The kidney and excretion

Joanna George-Johnson, Contributor

Excretion: The removal of waste products of metabolism from the body. If they were allowed to accumulate, they would prevent the maintenance of a steady state.

NB: Secretion is different from excretion. Secretion is usually the release of useful substances, for example, hormones from cells.

The human kidney

The kidney is a major excretory organ. Its main functions are:

• Removal of metabolic waste products.
• Regulations of water content of body fluids. This is called osmoregulation.
• Regulation of pH levels in body fluids.
• Filters the blood and excretes toxic wastes like urea.

Since the kidney has a rich blood supply where it keeps the composition of the blood at a steady state, it therefore contributes to homeostasis.

How the kidney filters blood

1. A large artery brings blood into the kidney. (renal artery)

2. Tiny blood vessels branch off the main artery and go through each nephron.

3. Water, salts, urea and glucose are forced out of the blood into the nephrons.

4. As these substances flow through the nephron, most of the water, all of the glucose, some of the salts and urea move back into the blood. A concentrated mixture of waste materials is left in the nephrons.

5. The cleaned blood, now with slightly less water and much less waste material, leaves the kidney via the large renal vein.

6. The yellow fluid that remains in the nephron is called urine. The urine is sent out of the kidney through the ureter into the urinary bladder to be stored.

7. Urine leaves the body through the urethra.

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 one million nephrons, each about three cm in length.

They make up a very large surface area.

About 125 cm3 of fluid is out of the blood per minute. About 99 per cent of the water is returned to the blood, so only about 1 cm3 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 and 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.

Questions

1. Using the picture on the left, draw a diagram of the kidney and label it. Make sure to use the SBA rules for biological drawings.

2. What does the glomerulus consists of?

3. For each of the flowing parts of the nephron, use one term to describe its function:

a. Glomerulus

b. Proximal convoluted tube

c. Collecting duct

• The imaginative Matthew Coley (right) shows his teacher, Lurlene Bryan-Davis (second right), Crystalee Sterling (second left) and Shannon Murray his scenery masterpiece 'Night Life', during an art and craft exhibition at Bridgeport High School on Monday, January 28. -Anthony Minott/Freelance Photographer

Joanna George-Johnson teaches at Ardenne High School.