Tubular Reabsorption – Urine Formation (Nursing)

00:01 So following glomerular filtration, we have tubular reabsorption.

00:07 Tubular reabsorption is going to quickly reclaim most of the tubular contents and return them back to the blood.

00:17 This is a selective trans epithelial process where almost all of our organic nutrients are going to be absorbed and water and ion reabsorption will be controlled by way of hormones.

00:33 This includes both active and passive transport of substances through the tubes.

00:42 So substances can follow two routes during tubular reabsorption.

00:48 They can either be reabsorbed in a transcellular route or reabsorbed by way of a paracellular route.

00:56 In the transcellular route, the solutes are going to enter the apical membrane of a tubule cell, travel through the cytosol of that cell and then exit out of the basolateral membrane of the cell.

01:09 From there, The filtrate will then enter the blood through the endothelium of the peritubular capillaries.

01:16 The other route, the paracellular route is going to occur between the tubule cells.

01:22 This route is limited by tight junctions found in different parts of the Nephron but is leaky and the proximal nephron.

01:31 Water, calcium, magnesium, potassium, and even some sodium are able to move in the proximal convoluted tubule by way of the paracellular route.

01:46 So looking at the two different routes, again, the transcellular route involves substances being taken up at the apical membrane being transported through the cytosol to the basolateral membrane, and then from the basolateral membrane being taken into the peritubular capillaries.

02:07 On the other hand, the paracellular route involves substances moving between the cells.

02:14 This is limited by the presence of tight junctions.

02:20 So when we're talking about the transcellular route transport is going to start at the apical membrane of the tubule cells.

02:29 Sodium enters the tubule cell at the apical surface by way of secondary active transport or sometimes by facilitated diffusion through channels.

02:40 The act of pumping of sodium at the other membrane at the basolateral side of the cell results in a strong electrochemical gradient within the tubule cell.

02:53 This results, in a low intracellular sodium levels that then facilitate sodium diffusion into the cell.

03:02 Also potassium leaks out of the cell into the interstitial fluid.

03:07 This leaves a negative charge inside of the cell.

03:11 Which also allows for the pull of sodium inward to the cell.

03:17 On the other side, the basolateral side sodium is going to be our most abundant cation.

03:24 So transport of sodium across this layer is going to require primary active transport.

03:31 In this process, sodium potassium atpase pumps are going to pump the sodium into the interstitial space.

03:40 The sodium is been swept by bulk flow into the peritubular capillaries that surround this area.

03:51 Sodium reabsorption, again, by primary transport is going to provide the means for reabsorbing almost every other substance.

04:00 So again, the sodium reabsorption is really the most important of all the solutes.

04:07 Secondary active transport is going to lead to electrical chemical gradients being created by pumps at the basolateral surface.

04:17 This gives a push that is needed for other solutes to be transported as well.

04:23 Usually organic nutrients are going to be reabsorbed by secondary active transport and co transported with the sodium molecules that are being actively transported.

04:37 Examples of organic nutrients that move in this manner include glucose, amino acids, some ions as well as some vitamins.

04:48 Also, there is some passive reabsorption that occurs as well.

04:53 Water is going to be passively reabsorbed into the blood.

04:58 Movement of sodium and other solutes are going to create an osmotic gradient for water.

05:05 Remember water will follow the salt.

05:08 Water is reabsorbed by osmosis and is aided by proteins found in the membrane known as aquaporins.

05:18 There are two types of water re-absorption in the kidneys.

05:22 We have obligatory water re-absorption.

05:25 And which the aquaporins are present in the proximal convoluted tubule.

05:30 And this is a passive process that is going to occur no matter what.

05:36 Then further along in the nephron we have facultative water re-absorption.

05:43 These are going to be by aquaporins that are inserted or removed from the collecting ducts in the presence of antidiuretic hormone.

05:54 So whereas obligatory is going to happen no matter what facultative is under hormonal control.

06:03 The solute concentration in the filtrate is going to increase as water is reabsorbed.

06:09 So if you think about it, the water in the filtrate is moving out leaving more salt in the filtrate.

06:16 So the filtrate is becoming more concentrated.

06:20 This creates concentration gradients for solute, which will then drive their entry into tubular cells, and peritubular capillaries.

06:31 Fat soluble substances, some ions, and urea will also follow water into the peritubular capillaries down their concentration gradients.

06:42 And it is for this reason that lipid soluble drugs and environmental pollutants are often reabsorbed even though they are not desirable and we would rather get rid of them.