Formation of urine 2 Flashcards
Descending limb of the Loop of Henle
This part is very permeable for water- allows extraction of water into blood.
Features:
- Thin walls
- No proteins to actively transport ions
- Walls packed with aquaporins.
- Some tight junctions for water movement.
Ascending limb of the Loop of Henle
This part is very impermeable to water and permeable to Na+ and Cl-.
- Water can be secreted but not absorbed.
Features:
- Thick walls
- NKCC2 co-transporters to absorb Na+, K+ and Cl-
Solute transport in the ascending loop of Henle
The apical membrane of the tubular cells contain NKCC2 co-transporter.
- Na+, K+ and 2Cl- transporter into cells.
Na+ and Cl- secreted into the medulla draws water out in the ascending limb into the blood.
Na+ and K+ repels Ca2+ and Mg2+ due to the positive charge.
Inhibition of NKCC2 co-transporter
Inhibited by furosemide and other loop diuretics.
Prevents Na+ and Cl+ absorption so water is not drawn out.
The keeps water, Na+ and Cl- in tubules- promoting natriuresis.
Osmolarity changes in the Loop of Henle
Fluid entering the loop of Henle is isotonic.
The deeper down the descending limb, the more hypertonic the tubular fluid becomes.
- Due to the absorption of water and retention of Na+ and Cl-.
Moving through the ascending limb, the tubular fluid becomes more hypotonic:
- Na+ and Cl- actively reabsorbed.
- Water retained
Countercurrent multiplication
Large osmotic gradient is created within medulla which creates deep concentration gradients for filtrates.
Caused by NKCC co-transporter in ascending limb of the Loop of Henle.
Urea also diffuses from the collecting duct to make the medulla more hypertonic at the loop of Henle- draws out more water
Main processes in the Distal tubule
Na+ and Cl- actively reabsorbed.
K+ and H+ secreted into tubular fluid in exchange for other ions.
Na+ and Cl- reabsorption in DT
Actively reabsorbed by exchanging with K+.
- Na+ and Cl- co-transported into cell
- Cl- leaves cell via channel basolaterally.
- Na+ leaves cell via sodium-potassium pump
Na+ exchange occurs in the late DT and early CD at principal cells.
Principal cells
Cell of the DT that is sensitive to aldosterone.
K+ is secreted in exchange for Na+ at the basolateral membrane.
- K+ secreted into lumen via channel
- Na+ enters cell via channel
- Exchanged at basolateral membrane
RAAS activated Na+ reabsorption at DT
Macula densa senses low Na+ at DT.
- Signals to juxtaglomerular cells to release renin.
Renin causes eventually secreted of angiotensin II= increased aldosterone.
Aldosterone increases Na+ reabsorption/ K+ secretion at DT.
- This inhibits juxtaglomerular cells
- Increases water absorption and blood pressure
Intercalated cells
- Location
- Different types
Cells located in the DT and early collecting cells
- Acid/base regulation
Exchanges Na+ for H+.
Alpha cells:
- H+ secreted in exchange for K+ or Na+.
- H+ secreted via H+ ATPase
- Reabsorbs HCO3-
Beta cells:
- Secrete HCO3- via pendrin
- Reabsorbs H+
Alpha intercalated cells
Cells located in early CD and DT.
Secretes H+ via:
- Exchange with K+
- Exchange with Na+
- H+ ATPase
Reabsorbs:
HCO3-
Beta intercalated cells
Cells located in early CD and DT.
Secretes:
-Bicarbonate through pendrin (exchanged with Cl-)
Absorbs:
H+
ADH - Secretion location - Half life - Metabolism -
Anti-diuretic hormone / Vasopressin.
- Regulates water balance
Secreted from posterior pituitary gland after stimulation from the hypothalamus.
Short half life- 10-15mins
Metabolised in the liver and kidneys
ADH mechanism
Acts on vasopressin 2 receptors (V2)
- Receptors are on basal membrane of DT principal cells.
Activation of V2 activates Aquaporin channels on apical membrane.
- Uses G-protein cell signaling to increase intercellular cAMP.
Allows water to move into blood
