f214 excretion

F214 Module 2 Excretion

a. Define the term excretion

·         Excretion is the removal of metabolic waste from the body which are by products or unwanted substances from cell processes.

b. Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste from the body

·         Carbon dioxide:

Excess carbon dioxide is toxic and has three main effects:

The majority of carbon dioxide is carried in the blood as hydrogencarbonate ions but in the process hydrogen ions are made inside red blood cells with carbonic anhydrase. These hydrogen ions compete with oxygen for space on the haemoglobin. Therefore too much carbon dioxide can cause a reduction in oxygen transportation.

Carbon dioxide can also combine with haemoglobin to make carbaminohaemoglobin which has a lower affinity for oxygen.

Excess carbon dioxide can also cause respiratory acidosis by dissolving in the blood plasma and reacting with water to make carbonic acid which dissociates and releases hydrogen ions. This lowers the pH by making the blood more acidic. If the change is small is leads to an increased breathing rate but if it is larger it can cause difficult breathing, drowsiness, headaches and confusion. It could cause a rapid heart rate and changes in blood pressure.

·         Nitrogenous compounds:

The body cannot store proteins or amino acids.

But because they contain a lot of energy, amino acids are transported to the liver where they get the toxic amino acid group removed via deamination.

The amino acid then forms the very soluble but highly toxic ammonium before being converted to urea and is then transported to the liver for excretion.

There is some remaining keto acid which can be respired or converted to a carbohydrate or fat for storage.

c. Describe the histology and gross structure of the liver

Hepatic artery supplies the liver with oxygenated blood from the heart. The oxygen supplied is need for aerobic respiration.

Hepatic portal vein carries oxygenated blood to the liver. The blood is rich in the products of digestion some of which may be toxic compounds.

Hepatic vein: This is where blood leaves the liver which then re-joins the vena cava.

Bile duct: Bile is secreted from the liver. Bile has a digestive and excretory function. It carries bile from the liver to the gall bladder where it is stored until needed for the digestion of fats in the small intestine.

Inter-lobular vessels are branches from the hepatic artery and the hepatic vein enter.

Sinusoid: This is a special chamber where blood from the hepatic artery and hepatic portal vein mix.

It is lined by liver cells so molecules can be removed from the blood and pass molecules into the blood.

They empty into the inter-lobule vessels.

Liver cells/Hepatocytes

They have a simple cuboidal shape with microvilli on the surface.

They have many metabolic functions:

Protein synthesis.

Transformation and storage of carbohydrates.

Synthesis of cholesterol and bile salts.


The cytoplasm is very dense.

Kupffer cells

Specialist macrophages.

Move about in


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