Renal Nephron

The renal nephron is the main structure filtering the bloodstream and recycling important molecules and ions before discarding metabolites in the urine collecting duct. The renal nephrons are located in both the medulla (in the renal pyramids) and the cortex tissues of the kidney. It has been estimated a single human kidney contains up to one million nephrons.

Structurally, a nephron is a long tubule with one extremity ending in a ball-like shape, called the Bowman's capsule. The other end connects to the renal pelvis that leads to the ureter. A nephron possesses several and distinctive sections with different purposes, sometimes even located in different tissues.

Following the flow of filtration and recycling, the structures are as follows:

-Renal corpuscle: this structure includes the Bowman's capsule, and an intricate network of blood vessels called the glomerulus. The blood arrives from the arterioles into the glomerulus, where the pressure pushes fluid from the vessel through a three-layered filtering barrier. Most of the fluid ends up in the Bowman's capsule and inside the consecutive tubule, while red blood cells and large molecules such as proteins are kept in the bloodstream and leave the corpuscle through the arterioles.

-Proximal convoluted tubule: immediately following the renal corpuscle (hence the name proximal), this section of the nephron is responsible for the large majority of reabsorption of molecules and ions back into the bloodstream. Glucose, amino acids, and ions such as sodium, phosphate, potassium, magnesium, and calcium are mainly reabsorbed here. The proximal convoluted tubule also secretes urea and ammonium, which will be discarded in the urine, as well as creatine.

-Proximal straight tubule: this section mainly reabsorbs the remaining phosphate.

-Loop of Henle: this structure includes the thin descending loop, the bend, and the thin ascending loop. It passively reabsorbs water but not ions, and thus concentrates the urine. This section becomes increasingly important in warmer climates.

-Thick ascending limb: in this section, Na⁺, K⁺, and Cl^- the sodium cation, the potassium cation, and the chloride anion are actively reabsorbed through the epithelial walls.

-Distal convoluted tubule: the farthest structure from the renal corpuscle (hence the name distal), it reabsorbs sodium and chloride, and its water permeability is depending on the action of the antidiuretic hormone (ADH).

-Collecting duct: this final structure reabsorbs water from the filtrate as needed to maintain body fluid homeostasis.

Overall, the nephrons are an extremely efficient recycling station preserving more than 90% of important nutrients and ions and water to preserve the homeostasis and contributing to body fluid regulation.