OCR - Excretion

  • Created by: Yetunde
  • Created on: 26-11-11 21:28

define the term excretion 

the removal of metabolic waste from the body. That is, removal of by-products or unwanted substances from normal cell processes.

Produced in large amounts: C02 - Respiration and Nitrogen containing compounds (urea)

explain the importance of removing metabolic wastes, including Carbon Dioxide and nitrogenous waste from the body


produced from every living cell in the body by respiration C6H1206 + O2 ---> H20 + CO2.

passed into the bloodstream and transported to the lungs in the form of HCO- Diffuse into the alveoli and breathed out.

Excess CO2 is toxic and in high levels lead to 3 main effects:

  • forming hydrogen carbonate ions also form H+ ions in the RBC under the influence of carbonic anhyrdase. H+ combine to RBC and compete with oxygen. Too much carbon dioxide  can reduce oxygen transport 
  • carbon dioxide can combine directly with rbc -> carbaminohaemoglobin. This molecules has a lower affinity for oxygen, lesss oxygen transport.
  • Respiratry acidosis -  CO2 dissolves in blood plasma and combine with water to produce carbonic acid : CO2 + H20 --> H2CO3. Carbonic acid dissociated to form HCO- and H+. The extra H+ ions lower PH. protein in the blood act a buffers trying to minimise the change. respiratory  centre in the medulla oblongata  detects the changes and causes increased breathing and heart rate to remove excess CO2.

Nitrogenous compounds

  • he body cannot store amino acids/ excess protein however contain lots of energy. To avoid waste, the are transported to the liver to undergo de-animation.

describe with the aid of diagrams and photographs, the histology and gross structure of the liver. 


Hepatic artery - oxygenated blood from the heart (aorta) into the liver supplying cells with oxygen needed for aerobic respiration, proving ATP needed for the highly metabolically active hepatocytes.

Hepatic portal vein - deoxygenated blood from the digestive system enters the liver rich in the products of digestion (protein, alcohol etc) 

Hepatic vein - blood leaves the liver to rejoin the vena cava

Bile duct - not a blood vessel - carries bile from the liver into the bile duct where it is stored until needed to aid the digestion of fats.


Inside the liver, the vessels are divide into lobes which are further divided into lobules 

Hepatocytes - full of specialised organelles to help with with metabolism (dense)

   microvilli on the surface

Sinusoids - blood from hepatic portal vein/hepatic artery drained into.

lined with hepatocytes

close for easy exchange

Kupffer Cells - macrophages that move inside the sinusoids engulfing bacteria. Break down and recycle RBC -> bulirubin  

Describe the formation of urea in the liver, including an outline of the ornithine cycle.

liver roles

  • detoxification of alcohols and drugs 
  • control of blood glucose/insulin levels.
  • breakdown of RBC

Formation of urea - deamination and ornathine cycle.


amino acid + 02 ----->  ammonia  + keto acid

ammonia is soluble + highly toxic and must be converted quickly

keto acid used in respiration 

Ornathine cycle 

ammonia + CO2 ------>  CO(NH2)2 (urea) +


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