifestations.

Massive proteinuria, hypoalbuminemia, generalized edema, hyperlipidemia, and lipiduria.

Massive proteinuria is caused by derangement in glomerular capillary walls, increasing permeability to plasma proteins.

Due to heavy proteinuria, serum albumin decreases.

Loss of osmotic pressure reduces intravascular volume and renal blood flow, activating the renin-angiotensin system and promoting salt and water retention.

Hyperlipidemia may be due to increased lipoprotein synthesis from hypoalbuminemia or loss of synthesis inhibitor, and lipiduria results from increased permeability to lipoproteins.

A clinical syndrome characterized by acute glomerular inflammation.

Acute onset of hematuria, oliguria, azotemia, and hypertension.

Children often recover fully, whereas adults are less likely to fully recover.

It occurs after streptococcal infection due to immune-mediated injury to the kidneys.

Hematuria with dysmorphic red blood cells and red cell casts in urine.

Reduced urine output and elevated nitrogenous waste in blood.

Elevated blood pressure due to renal injury.

They are typically less severe than in nephrotic syndrome.

Proliferation of glomerular cells with inflammatory leukocytic infiltrate.

Inflammatory injury to capillary walls allows blood into urine and reduces GFR.

Oliguria, fluid retention, and azotemia.

Hypertension results from fluid retention and increased renin release from ischemic kidneys.

A form of nephritic syndrome following infection.

Streptococcal infection triggers immune-mediated glomerular injury.

Deposition of immune complexes causing proliferation and damage of glomerular cells with leukocytic infiltration.

Streptococcus (Group A).

Increased glomerular cellularity (proliferation/swelling of endothelial and mesangial cells), infiltrating neutrophils/monocytes, and red cell casts in tubules.

Abrupt onset with nephritic syndrome.

Most children recover, while affected adults may develop end-stage renal disease.

Inflammation involving renal tubules and interstitium.

unilateral Acute infection of one kidney.

Ascending bacterial infection from the urinary tract.

Urinary tract infection involving lower or upper tract.

E. coli.

Recurrent or persistent infection can lead to chronic pyelonephritis.

Sudden flank pain at the costovertebral angle.

Chills, fever, and malaise.

Dysuria, frequency, and urgency.

Often benign and self-limited; symptomatic phase typically lasts no longer than a week, though bacteriuria may persist.

Bilateral Chronic pyelonephritis typically affects both kidneys.

Recurrent infections superimposed on diffuse or localized obstructive lesions, leading to repeated inflammation and scarring.

Scarring involving the pelvis or calyces.

Bilateral cases often involve congenital anomalies (e.g., posterior urethral valves) and can cause fatal renal insufficiency if uncorrected.

Unilateral cases may be due to calculi or unilateral obstructive lesions of the ureter.

A form of interstitial nephritis caused by adverse drug reactions.

Early recognition and withdrawal of the offending drug are necessary to prevent prolonged renal damage.

Adverse reactions to an increasing number of drugs.

Synthetic penicillins, other synthetic antibiotics, diuretics (thiazides), NSAIDs, phenindione, cimetidine.

Likely immune-mediated; drugs bind to tubular cells, become immunogenic, and cause cell-mediated immune reactions to tubular cells or basement membranes.

Onset is quickly, about 15 days after drug exposure.

Withdrawal of the offending drug leads to recovery, though renal function may take several months to normalize.

A severe form of hypertension causing vascular and renal damage.

Pathological activation of the renin-angiotensin system constricts the efferent arteriole, reducing renal blood flow and lowering GFR.

Renal ischemia activates the renin-angiotensin system, causing efferent constriction to raise GFR, but reduced renal blood flow prevents effective increase in GFR.

Activation increases sodium and water retention.

ACE inhibitors reduce renin-angiotensin system activation and help control blood pressure and fluid retention.

Blood pressure usually greater than 200/120 mm Hg.

It may arise de novo or suddenly in individuals with mild pre-existing hypertension.

Vascular damage to kidneys injures arteriolar walls, reducing renal ischemia and activating the renin-angiotensin system.

Reduced renal ischemia triggers renin release.

It causes intrarenal vasoconstriction, enhances ischemia, and increases renin secretion.

Elevated aldosterone causes salt retention and further exacerbates high blood pressure.

A major renal complication of diabetes.

Renal failure is the second leading cause of death in diabetics.

(1) Thickening of basement membrane, (2) diffuse mesangial sclerosis, and (3) nodular glomerulosclerosis.

Structural changes affecting glomeruli in diabetes.

Thickening of the capillary basement membrane is a key glomerular lesion.

Diffuse mesangial sclerosis involves increased mesangial matrix and mesangial cell proliferation and is always associated with basement membrane thickening.

It may lead to nephrotic syndrome.

Deposits of matrix in the periphery of the glomerulus.

Observed in long-term diabetics, unique to diabetes, and may cause renal ischemia and inability to form urine.

Vascular changes affecting renal blood vessels in diabetes.

Vascular lesions affecting renal arteries and arterioles.

Both afferent and efferent arterioles are affected.

Interstitial changes associated with diabetic kidney disease.

Diabetic nephropathy can predispose to acute or chronic pyelonephritis.

Regulation of body pH and fluid status.

Approximately 6.8 (lower limit) to 8.0 (upper limit).

Chemical acid-base buffers, respiratory center, and kidneys.

They act instantly within seconds to prevent excessive H⁺ concentration changes.

It regulates CO₂ removal (and thus H⁺ concentration) from extracellular fluid within minutes.

They excrete acid or alkaline urine to readjust extracellular fluid H⁺ concentration more slowly but comprehensively.

Regulation of acid-base balance through respiratory control of CO₂.

Increased pCO₂ increases respiratory rate to reduce acidosis.

Decreased pCO₂ decreases respiratory rate to reduce alkalosis.

It cannot return H⁺ concentration all the way to normal for non-respiratory disturbances but is effective within 3-12 minutes.

Its buffering power is 1-2 times greater than all other chemical buffers in extracellular fluid combined.

Regulation of acid-base balance through renal mechanisms.

(1) Secretion of H⁺, (2) reabsorption of filtered HCO₃⁻, and (3) production of new HCO₃⁻.

Approximately 4320 mEq of HCO₃⁻ daily.

Because HCO₃⁻ must first form H₂CO₃ before it can be reabsorbed.

Approximately 4320 mEq of H⁺ secretion daily.

Approximately 80 mEq/day.

In the proximal tubule, thick segment of loop of Henle, and early distal tubule.

Mainly by combining with phosphate and ammonia buffers; only a small amount is excreted as ionic H⁺.

H⁺ combines with phosphate and ammonia buffers in tubular fluid to be excreted.

H⁺ combines with phosphate and is excreted; HCO₃⁻ formed in renal cells is absorbed into blood (gaining bicarbonate).

Approximately 30-40 mEq/day.