9. Cellular Mechanisms of Reabsorption

Question 1

Describe the transcellular and paracellular pathways of reabsorption.

Answer 1

• Transcellular = (Through the cell) E.g. sodium transport in the proximal convoluted tubule: Sodium enters cell through membrane proteins, moving down its electrochemical gradient. Sodium is then pumped out the basolateral side (Between ICF and ISF) by the sodium potassium pump against its concentration gradient. - Paracellular = (between the cells) E.g. the transport of calcium, magnesium and potassium between cells VIA the lateral intercellular space by solvent drag.

Question 2

Describe first phase Na+ (NaCl) transport in the PCT.

Answer 2

Sodium moves down its concentration gradient via sodium channels from the lumen of the proximal convoluted tubule (high concentration gradient) to within the proximal convoluted tubule cell (low concentration gradient) via secondary active transport. This concentration gradient is maintained by the use of ATP by the basolateral sodium potassium pump. This strong desire for sodium to move into the cell via the apical membrane is employed to cotransport various substances into the cell through the use of symporters. Two examples of the substances are glucose and amino acids - all of which are completely reabsorbed (provided they don’t exceed the Tm of their carriers) by the time they are half way through the PCT

Question 3

Describe second phase Na+ (NaCl) transport in the PCT.

Answer 3

Second phase is all about Na and Cl moving together. There are 2 principle ways that sodium can move into the cell: transcellular or via the paracellular route. Paracellular: The movement of large amounts of Na into the ECF due to the activity of Na-K-ATPase induces an electrical gradient that convinces Cl to enter the LIS. The movement of Cl into the LIS then makes a negatively charged area that entices Na to follow. Basically it’s a cycle of Na causing Cl to follow, which then gets negative, and causes Na to follow the Cl. Transcellular: This is a complex phase covered ad nauseam in integrative physiology whereby there is a Na-H antiporter and a Cl-base antiporter that pushes an acid and a base into the lumen of the proximal convoluted tubule against their concentration gradient by tapping into the energy stored in the sodium gradient. Once inside the lumen, the acid and base recombine and move into the cell down its concentration gradient

Question 4

Describe water reabsorption in the PCT.

Answer 4

• Always passive via osmotic pressure gradients - Water follows sodium, in effect. Reabsorption of 67% of the filtered load of sodium translates to 67% of water reabsorbed in the first half of the PCT

Question 5

Using the terms “renal threshold” and “tubular maximum”, describe glucose reabsorption in the PCT?

Answer 5

TLDR: When all glucose carriers are working at maximum rate they are said to be saturated. The tubular maximum is the value at which all carriers are working at said rate. This is also termed the renal threshold for that substance, for it is the point at which any further increase in the filtered load of that substance/min will result in all additions beyond the Tm being lost to the urine and excreted. Many ‘goodies’ in the filtrate are non-specifically reabsorbed (e.g. glucose or amino acids), wherein these substances are completely reabsorbed irrespective of their concentration within the body. For glucose, the Tm (or transport maximum) is 375 mg/min, meaning that plasma glucose concentrations can rise up to 3x their normal levels before glucose will be lost to the filtrate. Up until 375 mg/min, 100% of filtered glucose is reabsorbed by the PCT and none is lost to the filtrate. However, once the filtered load exceeds 375 mg/min, all subsequent glucose that is filtered will be lost to the urine and excreted. This is because glucose’s renal threshold has been reached, after this point, the filtered load of glucose - 375 mg/min = the excreted amount of glucose. This is because glucose is transported from the lumen back into the proximal tubular cells via facilitated diffusion, in which the energy stored in the desire for sodium to enter the cell is used to move glucose against its concentration gradient into the cell. However, once all of these carriers are ‘saturated’ and working to their maximum capacity, then no more glucose can be ferried across.

Question 6

Draw a graph to show glucose reabsorption dynamics in the kidney.

Answer 6

Renal threshold = plasma concentration (mg/100ml), whilst tubular maximum is mg/min.

Question 7

Describe fat absorption in the small intestine?

Answer 7

Short chains (less than 10-12 carbons): diffuse directly into mucosal cells (because they lipophilic as fucc). The short chains are then able to leave the mucosal cell into the ECF then the blood via simple diffusion from the basal/basolateral? surface.

Long chains: emulsified by bile salts and lipases break them down into free fatty acids and glycerol. They are then transfered into mucosal cells via simple diffusion within micelles. The way this shit gets out is as follows: Free Fatty Acids + Glycerol give up triglycerides, which will then be surrounded by endoplasmic reticulum to give chylomicron vessicles, which will then move via exocytosis to the ECF and ultimately the lacteals of the villi.