The Nephron

1. Loop of Henle 1a. Descending loop of Henle 1b. Ascending loop of Henle 2. Afferent arteriole 3. Efferent arteriole 4. Proximal convoluted tubule 5. Distal convoluted tubule 6. Glomerulus 7. Renal corpuscle 8. Renal (Bowman's) capsule 9. Collecting duct

Structures of the nephron. Image courtesy of commons.wikimedia.org/wiki/File:Nephron_blank.svg under the terms of the GNU Free Documentation License.

The functional unit of the kidney is called nephron. In humans, each kidney consists approximately 1.2 million nephrons, each around 3 cm in length. Having a total length of about 120 km from tubules in each kidney, it offers an enormous surface area for the exchange of materials.  Approximately one fifth of the blood passes through the kidney for each circuit of the body and about 125 cm³ of fluid is filtered out of the blood per minute. Having 99% of water returning to the blood, only about 1 cm³ of urine is produced per minute.

 

The nephron has two principle parts: a renal corpuscle and a long renal tubule.

Renal Corpuscle

Blood plasma is filtered here. Renal corpuscle consists of a ball of capillaries called glomerulus which is enclosed in glomerular capsule made up of a parietal layer with simple squamous epithelium and a visceral layer rich in cells called podocytes.   Glomerular filtrate, the fluid that is filtered from the glomerulus collects in the capsular space between the two layers of glomerular capsule.

Renal Corpuscle. Image courtesy of en.wikipedia.org/wiki/File:Gray1130.svg

Renal Tubule

Renal tubule is also called the uriniferous tubule. It starts from the glomerulus capsule and ends at the renal papilla. Renal tubule is divided into four parts, namely the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. Each region has unique role in urine production.

 

Proximal Convoluted Tubule  (PCT)

PCT is the longest and most coiled compared to the other regions of renal tubule. It arises from the glomerular capsule, and is made up of cuboidal epithelium. A characteristic of PCT is the microvilli, a brush border which is important in a mechanism called selective reabsorption.

 

Loop of Henle

The U-shaped nephron loop is divided into two portions: a descending limb that passes from cortex into the medulla; an ascending limb that passes from the medulla back to the cortex. Besides, loop of Henle is also divided into thick and thin segments. The thick segment is made up of simple cuboidal epithelium whereas the thin segment is made up of simple squamous epithelium.

 

Distal Convoluted Tubule (DCT)

The DCT is the end of the nephron, and is made up of cuboidal epithelium. It is shorter and less convoluted than PCT.

 

Collecting Duct

The collecting duct passes down into the renal medulla and it receives fluid from many nephrons. Collecting ducts merge to form a papillary duct when they are near the renal papilla. Papillary ducts then drain into the minor calyxes.

 

There are two types of nephrons:

(1) Cortical nephrons:  

- found in the cortex

- have relatively short loops of Henle which just extend into the outer medulla

- some cortical nephrons has no nephron loops at all

- accounts for 80% of the nephrons

 

(2) Juxtamedullary nephrons:

- have their renal corpuscle (composed of Bowman’s capsule and glomerulus) close to the

   junction of the cortex and medulla

- have long loops of Henle which extend deep to the apex of the renal pyramid

- accounts for 20% of the nephrons

 

Blood enters the glomerulus of the kidney through afferent arteriole, which branches from the renal artery. Efferent arterioles then drain filtered blood out of the glomerulus, into a network of capillaries in renal cortex and renal medulla. Ions and substances in the blood that are needed to be saved in the body are reabsorped into peritubular capillaries. Blood then flows into veins and back to the general circulatory system.

 

The capillary system called the vasa recta run parallel to the loops of Henle and the collecting ducts. They participate in salt gradient formation and maintenance. They return blood that contains useful substances to the general circulation, leaving only 1-2% of total renal blood flow in the renal medulla. Blood flow through the vasa recta is lesser than through the capillaries around the tubules and thus enables a water potential gradient to be maintained in the medulla.

 

See an illustration of Vasa Recta

 

Below is a diagram of fluid flow from the point glomerular fluid is formed to the point where urine leaves the body via urethra.