Urinary System – Week 7 Comprehensive Notes

A comprehensive set of question-and-answer flashcards covering kidney structure, function, filtration, reabsorption, concentration mechanisms, hormonal regulation, acid-base balance, and clinical diuretics based on Week 7 urinary system lecture notes.

What are the five primary functions of the kidneys in regulating the extracellular fluid?

Regulation of blood plasma volume (blood pressure), removal of wastes, regulation of electrolytes, regulation of pH, and secretion of erythropoietin.

Which two gross anatomical regions make up the kidney?

Renal cortex and renal medulla (which contains renal pyramids and renal columns).

Into which structures does a renal pyramid drain, in order, before urine reaches the renal pelvis?

Minor calyx → major calyx → renal pelvis.

Which muscle forms the wall of the urinary bladder and how is it innervated?

The detrusor muscle, innervated by parasympathetic neurons that release acetylcholine onto muscarinic receptors.

Name the two urethral sphincters and their muscle types.

Internal urethral sphincter – smooth muscle; External urethral sphincter – skeletal muscle.

What is the ‘guarding reflex’?

A spinal reflex in which stretch receptors inhibit parasympathetic nerves to the detrusor and stimulate somatic motor neurons to the external sphincter, preventing involuntary urination.

Which brain center coordinates the voiding reflex?

The micturition center in the pons.

Define a nephron.

The functional unit of the kidney, consisting of small tubules and associated blood vessels that filter blood and form urine.

Approximately how many nephrons are in each kidney?

More than one million.

Trace the renal blood flow from renal artery to renal vein.

Renal artery → Interlobar arteries → Arcuate arteries → Interlobular arteries → Afferent arterioles → Glomerulus → Efferent arterioles → Peritubular capillaries → Interlobular veins → Arcuate veins → Interlobar veins → Renal vein.

What structures form the renal corpuscle?

The glomerulus and the surrounding Bowman's (glomerular) capsule.

List the sequence of nephron tubules from the renal corpuscle to the collecting duct.

Proximal convoluted tubule → Descending limb of loop of Henle → Ascending limb of loop of Henle → Distal convoluted tubule → Collecting duct.

Which type of nephron is specialized for producing concentrated urine?

Juxtamedullary nephron.

What feature of glomerular capillaries allows high filtration?

They are fenestrated capillaries with large pores that allow water and solutes to pass while retaining blood cells and most proteins.

What are podocytes and pedicels?

Podocytes are cells of the visceral layer of Bowman's capsule whose foot-like extensions (pedicels) form filtration slits around glomerular capillaries.

What condition results from a defect in the slit diaphragm pores?

Proteinuria – the presence of proteins in urine.

What is the normal net filtration pressure in the glomerulus?

About 10 mm Hg.

Define glomerular filtration rate (GFR) in adults.

The volume of filtrate formed by both kidneys each minute, normally 115–125 ml/min (~180 L/day).

How does sympathetic activation affect GFR?

It causes vasoconstriction of afferent arterioles, decreasing GFR and urine formation to conserve blood volume during fight/flight situations.

What is renal autoregulation?

Intrinsic mechanisms within the kidney that keep GFR relatively constant despite wide fluctuations in arterial blood pressure.

How do afferent arterioles respond when blood pressure falls below 70 mm Hg?

They dilate to maintain GFR.

Which cells detect increased Na⁺ and water in the filtrate and signal afferent arteriole constriction?

Macula densa cells in the ascending limb of the loop of Henle (tubuloglomerular feedback).

What percentage of filtered water is obligatorily reabsorbed and where?

85 % (65 % in the proximal tubule and 20 % in the descending limb of Henle); this reabsorption is unregulated.

What is obligatory water loss and its minimum volume?

The minimum urine output needed to excrete wastes, about 400 ml/day.

Why is the ascending limb of Henle essential for the countercurrent multiplier?

It actively pumps NaCl into the interstitial fluid but is impermeable to water, generating the medullary osmotic gradient.

Why does water leave the descending limb of Henle?

Because the surrounding interstitial fluid is hypertonic, drawing water out by osmosis; the descending limb is permeable to water but not to salt.

What specialized vessels help maintain the medullary osmotic gradient?

The vasa recta, which function as countercurrent exchangers.

How does urea contribute to the medullary concentration gradient?

Urea diffuses out of the collecting duct into the interstitium, then back into the loop of Henle, recycling to add solute to the medulla.

Which hormone regulates aquaporin insertion in collecting duct cells?

Antidiuretic hormone (ADH, vasopressin).

Where is ADH produced and from where is it released?

Produced by neurons in the hypothalamus and released from the posterior pituitary gland.

What stimulus triggers ADH release?

An increase in blood osmolality (concentration) detected by hypothalamic osmoreceptors.

Define renal clearance.

The volume of plasma from which a substance is completely removed by the kidneys per minute through filtration, secretion, and reabsorption.

Why is inulin the gold standard for measuring GFR?

Because it is freely filtered but neither reabsorbed nor secreted, so its clearance equals the true GFR.

Provide the formula for calculating GFR using inulin.

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

What happens to glucose in a healthy nephron?

It is completely reabsorbed in the proximal tubule via Na⁺-glucose cotransport, facilitated diffusion, and simple diffusion.

What is glycosuria and what condition commonly causes it?

Presence of glucose in urine, typically caused by diabetes mellitus when glucose exceeds the transport maximum.

Why are Na⁺ and K⁺ homeostasis critical?

