Renal tubules are essential structures in the kidneys. This lesson explores the function of the renal tubule and its parts, including the proximal and distal convoluted tubules, the loop of Henle, and the collecting duct. Learn how these components work together to absorb materials and form urine.
The Ultrafiltrate in the Renal Tubule
As blood enters your kidneys, it is filtered by the first section of the functional unit of your kidneys, the nephron. Believe it or not, there are about 1,000,000 of these nephrons in each kidney! Regardless, the first section of each individual nephron is called the renal corpuscle and is made up of the glomerulus, which filters the blood, and Bowman’s capsule, which collects the ultrafiltrate.
The ultrafiltrate produced by the glomerulus initially contains small molecules like water, glucose, amino acids, bicarbonate, urea, and creatinine as well as ions such as potassium, sodium, chloride, calcium, and phosphate. All of this filtrate is channeled from the Bowman’s space inside of the Bowman’s capsule into the first part of a long series of ducts in the nephron, which are collectively called the renal tubule. During this journey, many things will be added and subtracted to and from the ultrafiltrate by the cells lining these ducts in order to finally form the urine your body excretes.
Proximal Convoluted Tubule
The filtrate in the Bowman’s space I just mentioned enters the first duct in the renal tubule. This duct is called the proximal convoluted tubule, which is a section of the renal tubule located in the kidney’s cortex that is responsible for the reabsorption of the majority of ultrafiltrate.
The word ‘proximal’ in ‘proximal convoluted tubule’ implies it’s near or attached to the renal corpuscle, and if you look at the image above, you can also see how convoluted this duct is due to a bunch of twists and turns.
Again, it’s really important to remember that under normal conditions, the majority of water (H2O), sodium (Na +), chloride (Cl -), calcium (Ca 2+), potassium (K +), phosphate (PO4 3-), bicarbonate (HCO3 -), and urea, as well as all of the glucose and amino acids, are reabsorbed from the ultrafiltrate made by the glomerulus back into the bloodstream thanks to the proximal tubule. You can think of the proximal tubule as the first person in a long line in a school kitchen. The kitchen is the glomerulus producing the ultrafiltrate, the first person in line is our proximal tubule, and the line is the entire collection of ducts making up the long renal tubule.
Since the proximal tubule is the first in line, and it’s a greedy little thing, it will take up as much of the ultrafiltrate back into the bloodstream as possible and leave the rest of the ducts with scraps for lunch.
It’s really easy to remember what is reabsorbed by the proximal tubule. Think of it this way: the proximal tubule is hungry in that lunch line! It wants to eat a lot of food. Therefore, the glucose that makes up carbs and the amino acids that make up proteins are all gobbled up! A lot of food has salt, which is sodium chloride; hence, sodium and chloride are also reabsorbed.
Don’t forget that because you need to drink when you eat a lot of salt and because water follows sodium wherever it goes in the body, the majority of water is also reabsorbed! Finally, after drinking all of that water, you need to urinate, or pee – hence, urea and the two ‘p’s (potassium and phosphate) are also reabsorbed there. If you can recall this lunch line example, you’ll only have to memorize bicarbonate and calcium in the list above as stand-alone words.
The Loop of Henle
The next duct of the renal tubule in line to collect the scraps left by the proximal tubule is called the loop of Henle. The loop of Henle is a long duct mainly located in the kidney’s medulla that creates a concentration gradient, called a countercurrent exchange system, that allows for the reabsorption of water. While the majority of water, like the rest of the ultrafiltrate, was reabsorbed by the proximal convoluted tubule, the loop of Henle is involved in absorbing even more water in order to create concentrated urine.
The entire loop of Henle has three different parts. One is called the thin descending limb of Henle, the next in line is called the thin ascending limb of Henle, and the final part is called the thick ascending limb of Henle.
The thin descending limb, which descends into the medulla, is very permeable to water. This allows for the reabsorption of water back into the blood. The thin and thick ascending limbs are not permeable to water and instead reabsorb ions like sodium, chloride, calcium, and potassium.
You can remember which part of the loop of Henle is important in the reabsorption of water really easily. Recall that the proximal tubule reabsorbs the majority of water, and the first structure of the loop of Henle to connect to the proximal tubule is the thin descending limb. Therefore, this thin descending limb simply continues the reabsorption of water just like the preceding proximal tubule, whereas the rest of the loop of Henle that follows the thin descending limb does not reabsorb water. This difference in functionality is what establishes the kidney’s medullary osmotic gradient for maximal conservation of water.
