Human A&PBIO 304 · American River College ← Week 08 pre-work

BIO 304 · Week 08 · Interactive Workbook

Tubular Function and Urine Concentration

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Part 1 of 4 · Recall

Fill in the blanks

Type the term that completes each statement, using the word bank. Pull it from memory first.

Word bank

Proximal convoluted tubule (PCT)Descending limb of HenleANPADH (antidiuretic hormone)Thick ascending limbLate DCT and collecting ductAldosteroneEarly DCT

  1. Bulk reabsorption: about 65% of filtered Na+, water, glucose, amino acids reabsorbed here. · SGLT2 cotransporter reabsorbs glucose with sodium. (SGLT2 inhibitors are diabetes drugs.) · Brush border maximizes surface area.
  2. Permeable to water, not solute. Water exits into the increasingly hypertonic medulla. Filtrate becomes more concentrated as it descends.
  3. Permeable to solute, not water. NKCC2 cotransporter pumps Na+, K+, and 2 Cl- out into the interstitium. Filtrate becomes more dilute. Builds the medullary gradient. · This is where loop diuretics (furosemide) act.
  4. NCC (Na-Cl cotransporter) reabsorbs sodium and chloride. Thiazide diuretics block this.
  5. Principal cells: ENaC channels for Na+ reabsorption (under aldosterone control); aquaporin-2 channels for water (under ADH control). · Intercalated cells: handle acid-base by secreting H+ or HCO3-.
  6. Released from posterior pituitary when plasma osmolarity rises. · Inserts aquaporin-2 into collecting duct cells. Water reabsorbed, urine concentrated.
  7. Released from adrenal cortex (zona glomerulosa) when blood pressure or Na+ drops. · Acts on principal cells: Na+ in, K+ out, water follows Na+. Blood volume and pressure rise.
  8. Released from atrial myocytes when stretched. Opposes aldosterone: promotes Na+ excretion and lowers blood pressure.

Define it: high-yield vocabulary

Write a clear definition in your own words for each term.

  1. Proximal convoluted tubule (PCT)
  2. Descending limb of Henle
  3. Thick ascending limb
  4. Distal convoluted tubule (DCT)
  5. Collecting duct
  6. ADH (antidiuretic hormone)
  7. Aldosterone
  8. ANP (atrial natriuretic peptide)

Part 2 of 4 · Anatomy lab

Draw and label

Box A. Nephron with segment functions

Directions

  1. Draw a nephron in the same orientation as Day 29: glomerulus, PCT (in cortex), loop of Henle (descending and ascending limbs going into medulla), DCT (back in cortex), collecting duct (going down through medulla to papilla).
  2. Beside each segment, write what is REABSORBED (into blood) and what is SECRETED (into filtrate).
  3. PCT: ~65 percent of water and Na+, all glucose and amino acids (via co-transport), bicarbonate (HCO3-). Reclaims most filtered nutrients.
  4. Loop of Henle, descending limb: water reabsorbed (permeable to water, not solute). Loop of Henle, ascending thick limb: Na+ and Cl- reabsorbed (not permeable to water). This creates the medullary gradient.
  5. DCT: fine-tuning. Na+ reabsorbed (aldosterone-regulated), Ca2+ reabsorbed (PTH-regulated), K+ secreted.
  6. Collecting duct: water reabsorption controlled by ADH (vasopressin). Urea reabsorbed in deep medulla.
  7. Label each segment and its key activity.
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Box B. Medullary osmotic gradient

Directions

  1. Draw the kidney section showing cortex at top and deep medulla at bottom.
  2. Mark interstitial osmolarity at different depths: 300 mOsm/L at cortex (same as plasma), rising to about 1200 mOsm/L deep in the medulla.
  3. Show how the loop of Henle creates this gradient (countercurrent multiplier).
  4. Now show two scenarios for the collecting duct passing through this gradient:
  5. ADH ABSENT (diabetes insipidus or overhydration): collecting duct is impermeable to water; urine stays dilute (about 50 to 100 mOsm/L); large urine volume.
  6. ADH PRESENT (dehydration or normal): aquaporins inserted in collecting duct; water leaves to enter the concentrated medullary interstitium; urine becomes concentrated (up to 1200 mOsm/L); small urine volume.
  7. Note: the medullary gradient is what makes concentrated urine POSSIBLE; ADH controls whether the body USES it.
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Structures to label

Label each on your drawing.

  1. Proximal convoluted tubule (PCT)
  2. Loop of Henle
  3. Descending limb
  4. Ascending thick limb
  5. Distal convoluted tubule (DCT)
  6. Collecting duct
  7. Reabsorption
  8. Secretion
  9. Aldosterone
  10. ADH (vasopressin)
  11. Parathyroid hormone (PTH)
  12. Aquaporin (water channel)
  13. Medullary osmotic gradient
  14. Countercurrent multiplier

Part 3 of 4 · Physiology lab

Reason it through

A. Trace one molecule of filtered glucose

Explain the main structure-function relationship for this topic.

B. Synthesis

1. A patient with diabetes mellitus has blood glucose of 350 mg/dL (well above the renal threshold of ~200). Predict the urine composition and explain why these patients experience polyuria (excessive urine) and polydipsia (excessive thirst).
2. A patient takes a loop diuretic (e.g., furosemide), which blocks Na+/K+/2Cl- reabsorption in the thick ascending limb. Predict the immediate effects on (a) the medullary gradient, (b) urine volume, (c) serum potassium, and (d) blood pressure. Why is this drug used to treat heart failure?
3. SIADH (syndrome of inappropriate ADH) causes ADH release even when blood is dilute. Predict the urine osmolarity, blood sodium, and the patient's symptoms. Why is excess water retention more dangerous than excess water loss in terms of brain function?

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