Excretion

Define the term excretion.
Excretion: The removal of metabolic wastes from the body
Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body.
Carbon dioxide must be removed as, when it dissolves in water is produces hydrogencarbonate ions. These
ions compete with oxygen for space on the haemoglobin. This causes a reduction in oxygen transport.
Carbon dioxide can also combine directly with haemoglobin to form carbaminohaemoglobin, which has a
low affinity for oxygen.
Can cause respiratory acidosis; breathing difficulties, headaches, drowsiness , restlessness etc 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 orthinine 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 is transported to the kidneys for excretion.
Describe, with the aid of diagrams and photographs, the histology and gross structure of 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 until 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 vein are connected to the central vein via capillaries called sinusoid.
The blood flows past every hepatocytes via the sinusoid, this ensures that the harmful stuff are broken
down quickly. Also the blood provides the liver cells with oxygen.
The central veins from all the lobules join up to form the hepatic vein.
Describe the formation of urea in the liver, including an outline of the ornithine cycle.
Amino acid + Oxygen →Keto acid + Ammonia
Ammonia + Carbon dioxide → Urea + Water
2NH3+CO2→CO(NH2)2+H2O
Describe the roles of the liver in detoxification.
Catalase can convert 5 million molecules of H2O2 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.
Ethanol →Ethanal → Ethanoic Acid → Acetyl CoA
Ethanal and Ethanoic acid are dehydrogenated, and the hydrogen reduces NAD. If too many NADs are busy
detoxifying alcohol, there will be too few NAD to break down fatty acids for use in respiration, so the fatty
acids are converted back to lipids, which are stored in hepatocytes, making the liver enlarged- Fatty liver.
Describe, with the aid of diagrams and photographs, the histology and gross structure of the kidney.
Supplied with blood from the renal artery and is drained by the renal vein. The Kidney is surrounded by a
tough capsule, the outer region is the cortex and the inner is the medulla. The central region if the pelvis,
which leads into the ureter.