EXCRETION AND THE LIVER

EXCRETION:

Main substances:

Product: Carbon dioxide from respiration Where it is produced: This is produced by all living cells in the body as a result of respiration. Where it is excreted: CO2 is passed from the cells of respiring tissues into the bloodstream. It is transported in the blood (mostly in the form of hydrogen carbonate ions) to the lungs. In the lungs the carbon dioxide diffuses into the alveoli to be excreted as we breath out.

Product: Nitrogen containing compounds (Urea)Where it is produced: Urea is produced in the liver from the break down of excess amino acids. This process is called deamination. Where it is excreted: The urea is passed into the bloodstream to be transported to the kidneys. It is transported in solution (dissolved in plasma). In the kidneys the urea is removed from the blood to become part of urine. Urine is stored in the bladder before being excreted via the urethra.

This is the removal of metabolic waste from the body

Why these substances must be excreted…

Excess carbon dioxide is toxic. A high level of carbon dioxide has three main effects:

1. Carbon dioxide is carried in the blood as hyrogencarbonate ions. This leads to the formation hydrogen ions. This occurs inside red blood cells, under the influence of the enzyme carbonic anhydrase. Hydrogen ions combine with haemoglobin. They compete with oxygen for space on the haemoglobin if there is too much carbon dioxide(too many H+) in the blood it can reduce oxygen transport.

Carbon Dioxide:

HCO3-

HCO3-

HCO3-

H+

H+

H+Carbonic Anhydrase

Haemoglobin

O2

H+ H+ H+ H+ H+

H+H+ H+H+ H+

O2

O2

Haemoglobin

2. The carbon dioxide also combines directly with haemoglobin to form carbaminohaemoglobin. This molecule has a lower affinity for oxygen than normal haemoglobin.

3. Excess carbon dioxide can also cause respiratory acidosis. Carbon dioxide dissolves in the blood plasma. Once dissolved it can combine with water to produce carbonic acid:

CO2 + H2O H2CO3

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 change in pH. If the change in pH 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 the breathing rate to help remove excess CO2.

If the blood pH drops below 7.35 it results in slow or difficult breathing, headaches, drowsiness , restlessness, tremor and confusion. There may 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 emphysema, chronic bronchitis , asthma or severe pneumonia. Blockage of the airway due to swelling a foreign object or vomit can also induce respiratory acidosis.

CO2 CO2

CO2 CO2CO2

CO2Carbonic Acid

Carbonic Acid

IIIII

HCO3-

HCO3-

H+

H+ H+

H20 H20

H20

Why these substances must be excreted…

Acid

AminoAmino

Acid

} DeaminationAmmonia

Urea

Keto Acid

The remaining keto acid may be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.

Nitrogenous Compounds:

The body cannot store proteins or amino acids. However, amino acids contain almost as much energy as carbohydrates. Therefore it would be wasteful to simply excrete excess amino acids.

Amino acids are transported to the liver and potentially toxic amino group is removed (deamination). The amino group initially forms the very soluble and highly toxic compound, ammonia.

This is converted to a less soluble and less toxic compound called Urea. This can be transported to the kidneys for excretion.

DEAMINATION: Amino Acid + Oxygen Keto acid + ammonia

UREA FORMATION: Ammonia+ Carbon Dioxide Urea + Water2NH3 + 2CO2 CO(NH2)2 + H20

Blood leaves the liver and retuning to the heart

Hepatic vein Diaphragm

Left lobe

Right lobe

Hepatic artery: Receives oxygenated blood from the aorta

Gall Bladder

Common bile duct

Bringing blood from the intestines

Portal vein: Deoxygenated blood enters from the digestive system.

Bringing fresh blood from the heart

THE LIVER

BLOOD FLOW TO AND FROM THE LIVER

Deoxygenated blood from the digestive system: This enters the liver via the hepatic portal vein. The blood is rich in products of digestion, the concentrations of various compounds will be uncontrolled. The blood may contain toxic compounds that were previously absorbed in the intestine.

Oxygenated blood from the heart:Blood travels from the aorta to the hepatic artery into the liver. This supplies the oxygen that is essential for aerobic respiration. The liver cells are very active and carry out metabolic processes which require energy in the form of ATP.

The liver is involved in homeostasis it therefore requires a good blood supply. The liver is supplied with blood from two sources:

Blood leaves the liver via the hepatic vein, this re-joins the vena cava and the blood returns to normal circulation.

Bile is a secretion from the liver. It has both a digestive function and an excretory function. The bile duct carries bile from the liver to the gall bladder where it is stored until required to aid the digestion of fats in small intestines.

