Reabsorption in Kidney

The kidney is the organ that helps to eliminate harmful substances from the body. The process that helps to eliminate those substances is known as the excretion process. So, the kidney is an excretory organ. This organ helps to balance the substances in the body. The human body also has a kidney. In the human body, there are two kidneys are present. They are situated in the posterior wall of the abdomen cavity. They are situated on both sides of the vertebral column. These bean-shaped organs play an important role in the human body.

If the kidneys are intersects in two parts, there two major divisions can be witnessed. They are the Cortex & Medulla. The cortex is the region outside of the kidney. They are the outer portion of the kidney. And the inner part is known as the Medulla. There are some pyramids-like structures are present. They are known as the Medulla Pyramid. The kidney has a functional unit. This is known as the Nephron. There are millions of nephrons are present in the Kidney. Whatever function is being executed by the kidney; the whole functions are executed by the Nephrons.

What is Reabsorption?

Reabsorption is a process in the kidney. Also, if more specific, then it is the process of the Nephrons. As the process of the kidney is the combination process of the Nephrons. As Nephrons are the functional unit of the kidney, so it is being used to excrete unessential elements from the body. And Reabsorption is one type of subprocess. There are mainly three sub-processes in the urine formation process. Reabsorption is one type of subprocess. Other than the Reabsorption, there is a Filtration & Secretion process present.

The word “Reabsorption” means to again absorb all the elements. After the filtration process, this process starts in the kidney or the Nephrons. In the filtration process, polluted blood comes to the Glomerulus via the Afferent Blood Vessels. There all the elements from the blood come into the renal corpuscles region. Only the elements of the blood remain there. So, the blood is now pure. And it goes out by the Efferent Blood Vessels.

Now, all the substances that come into the Renal Corpuscles are not harmful. Among them, there are a large number of substances that are very useful for the body. The filtration process can’t pick up only the harmful elements of the body. So, all the elements come to the Nephrons. Now, Nephrons return those helpful elements to the body by absorbing those elements. This process is known as Reabsorption. It is seen that 180 Liter/Day of substances are comes to the Nephrons. But among them, only 1.8 Liter/Day substances are selected as harmful. So they are only removing from the body. So, the process that helps to return the essential elements from the Nephrons to the body is known as Reabsorption.

Process of Reabsorption in Kidney

Mainly the Reabsorption process is executed in the tube of the Nephrons. This means this process can be highly visualized from the Proximal Convoluted Tubule to the Distal Convoluted Tubule. So, Proximal Convoluted Tubule, Henley’s Loop & Distal Convoluted Tubule are the area where the Reabsorption takes place. Also, there is one more important part is the blood vessels. The Efferent Blood Vessels are situated very near to these tubes. There is a capillary network near this tube. This capillary structure is made up in such a way that the tubes & the layer of the blood vessels are close to each other. So, the substances can easily diffuse by the layer of the blood vessels. Hence, they can able to return to the body using the blood vessels.

• Reabsorption in the Proximal Convoluted Tubule (PCT): Reabsorption mainly takes part in the PCT region. Here, 65% of the elements are reabsorbed & returned to the body. Here, glucose, and amino acids are reabsorbed. Along with that, some ions are also reabsorbed here. Ions are the most important for the easy functioning of the body. Mainly, Na⁺, K⁺, Cl^–, Mg²⁺, etc, ions are reabsorbed from the PCT & return to the body. The main importance of PCT is that it can able to reabsorb Glucose. Glucose is the only element that can be absorbed by the PCT. 98% glucose is reabsorbed from here. Other tube-like structures don’t have this type of ability. And glucose is the main energy provider to the body. So, from this context, PCT is very important.
• Reabsorption in the Henley’s Loop: In Henley’s Loop, some more Reabsorption takes place. It is the region where mainly water is reabsorbed. Here mainly 15% of the substances are reabsorbed. Among them, the large amount is water. Along with the water, it can reabsorb other elements also. Nearly, 25% of Sodium, Calcium, Potassium, and 55% of Magnesium are reabsorbed from here. Water is reabsorbed to concentrate the urine formation. Water is also an essential element in the body. Removing excess water from the body will reduce the body’s energy. This might create dehydration. So, during dehydration, the water is reabsorbed in high amounts from this region. So, the urine gets concentrated.
• Reabsorption in the Distal Convoluted Tubule (DCT): It is the region where there is little presence of the Reabsorption process. Mainly the DCT is known for another process Secretion. But, this place is known for the absorption of different ions. Like, Na⁺, K⁺, and HCO₃^– are reabsorbed from this region. Nearly, 10% of the remaining substances are reabsorbed from here. After the concentration of the urine, there is no chance to more reabsorb from that concentrated element. It is now becoming ready to remove from the kidney. That is the reason, the here Reabsorption process is very less in percentage.
• Reabsorption in the Collecting Duct: The collecting Duct doesn’t also perform in the Reabsorption process. In a very less amount, this area can reabsorb the ions from the substances. Otherwise, there is no Reabsorption function in the collecting duct. Some ions like Na⁺, K⁺, Cl^–, and HCO₃^– are reabsorbed here. Nearly, 4% of the remaining substances are reabsorbed from here.

