The kidney and excretion - Pt 11
Adrian Whyte and Joanna Johnson, Contributor

Fifth-form Montego Bay High students during a Spanish class. - Claudine Housen Photo

THE NEPHRON

THE BASIC unit of structure and function of the kidney is the nephron and its associated blood supply.

Each human kidney has an estimated 1000000 nephron, each about 3cm in length. They make up a very large surface area.

About 125cm/3 of fluid is out of the blood per minute. About 99 per cent of the water is returned to the blood, so only about 1cm/3 of urine is made per minute, although this may vary with other factors like drinking.

Each nephron consists of five main regions, each having its own particular function.

1. Renal corpuscle composed of Bowman's capsule and glomerulus

2. First convoluted tubule

3. The loop of Henle

4. Second convoluted tubule

5. Collecting duct.

There are two types of nephrons, cortical and juxtamedullary nephrons.

Cortical nephrons are found in the cortex and have relatively short loops of Henle which just extend into the medulla.

Juxtamedullary nephrons have their renal corpuscle close to the junction of the cortex and medulla. They have long loops of Henle that extend deep into the medulla.

The two types of nephrons have different uses. Under normal conditions of water availability the cortical nephron deals with the control of blood volume.

When water is in short supply, increased water retention occurs through the juxtamedullary nephrons.

Blood enters the kidney by the renal artery which branches into finer arteries before entering the glomerulus. The blood vessel that enters the glomerulus is called the afferent arteriole. The blood vessel that leaves the glomerulus is called the efferent arteriole.

Blood is filtered in the glomerulus. The filtered blood leaves the glomerulus through the efferent arteriole.

STRUCTURE OF A NEPHRON

Ultrafiltration

St. Andrew High's Ebony McLean (left) positions her stick to play the ball before Wolmer's Girls' Olivina Holness (right), while St. Andrew's Yanique Holness prepares to intervene at the Mona Hockey Turf. McLean scored a goal in St. Andrew's 2-0 victory. - Junior Dowie Photo

Ultrafiltration is actually filtration under pressure. This is basically what happens to the blood while it is in the glomerulus.

The glomerulus is a knot of blood capillaries in the Bowman's capsule. The blood enters it at high pressure coming directly from the heart.

Since the capillaries of the glomerulus are much smaller than the afferent arteriole as the blood enters the glomerulus, the pressure rises.

After the filtering the larger molecules like proteins, as well as blood cells and platelets, are left behind in the blood.

The filtered fluid is now called glomerular filtrate. It has a similar composition of blood plasma. It contains glucose, amino acids, vitamins, ions, nitrogenous waste, some hormones and water. N.B. NO PROTEIN!

Blood leaving the glomerulus has a higher concentration.

Selective Reabsorption

Ultrafiltration produces about 180L of filtrate per day yet only about 1.5L of urine is produced each day. This means that a great deal of reabsorption must occur.

During ultrafiltration, substances which are useful and vital are lost from the plasma along with excretory substances.

Apart from filtering the blood the nephron is also responsible for selectively reabsorbing the useful materials back into the blood and also to have waste actively secreted from the blood capillaries surrounding the tubules.

The proximal convoluted tubule cells are adapted for reabsorption:

• They have a large surface area due to microvilli.
• Numerous mitochondria.
• Closeness of blood capillaries.

Over 80 per cent of the glomerular filtrate is reabsorbed here. All the glucose, amino acid, vitamins, hormones and about 80 per cent of the sodium chloride and water are reabsorbed here.

Loop of Henle

The function of the loop of Henle is to converse water. The longer the loop of Henle, the more concentrated the urine that can be produced will be.

Birds and mammals are the only vertebrate with loops of Henle. They are also the only vertebrate that produces urine that is more concentrated than blood.

The drier the habitat, the longer the loop of Henle would be. The wetter the habitat, more urine is produced, thus shorter the loop of Henle.

The descending limp is highly permeable to water and permeable to most solutes. Its function is to allow substances to diffuse easily through its walls.

Both parts of the ascending limb are almost totally impermeable to water.

The cells in the thick part are able to selectively reabsorb sodium, potassium, chloride and other ions from the tubule.

Normally water would follow by osmosis, but this cannot happen because the loop is impermeable to water. The fluid in the loop of the Henle becomes very diluted by the time it reaches the second convoluted tubule.

The second convoluted tubule and the collecting duct are responsible for the fine-tuning of the body fluid composition. They also control pH levels in the body.

The cells of the second convoluted tubule are similar to that of the first convoluted tubule. They have microvilli lining the inner surface, thus increasing the surface area for reabsorption. There are also numerous mitochondria for active transport.

The collecting duct carries the fluid towards the ureter. As the fluid moves down the collecting duct, the tissue fluid surrounding the duct gets more and more concentrated, water therefore leaves the collecting duct by osmosis.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.