THE LIVER

CELLSLiver cells are known as hepatocytes. They are relatively unspecialised and have a simple cuboidal shape with many microvilli on their surface. The cytoplasm however is very dense and specialised in the amounts of organelles it contains. Their functions include:

• Transformation and storage of carbohydrates

• Synthesis of cholesterol and bile salts

• Detoxification

• Protein Synthesis

LIVER CELLS

KUPFFER CELLSKupffer cells are specialised macrophages. They move around within the sinusoids and are involved in the breakdown and recycling of old red blood cells. One of the products of haemoglobin breakdown is bilirubin, which is excreted as part of the bile and in faeces.

Liver lobuleInterlobular vessel

Intralobular vesselCells lining the sinusoid

ARRANGEMENT OF CELLS IN THE LIVER

Cells, blood vessels an chambers inside the liver are arranged to ensure the best possible contact between the blood and the liver cells. The liver is divided into lobes, these are further divided into cylindrical lobes. As the hepatic artery and hepatic portal vein enter the liver they split into small and smaller vessels. These vessels run between and parallel to the lobules (inter lobular vessels).

SinusoidHepatic vein

(Hepatic vein)

ARRANGEMENT OF CELLS IN THE LIVER

One of the many functions of the liver cells is to manufacture bile. This is released into thee bile canaliculus. These join together to form the bile duct which transports the bile too the gall bladder.

At intervals, branches from the two blood vessels mix and pass along a chamber called a sinusoid. The sinusoid is lined by livercells. The sinusoids empty into the intralobular vessel, a branch of the hepatic vein.

The branches of the hepatic vein from different lobules join to form the hepatic vein which drains from the liver.

FORMATION OF UREAExcess amino acids cannot be stored as the amine groups make them toxic. However, the amino acid molecules contain a lot of energy so it would be wasteful to excrete the whole molecule.

It goes through the following processes before the amino component is excreted:

Amino acid Keto acid + ammonia Urea Deamination Ornithine Cycle

Deamination: Ammonia is produced which is highly toxic. The ammonia must not be allowed to accumulate. It also produces an organic compound called keto acid which can enter respiration directly to release energy.

Ornithine Cycle: The ammonia is combined with carbon dioxide to produce urea. Urea is both less soluble and less toxic than ammonia. It can be passed back into the blood and transported around the body into the kidneys. The kidneys filter the urea of out of the blood and concentrated in the urine. Urine can be stored in the bladder until it is released from the body

2NH3 + CO2 CO(NH2)2 +H20Ammonia + carbon dioxide urea + water

Keto Acid

ORNITHINE CYCLE

Ammonia (NH3)

UreaCO(NH2)2

H20

H20

H20

NH3

CO2

Arginine

Ornithine Citrulline

} Formation of water from ammonia

} Formation of urea from ammonia

FUNCTIONS OF THE LIVER

Control of: blood glucose levels, amino acid levels, lipid levels.

Synthesis of: red blood cells in the fetus, bile, plasma proteins, cholesterol Storage of:

vitamins A, D and B12, Iron, glycogen

Detoxification of: alcohol, drugs Breakdown of

hormones Destruction of red blood cells

DETOXIFICATIONThe liver is able to detoxify many compounds. Some of these compounds such as hydrogen peroxide are produced in the body. Some compounds, such as alcohol may be consumed as part of our diet. Others, such as drugs may be taken for health reasons or for recreational purposes. Toxins can be rendered harmless by oxidation, reduction, methylation or combination with another molecule.

Example: Liver cells contain enzymes that make toxic molecules less toxic. These include catalase which converts hydrogen peroxide to oxygen and water.

DETOXIFICATION OF ALCOHOLDetoxification of alcohol:

Ethanol is a drug that depresses nerve activity. In addition it contains chemical potential energy which may be used for respiration. It is broken down in the hepatocytes by the action of the enzyme ethanol dehydrogenase. The resulting compound is ethanal. This is dehydrogenated further by the enzyme ethanol dehydrogenase. The final compound produced is ethanoate (acetate), which is combined with coenzyme A to form acetyl coenzyme A, which enters the process of respiration.

2H 2H

NAD NAD Reduced NAD NAD Reduced

Ethanol Ethanal Ethanol Dehydrogenase Ethanol Dehydrogenase Ethanoate(Acetate ) Coenzyme A

Acetyl coenzyme A

Ethanol Ethanal Ethanoic acid-> Acetyl CoA

Hydrogen atoms released in this process combine with another coenzyme called NAD to form reduced NAD.

NAD is also required to oxidise and breakdown fatty acids for the use in respiration. If the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids. These fatty acids are then converted back to lipids and are stored in hepatocytes causing the liver to become enlarged. This is a condition known as fatty liver, which may lead to alcohol related hepatitis or cirrhosis.

This is used for respiration.