Excretion Flashcards
What is excretion
This is the removal from the body of these substances such as water and ions thatare excess in body.
Function of a liver
It breaks down unwanted substances and produces excretory waste.
Controls glucose concentration in the blood
It absorbs and metabolises much of the nutrients that are absorbed in the small intestine.
It makes bile salts.
What are bile salts needed for
They neutralise excess stomach acid.
They help digest fat and bile pigments (which are a waste product). These are stored in the gall bladder as part of the bile and released into the duodenum via the bile duct.
From where does the liver get a blood supply from
Hepatic artery and the hepatic portal vein.
Oxygenated blood flows from the hepatic artery to the liver
And deoxygenated blood flows from the hepatic portal vein.
Where does the blood go from the liver to
The deoxygenated blood flows from the hepatic vein.
Structure of the liver
It is divided into many lobules that are separated by connective tissue. The liver is only made of hepatocytes (liver cells). Each lobule is supplied blood from the branches of the hepatic artery and the hepatic portal vein. The blood flows through wide capillaries known as sinusoids that are lined by an incomplete layer of endothelial cells which allows blood to reach hepatocytes. This facilitates the exchange of substances between blood and cells. The lobule carries a bunch of chemical reactions. Each hepatocyte has a large surface area in contact with blood for exchange of substances.
What is a heptocyte
They store glucose as the polysaccharide glycogen. They also make bile (a digestive secretion) that is stored in the gall bladder and enters the duodenum. It contains bile pigments and bile salts.
What are bile pigments
These are excretory products made from haemoglobin.
How are bile salts
These are formed from the emulsification of fats.
How was the bile duct formed
Little channels (canaliculi) that join together which drains into the gall bladder and the duodenum. The blood flows along the sinusoids from the branches of the hepatic artery and the hepatic portal vein to drain into a branch of hepatic vein. This deoxygenated blood flows back to the heart through the vena cava.
How are proteins digested
Protein in the diet is digested into amino acids which are absorbed into the blood and taken directly to the liver. Excess amino acids are not excreted.
What happens to the excess amino acids
As these are good sources of energy, the amine group is removed. After the deamination, ammonia is formed. This is converted to ammonium ions in the cytoplasm. After the removal, the organic acid left is called keto acid. This may be respired aerobically through the Krebs cycle in mitochondria. Or else, it can be in the synthesis of other compounds.
How is ammonia made less harmful
Through the ornithine cycle.
Ammonia and carbon dioxide reacts together to form Carbamyl phosphate (1N). Two ATPs are needed. This carbamyl phosphate is then converted to Citruline (3N). This compound then reacts with an amine group to form Arginine (4N). This is then converted to Ornithine (2N). To do this, water needs to react with arginine to also form urea.
Why is urea excreted instead of ammonia
It is soluble in water and less toxic than ammonia.
Urea readily diffuses through the phospholipid bilayer of the membranes and so leaves the hepatocytes. This is then transported to the kidney dissolved in the blood plasma.
What are other two nitrogenous waste products
Uric acid- made from excess purine bases
Creatinine- made from creatine phosphate
What is detoxification
The liver breaks down many substances that are no longer required. These include Lactate Alcohol Hormones Medicinal drugs
What is lactate metabolism
Glycogen is broken down to glucose in the muscle. This glucose goes through glycolysis and links reaction to form Triose phosphate and then to pyruvate. This then goes through anaerobic respiration to produce Lactate. Pyruvate accepts two hydrogen from redNAD to form lactate. This causes NAD to become oxidised; the enzyme, lactate dehydrogenase catalyses this reaction.
The lactate enters the blood plasma and is transported to the liver,
After it does enter the liver, the lactate is converted back to pyruvate. This reaction involved reducing NAD. The pyruvate enters the matrix of the mitochondria and goes through aerobic respiration. Carbon dioxide and water is produced.
The pyruvate is then converted back to triose phosphate and then to glucose. The triose phosphate can also be stored as glycogen in the liver. The tlucose enters the plasma and is transported to the muscles.
How does alcohol metabolism work
Ethanol enters the cell through the cell surface membrane and is converted to ethanal with the help of the enzyme alcohol dehydrogenase as well as NAD. NAD becomes reduced.
The ethanal is converted to acetate with the help of the enzyme, ethanal dehydrogenase. The NAD is reduced here again. Otherwise, the ethanal can enter the matrix of a mitochondria and can be converted to acetate here. It will go through the aerobic pathway. So, it will be converted to acetyl coenzyme A. From here, it can used to form fatty acids or go into the Krebs cycle to ultimately produce carbon dioxide and water. Ethanal dehydrogenase would catalyse this reaction.
