1) Excretion in Liver, Lungs and Kidneys

• Liver:  excess amino acids cannot be converted to proteins and stored in the body, so has to be removed in a process called de-amination. De-amination takes place in the liver and involves the removal of the nitrogen-containing section of the amino acids, to form urea.
• Lungs: carbon dioxide is a waste product produced in cells during respiration, it dissolves in the blood, and is transported to the lungs to be excreted.
• Kidney: removes urea and other nitrogenous waste from the blood, and filters out excess water, salts, hormones and drugs to be excreted as urine. 

2) Describe Role of Liver

• Plays an important role in assimilation (absorbing) amino acids.
• Removes amino acids from the plasma of the bloodstream and builds them up into proteins.
• Proteins are long chains of amino acids, joined together by peptide bonds.

3) Explain Need for excretion

• Some of the compounds made in reactions in the body are potentially toxic if their concentrations build up.
• CO2 dissolves in fluids such as tissue fluid and blood plasma to form carbonic acid. This increase in acidity can affect the actions of enzymes and can be fatal.
• Ammonia is made in the liver when excess amino acid are broken down. However, ammonia is very alkaline and toxic. It is converted to urea which is much less poisonous, making it a safe way of excreting excess nitrogen.​

4) Key structure and Functions of Kidney

Source: Pearson

• The kidney’s role is to filter waste and excess substances from the blood to be excreted from the body as urine.
• Ureters transport urine from the kidneys to the bladder.
• Urethra is the tube that carries urine out of the body.

5) Outline the Structure and Function of a Kidney Tubule

Source: NIDDK

• The kidneys are made up of thousands of tiny tubule, or nephrons.
• Each nephron begins in the cortex, loops down into the medulla, back into the cortex, and then goes down again through the medulla to the pelvis.
  
• In the pelvis, the nephrons join up with the ureter.

• ​Each nephron includes a filter, called the glomerulus, and a tubule. The nephrons work through a two-step process:
• The glomerulus filters your blood, where glucose, urea, water and salts are removed.
  
• The tubule reabsorbed all of the glucose, most of the water and some salts back into the blood to prevent loss of too much water. Urea is not reabsorbed, leading to high concentration of urea in the urine.​

5) Explain Dialysis

• Dialysis is a treatment that filters and purifies the blood using a machine.
  
• Dialysis fluid contains: a glucose concentration similar to a normal level in the blood & a concentration of ions similar to that found in normal blood plasma & no urea.
  
• As the dialysis fluid has no urea in it, there is a large concentration gradient – meaning that urea moves across the partially permeable membrane, from the blood to the dialysis fluid, by diffusion.
  
• As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists.
  
• As the dialysis fluid contains an ion concentration similar to the ideal blood plasma concentration, movement of ions across the membrane only occurs where there is an imbalance.

6) Advantages and Disadvantages of Kidney Transplant, compared with Dialysis

• Kidney transplants are an alternative to constant dialysis.
  
• Although this comes with the risks associated with major surgery, as well as the risk of rejection to the organ, a successful kidney transplant can raise the quality of life of the patient and patients are not required to undergo hours of dialysis.
  
• Most humans are born with two kidneys, although they can survive with one, thus kidney transplants are from family members, as there is less chance of rejection.