Renal physiology

Question 1

What are the main zones of the kidney?

Answer 1

Cortex and medulla (which includes medullary pyramids).

Question 2

What is the vertical osmotic gradient?

Answer 2

A gradient in the medulla where osmolarity increases from cortex (~300 mOsm/L) to papillary tip (~1200–1800 mOsm/L).

Question 3

What structures create the vertical osmotic gradient?

Answer 3

Loop of Henle, collecting duct, and surrounding medullary interstitium.

Question 4

What is the key function of the loop of Henle?

Answer 4

Creates a concentration gradient via selective permeability:

Descending limb: Permeable to water
Ascending limb: Impermeable to water, actively pumps Na⁺/Cl⁻

Question 5

What is the role of the thick ascending limb of the loop of Henle?

Answer 5

“Engine room” – actively pumps Na⁺ and Cl⁻ into the interstitium, creating the initial osmotic gradient.

Question 6

Why is water reabsorbed in the descending limb?

Answer 6

Osmotic gradient causes water to leave the tubule (osmosis), concentrating the filtrate.

Question 7

What happens to the filtrate in the ascending limb?

Answer 7

Na⁺/Cl⁻ is pumped out; filtrate becomes progressively more dilute.

Question 8

What enhances the medullary concentration gradient beyond NaCl?

Answer 8

Urea diffuses out of the inner medullary collecting duct, further increasing osmolarity.

Question 9

. What is the maximum osmolarity achieved in the medulla?

Answer 9

~1200–1800 mOsm/L (depending on loop length and urea recycling).

Question 10

What is the role of vasopressin (ADH/AVP)?

Answer 10

Increases water reabsorption in the distal tubule and collecting duct by inserting aquaporin water channels.

Question 11

Where is vasopressin produced and released?

Answer 11

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

Question 12

What stimulates vasopressin release?

Answer 12

• Increased plasma osmolarity
  → - Decreased blood pressure or volume
  → - Low Na⁺ detected by macula densa

Question 13

What molecular mechanism does vasopressin use to act on tubule cells?

Answer 13

Activates receptors → ↑cAMP → insertion of aquaporins into apical membrane → ↑water permeability.

Question 14

Why is controlled water permeability important in the collecting duct?

Answer 14

Enables precise regulation of urine concentration (dilute or concentrated) depending on hydration status.

Question 15

What protein forms water channels in tubule cells?

Answer 15

Aquaporins (especially AQP2 in collecting ducts under ADH control).

Question 16

What triggers vasopressin release?

Answer 16

Increased plasma osmolarity (detected by hypothalamic osmoreceptors) and decreased blood pressure (detected by baroreceptors in the left atrium).

Question 17

What are the two main effects of vasopressin (ADH)?

Answer 17

Increases water reabsorption in the collecting duct by inserting aquaporins.
Causes vasoconstriction to maintain blood pressure (hence the name “vasopressin”).

Question 18

How does the kidney maintain the osmotic gradient in the medulla?

Answer 18

The vasa recta (capillaries) follow a hairpin loop structure, descending and ascending alongside the tubules, preserving the medullary gradient via countercurrent exchange.

Question 19

What part of the nephron is impermeable to water even with vasopressin?

Answer 19

The thick ascending limb of the loop of Henle.

Question 20

What is the maximum urine concentration humans can achieve?

Answer 20

: Approximately 1200 mOsm/L.

Question 21

What happens when vasopressin is repressed?

Answer 21

Aquaporins are not inserted in the collecting duct, making it impermeable to water, leading to the excretion of dilute urine.

Question 22

what is the normal urine production rate?

Answer 22

Around 1 mL/min under isotonic conditions (about 300 mOsm/L).

Question 23

How does the body expel urine?

Answer 23

Urine is transported by ureter peristalsis into the bladder, stored until stretch receptors trigger involuntary (internal sphincter) and voluntary (external sphincter) urination.

Question 24

Why is the regulation of plasma volume crucial?

Answer 24

It’s the only fluid compartment the body can directly regulate, which in turn influences all other body fluid compartments.

Question 25

What are the two key variables the kidney regulates to control body fluid balance?

