2.68 Recall that the lungs, kidneys and skin are organs of excretion

Lungs
What? The lungs excrete carbon dioxide, which is a waste product of respiration. It diffuses into the lungs and is breathed out. 

Why? Carbon dioxide diffuses into blood plasma and makes it more acidic. It needs to be excreted so it doesn't reach toxic levels.As blood flows through the lungs, the carbon dioxide diffuses from the red blood cell into the alveoli and is breathed out.

Higher tier
If the levels of carbon dioxide are too high, the brain will detect this. It sends signals to increase the breathing rate so carbon dioxide can be removed from the blood more quickly, and the levels return back to normal.

Figure 1

Kidneys
What? The kidneys excrete excess water, salt and urea through urine. 

Why? (urea) Proteins are broken down into amino acids. Excess amino acids are broken down to form ammonia in the liver. It's then converted into urea (still in the liver). Although it is less toxic than ammonia, it still needs to be excreted from the body. 

(water) Water is a waste product of respiration and is also taken in by food or drink. Excess water could cause the plasma to be too dilute and cause blood cells to swell and burst :( while too little water could cause the plasma to be too concentrated and the blood cells to shrivel up / shrink and stop working. It's important to keep the water concentration in the blood plasma constant.

Skin
What? The skin excretes urea, water, salt and ammonia. This is released through sweat, which is a combination of urea, salts and water and is produced by the sweat glands.

Why? Things like ammonia and urea could be toxic if left in the body for too long. (see why water needs to be excreted by the kidneys)

Structure of the kidney: see 'pages' menu on right hand side

Source: BBC Bitesize http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/waste_disposal/revision/4/