Nephrons: Function of the Various Segments of the Renal Tubule
- Proximal convoluted tubule
- Proximal straight tubule
- Loop of Henle divided into the:
- thin descending limb
- thin ascending limb
- thick ascending limb
- Distal convoluted tubule
- 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 corpusle. 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.
After filtrats 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
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 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 casuing a low volume concentrated unrine. Decrease ADH will result in a diluted high volume urnine.
- secretes hydrogen ions
- reabsorbs K by a H+,K+-ATPase