a) excretion is the removal or metabolic waste from the body. Metabolic waste consists of waste substances that may be toxic or are produced n excess by reactions inside cells.
b) Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body.
The are many substances that need to be excreted - almost any cell product that is formed in excess by the chemical process occurring in the cells must be excreted. However there are two products that are produced in very large amounts. Carbon dioxide from respiration.
Nitrogen containing compounds such as urea.
Where are these substances produced.
Carbon dioxide is produced by every living cell in the body as a result of respiration. Urea is produced in the liver from excess amino acids.
Where are the substances excreted?
Carbon dioxide is passed from the cells of respiring tissues into the blood stream. It is transported in the blood (mostly in the form of hydrogen carbonate ions) to the lung. In the lungs the carbon dioxide diffuses into the alveoli to be excreted as we breathe out.
Urea is produced by breaking down excess amino acids in the liver. This process is known as deamination. The urea is then passed through the blood stream to be transported to the kidneys It is transported in solution - dissolved in the plasma. In the kidneys the urea is removed from the blood to become part of urine. Urine is stored in the bladder until it is removed.
Why must the substances be removed?
Excess CO2 is very toxic. A high level of carbon dioxide has three main effects. Majority of carbon dioxide is carried in the blood as hydrogen carbonate ions. However, forming hydrogen carbonate ions forms hydrogen ions. This occurs inside the red blood cells, under the haemoglobin. They compete with oxygen for space on the haemoglobin. If there is too much carbon dioxide in the blood it can reduce oxygen transport. The carbon dioxide also combines directly with the haemoglobin and produces carbaminohaemoglobin - this molecule has lower affinity for oxygen than normal haemoglobin. Excess carbon dioxide can also 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 and make the blood more acidic. Proteins in the blood act as buffers to resist the pH change. If the pH change is small then the extra hydrogen ions are detected by the respiratory centre in the medulla oblongata of the brain. This causes an increase in breathing rate to help remove the excess carbon dioxide. However, if the blood pH drops below 7.35 it results I slowed or difficult breathing, headache, drowsiness, confusion. There may also be a rapid heart rate and changes in blood pressure. This is respiratory acidosis. It can be caused by diseases or conditions that affect the lungs themselves, such as asthma. Blockage of the airway due to swelling, a foreign object, or vomit can also induce respiratory acidosis.
The body cannot store proteins or amino acids.
Amino acids contain almost as much energy as carbohydrates, therefore would be wasteful. Amino acids are broken down through deamination in the liver. - the toxic amino group is removed. The amino group initially forms the toxic, soluble ammonia. This is converted to less soluble and toxic urea, which can be transported to the kidneys for excretion. The remaining keto acid can be used directly in respiration to release energy or it may be converted to a carbohydrate or fat for storage,
Deamination: amino acid +oxygen -> keto acid +ammonia.
Formation of urea: 2NH3 +CO3 -> CO(NH2)2 +H2O
c) the gross structure and histology of the liver.
The liver is made up of cells called hepatocytes.
Hepatocytes carry out...