5.2.1 The Importance of Excretion

Question 1

What is excretion?

Answer 1

Excretion is the process by which toxic waste products of metabolism and substances in excess of requirement are removed from the body
For example:
The lungs excrete the waste product carbon dioxide by gas exchange and the act of breathing out (exhalation)
The kidneys produce urine that contains the waste product urea in solution
Excretion is a key process in homeostasis and is important in maintaining metabolism, as metabolic waste can have serious negative consequences on the body if allowed to accumulate

Question 2

What is metabolic waste?

Answer 2

As mammals are active, warm-blooded animals, they have high metabolic rates, which means they also produce relatively large amounts of metabolic waste
This metabolic waste includes:
Carbon dioxide
Nitrogenous waste (ammonia, urea and uric acid)
Bile pigments (produced during the breakdown of haemoglobin)
Carbon dioxide is produced from the decarboxylation of respiratory substrates
Ammonia is produced from the deamination of excess amino acids
If these two waste products are not excreted properly, they can accumulate and change the cytoplasm and body fluid pH, which can cause enzymes to work less efficiently
The effects that different metabolic wastes can have on the body if allowed to accumulate are summarised in the table below
Note that the liver is a key organ in producing all of these excretory substances (except for carbon dioxide)

Look at SME table