F214: Communication, Homeostasis and Energy 4.2.1 Excretion define the term excretion; explain the importance of removing metabolic wastes, including carbon

F214: Communication, Homeostasis and Energy4.2.1 Excretion

• define the term excretion;• explain the importance of removing metabolic

wastes, including carbon dioxide and nitrogenous waste, from the body;

• describe, with the aid of diagrams and photographs, the histology and gross structure of the liver;

• describe the formation of urea in the liver, including an outline of the ornithine cycle

• describe the roles of the liver in detoxification;

Excretion“The removal of metabolic waste (unwanted

substances from cell processes) from the body”

• Urea from excess amino acids in the liver

• Carbon dioxide from respiration

Task

• Using textbooks, research the importance of the excretion of both carbon dioxide and nitrogenous compounds

Q. Why must these substances be removed and where/how are these substances excreted?

Why Excrete Nitrogenous Compounds?• The body can’t store them• They have almost as much energy as carbohydrates• Transported to liver• Toxic amino acid group removed (deamination)• Amino group forms ammonia• Ammonia converted to urea• Urea transported to kidneys for excretion• Remaining Keto acid used in respiration or stored

Deamination: amino acid + oxygen keto acid + ammonia

Urea formation: 2NH3 + CO2 CO(NH2)2 + H2O

ammonia + carbon dioxide urea + water

Why Excrete Carbon Dioxide?• Excess carbon dioxide is toxic• Carbon dioxide is carried in blood as hydrogencarbonate ions

producing hydrogen ions that compete with oxygen for haemoglobin

• It combines with haemoglobin to form carbinohaemoglobin with a lower affinity for oxygen than normal haemoglobin

• Excess carbon dioxide can cause respiratory acidosis- carbon dioxide dissolves in blood plasma producing carbonic acid with dissociates forming hydrogen ions which lower the pH of blood

Carbonic acid production: CO2 + H2O H2CO3

Dissociation of Carbonic acid: H2CO3 H+ + HCO3-

Why Excrete Carbon Dioxide?• Hydrogen ions lower the pH and make the blood more acidic• Proteins in the blood act as buffers to resist the pH change, by

adding a hydrogen onto their COO (carboxyl group) to become COOH (They can exist as COO in a neutral solution)

• The extra hydrogen ions are detected by the respiratory centre in the medulla oblongata of the brain

• The brain causes an increase in breathing rate to remove the excess carbon dioxide

• If the pH drops below 7.35, the result is slow, difficult breathing, headache, drowsiness, restlessness, tremor, confusion, rapid heart rate and blood pressure changes

• This is called respiratory acidosis• Emphysema, chronic bronchitis, asthma and pneumonia, choking

or vomiting can also lead to it

Where is the liver located?

Where is the liver located?

Where is the liver?

The liver lies just beneath the diaphragm towards the right hand side of the body- it is made up of several lobes

• one of the largest organs in the body• 1450cm3 of blood passes through it in one minute• Enormous variety of functions• Uniform structure

The Liver

1. Stick in the liver diagram showing the blood vessels

2. Give the function of:• Hepatic artery• Hepatic vein• Hepatic portal vein• Describe the location of the liver in the body

The Structure of the LiverTask:

Your task is to describe, with the aid of diagrams and photographs, the histology and gross structure of the liver.

You must then present your findings to the rest of the class. You can choose how you would like to do this, and what format you would like to use e.g. PowerPoint, information board, poster etc.

Task Guide

• Decide on the format• Research the liver on the internet using the

checklist provided (approx 1-3 hours)• Gather appropriate images (approx 1 hour)• Complete the project including the images and

information you have gathered (1-3 hours)• Present your project• You will be assessed by your ability to annotate

(label with descriptions) a diagram of the liver

Checklist

Blood flow to and from the liver including the hepatic portal vein

Arrangement of cells inside the liverStructure of liver cells including microvilliThe role of Kupffer cells

Blood Supply• Blood arrives in two different blood vessels• Hepatic artery leads from the aorta and delivers

oxygenated blood to the liver• The hepatic portal vein leads from the small intestine and

delivers blood rich in absorbed nutrients (like glucose and amino acids from the intestine) and insulin and glucagon from the pancreas

• The hepatic portal vein carries about 3 times more blood per minute than the hepatic artery

• Blood from the hepatic portal vein has already travelled through a set of capillaries so is at a much lower pressure than blood in the hepatic artery

• The hepatic vein carries blood away from the liver to the vena cava which then transports it back to the heart- it returns glucose to circulation and allows amino acids to enter circulation