Distal Convoluted Tubule and Connecting Tubule
Once the filtrate passes through the thick ascending limb of Henle, it enters the distal convoluted tubule, which is a duct of the renal tubule located in the kidney’s cortex that reabsorbs calcium, sodium, and chloride and regulates the pH of urine by secreting protons and absorbing bicarbonate. The reason this is called the distal, as opposed to proximal, convoluted tubule is because it is the segment farthest away in terms of position and connection to the renal corpuscle.
The distal convoluted tubule will then connect to a section called the connecting tubule, which will do many of the same things as the distal convoluted tubule, but it is not, however, considered to be a part of the renal tubule.
The Role of PTH in the Regulation of Calcium
It’s important to know that a hormone in your body, called parathyroid hormone, or PTH for short, plays a role in the distal and proximal convoluted tubules.
Recall that the majority of calcium is reabsorbed, through passive means, in the proximal convoluted tubule. This is the case under normal conditions. However, if PTH is secreted in response to low levels of calcium in the blood, then the proximal convoluted tubule will not be allowed to reabsorb calcium. Instead, the majority of calcium will be reabsorbed actively in the distal convoluted tubule and connecting segment as well as part of the loop of Henle. In addition, PTH will prevent the proximal tubule from reabsorbing phosphate, causing a net excretion of phosphate out of the body via urine.
Finally, PTH will activate in the kidneys a prehormone produced in the liver called calcifediol into the active form of vitamin D, called calcitriol. This will lead to further reabsorption of calcium from the glomerular ultrafiltrate passing through the renal tubule.
Conversely, the hormone calcitonin decreases the reabsorption of calcium in these two segments while simultaneously preventing the proximal convoluted tubule from reabsorbing calcium as well, leading to a net calcium excretion out of the body.
The Collecting Duct
After passing through the connecting tubule, the filtrate then enters a structure called the collecting duct. This is a duct involved in regulating the urine’s pH, water, and electrolyte balance as well as the channeling of urine out of the kidneys. The collecting duct is not part of the renal tubule. Instead, it funnels the filtrate out of the renal tubule into further structures, which leads to the excretion of urine out of the body.
Hormones in the Distal Nephron and Collecting Duct
As an important side note, the last three segments I talked about – the collecting duct, distal convoluted tubule, and the connecting tubule – are all involved in the water and acid-base balance of the urine. For our purposes, I’ll be brief.
Specifically, the hormone aldosterone will cause the reabsorption of sodium, and therefore water, in the distal tubule and collecting ducts. This hormone will also cause these two segments to secrete potassium and acid, in the form of hydrogen ions, into the urine and out of the body.
In addition, a hormone called vasopressin, also known as antidiuretic hormone (or ADH for short), will cause the collecting duct and the distal convoluted tubule to become permeable to water, leading to the absorption of more water in times of need.
I’ll stop right there as your brain has probably been overwhelmed by all of that important information. Perhaps not surprisingly, that’s unfortunately just the very tip of the complex but beautiful iceberg we call renal physiology. Therefore, let’s hit on the main points of this lesson.
Ultrafiltrate flows through the first part of the renal tubule, called the proximal convoluted tubule, which is a section of the renal tubule mainly located in the kidney’s cortex that is responsible for the reabsorption of the majority of ultrafiltrate.
The ultrafiltrate will then flow into the loop of Henle. This loop is a long duct mainly located in the kidney’s medulla that creates a concentration gradient, called a countercurrent exchange system, that allows for the reabsorption of water.
The entire loop of Henle has different parts: one is called the thin descending limb of the loop of Henle, and the others are called the thin and thick ascending limbs of Henle.
Once the filtrate passes through the thick ascending limb of Henle, it enters the distal convoluted tubule, which is a duct of the renal tubule located in the kidney’s cortex that reabsorbs calcium, sodium, and chloride and regulates the pH of urine by secreting protons and absorbing bicarbonate.
The distal convoluted tubule connects to a section called the connecting tubule, and this connects to the collecting duct, which is a duct involved in regulating the urine’s pH, water, and electrolyte balance as well as the channeling of urine out of the kidneys.
After exploring this lesson, you should have the ability to:
- Describe the composition of ultrafiltrate
- Trace the flow of ultrafiltrate through the renal tubule
- List and characterize the three parts of the loop of Henle
- Explain which structures in the renal tubule help regulate pH, water and electrolyte balance