As we have learned from my previous article on nephrons, the renal tubule is the part of the nephron in which the filtrate from the glomerulus enter. It consists of the following:

1. Proximal convoluted tubule
2. Loop of Henle divided into the:
   • thin descending limb
   • thin ascending limb
3. Distal convoluted tubule
4. The collecting ducts

Proximal Convoluted Tubule (PCT)

This is the first part of the renal tubule and it lies in the cortex of the kidney, close to the renal corpuscle.  Fluid that is filtered from the Bowman’s Capsule enters into the proximal tubule. It contains brush border, and is made up of cubiodal epithelium. It has an acidophilic cytoplasm due to the number of mitochondria.

Functions of PCT

• PCT reabsorbs 2/3 of the filtered Na or (65-80% of Na) and H2O
• It reabsorbs all of the glucose,  and amino acids  
• glucose is reabsorbed via Na-Glu cotransporter
• It also reabsorbs a fraction  of the bicarbonate, potassium, phosphate and calcium
• It secretes ammonia, which functions as a buffer for secreted H+. It also secretes creatine, which  is used to access the function of the kidney.

Loop of Henle

After filtrates leave the PCT, it enters the Loop of Henle. This is a u-shaped structure tube and it contains different segments which perform different functions.

Thin Descending Loop (TDL)

• reabsorbs water(H2O) passively
• it is impermeable to sodium(Na)
• it allows the urine to be concentrated, the urine in the TDL is hypertonic

Thick Ascending Loop

• In this part of the loop, Na, K and Cl is actively reabsorbed
• It is impermeable to H2O
• this segment makes the urine less concentrated

The Loop of Henle reabsorbs 10-20% sodium and chloride and 10% of the filtered water.

Distal Convoluted Tubule

• this segment of the renal tubule is lined with simple cuboidal epithelium and has no brush border
• is actively reabsorbs sodium and chloride
• it is relatively impermeable to water, but in the present of antidiuretic hormone (ADH) its permeability to water increases making urine concentrated.
• it secretes ammonium ions and hydrogen ions.
• forms part of the juxtaglomerular apparatus

The Collecting Ducts

The CD runs through the cortex into the medulla and opens into the renal papilla. Its epithelium varies from cuboidal to columnar epithelium(near the papilla).  It is the final segment of the renal tubule. It has two types of cells:

1. Principal cells

• Reabsorbs sodium and water
• Secretes potassium

These functions are regulated by ADH and aldosterone. Aldosterone increases sodium reabsorption and increases potassium secretion. ADH on the other hand enhances water  reabsorption. Therefore one can say that when ADH is increase water in the tubule is reabsorbed causing a low volume concentrated urine. Decrease ADH will result in a diluted high volume urine.

2. Intercalated cells

• Secretes Hydrogen ions
• Reabsorbs K by a H+,K+-ATPase

For a detailed illustration of their functions, you can check this page here.

Infographic