How is water transported?
Through transcellular and paracellular routes. These assist water in moving from the lumen to the interstitial space.
How many aquaportin channels transport water?
Where is aquaporin 1 found?
In most regions of the nephron, including renal tubules.
Where are aquaporin-2 channels located?
The apical membranes of collecting tubule cells. They are controlled by ADH.
Where are aquaporin-3 channels found?
In the basolateral membranes of collecting tubule cells.
What are the roles of ATPase enzymes in the nephron cell memranes?
They establish ionic gradients. This drives reabsorption or secretion of many other solutes.
What is the ENaC channel?
A channel found in the apical membrane. IT is closed by amiloride and opened by a number of hormones.
What are the 3 types of channel found in the apical membrane?
ENaC, CFTR (chloride) and K+ channels.
What type of transport occurs through channels or uniporters?
Facilitated transport. This includes the transport of glucose.
What are the primary active transports in a nephron membrane?
Na/K, H/ATPase, H/K ATPase, Ca ATPase
The reabsorption of glucose or amino acids by rental tubules is an example of what type of transport?
secondary active transport. SGLT2 is the primary transporter of glucose in the early proximal tubule.
What two substances are actively secreted into the renal tubules?
creatinine and para=aminohippuric acid.
That is the transport maximum?
Limit to the rate at which the solute can be transported. It is due to saturation of a specific transport system.
What is the maximum rate at which glucose can be filtered out?
What is the rate of maximum transport for glucose?
When glucose concentration reaches a maximum of 200, what occurs?
Extra glucose is excreted.
Why do some passively reaabsorbed substances not have a transport maximum?
The rate of diffusion may be determined by the electrochemical gradient of the substance. The permeability of the membrane for the substance may vary. Time that the fluid containing the substance remains within the tubule.
what is the rate of transport dependent on?
The electrochemical gradient and the time the subusance is in the tubule. This is characteristic of some passively absorbed substances.
What is solvent drag?
The osmotic movement of water that also carries some solutes.
Generally, Na+ reasorption leads to ___ reabsorption.
As water and sodium are reabsorbed, what occurs in the lumina?
The lumen negative potential increases, as well as the chlorine and urea concentrations.
What does the proximal tubule reabsorb?
All filtered glucose and amino acids. It also reabsorbs 65% of filtered sodium, chloride, bicarbonate and potassium.
What does the proximal tubule secrete?
Protons, organic acids and bases
What does the proximal tubule secrete?
organic acids, bases and hydrogen ions into the tubular lumen.
How is the reabsorption of sodium in the first and second halves of the proximal tubule different?
In the first half, reabsorption is via co-transport along with glucose, amino acids and other solutes. In the second half, reabsorption is mainly with chloride ions.
In the proximal tubule, what is the significance of hydrogen and bicarbonate ions?
Hydrogen ions in the lumen combine with bicarbonate to form carbonic acid. This is then split into carbon dioxide and water.
In the proximal tubule, what is the significance of arbon dioxde and water?
They combine to form carbonic acid, which dissociates into bicarbonate and H+. Bicarbonate diffuses out of the cell into the interstitial space, and H+ is removed via antiport and H/ATPase.
What is the thin descending segment of the loop of henle permeable to?
Water (high) and urea & sodium (moderate).
What is the thin descending segment of the loop of henlse permeable to?
water (high) and urea & sodium (moderate)
In the ascending loop of Henle, the positive potential in the lumen causes what?
The diffusion of cations through the paracellular pathway.
In the ascending loop of Henle, what maintains a negative electrical potential in the cell?
An ATP pump.
What is the thin ascending segment of the loop of henle impermeable to?
The Na/K ATPase pump in the basolateral membranes does what?
Drives reabsorption of K+ into cell against its concentration gradient.
At the thick ascending segment of the loop of Henle, there is a slight leak of what ion back into the lumen?
What is the thick ascending segment of the Loop of Henle impermeable to?
Water; it is also the site of action of powerful diuretics.
What are 3 diuretics that function at the thick ascending segment of the Loop of Henle?
Furosemide, ethacrynic acid and bumetanide.
What is the first portion of the distal tubule?
A region that forms macula densa.
What is the other portionof the distla tubule?
A highly convoluted region that has characteristics similar to the thick ascending segment of the loop of Henle.
What is the distal tubule permeable to?
most ions (Na, Cl, K)
What is the distal tubule impermeable to?
water and urea.
What is the function of the late distal tubule/cortical collecting tubule?
to reabsorb Na and water from tubular lumen. It also secretes K+ into the tubular lumen and is the site of K+ sparing diuretics.
What are the 4 diuretics used at the late distal tubule?
spironolactone, epelerenone, amiloride, triameterene.
What is reabsorbed and secreted at the late distal tubule?
sodium is reabsorbed and potassium is secreted.
What do the intercalated cells at the late distal tubule/cortical collecting tubule do?
reabsorb K+ from the lumen and secrete H+ into the lumen.
Where is aldosterone produced?
The adrenal cortex.
What is the function of aldosterone?
It increases sodium reabsorption and stimulates potassium secretion.
Where does aldosterone act?
On the principal cells of cortical collecting ducts.
What causes aldosterone to be secreted?
Increased extracellular potassium and increased levels of angiotensin II.
What occurs in the absence of aldosterone?
Marked loss of sodium and accumulation of potassium.
What is the function of angiotensin II?
Increased sodium and water reabsorption. It helps to return blood pressure and extracellular volume toward normal.
What does angiotensin II stimulate the secretion of?
aldosterone, which constricts efferent arterioles and stimulates sodium reabsorption in proximal tubules.
The release of angiotensin II directly stimulates absorption of what ion in the proximal tubules, loops of Henle, distal tubules and collecting tubules?
Where is ADH produced, and what is its function?
Posterior pituitary; it increases water reabsorption.
Where does ADH bind?
To V2 receptors in late distal tubules, collecting tubules and collecting ducts. It increases the formation of cAMP.
What is the source and function of atrial natriuretic peptide?
Cardiac atrial cells; it inhibits the reabsorption of sodium and water.
What is the source and function of parathyroid hormone?
Parathyroid glands; it increases calcium reabsorption.
What is the renal clearance of a substance?
The volume of plasma that is completely cleared of the substance by the kidneys per unit time.
What is the formula for renal clearance?
Cs x Ps = Us x V; where V = urine flow.
What is inulin?
A polysaccharide not produce in the body. It has a GFR of 125 mL/min, which means that 125 mL of plasma flowing through the kidneys must be filtered to deliver the inulin that appears in the urine.
For a substance to be reabsorbed, where must it be transported?
Across the tubular epithelial membrane in to the renal interstitial fluid, and then through the peritubular capillary membrane into the blood.