Lecture 3.pdfdrhsdysehsryryrdyryryryrdyr
- 2. Loop of Henle • Cells of the loop of Henle and the distal regions of the nephron are thinner they are no less important in reabsorption and secretion.
- 3. • The descending limb of the loop is permeable to water but not to solutes, thus when the tubular fluid reaches the bend in the loop it has a higher osmolality than when it left the PCT.
- 4. • The ascending limb is thicker than the descending limb, reflecting its role in further reabsorption of solutes, for example sodium chloride, potassium, calcium and magnesium.
- 5. The loop of Henle functions basically in regulating the salt (electrolyte) concentration in urine. Let us see the anatomy of the loop first – it consists of three parts – the descending thin segment, the ascending thin segment and the thick ascending segment.
- 6. In the thick ascending loop Na+ and Cl- reabsorption is accomplished by a Na+, K+, Cl- symporter. The thick ascending limb has a high reabsorptive capacity and is responsible for reabsorbing 25% of the filtered load of Na+.
- 7. • The descending part of the thin segment is highly permeable to water and moderately permeable to most solutes. About 20% of the filtered water is reabsorbed in the descending limb, because, the ascending limb (both thin and thick parts) are impermeable to water – a characteristic which is important for concentrating urine. • Because the thick segment of the ascending limb is virtually impermeable to water, most of the water delivered to this segment remains in the tubule, diluting the urine, as most of the solutes are reabsorbed. This feature is important in allowing the kidneys to dilute or concentrate urine under different conditions.
- 9. • And when there is a water deficit in the body, the kidney forms a concentrated urine by continuing excrete solutes while increasing water reabsorption and decreasing the volume of urine formed.
- 11. • So, what ultimately happens is that, of the urine filtered through the nephron, most of it is reabsorbed (water as well as solutes) by the proximal tubule and the loop of Henle. • When urine reaches the distal tubule, urine is essentially diluted. • The final concentration of the urine formed depends on the condition of the body, and the distal tubules, accordingly dilute or concentrate urine.
- 13. Distal tubule and collecting duct
- 15. The distal tubule and collecting duct • The volume of fluid entering the distal tubule is only about 90% of the original filtered load and is approximately isotonic with respect to plasma. • It is the distal nephron that controls the fine tuning of the chemical composition of the blood by reabsorbing, for example, more or less sodium (under the influence of aldosterone), water (controlled by ADH, in the collecting duct), calcium (via parathyroid hormone, PTH).
- 16. • The DCT is also very important in achieving subtle pH regulation. • A number of substances, for example some drugs and urate (uric acid) are secreted directly into the lumen of the distal tubule
- 18. “Osmolality” refers to the concentration of dissolved particles of chemicals and minerals -- such as sodium and other electrolytes -- in your serum. Higher osmolality means more particles in your serum. Lower osmolality means they're more diluted.
- 19. Sodium reabsorption Much less than 10% of the filtered load of NaCl reaches the distal nephron. Regulation of Na uptake, occurring mainly in the principal cells of the cortical collecting tubule, is controlled by the steroid hormone aldosterone. The net effect of aldosterone is the reclamation of NaCl and potassium excretion in to the luminal fluid.
- 20. • Like all steroids, aldosterone enters the target cell and combines with cytosolic mineralocorticoid receptor. • Such receptors are not entirely specific for aldosterone and will also bind cortisol, the principal glucocorticoid hormone. • The receptors are ‘protected’ from cortisol activation by 11 β hydroxysteroid dehydrogenase which converts cortisol into cortisone, a steroid which does not engage the aldosterone receptor and so does not influence sodium and potassium transport.
- 23. Two cell types in the cortical collecting duct. A, The principal cells mediate sodium (Na+) reabsorption energized by the basolateral Na+, K+ pump. Entry from the lumen is through the epithelial Na+ channel (ENaC), which renders the lumen negatively charged (–mv). This transepithelial voltage stimulates secretion of potassium (K+) through K+ channels. Reabsorbable anions such as chloride (Cl–) lessen the luminal negativity and decrease K+ secretion. Bicarbonate (HCO3 –) has an effect to increase K+ secretion. High flow rates increase net K+ secretion by preventing development of high lumen K+ concentrations. The effects of the renin-angiotensin-aldosterone axis are shown: increased mineralocorticoid receptor (MR) activation increases ENaC, the Na+, K+ pump, and K+ channels, thereby increasing Na+ reabsorption and K+ secretion. Cortisol would also increase MR activity, but it is inactivated by the enzyme, 11β-hydroxysteroid dehydrogenase (11β- HSD). AI = angiotensin I; AII = angiotensin II; ACE = angiotensin-converting enzyme; mv, millivolts.
- 26. B, Intercalated cells are carbonic anhydrase–rich cells that secrete acid and reabsorb HCO3 –. The H+ ATPase secretes H+ in a way favored by the negatively charged lumen in conjunction with the aldosterone-stimulated effect on Na+ reabsorption in neighboring principal cells. The K+, H+-ATPase is an electroneutral pump. The K+/H+ exchanger, reabsorbing K+, is important in states of K+ depletion when urinary K+ is decreased. ADP = adenosine diphosphate; ATP = adenosine triphosphate."