Renal Transporters

Most of the Reabsorption process takes place with the help of diffusion. As the epithelial cells of the renal tube are closely placed in the blood vessels. So, there is no need to use any more methods to reabsorb. Water follows this methodology. When there is a concentration gradient in the water level. Water will flow from the renal tube to the blood vessels using the diffusion process. There are Tight junction channels are present in the epithelial cells. This creates some pores in the layer of the renal tube. These tiny pores are used to diffuse. But for the ions, diffusion is not the process to reabsorb. For that there are some transporters are present.

These transporters are found in the Nephrons. That is the reason it is called the Renal Transporters. There are different types of transporters are present. Based on their location, the form of the transporters is changed. Their method of working will be the same. But the Reabsorption of the ions is not the same at the entire place in the renal tube. Based on the function, the transporters are changed. Their name convention is based on their work. As these transporters perform the same operation, there will be a similarity in the channels. These transporters use the same type of channels.

Sodium-Glucose Linked Transporter (SGLT)

As the name suggests, this transporter is used to reabsorb sodium ions & glucose. And glucose is only reabsorbed at the Proximal Convoluted Tubule (PCT). So, these types of transporters are available in the PCT. There are two membrane junctions. The layer of the epithelial cells of the PCT & the lumen outside of the PCT. This is one membrane junction. This transporter is present there. There is another membrane junction is present. The membrane of the blood vessels & the lumen acts as the junction. This transporter uses ion-based channels to reabsorb sodium ions & glucose.

In the membrane layer of the blood vessels & the lumen, there is a Na⁺/K⁺ ATPase present. This throws the sodium ions to the blood vessels from the lumen. Instead of that, it takes the potassium ions from the blood. So, there will be a deficiency of sodium ions in the lumen. To balance that, sodium inside of the PCT uses this channel. The excess potassium in the lumen helps to open the ion-gated channels in this transporter. After the channel is open, the sodium ion can easily move out from the PCT to the lumen.

But this deficiency brings the required energy to move out the glucose from the PCT. Glucose then binds with the sodium ions. And it is now ready to reabsorb. With the help of the sodium ions, glucose will come to the lumen space from the PCT. Now, it will come to the blood vessels using the ATPase gate. In the blood, glucose will be degraded from the sodium ion. Hence, sodium & glucose is successfully reabsorbed in PCT.

Na-K-Cl Co-transporter (NKCC)

This transporter is used to reabsorb three different ions. They are the sodium, potassium & chloride ions. These transporters are present in Henley’s Loop. But this transporter is not present in the whole length of Henley’s Loop. The descending limb of the Henley’s Loop is permeable to water. There is no existence of the transporters or the ion channels. The descending limb of the Henley’s Loop is present in the Medulla of the Kidney. The medulla region has a very less density of water. So, as per the theory of diffusion, water gets outside Henley’s Loop. There are small pores that act as channels. It is just diffused from Henley’s Loop to the blood vessels. So, this type of transporter is present in the ascending limb of Henley’s Loop.