Histology of the Liver

• Made up of many lobules• In the centre of each lobule is a branch of the

hepatic vein• Between the lobules are branches of the

hepatic artery and the hepatic portal vein- blood flows from here, through the lobules and into the branch of the hepatic vein

• The lobules are made up of many liver cells called hepatocytes arranged in rows that radiate out from the centre like spokes of a wheel

• The channels which carry blood between these rows of cells are called sinusoids

• Other channels carry bile, which is produced from some of the hepatocytes: these channels are called bile canaliculi


• The bile flows from the centre of the lobule towards the outside – the opposite direction to the blood flow, where it enters a branch of the bile duct

• The rows of hepatocytes are never more than two cells thick, so that each cell is close to the blood in the sinusoids


• The sinusoids are lined with large, phagocytic macrophages which capture and destroy bacteria entering the liver via the hepatic portal vein from the intestine

• These cells are sometimes called kupffer cells.

• They are very efficient- if a bacterium comes into contact with the membrane of a kupffer cell, it is taken into the cell by phagocytosis within 0.01 second!!



















Urea Formation

• Excess amino acids cannot be stored as the amine groups are toxic

• Therefore it goes through two treatments in the liver: Deamination and The Ornithine Cycle

Deamination• Ammonia is produced which is soluble and

toxic• Keto acid is produced which enters respiration

directly to release energy

O2NH2 C

H

COOH

R

+ C

O

COOH

R

+ NH3

Amino Acid AmmoniaKeto Acid

The Ornithine Cycle(ammonia’s conversion to urea)

• Ammonia is combined with carbon dioxide to produce urea• Urea is less soluble and less toxic than ammonia so it can be

passed back into the blood to the kidneys• The kidneys filter out the urea from the blood and store it as

urine in the bladder

2NH3 + CO2 CO(NH2)2 + H2O

Ammonia + carbon dioxide urea + water

As you can see above, ammonia and carbon dioxide enter the cycle. ATP is required and Urea is produced. Water is also produced as a by-product.

The body can manufacture ornithine, but it is abundant in meat, fish, dairy and eggs

Detoxification• Many dangerous substances are broken down by

the liver• Some are made harmless, some are excreted into

bile• Most of these processes take place in the smooth

endoplasmic reticulum in the hepatocytes• Toxins can be made harmless by oxidation,

reduction, methylation or combination with another molecule

Detoxification• Oxidation: loss of electrons• Reduction: gain of electrons• Methylation: the addition of a methyl group• Combination with another molecule

Methylation is the addition of a methyl group

Metabolism of Alcohol

• Alcoholic drinks contain ethanol C2H5OH

• Ethanol molecules are small and lipid soluble so diffuse easily across the plasma membranes and enter cells

• Ethanol is a toxic substance and can cause damage to liver cells

• The liver avoids damage by breaking down ethanol into harmless substances

Metabolism of Alcohol• The enzyme alcohol dehydrogenase converts

ethanol into ethanal (acetaldehyde)• The enzyme aldehyde dehydrogenase then

converts ethanal (acetaldehyde) into ethanoate (acetate)

• This can then be combined with coenzyme A to form Acetyl Coenzyme A, which then enters the Kreb’s cycle and be metabolised to produce ATP

• Ethanol is therefore a source of energy for cells

Ethanol Ethanal Ethanoate

NAD ReducedNAD

NAD ReducedNAD

To Respiration

Acetyl Coenzyme A

• The Hydrogen atoms released are combined with another coenzyme called NAD to form reduced NAD

• NAD is also needed to oxidise and break down fatty acids for respiration

• If the liver has to detoxify too much alcohol, it does not have enough NAD to deal with the fatty acids, so they are changed back to lipids and are stored in hepatocytes (liver cells), causing the liver to become enlarged- this is called ‘fatty liver’ and can lead to liver cirrhosis.

Discuss in pairs...

1. Why must ammonia be converted to urea?2. Explain why excess amino acids and alcohol

should not be secreted?3. Suggest why the liver cells have large

numbers of mitochondria and ribosomes?

Write down your responses in pairs and pass it to another pair to look at- do you have the same responses?

Discuss in pairs...

1. Why must ammonia be converted to urea?Ammonia is highly soluble and very toxic- urea is less soluble and less toxic

2. Explain why excess amino acids and alcohol should not be secreted?They contain valuable energy that can be used. Amino acids can be converted into others

3. Suggest why the liver cells have large numbers of mitochondria and ribosomes?Mitochondria provide ATP for processes e.g. protein synthesis. Ribosomes manufacture the enzymes needed by liver cells







Documents

F214: Communication, Homeostasis and Energy 4.2.1 Excretion define the term excretion; explain the importance of removing metabolic wastes, including carbon