Answer
Renal tubules are essential structures in the kidneys. The components of the renal tubule are the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT), and the collecting duct all working 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.
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.
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 counter current 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.
Distal Convoluted Tubule and Connecting Tubule
The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule. It has been used by anatomists to denote the region of the nephron that extends downstream from the macula densa to the confluence of another tubule (i.e., the collecting system). It includes two nephron segments, the DCT and the connecting tubule (CNT).