After water gets removed from the urine, it becomes more concentrated. In the concentrated urine, there is a high level of sodium is present. The present sodium amount is greater than the normal case. So, there is a need to reabsorb extra sodium. Here, again the ATPase will drain the sodium ions from the lumen. As there will be a deficiency of the sodium ion. But there is a difference with PCT. Here, ATPase removes three sodium ions from the blood vessels. But instead of that, it provides one potassium ion. So, there will be a misbalance in the ion. To remove that misbalance, with one sodium ion, another potassium ion is reabsorbed from the urine. But in this case, there will be more negative charge in the urine. So, it reabsorbed two chloride ions also. So, one Na⁺ ion, one K⁺ ion & two Cl^– ions will be reabsorbed. So, the ionic balance will be the same.

These ions will come to the lumen. Then using the diffusion method, these ions will come to the blood vessels. In the blood, the Na⁺ & Cl^– ions will be bonded. It will create the NaCl. This will help to maintain the blood pressure in the body. So, this Reabsorption is important for the body’s blood pressure also.

Sodium-Chloride Co-transporter (SLC)

This transporter is being used to reabsorb sodium & chloride ions from the Distal Convoluted Tubules (DCT). In the DCT, there is a very small amount of the Reabsorption proceed. So, there is a very small amount of the transporter is present. The DCT is known for the secretion process of Nephrons. So, there is likely a very small amount of Reabsorption happening. Here, the main ion sodium is reabsorbed. But along with that, due to some processes, chloride ions are also reabsorbed.

Here, first, the ATPase starts pumping out the sodium ions from the lumen to the bloodstream. This causes a deficiency gradient of sodium in the lumen. To balance that, sodium ions come out from the DCT to the lumen. But there is some change in the ion balancing. The ATPase removes three Na⁺ ions from the lumen. Instead of that, there will be two potassium ions coming from the blood. So, the one sodium ion comes from the DCT. To balance the positive-negative ions, one chloride ions also join that.

This means, one Na⁺ ion & one Cl^– ion will simultaneously come out from the DCT using this transporter & the ion-gated channels. Now, these ions will bind with each other & creates the salt. This is the salt Reabsorption in the DCT. This is the excess salt that comes into the urine through the filtration process. This Reabsorption helps to maintain the salt balance in the body. It is the only Reabsorption process in the DCT.

Importance of Reabsorption

Reabsorption is an important process in the kidney. Maybe this is the process of the Nephrons, but still, it is considered one of the most important processes in the kidney. It is seen that daily kidney helps to return a good amount of the essential elements to the body. Daily, nearly 99% of the filtered substances are returned to the body. Only one percent of the element that is filtered is removed. So, if those elements are not returned to the body, then there will be a serious essential elements crisis in the body. The body will not perform well. And there will be an intense need for the elements inside of the body. So, Reabsorption helps to maintain balance in the body. This is the largest process of urine formation than the other processes. Without the Reabsorption process, instead of 1.8 Littre/Day substances, 180 Littre/Day substances will be removed from the body. And this is a very high loss of the body.

FAQs on Reabsorption Process

Question 1: What is the Reabsorption process?

Answer:

Reabsorption is a process of the kidney or the process of the Nephrons, where filtered essential elements return to the body from the Nephrons with the help of the diffusion process. This process is flowed by the bloodstream. This process is known as Reabsorption.

Question 2: Which part of the Nephrons can able to reabsorb glucose?

Answer:

The Proximal Convoluted Tubule can able to reabsorb glucose. Mainly, 99% of glucose can be reabsorbed from here. This is the only region in the Nephrons that can reabsorb glucose.

Question 3: Which part of the Nephrons is responsible for the concentration of the urine?

Answer:

The Henley’s Loop is responsible for the Reabsorption of the water. As the water is reabsorbed from this region, so the urine gets concentrated here. So, Henley’s Loop is responsible for the concentration of urine.

Question 4: Which part of the Nephrons mostly does the Reabsorption process?

Answer:

The Proximal Convoluted Tubule is the part of the Nephrons that can able to reabsorb more than other places. Nearly, 65% of the elements are reabsorbed & returned to the body from this region. No other place can reabsorb more than this percentage.

Question 5: How much urine is reduced with the help of the Reabsorption process?

Answer:

Reabsorption can reduce urine formation. In absence of the Reabsorption process, 180 Liter/Day of urine will form. But Reabsorption reduces this amount to 1.8 Liter/Day amount. This is the savings of the essential elements of the body using the Reabsorption process.