Urinary System – Week 7 Lecture Review

70 question-and-answer flashcards that cover the anatomy, physiology, regulation, and clinical aspects of the urinary system as presented in the Week 7 lecture notes.

What are the five main functions of the kidneys?

Regulate blood plasma volume, remove metabolic wastes, regulate electrolyte balance, regulate blood pH, and secrete erythropoietin.

Which two gross regions form each kidney?

Renal cortex (outer) and renal medulla (inner, containing pyramids and columns).

Trace urine flow from renal pyramid to ureter.

Renal pyramid → minor calyx → major calyx → renal pelvis → ureter.

Which muscle forms the bladder wall and what innervates it?

Detrusor muscle; parasympathetic neurons releasing acetylcholine onto muscarinic receptors.

Differentiate the internal and external urethral sphincters.

Internal: smooth muscle (involuntary). External: skeletal muscle (voluntary).

What is the ‘guarding reflex’?

Spinal reflex that inhibits detrusor contraction and stimulates external sphincter contraction to prevent involuntary urination.

Where is the micturition center located?

In the pons of the brainstem.

Define nephron.

The functional unit of the kidney consisting of renal tubules and associated blood vessels; each kidney has >1 million.

List the order of blood vessels supplying a nephron beginning with the renal artery.

Renal artery → interlobar a. → arcuate a. → interlobular a. → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → interlobular v. → arcuate v. → interlobar v. → renal vein.

Name the segments of a nephron tubule in order.

Glomerular (Bowman’s) capsule → proximal convoluted tubule → descending limb of loop of Henle → ascending limb → distal convoluted tubule → collecting duct.

Which type of nephron is specialized for producing concentrated urine?

Juxtamedullary nephron.

What type of capillaries make up the glomerulus and what is the filtrate called?

Fenestrated capillaries; fluid that enters the capsule is called filtrate or ultrafiltrate.

Through which three layers must filtrate pass in the renal corpuscle?

Capillary fenestrae, glomerular basement membrane, and the slit diaphragm between podocyte pedicels.

What pathology results from a defect in slit diaphragm pores?

Proteinuria – presence of proteins in urine.

State the normal glomerular filtration rate (GFR).

115–125 mL/min (~180 L/day).

How does sympathetic activation affect GFR?

Causes vasoconstriction of afferent arterioles, decreasing GFR to conserve plasma volume during fight-or-flight.

Describe renal autoregulation when blood pressure drops below 70 mm Hg.

Afferent arterioles dilate to maintain a nearly constant GFR.

What is the role of the macula densa in tubuloglomerular feedback?

Senses elevated NaCl/water in ascending limb; releases ATP to constrict afferent arteriole, lowering GFR.

Define reabsorption in the nephron.

The return of filtered molecules from the tubular fluid to the blood.

What percentage of filtered water is reabsorbed in proximal tubule and descending loop together?

Approximately 85% (65% PCT + 20% descending limb).

How is Na⁺ reabsorbed in the proximal convoluted tubule?

Diffuses into tubule cells then actively pumped (Na⁺/K⁺ ATPase) to interstitial space; drives passive Cl⁻ and water movement.

Explain why filtrate remains iso-osmotic after the proximal tubule.

Salt and water are reabsorbed proportionally, keeping osmolality equal to plasma.

Why can water not leave the ascending limb of the loop of Henle?

Its walls are impermeable to water.

What transport occurs in the thick ascending limb?

Active extrusion of Na⁺ (Na⁺/K⁺/2 Cl⁻ cotransport) into interstitial fluid; Cl⁻ follows passively; K⁺ recycles.

Describe the countercurrent multiplier positive feedback loop.

Salt pumped from ascending limb makes medulla hypertonic → water leaves descending limb → filtrate entering ascending limb becomes saltier → more salt pumped out, repeating until maximum gradient achieved.

What is the function of the vasa recta?

Countercurrent exchanger that maintains medullary concentration gradient by absorbing water and recycling salt/urea without dissipating gradient.

How does urea contribute to medullary hypertonicity?

It diffuses from collecting duct into interstitium and re-enters loop of Henle, continuously recycling and adding solute.

Why is the collecting duct’s water permeability variable?

It depends on the number of aquaporin channels inserted in response to ADH.

Where is ADH synthesized and stored?

Synthesized in the hypothalamus; stored and released from posterior pituitary.

What triggers ADH release?

Increased plasma osmolality (dehydration) sensed by hypothalamic osmoreceptors.

Define renal clearance.

Volume of plasma from which a substance is completely removed by kidneys per minute.

Give the formula for GFR using inulin.

GFR = (V × U) / P, where V = urine flow rate, U = urine inulin concentration, P = plasma inulin concentration.

How does secretion affect a substance’s renal clearance relative to GFR?

If secreted, clearance exceeds GFR.

Why can glucose appear in urine (glycosuria)?

When plasma glucose exceeds the transport maximum (Tm) of glucose/Na⁺ cotransporters, excess is not reabsorbed.

List five key electrolytes whose balance is regulated by kidneys.

Na⁺, K⁺, Cl⁻, HCO₃⁻, and phosphate.

What hormone controls Na⁺ reabsorption and K⁺ secretion in distal nephron segments?

Aldosterone.

Explain the direct stimulus for aldosterone secretion.

Elevated blood K⁺ concentration directly stimulates adrenal cortex.

Which system indirectly increases aldosterone when Na⁺ or blood volume falls?

Renin-angiotensin-aldosterone system (RAAS).

Where is renin released and what converts angiotensin I to II?

Renin from granular (juxtaglomerular) cells of afferent arteriole; angiotensin-converting enzyme (ACE) converts I to II.

What is the macula densa’s role in RAAS regulation?