Answer 25

Water content (affects ECF osmolarity) Sodium concentration (controls ECF volume)

Question 26

What percentage of body fluids can the kidney directly regulate?

Answer 26

About 1/15 (plasma), as the rest is mostly intracellular or interstitial fluid.

Question 27

: What happens to potassium if it's not properly regulated?

Answer 27

Rapid and dangerous consequences—cardiac arrhythmias, neurological dysfunction, and potentially death.

Question 28

Where does most sodium reabsorption occur?

Answer 28

In the proximal tubule and loop of Henle (especially the thick ascending limb).

Question 29

What powers sodium reabsorption in the nephron?

Answer 29

The Na⁺/K⁺ ATPase on the basolateral membrane, pumping sodium out of cells into the interstitium in exchange for potassium.

Question 30

What portion of sodium reabsorption is hormonally regulated?

Answer 30

A small portion in the distal tubule and collecting duct, regulated by hormones like aldosterone.

Question 31

Where is sodium reabsorbed in the nephron and how is it regulated?

Answer 31

Proximal tubule: ~60–65% reabsorbed, unregulated, constant. Distal tubule/collecting duct: Regulated by hormones, mainly aldosterone via RAAS.

Question 32

What triggers the Renin-Angiotensin-Aldosterone System (RAAS)?

Answer 32

Low sodium concentration Low extracellular fluid volume Low blood pressure Macula densa cells detect low sodium in distal tubule and stimulate renin release.

Question 33

What is the sequence of hormone activation in RAAS?

Answer 33

Macula densa cells release renin → Renin converts angiotensinogen (from liver) into angiotensin I → ACE enzyme in lungs converts angiotensin I into angiotensin II (active hormone) → Angiotensin II stimulates adrenal cortex to release aldosterone.

Question 34

What does aldosterone do in the kidney?

Answer 34

Inserts sodium channels into distal tubule & collecting duct luminal membranes. Upregulates Na⁺/K⁺ ATPase pumps on basolateral membrane. Promotes sodium (and chloride passively) reabsorption, increasing blood volume and pressure.

Question 35

How does angiotensin II contribute to blood pressure regulation besides aldosterone release?

Answer 35

Causes vasoconstriction (powerful pressor effect). Stimulates thirst and vasopressin release to retain water.

Question 36

How is potassium reabsorbed or secreted in the nephron?

Answer 36

Proximal tubule: unregulated potassium reabsorption. Distal tubule & collecting duct: potassium secretion regulated by aldosterone. Aldosterone stimulates potassium channels → promotes potassium secretion when plasma potassium is high.

Question 37

What clinical relevance does RAAS have?

Answer 37

Angiotensin II is a potent vasoconstrictor → contributes to hypertension. ACE inhibitors are widely used to lower blood pressure by blocking angiotensin II production.

Question 38

. What is the relationship between potassium and acid-base balance?

Answer 38

Potassium and hydrogen ions exchange in tubular cells. Acidosis can cause hyperkalemia due to proton-potassium exchange.

Question 39

Why is tight regulation of sodium and potassium critical?

Answer 39

Sodium: crucial for fluid balance and blood pressure control. Potassium: essential for nerve and cardiac function; imbalance can cause cardiac arrest.

Question 40

What happens if there's too much potassium in the circulating fluid?

Answer 40

Potassium channels are inserted, allowing potassium to pass down its concentration gradient quickly to regulate potassium levels.

Question 41

How does the body regulate sodium and potassium balance via the kidneys?

Answer 41

Detection of arterial pressure or sodium concentration triggers renin release → activates angiotensin II → stimulates aldosterone → causes sodium reabsorption and potassium secretion.

Question 42

: Why must sodium and potassium levels be regulated together?

Answer 42

Because their relative proportions must be maintained; an imbalance in one affects the other and overall fluid balance.

Question 43

What are the two primary routes for regulating acid-base balance?

Answer 43

1) Excretion of CO₂ through the lungs 2) Excretion of acid through the kidneys

Question 44

What defines an acid and a base in terms of hydrogen ions?

Answer 44

Acids release hydrogen ions (H⁺), increasing their concentration, lowering pH; bases sequester hydrogen ions, raising pH.

