1.2.1 ExcretionOCR – A2 – Module 2

10/04/20

151By Piril Erel

What is excretion?

Excretion = the removal of metabolic waste from the body

Metabolic waste = consists of waste substances that may be

toxic or are produced in excess by the reactions inside cells.

There are 2 substances that are produced in very large

amounts:

Carbon dioxide

Nitrogen containing compounds such as urea

Note: EGESTION is when your body removes undigested food

by the process of defecation. EXCRETION is when your body

removes metabolic waste which has entered your cells.

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152By Piril Erel

Where are these

substances excreted?

Carbon dioxide is passed from the cells of respiring tissues into the bloodstream. It is transported in the blood (mostly in the form of hydrogencarbonate ions) to the lungs. In the lungs the carbon dioxide diffuses into the alveoli to be excreted as we breathe.

Urea is made from the break down of excess amino acids in the liver. This process is called deamination. Urea is passed into the bloodstream to be transported to the kidneys.

it is transported in solution (dissolved in plasma)

In the kidneys, urea is removed from the blood to become a part of urine.

Urine is stored in the bladder before being excreted via the urethra.

10/04/20

153By Piril Erel

Why must these substances be

remove?

Part ICarbon dioxide:

Excess carbon dioxide can lead to toxicity via three main effects:

1. Carbon dioxide is transported in the blood as hydrogencarbonate ions. This occurs inside the red blood cells under the influence of carbonic anhydrase

H2CO3 H+ + HCO3-

The hydrogen ions combine with haemoglobin. They compete with oxygen for space on the haemoglobin. If there is too much carbon dioxide in the blood it will reduce oxygen transport via:

2. Carbon dioxide when bound to haem forms carbaminohaemoglobin. (Carbon dioxide has a higher affinity for haem, when the compound is formed carbaminohaemoglobin will have a lower affinity for oxygen than normal haem) Therefore it becomes difficult for oxygen to attach onto haem as carbon dioxide is strongly associated with haem.

10/04/20

154By Piril Erel

Why must these substances be

removed?

Part II3. Excess carbon dioxide can cause respiratory acidosis. The

carbon dioxide dissolves in the blood plasma. Once dissolved it

can combine with water to produce carbonic acid:

CO2 + H2O H2CO3

The carbonic acid dissociates to release hydrogen ions

H2CO3 H+ + HCO3-

The hydrogen ions lower the pH acidity within the blood

circulation. Proteins in the blood act as buffers to resist the

change in pH.

If the pH change is small the extra H+ ions are detected by the

respiratory centre in the medulla oblongata of the brain resulting in

Increased breathing rate removing excess carbon dioxide

10/04/20

155By Piril Erel

Why must these substances be

removed?

Part III If the pH drops below 7.3 it

results in

slowed or difficult breathing, headache, drowsiness, restlessness, tremor and confusion.

May also be a rapid heart rate and changes in blood pressure.

THIS IS RESPIRATORY ACIDOSIS (A MEDICAL EMERGENCY, NEEDS TREATMENT IMMEDIATELY)

Respiratory acidosis can also be caused by diseases or conditions that affect the lungs themselves due to blockage of airway leading to increase in carbon dioxide. These diseases include; emphysema, chronic bronchitis, asthma or severe pneumonia. Conditions such as swelling, a foreign object or vomit in the airways are also causative.

10/04/20

156By Piril Erel

Why must these substances be

removed?

Part IV Nitrogenous Compounds:

The body cannot store proteins or amino acids(aa). However, aacontain as much energy as carbohydrates, therefore we do not want to waste this energy source. AA are transported to the liver and the potentially toxic amino group is removed (deamination).

Step 1 -The amino group initially forms a very soluble and highly toxic compound = ammonia

Amino acid + oxygen keto acid + ammonia

Step 2 - Ammonia is converted to a less soluble and less toxic compound = urea

Ammonia + carbon dioxide urea + water2NH3 + CO2 CO(NH2)2 + H2O

Urea is then transported to the kidneys for excretion. The remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.

10/04/20

157By Piril Erel