High Na⁺/flow in distal tubule causes macula densa to inhibit renin release; low Na⁺ stimulates renin via reduced inhibitory signal.

What hormone opposes RAAS by promoting natriuresis?

Atrial natriuretic peptide (ANP).

Describe potassium handling in cortical collecting ducts when plasma K⁺ rises.

Increased K⁺ channels (aldosterone-independent) and higher aldosterone (dependent) enhance K⁺ secretion.

How are H⁺ and K⁺ secretion interrelated?

Na⁺ reabsorption stimulates secretion of either K⁺ or H⁺; acidosis favors H⁺ secretion (reduces K⁺), alkalosis favors K⁺ secretion.

State the two main renal mechanisms to maintain blood pH.

Reabsorbing filtered bicarbonate and secreting H⁺ into tubule fluid.

Why must bicarbonate be converted to CO₂ before reabsorption?

The tubule membrane is impermeable to bicarbonate; carbonic anhydrase converts it to CO₂ which diffuses into cells, where it reforms HCO₃⁻.

What urinary buffers allow H⁺ excretion below tubular pH 4.5?

Filtered phosphate (HPO₄²⁻ ⇌ H₂PO₄⁻) and ammonia (NH₃/NH₄⁺).

How does the kidney compensate for acidosis with glutamine metabolism?

Proximal tubule converts glutamine to bicarbonate (reabsorbed) and ammonia (buffers urine H⁺).

Define obligatory water loss.

Minimum 400 mL/day of urine required to excrete metabolic wastes.

Which diuretic class is most potent and where does it act?

Loop diuretics (e.g., furosemide/Lasix); inhibit NaCl transport in thick ascending limb.

What is the mechanism of thiazide diuretics?

Inhibit NaCl reabsorption in early distal tubule.

How do carbonic anhydrase inhibitors promote diuresis?

By preventing bicarbonate reabsorption in proximal tubule, reducing water reabsorption.

Give an example and action site of an osmotic diuretic.

Mannitol; acts throughout tubules by increasing filtrate osmolality, reducing water reabsorption.

What makes potassium-sparing diuretics distinct?

They block aldosterone receptors or Na⁺ channels in cortical collecting duct, reducing Na⁺ reabsorption without promoting K⁺ loss.

Why is glucose clearance normally zero?

Because filtered glucose is 100 % reabsorbed in proximal tubule under normal plasma levels.

If a substance’s clearance equals renal plasma flow (~625 mL/min), what does that indicate?

It is completely cleared from plasma by filtration and secretion (e.g., PAH), none returns via renal vein.

What minimum blood osmolality change stimulates ADH release?

As little as a 1 % rise in plasma osmolality.

How does ADH influence aquaporin insertion mechanistically?

ADH binds V₂ receptor → activates cAMP → protein kinase A → exocytosis of vesicles containing aquaporin-2 into luminal membrane.

What is the typical osmolality of filtrate entering the loop of Henle?

Approximately 300 mOsm/kg (iso-osmotic to plasma).

During dehydration, urine osmolality can reach what value?

Up to about 1,400 mOsm/kg due to maximal ADH and medullary gradient.

What is the effect of vasoconstriction of afferent arterioles on filtration pressure?

Reduces hydrostatic pressure in glomerulus, decreasing GFR.

Explain obligatory vs facultative water reabsorption.

Obligatory: unregulated (85 %) in PCT & descending limb; facultative: regulated (remaining 15 %) in distal nephron via hormones.

Why does protein remain in plasma during filtration?

Slit diaphragm and basement membrane present size/charge barrier preventing large plasma proteins from passing.

Which part of nephron creates dilute (hypotonic) tubular fluid?

Thick ascending limb removes NaCl without water, making filtrate hypotonic (~100 mOsm) before distal tubule.

What is proteinuria and a potential cause related to filtration barrier?

Presence of protein in urine; caused by defects in slit diaphragm pores of podocytes.

How is renal plasma clearance of a reabsorbed substance compared to GFR?

Clearance is less than GFR because some filtered amount returns to blood.

Why does sympathetic activity decrease urine formation during exercise?

Diverts blood to muscles, constricts renal afferent arterioles, lowering GFR and urine output.

Name two ways aldosterone affects ion movement in the distal nephron.

Increases Na⁺ reabsorption and K⁺ secretion.

What happens to renin release when macula densa detects high NaCl?

It decreases renin secretion, lowering aldosterone and promoting Na⁺ excretion.

Which cells act as intrarenal baroreceptors for blood volume?

Granular (juxtaglomerular) cells in the afferent arteriole.

State the normal minimal urine pH and why it cannot be lower.

About 4.5; below this H⁺ gradient would halt secretion without buffering.

What are organic anion and cation transporters (OATs/OCTs)?

Polyspecific membrane carriers that secrete foreign substances, including drugs, from peritubular blood into renal tubules.

Define excretion rate formula.

Excretion rate = (filtration + secretion) – reabsorption.

How do loop diuretics potentially cause hypokalemia?

By increasing flow and Na⁺ delivery to distal nephron, stimulating K⁺ secretion and loss.

Which hormone reduces blood volume by enhancing Na⁺ and water excretion and where is it released?

Atrial natriuretic peptide (ANP); released from atria when stretched.

Why is a minimum urine output necessary even during severe dehydration?

To eliminate metabolic wastes (obligatory water loss).

What segment of nephron reabsorbs bicarbonate via Na⁺/H⁺ antiport?

Proximal convoluted tubule.

Name a clinical condition resulting if ADH is absent or receptors are non-functional.

Diabetes insipidus – characterized by large volumes of dilute urine.

What is the effect of alkalosis on K⁺ handling?

Stimulates K⁺ secretion/excretion, potentially leading to hypokalemia.