Nephrons

A single kidney may have more than a million nephrons. Nephrons are the structural and functional units of the kidneys. The diagram represents an individual nephron and shows its main structures and functions. The structures include the glomerulus, Bowman’s capsule, and renal tubule.
Nephron structures.

• The renal corpuscle that consists of two structures:

• The glomerulus is a cluster of capillaries that filters substances out of the blood.
• Bowman’s capsule is a cup-shaped structure around the glomerulus that collects the filtered substances.

• The renal tubule is a long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances. It is divided into three parts: the proximal tubule, the Loop of Henle, and the distal tubule.

Tubules of several nephrons join to form a single collecting tubule.
Nephron function.
The nephron function is to produce urine. In this process we can consider two stages
FILTRATION
Filtration is the process of filtering substances from blood in the glomerulus. The renal arteries, which carry blood into the kidneys, branch into the capillaries of the glomerulus of each nephron. The pressure of blood moving through these capillaries forces some of the water and dissolved substances in the blood through the capillary walls and into Bowman’s capsule.
The fluid that collects in Bowman’s space is called filtrate. It is composed of water, salts, glucose, amino acids, and urea. Larger structures in the blood—including protein molecules and blood cells—do not pass into Bowman’s space. Instead, they return to the main circulation.
REABSORPTION AND SECRETION.
From the space inside the Bowman’s capsule the filtrate passes into the renal tubule. The main function of the renal tubule is reabsorption. Reabsorption is the return of needed substances in the filtrate back to the bloodstream.
At the beginning, in the proximal tubule, salts, glucose and amino acids are picked up from the filtrate. In the rest of the tubule the main reabsorbed substance is water. Before the tubule arrives to the collecting tubule, some substances can be secreted in the distal tubule.
The collecting tubule reabsorbs water from tubular fluid and return it to the blood. The remaining fluid, called urine, has a smaller volume and a greater concentration than tubular fluid. From the collecting ducts, urine enters a ureter and is eventually excreted from the body.