F214: Communication, Homeostasis and Energy 4.2.1 Excretion

1
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

2
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

3
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?

4
Why Excrete Nitrogenous Compounds?

• The body cant 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

5
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-

6
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

7
Where is the liver located?
8
Where is the liver located?
9
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
10

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

11
(No Transcript)
12
(No Transcript)
13
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

14
The Structure of the Liver

• Task
• 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.

15
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

16
Checklist

• Blood flow to and from the liver including the
  hepatic portal vein
• Arrangement of cells inside the liver
• Structure of liver cells including microvilli
• The role of Kupffer cells

17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
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

21
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

22

• 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

23
Histology of the Liver

• 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

24
Histology of the Liver

• 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!!

25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
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

29
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

30
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

31
(No Transcript)
32
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

33
Deamination

• Ammonia is produced which is soluble and toxic
• Keto acid is produced which enters respiration
  directly to release energy

Amino Acid
Ammonia
Keto Acid
34
The Ornithine Cycle(ammonias 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

35
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
36
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

37
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
38
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

39
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 Krebs
  cycle and be metabolised to produce ATP
• Ethanol is therefore a source of energy for cells

40
To Respiration
Ethanol
Ethanal
Ethanoate
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.

41
(No Transcript)
42
(No Transcript)
43
(No Transcript)
44
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?
45
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
• 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
• 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

46
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