Excretion

Excretion

is the removal of metabolic waste from the body

Deamination

is the removal of the amine group from an amino acid to produce ammonia

Why must carbon dioxide be removed?

Forming hydrogen carbonate ions also forms hydrogen ions. This occurs inside the red blood cells, under the influence of the enzyme carbonic anhydrase. The H ions combine with heamoglobin and compete with oxygen for space.

The liver

Oxygenated blood from the heart

Blood travels from the aorta via the hepatic artery into the liver. This supplies the oxygen that is essential for aerobic respiration

The liver

Deoxygenated blood from the digestive system

This enters the liver via the hepatic portal vein. This blood is rich in the products of digestion.

The liver

Blood leaves

Via the hepatic portal vein. This rejoins the vena cava and the blood returns to normal circulation.

Bile duct

Bile is a secretion from the liver. It has both a digestive function and a secretory function. The bile duct carries bile from the liver to the gall bladder where it is stored until required

The arrangement of cells inside the liver

- HA and HPV enter the liver and split into smaller and smaller vessels (inter-lobular vessels)
- At intervals, branches from the HA and the HPV enter the lobules.
- The blood is mixed and passes along the sinusoid. The sinusoid is lined by liver cells
-The sinusoids enter into the intra-lobular vessel, a branch of the HV.
-the branches of the HV from different lobules join together to form the HV, which drains blood from the liver.
-As blood flows along the sinusoid it is in very close contact with the liver cells. They are able to remove molecules from the blood and pass molecules into the blood.

Hepatocytes

-appear to be relatively unspecialised.
- simple cuboidal shape
- surface microvilli
- metabolic functions: protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification etc.
-dense cytoplasm due to the amount of organelles.

Kupffer cells

-specialised macrophages
-involved in the breakdown and recycling of old red blood cells.

Urea

is an excretory product formed from the breakdown of excess amino acids.

Formation of urea

-Excess amino acids cannot be stored.
-amine groups make them toxic
-The amino acid molecules contain a lot of energy, so it would be wasteful to excrete the whole molecule.
-The treatment consists of 2 processes, deamination and the ornithine cycle.

Deamination

-The process of deamination produces ammonia
-soluble, highly toxic
- ammonia must not be allowed to accumulate
-it also produces keto acid , which can enter respiration directly to release its energy.

amino acid + oxygen ---- keto acid + ammonia

The ornithine cycle

is the process in which ammonia is converted to urea. It occurs partly in the cytosol and partly in the mitochondria, as ATP is used.

-The ammonia is combined with CO2 to produce urea.
-Urea is both less soluble and less toxic than ammonia.
-It can be passed into the blood where it can then be transported to the kidneys.
- In the kidneys the urea is filtered out of the blood and concentrated in the urine.

ammonia + carbon dioxide----- urea+ water

Detoxification

is the conversion of toxic molecules to less toxic or non-toxic molecules.

Detoxification of alcohol

-alcohol contains chemical potential energy, which can be used for respiration.
-It is broken down in the hepatocytes by the action of the enzyme ethanol dehydrogenase resulting in ethanal.
-Ethanal is the dehydrogenated further (by ethanal dehydrogenase) resulting in ethanoate (acetate).
-Ethanoate is then combined with coenzyme A to form acetyl coenzyme A, which enters the process of respiration.

NAD and reduced NAD

The hydrogen atoms released in the process of detoxification are combined with a conenzyme called NAD to form reduced NAD.
- NAD is also required to breakdown fatty acids for use in respiration.
-It the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids.
-The fatty acids are then converted to lipids and stored in the hepatocytes-- cirrhosis