Facts
What percentage of sodium, chloride, and water undergo tubular reabsorption?
>99%
What percentage of Na is filtered in the various parts of the kidney?
- Proximal Tubule
- Thick/Thin Loops of Henle
- DCT and collecting duct
- Proximal Tubule = 65%
- Thick/Thin Loops of Henle = 25%
- DCT and collecting duct = 10%
What percentage of K+ is reabsorbed in the various parts of the kidney?
- Proximal Tubule
- Thick Loops of Henle
- Cortical and medullary collecting duct
- Proximal Tubule - 65%
- Thick Loops of Henle -25%
- Cortical and medullary collecting duct - 10-15%
The normal filtrate contains equal parts of anions and cations. What are the two prominent anions?
Chloride and bicarbonate
What percent of water is reabsorbed in the various parts of the kidney?
- Proximal Tubule
- Descending thin loop of Henle
- Ascending loops loop of Henle
- DCT and collecting duct
- Proximal Tubule - 65%
- Descending thin loop of Henle - 10%
- Ascending loops loop of Henle - 0
- DCT and collecting duct - 5-24% depending on hydration status
The osmolarity of the tubular fluid in the proximal tubule is _____ to plasma and interstitial fluid.
H+ ions via the NHE-3 antiporter
The filtered load of sodium is ____ times higher than potassium?
30-40%
A major percentage of Cl reabsorption occurs via ___ in the PT?
Paracellular diffusion
How does the majority of water cross the tubular epithelium?
Via aquaporins or through tight junctions between cells
In the loop of Henle:
The descending limb absorbs ___ but not ____.
The ascending limb absorbs ____ but not ____.
The descending limb reabsorbs water but not sodium chloride. The ascending limb reabsorbs sodium chloride but not water.
In the PCT, sodium is symported with which substances?
Glucose, amino acids, phosphate, and bicarbonate
In what parts of the nephron is the majority of potassium excreted?
distal convoluted tubule cells, principal cells, connecting tubule, and cortical collecting duct
How is Cl actively transported from the lumen to the cell in the late proximal tubule?
Parallel Na-H and Cl-base antiporters
Explain the concept of obligatory water loss.
The sum of the urea, sulfate, phosphate, other waste products, and nonwaste ions excreted each day = ~600mOsm/day. Therefore, the minimum volume of water in which this mass of solute can be dissolved is roughly 600 mmol/1400 mOsm/L (maximal urinary concentration) = 0.43 L/day.
The continuous excretion of organic waste obligates the loss of water
In the distal tubule, resorption of sodium and water occurs in which cells?
What other electrolyte do they interact with to maintain hemostasis?
Principal cells
Potassium
What is the major luminal entry for Na and Cl in the thick ascending limb of the loop of Henle?
Na-K-2Cl symporter
What are the two essential potassium-secreting channels found in the apical membrane of principal cells in the distal nephron?
ROMK (standing for renal outer medulla, because that is where they were first identified) and BK (since each channel has a “big” capacity to secrete potassium, also called maxi-K).
[T or F] Chloride reabsorption is always coupled with sodium?
True. Reabsorption of Cl- is both passive via paracellular diffusion and active (transcellular) and always coupled with sodium (directly or indirectly).
Cl- always goes with Na+ due to law of electroneutrality. Any finite volume of fluid reabsorbed must contain equal amounts of anion and cation equivalent.
Water permeability is controlled in the collecting duct via ____ and how does it work?
ADH - binds to V2 receptors on the basolateral membrane of principal cells. Activates adenylate cyclase --> catalyzes production of cAMP. cAMP induces migration of intracellular vesicles/fusion into the luminal membrane. AQP-2's are inserted into the luminal membrane.
What are the 3 main components that develop the medullary osmotic gradient?
(1) active NaCl transport by the thick ascending limb (2) the parallel arrangement of blood vessels and tubular segments in the medulla, with descending components in close apposition to ascending components
(3) the recycling of urea between the medullary collecting ducts and the deep portions of the loops of Henle
In the DCT what are the major transport pathways for sodium and chloride?
The apical membrane contains the Na-Cl symporter (NCC), which is the target for inhibition by thiazide diuretics. There is also some sodium reabsorption via apical sodium channels (ENaCs).
What factors increase and decrease potassium secretion into the distal nephron?
Increase: High K diet, High Na delivery to principal cells, Aldosterone, High plasma K
Decrease: Angiotensin II, Low K diet
The distal convoluted tubule contains a Na-Cl symporter within the apical membrane. What drugs inhibit this?
Thiazide diuretics
Describe circumventricular organs and their importance.
Tissues surrounding the third and fourth cerebral ventricles that contain fenestrated capillaries and osmoreceptors. These allow rapid interstitial composition changes secondary to plasma composition changes. Stretch-sensitive channels are located within their membranes, changing osmotic signals into electrical signals. These synapse with hypothalamic cells that synthesize ADH.