S3: reabsorption along the nephron Flashcards
Describe sodium absorption in the PCT
Basolateral 3Na/2K/ATPase Apical: -Na/H exchange -cotransport with glucose -cotransport with AA/carboxylic acids -cotransport with phosphate Aquaporin 1
Summarise reabsorption in the PCT
ISOSMOTIC
PCT responsible for ‘bulk’ reabsorption of many solutes
Very metabolically active, high concentration of mitochondria
Provides energy for Na/K/ATPase
Describe the reabsorption of glucose in the kidney
Virtually all filtered glucose is reabsorbed in the proximal convoluted tubule
Reabsorption by secondary active transport, driven by energy released from transport of Na+ down its concentration gradient
Limited number of Na+/glucose carriers, so glucose reabsorption is limited
-plasma glucose > 10mmol/L glycosuria will develop
-common in pregnancy as the Tm for glucose falls & glucose is excreted in the urine
Describe reabsorption of amino acids in the kidney
Filter easily through the glomerulus
Reabsorbed in the PCT by secondary active transport
At least 5 different transport systems coupled with AA reabsorption
-this is a Tm limited process
Describe sodium ion reabsorption in the loop of Henle
Reabsorbs 20% of the filtered Na+ and 15% of tubular water
Reabsorption of NaCl in the thick ascending limb produces a hypertonic interstitial fluid in the surrounding medulla -> creates a conc. gradient and water move passively out of the thin descending limb
Tubular fluid becomes hypotonic due to ion reabsorption
Describe the function of the thin descending limb
Lined with thin, flat cells
Permeable to water, Na+ and Cl-
Water is passively reabsorbed down a concentration gradient caused by the hypertonic interstitium of the medulla
Describe the function of the thin ascending limb
Similar structure to the thin descending limb BUT
- it is impermeable to water
- minimal NaCl transport occurring within the cells
Describe the function of the thick ascending limb
Consists of large cells with mitochondria which generate energy for the active transport of Na+ and Cl- ions from the tubular fluid into the interstitium
Filtrate becomes progressively diluted – this part of the tubule is impermeable to water
Describe sodium ion uptake by the early and late distal tubule
Water permeability of early DCT is low
Active Na+ reabsorption results in further tubular dilution (stimulated by aldosterone (RAAS))
Early part of CD and latter part of the DCT share many similarities
Water permeability is variable depending on ADH
Describe central diabetes insipidus
Impaired ADH synthesis or secretion by the hypothalamus
Causes: brain injury (fracture of the base of the skull), a tumour, sarcoidosis/tuberculosis, aneurysm, encephalitis/meningitis
Treatment: administering ADH (desmopressin)
Water is inadequately reabsorbed from the collecting ducts so a large quantity of urine is produced
Describe nephrogenic diabetes insipidus
Acquired insensitivity of the kidney to ADH
Water is inadequately reabsorbed -> large quantity of urine is produced
Difficult to manage clinically but a low salt, low protein diet reduces urine output
Describe a condition where too much ADH is produced
SIADH – syndrome of inappropriate antidiuretic hormone secretion
Characterised by excessive release of ADH from the PP gland/another source
Signs: dilutional hyponatremia in which the plasma Na+ concentration is lowered, total body fluid is increased
Describe the role of the loop of Henle in countercurrent multiplication
Acts as a countercurrent multiplier – produces a hypertonic medulla by pooling NaCl in the interstitium -> favours the subsequent movement of water out of the collecting ducts
Thick ascending limb can maintain a difference of 200mOsm/kg H20 between the tubular fluid & the interstitium at any point along its length
Normal plasma osmolality = 300mOsm/kg H20 (isotonic)
Maximum osmolality of interstitium is 1400mOsm/kg H20 (hypertonic)
Fluid leaving LoH is hypotonic (100mOsm/kg H20)
Describe the role of the vasa recta in countercurrent multiplication
Hairpin arrangement – permeable to solutes and water
Descend – absorb solutes such as Na+, Cl- & urea, water lost
Ascend – reabsorb water & loss of solutes
Slow flow enables vasa recta to equilibrate at each stratification level – minimises washout
Absorbs water released from CD in presence of ADH – maintains high osmolality of interstitium
Describe the process of urea recycling
Helps to maintain the medullary hypertonicity
Tubular conc. of urea increases as it diffuses down concentration gradient from medulla into lumen
Ascending limb and early DCT impermeable to urea
ADH causes increase of UT1 (urea transporter) on apical surface of medullary collecting tubules -> enables urea to flow down concentration gradient
Urea secreted back into LoH & goes back up to DCT/CD and is recycled
