Excretion: summary and addition understanding

The liver 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.
Therefore, kidneys are adapted to excrete Urea.

 *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.

 


Structure of the Kidney:


There are 3 main visible regions in a kidney: cortex, medulla (contains lots of blood capillaries), and renal pelvis that collects the urine to transfer it into the ureter.  Blood enters the kidney through the  renal artery and deoxygenated blood containing less urea leaves the kidney in the renal vein.

*Each kidney contains a million tiny microscopic tubules: nephrons. 




(refer to 2.72B for information on the role of kidneys on osmoregulation and excretion)
The diagram on the right hand side shows the structure of a nephron.

The production of Urine can be separated to 2 steps: Ultrafiltration and Selective Reabsorption.

1. Ultrafiltration:
- Blood is filtered in the kidneys under high pressure, called ultrafiltration. In chemistry, 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 is filtered from the blood forming glomerular filtrate.

 - Ultrafiltration happens in the glomerulus and the glomerular filtrate (GF) passes into the Bowman's capsule
- The high pressure is generated by thinner blood vessels.

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 selective re-absorption.

2. Selective Re-absorption
Selective Re-absorption is when useful substances are re-absorbed back into the blood, using osmosis (for water) and Active Transport (for glucose and amino acids)
Refer to 2.76B for reabsorption of water with osmosis in collecting duct, 2.77B for reabsorption of glucose with active transport in proximal convoluted tubules