The Nephron

Nephron is the basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine. A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone. In humans, a normal kidney contains 800,000 to one million nephrons.

Ultrafiltration

In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent arteriole and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:

- The renal artery contains blood at very high pressure which enters the glomerulus via the short afferent arteriole.

- The efferent arteriole has a smaller diameter than the afferent arteriole.

The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.

Once inside the lumen of the nephron, small molecules, such as ions, glucose and amino acids, get reabsorbed from the filtrate:

Specialized proteins called transporters are located on the membranes of the various cells of the nephron.

These transporters grab the small molecules from the filtrate as it flows by them.

Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium.

Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.

Some transporters require energy, usually in the form of adenosine triphosphate (active transport), while others don't (passive transport).

Water gets reabsorbed passively by osmosis in response to the buildup of reabsorbed Na in spaces between the cells that form the walls of the nephron.

Other molecules get reabsorbed passively when they are caught up in the flow of water (solvent drag).

Reabsorption

Reabsorption of most substances is related to the reabsorption of Na, either directly, via sharing a transporter, or indirectly via solvent drag, which is set up by the reabsorption of Na.

The reabsorption process is similar to the "fish pond" game that you see in some amusement parks or state fairs. In these games, there is a stream that contains different colored plastic fish with magnets. The children playing the game each have a fishing pole with an attached magnet to catch the fish as they move by. Different coloured fish have different prize values associated with them, so some children will be selective and try to grab the colored fish with the highest prize value. Now suppose our nephron is the stream, the filtered molecules are the various colored fish, and our children are the transporters. Furthermore, each child is fishing for a specific colored fish. Most children start at the beginning of the stream and some spread out further downstream. By the end of the stream, most of the fish have been caught. This is what happens as the filtrate travels through the nephron.

Two major factors affect the reabsorption process:

-Concentration of small molecules in the filtrate - the higher the concentration, the more molecules can be reabsorbed. Like our children in the fish pond game, if you increase the number of fish in the stream, the children will have an easier time catching them.

In the kidney, this is true only to a certain extent because:

There is only a fixed number of transporters for a given molecule present in the nephron.

There is a limit to how many molecules the transporters can grab in a given period of time.

- Rate of flow of the filtrate - flow rate affects the time available for the transporters to reabsorb molecules. As with our fish pond, if the stream moves by slowly, the children will have more time to catch fish than if the stream were moving faster.

To give you an idea of the quantity of reabsorption across the nephron, let's look at the sodium ion (Na) as an example:

Proximal convulated tubule - reabsorbs 65 percent of filtered Na. In addition, the proximal tubule passively reabsorbs about 2/3 of water and most other substances.

Loop of Henle - reabsorbs 25 percent of filtered Na.

Distal convulated tubule - reabsorbs 8 percent of filtered Na.

Collecting duct - reabsorbs the remaining 2 percent only if the hormone aldosterone is present.

Secretion

Secretion is a process in which waste and excess substances that were not initially filtered are secreted in renal tubule. Secretion takes place at the renal tubule and collecting ducts but is active at distal convulated tubule. Secretion occurs by passive diffusions and active transport.

Secreted substances include hydrogen ions, potassium ions, ammonia, urea, creatinine, toxins and certain drugs.