Na⁺ balance affects blood volume and pressure; K⁺ balance affects skeletal and cardiac muscle excitability.

Which hormone fine-tunes Na⁺ reabsorption and K⁺ secretion in the distal nephron?

Aldosterone.

Describe the aldosterone-independent response to elevated plasma K⁺.

High K⁺ increases the number of K⁺ channels in the cortical collecting duct, enhancing K⁺ secretion without aldosterone involvement.

What triggers renin release from granular cells?

Low blood pressure/volume, low Na⁺ at the macula densa, or sympathetic stimulation.

Outline the renin-angiotensin-aldosterone pathway.

Renin converts angiotensinogen to angiotensin I → ACE converts it to angiotensin II → angiotensin II stimulates aldosterone secretion → aldosterone increases Na⁺ reabsorption and K⁺ secretion.

What does atrial natriuretic peptide (ANP) do?

Released from atria when stretched; it increases Na⁺ and water excretion, lowering blood volume and pressure.

How does acidosis influence K⁺ handling?

Acidosis promotes H⁺ secretion and inhibits K⁺ secretion, potentially leading to hyperkalemia.

How do kidneys reabsorb filtered bicarbonate?

Filtered HCO₃⁻ combines with secreted H⁺ to form H₂CO₃, which carbonic anhydrase converts to CO₂ + H₂O; CO₂ diffuses into tubule cells, reforms HCO₃⁻, which is transported into blood.

Which pump actively secretes H⁺ in the distal tubule?

H⁺-ATPase (proton pump).

How can the proximal tubule generate new bicarbonate during acidosis?

By metabolizing glutamine to produce NH₃ (which buffers urine) and HCO₃⁻ (which enters blood).

What are the two main urinary buffers allowing H⁺ excretion below pH 4.5?

Phosphate (HPO₄²⁻/H₂PO₄⁻) and ammonia (NH₃/NH₄⁺).

Why are loop diuretics considered the most potent?

They inhibit NaCl transport in the thick ascending limb, blocking up to 25 % of water reabsorption.

Give an example of a loop diuretic and a thiazide diuretic.

Loop: Furosemide (Lasix); Thiazide: Hydrochlorothiazide.

What is the mechanism of carbonic anhydrase inhibitors as diuretics?

They inhibit bicarbonate reabsorption in the proximal tubule, reducing water reabsorption.

Which diuretics are termed ‘potassium-sparing’ and why?

Aldosterone antagonists (e.g., spironolactone) and Na⁺ channel blockers (e.g., triamterene) because they inhibit Na⁺ reabsorption and K⁺ secretion, preserving K⁺.

What is the minimum urine pH the kidneys can safely produce?

Approximately 4.5.

What transporter returns Na⁺ to blood and brings H⁺ into tubule cells in the proximal tubule?

The Na⁺/H⁺ antiporter (secondary active transport).

How does the macula densa reduce renin secretion when filtrate Na⁺ is high?

By signaling the afferent arteriole to constrict and directly inhibiting granular cell renin release, reducing Na⁺ reabsorption downstream.

Explain obligatory versus facultative water reabsorption.

Obligatory: unregulated reabsorption in proximal tubule and loop (≈85 % of water). Facultative: hormonally regulated reabsorption in distal nephron and collecting duct (≈15 %), primarily via ADH.

What is the effect of osmotic diuretics such as mannitol?

They add non-reabsorbable solute to filtrate, increasing its osmolality and reducing water reabsorption throughout the nephron.

How does sympathetic stimulation both decrease GFR and promote renin release?

α-adrenergic vasoconstriction of afferent arterioles lowers GFR, while β-adrenergic stimulation of granular cells increases renin secretion.

Why does clearance of a reabsorbed substance like urea fall below GFR?

Because reabsorption returns part of the filtered load to blood, decreasing the volume of plasma cleared per minute relative to pure filtration.

What is the transport maximum (Tm) concept?

The maximum rate at which a substance can be reabsorbed due to saturation of its carrier proteins; exceeding Tm leads to excretion of the excess.

State two conditions that will increase ADH secretion.

High plasma osmolality (dehydration) or low blood volume/pressure.

Why can’t water leave the ascending limb of Henle?

Its thick segment lacks water-permeable channels, making the limb impermeable to water.

What role does the vasa recta’s countercurrent exchange play?

It removes reabsorbed water from the medulla while preventing loss of medullary solutes, preserving the osmotic gradient.

Describe the positive feedback (countercurrent multiplication) in the loop of Henle.

More NaCl pumped out of ascending limb → interstitium hypertonic → more water leaves descending limb → tubular fluid becomes saltier → ascending limb pumps even more NaCl until gradient maximizes.

How does aldosterone secretion respond to elevated plasma K⁺?

Direct stimulation of the adrenal cortex increases aldosterone release, enhancing K⁺ secretion.

Which renal segment is impermeable to NaCl but permeable to water, making it the ‘last stop’ for urine concentration?

The collecting duct (specifically the medullary portion).

What is obligatory water loss and why is it necessary?

Minimum 400 ml/day urine required to excrete metabolic wastes and maintain homeostasis despite maximal water conservation.

How do organic anion transporters (OATs) contribute to drug clearance?

They actively secrete a wide range of foreign anions from peritubular capillaries into the renal tubules, enhancing elimination of many drugs.

What effect does alkalosis have on bicarbonate handling?

Less H⁺ is secreted, so less filtered bicarbonate is reabsorbed; kidneys may secrete excess bicarbonate to help normalize pH.