Question 45

What is the normal pH range for human blood?

Answer 45

Approximately 7.35 to 7.45; outside this range causes physiological dysfunction.

Question 46

Why is maintaining pH important at a cellular level?

Answer 46

Enzymatic functions, neural and muscular excitations, and cellular structures like the actin cytoskeleton depend on a tightly regulated pH.

Question 47

How do changes in pH affect potassium concentration?

Answer 47

: Potassium and hydrogen ions exchange; pH imbalances can lead to altered potassium levels in extracellular fluid.

Question 48

What is the role of carbonic anhydrase?

Answer 48

t catalyzes the reversible reaction of CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid), facilitating rapid regulation of acid-base balance.

Question 49

What is the function of biological buffers?

Answer 49

To mop up excess hydrogen ions and maintain a stable pH by chemically binding free H⁺ within certain pH ranges.

Question 50

Name some biological buffer systems.

Answer 50

: Carbonic acid-bicarbonate system, intracellular proteins, hemoglobin, and phosphate buffers.

Question 51

How does the carbonic acid-bicarbonate buffer system help regulate pH?

Answer 51

: Bicarbonate combines with H⁺ to form carbonic acid, which decomposes into water and CO₂ that is expelled by the lungs.

Question 52

Why is kidney function essential in acid-base balance?

Answer 52

Kidneys excrete acids that cannot be expelled via the lungs or feces, completing acid removal and maintaining pH.

Question 53

How fast is the blood filtered through the kidneys?

Answer 53

The entire blood volume is filtered approximately every 24 minutes.

Question 54

What is the primary way the kidneys restore pH balance over time?

Answer 54

By regulating proton (H⁺) excretion, bicarbonate reabsorption/excretion, and ammonium (NH₄⁺) excretion primarily in the proximal tubule and collecting duct.

Question 55

How does the proximal tubule primarily manage bicarbonate during acid-base regulation?

Answer 55

It reabsorbs filtered bicarbonate and converts dissolved CO₂ and water to bicarbonate and protons via carbonic anhydrase, then pumps bicarbonate back into circulation and secretes protons.

Question 56

What drives proton secretion and bicarbonate reabsorption in the proximal tubule?

Answer 56

The sodium-potassium ATPase pump creates the gradient needed for proton secretion via Na⁺/H⁺ exchange and bicarbonate reabsorption via sodium-bicarbonate cotransport.

Question 57

What are the two types of intercalated cells in the collecting duct and their main functions?

Answer 57

One type pumps protons (H⁺) out to reduce acidity; the other pumps protons into the blood to raise acidity (sequester H⁺).

Question 58

How does the collecting duct handle potassium in relation to acid-base balance?

Answer 58

Proton-potassium ATPase pumps exchange H⁺ for K⁺; in acidic states, proton secretion is linked with potassium retention, potentially worsening hyperkalemia.

Question 59

What happens in the collecting duct when the blood is too alkaline (low H⁺ concentration)?

Answer 59

Proton pumps move to the basolateral membrane to pump H⁺ back into circulation, and bicarbonate/chloride exchangers move to the apical membrane to excrete bicarbonate into urine.

Question 60

What role does ammonia (NH₃) play in acid-base balance in the kidney?

Answer 60

Ammonia binds free protons in the urine to form ammonium (NH₄⁺), which helps buffer acidic urine and prevent damage to the urinary tract.

Question 61

Why is carbonic anhydrase important in acid-base regulation?

Answer 61

It catalyzes the reversible reaction between CO₂ and water to form carbonic acid, which dissociates into bicarbonate and protons, crucial for acid-base balance.

Question 62

Why is pH regulation so critical for life?

Answer 62

Because even small deviations from the normal blood pH (7.35-7.45) disrupt enzyme function, cell structure (e.g., actin cytoskeleton), and neural/muscular excitation, risking catastrophic cellular failure.

Question 63

How do kidneys complement the lungs in maintaining acid-base balance?

Answer 63

Lungs remove CO₂ rapidly, while kidneys remove non-volatile acids and regulate bicarbonate, adjusting acid/base levels more slowly but completely.