13. Excretion in humans

Annotations:

• Excretion is the removal of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements

1. state that urea is formed in the liver from excess amino acids
   1. describe the role of the liver in the assimilation of amino acids

      Annotations:

      • by converting them to proteins, including plasma proteins, eg fibrinogen
      1. define deamination

         Annotations:

         • as the removal of the nitrogen-containing part of amino acids to form urea
2. state that carbon dioxide is excreted through the lungs

   Annotations:

   • Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells
3. describe that the kidneys excrete urea and excess water and salts
4. explain that the volume and concentration of urine produced is affected by water intake, temperature and exercise

   Annotations:

   • Water intake – the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced. Temperature – the higher the temperature the more water is lost in sweat and so less will appear in urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced. Exercise – the more exercise done, the more water is lost in sweat and so less will appear in urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced. (More concentrated means higher urea content)
5. identify on drawings, diagrams and images ....
   1. the ureters
   2. bladder
   3. urethra
6. explain the need for excretion

   Annotations:

   • limited to toxicity of urea and carbon dioxide Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells. This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions. For this reason, too much carbon dioxide in the body is toxic. Urea is also toxic to the body in higher concentrations and so must be excreted
7. outline the structure of the kidney
   1. cortex
   2. medulla
   3. ureter
8. outline the structure and functioning of a kidney tubule

   Annotations:

   • including: - the role of the glomerulus in the filtration from the blood of water, glucose, urea and salts - the role of the tubule in the reabsorption of all the glucose, most of the water and some salts back into the blood, leading to the concentration of urea in the urine as well as loss of excess water and salts (details of these processes are NOT required)
9. explain dialysis

   Annotations:

   • in terms of salt balance, the maintenance of glucose concentration and the removal of urea This is an artificial method of filtering the blood to remove toxins and excess substancesPatients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood
   1. describe the use of dialysis in kidney machines

      Annotations:

      • Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm. Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane  – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists
      1. discuss the advantages and disadvantages of kidney transplants, compared with dialysis

         Annotations:

         • Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including: Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced – but not removed –  by ‘tissue typing’ the donor and the recipient first) This has to be suppressed by taking immunosupressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections.  There are not enough donors to cope with demand. However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include: the patient has much more freedom as they are not tied to having dialysis several times a week in one place.Their diets can be much less restrictive than they are when on dialysis.Use of dialysis machines is very expensive and so this cost is removed.A kidney transplant is a long-term solution whereas dialysis will only work for a limited time.