Kidney (part I): A* understanding of kidney’s role in excretion

Excretion is defined as “the removal of waste molecules that have been produced in metabolism inside cells”.  So for example carbon dioxide is a waste product of respiration and is excreted in the lungs.

The liver too produces a waste molecule urea from the breakdown of amino acids. Amino acids and proteins cannot be stored in the body:  if you eat more than you use, the excess is broken down to urea.  Urea would certainly become toxic if it was allowed to accumulate in the body (patients with no kidney function will die within 3-4 days without treatment) and the organ that is adapted to excrete urea from the blood is the kidney.  Kidneys excrete urea by dissolving it in water, together with a few salts to form a liquid called urine.

Don’t confuse urine, the liquid produced in the kidney that is removed from the body, with urea, the nitrogen-containing chemical made in the liver that ends up as one component of urine.

Urine is produced in the kidneys continuously day and night.  It travels away from the kidney in a tube called the ureter.  Each kidney has a ureter coming out of it, and the two ureters carry the urine to the bladder.  The bladder is a muscular storage organ for urine.  Urine drains from the bladder through a second tube called the urethra.

Make sure you check your spelling:  ureter and urethra are easy to muddle and correct spelling is essential to ensure the meaning is not lost….

How is urine made in the kidney?

Well that’s the big question for this post.  How does the kidney start with blood and produce a very different liquid called urine from it…..  Urine is basically made of water, dissolved urea and a few salts.

Before I can explain how urine is made, I need to briefly look at the structure of a kidney.

You can see the structure of the kidney on this simple diagram.  There are three regions visible in a kidney: an outer cortex, an inner medulla which is often a dark red colour due to the many capillaries it contains, and a space in the centre called the renal pelvis that collects the urine to transfer it into the ureter.  Blood enters the kidney through the large renal artery and deoxygenated blood containing less urea leaves the kidney in the renal vein.

Each kidney contains about a million tiny microscopic tubules called nephrons.  The nephron has an unusual blood supply and an understanding of what happens in different regions of the nephron allows an understanding of how urine is made to be built up.
   
The nephron is the yellow tubule in the diagram above.  It starts in the cortex with a cup-shaped structure called the Bowman’s capsule.  This cup contains a tiny knot of capillaries called the glomerulus.  The Bowman’s capsule empties into the second region of the nephron which is called the proximal convoluted tubule.  The tubule then descends into the medulla and out again in a region called the Loop of Henle.  There is then a second convoluted region called the distal convoluted tubule before the nephron empties into a tube called a collecting duct.    The collecting ducts carry urine down into the renal pelvis and into the ureter.

Stages in the Production of Urine

1) Ultrafiltration
Blood is filtered in the kidney under high pressure, a process called ultrafiltration.  Filtration is a way of separating a mixture of chemicals based on the size of the particles and this is exactly what happens to the blood in the kidney.  Red blood cells, white blood cells and platelets are all too large to cross the filtration barrier.  Blood plasma (with the exception of large plasma proteins) is filtered from the blood forming a liquid called glomerular filtrate.  The kidneys produce about 180 litres of glomerular filtrate per day.

   
Ultrafiltration happens in the glomerulus and the glomerular filtrate (GF) passes into the Bowmans capsule.  The high pressure is generated by the blood vessel that takes blood into the glomerulus (afferent arteriole) being much wider than the blood vessel that takes blood out of the glomerulus (efferent arteriole).  The plasma of blood (minus the large plasma proteins) is squeezed out of the very leaky capillaries in the glomerulus and into the first part of the nephron.

What’s in Glomerular Filtrate?

• water
• glucose
• amino acids
• salts
• urea

As well as containing urea, water and salts, glomerular filtrate also contains many useful molecules for the body (glucose and amino acids for example) so these have to be collected back into the blood in the second stage…..

2) Selective Reabsorption

The useful substances in the glomerular filtrate are reabsorbed back into the blood.  This can be by osmosis (for water) or by active transport (glucose and amino acids).
All of the glucose and all of the amino acids in the GF are reabsorbed in the proximal convoluted tubule by active transport.  Remember active transport can pump substances against the concentration gradient using energy from respiration.  Almost all the water in GF is reabsorbed by osmosis in the proximal tubule too.
So that leaves the question, what is the rest of the nephron doing…?

  
Well this is where it gets much more complicated……  Extra urea and salts can be secreted into the nephron at certain points along the tubule.  The Loop of Henle allows the body to produce a urine that is much more concentrated than the blood plasma.  And much of the distal tubules and collecting ducts are used for the second function of the kidney: homeostasis.