Excretion - The removal of metabolic waste from the body.

Excretion and secretion: 
Differences - excretion produces metabolic waste, toxins or harmful substances that are to be removed from the body, and does not use vesicles. Secretion produces a useful product or is used in cell communication, and remains in the body. It uses vesicles or released from glands (ducts or ductless).
Examples - Excretion: urea, carbon dioxide, water. Secretion: hormones, enzymes, antibodies, muscus, neurotransmitters.
Similarities: Both require ATP, are involved in homeostasis, produced by cells, produced by metabolism, need to cross membranes, need to leave the cell, transported in the blood.

Carbon dioxide must be removed as it causes respiratory acidosis; breathing difficulties, headaches, drowsiness, restlessness, caused by carbon dioxide dissolving in the blood plasma and combining with water to produce carbonic acid, which dissociates to release hydrogen ions. This lowers the pH.
Nitrogenous wastes must be removed because the amino group is highly toxic, but proteins and amino acids are very high in energy, so it would be wasteful to excrete them. In the ornithine cycle, the amine group is removed to form ammonia, which forms urea, water and a keto acid when added to oxygen and carbon dioxide. The keto acid can be used in respiration and the urea (which is less toxic) is transported to the kidneys for excretion.

The liver.
The hepatic arteries supply the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy. 
The hepatic vein takes deoxygenated blood away from the liver - which rejoins the vena cava and normal circulation will proceed. 
Bile duct is where the substance bile is secreted, which is carried to the gall bladder where it is stored intil it is required in the small intestines. 
The hepatic portal vein brings blood from the small intestine. The blood is rich in the products of digestion, and this means that any harmful substances ingested will be broken down quickly by the liver cells (hepatocytes).
The liver is made up of lobules, which consists of cells called hepatocytes that are arranged in rows. Each lobule has a central vein in the middle that connects to the hepatic vein. Every single lobule has branches of the hepatic artery, hepatic portal vein and bile duct. Hepatic artery and hepatic portal vein are connected to the central vein by blood filled spaces called sinusoids. The blood flows past every hepatocyte via the sinusoids, this ensres that the toxins are broken down quickly. Also the blood provides the liver cells with oxygen. The central veins from all the lobules join up to the hepatic vein.

Amino acid + Oxygen --> Keto Acid + Ammonia
Ammonia + Carbon Dioxide --> Urea + Water

Catalase can convert 5 million molecules of hydrogen peroxide into harmless substances in a minute. Alcohol contains a lot of chemical potential energy which can be used in respiration. Ethanol dehydrogenase catalyses the detoxification of alcohol in hepatocytes.


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