The Nephron

The nephron, also referred to as the uriniferous tubule or kidney tubule, is a structure found in the kidney, which filters blood, allowing for useful substances to be taken back up during the filtration process, while those that are either harmful or in excess can be removed from the body.

It should be noted that each kidney contains about one million nephrons.

The following list gives the main parts of the nephron:

1.      The Glomerulus and Bowman’s/ Renal Capsule

2.      Proximal Convoluted Tubule (often shortened as PCT)

3.      The Loop of Henle

4.      The Distal Convoluted Tube (often shortened as DCT)

The following are not part of the nephron, but are closely associated with it:

a.      Afferent arteriole- Carries oxygenated blood to the glomerulus of nephrons. Notably, it possesses a larger diameter than that of the efferent arteriole, so that a larger pressure can be created in the glomerulus for pressure filtration/ ultrafiltration to occur.

b.      Efferent arteriole- Carries filtered blood away from the glomerulus, inclusive of larger molecules, such as proteins and red blood cells, which are not ‘squeezed out’ during ultrafiltration.

c.      The Collecting Duct- Some consider it a part of the nephron (reflected in annotated drawing). Its role is to send urine to the renal pelvis. Additionally, its permeability can vary depending according to the secretion of ADH, and can thus result in more or less water being reabsorbed.

Annotated Drawing:

Two main stages of urine formation:

Ultrafiltration (also referred to as pressure filtration):

In this first stage of urine formation, blood (oxygenated) enters the glomerulus ( through the afferent arteriole) under a high pressure (hence why this process is sometimes referred to as pressure filtration). At this high pressure, smaller substances are ‘squeezed’ out of the capillary walls of the glomerulus, as they are selectively permeable. These substances which are squeezed out include ions, amino acids, glucose and urea.

The remaining fluid, now known as glomerular filtrate, passes to the Bowman’s Capsule, where the next step of urine formation occurs.

As well, the filtered blood (not glomerular filtrate), leaves via the efferent arteriole.

Selective reabsorption-

Different substances are selectively reabsorbed back into the bloodstream in different locations of the nephron:

a.      Proximal Convoluted Tubule- reabsorbs glucose, amino acids, Na+ and Cl- ions.

b.      Loop of Henle- primarily reabsorbs water, glucose, amino acids and ions.

c.      Distal Convoluted Tubule- reabsorbs some remaining water and remaining salts.

After passage through these parts of the nephron, the remaining fluid can now be called urine. It consists of many excess substances, and importantly toxic substances as well.

Urine now passes through the collecting duct of the nephron. It goes to the renal pelvis of the kidney, and then travels through the ureters, where it is stored in the bladder, until urination occurs, where it passes through the urethra and leaves the body.