Excretion as an example of homeostatic control (5.1.2)
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|Content (from A-level)
The content from the specification that is covered by this delivery guide is:
|5.1.2 Excretion as an example of homeostatic control|
|(a)||the term excretion,and its importance in maintaining metabolism and homeostasis||To include reference to the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body.|
|(b)||(i) the structure and functions of the mammalian liver
(ii) the examination,and drawing of stained sections to show the histology of liver tissue
|To include the gross structure and histology of the liver
the roles of the liver in storage of glycogen, detoxification and the formation of urea (the ornithine cycle covered in outline only).
|(c)||(i) the structure, mechanisms of action and functions of the mammalian kidney
(ii) the dissection, examination and drawing of the external and internal structure of the kidney
(iii) the examination and drawing of stained sections to show the histology of nephrons
|To include the gross structure and histology of the kidney including the detailed structure of a nephron and its associated blood vessels
the processes of ultra filtration, selective reabsorption and the production of urine.
M0.1, M0.3, M1.1, M1.3, M2.1, M3.1
HSW4, HSW6, HSW8
|(d)||the control of the water potential of the blood||To include the role of osmoreceptors in the hypothalamus, the posterior pituitary gland, ADH and its effect on the walls of the collecting ducts.
|(e)||the effects of kidney failure and its potential treatments||To include the problems that arise from kidney failure including the effect on glomerular filtration rate (GFR) and electrolyte balance
the use of renal dialysis and transplants for the treatment of kidney failure.
HSW7, HSW9, HSW12
|(f)||how excretory products can be used in medical diagnosis.||To include the use of urine samples in diagnostic tests, with reference to the use of monoclonal antibodies in pregnancy testing and testing for anabolic steroids and drugs.
HSW7, HSW9, HSW11, HSW12
The structure of the liver (5.1.2b) and kidney (5.1.2c) should be examined using stained sections of each tissue. This can be prestained or unstained sections can be purchased from the Abcam website (see resource).
Students can perform a haematoxylin/eosin stain and observe their sections under a light microscope (PAG 1). Students could be encouraged to calculate the size of the cells to enforce their understanding of earlier topics (2.1.1d & e) and M0.2.
Both liver and kidneys could be dissected although only the lobular structure of the liver could be demonstrated with dissection (PAG 2).
The function of the liver in detoxification (5.1.2b) can be illustrated in a pharmacological context as many pharmaceutical companies have good websites on drug metabolism. For an illustration of drug metabolism see the Drug metabolism animation resource link.
The gross structure of the liver and kidney as well as detailed structure of a nephron could be literally pieced together by setting a diagram of the organs as a jigsaw puzzle. Many free jigsaw puzzle programmes are available from the Viscomsoft resource.
The Alcohol and your body resource contains useful information to construct a ‘drinking’ game which would combine well with the biochemistry of alcohol detoxification by the liver. Students can work in groups with role play cards (example below) to work out whether they would pass a breath test (HSW5 and HSW9).
Example role play card:
- You have had 3 small glasses of red wine.
- You weigh 65kg.
- You finished drinking 3 hours before the breath test
Importance of Excretion 5.1.2a
This topic combines many elements of medical physiology and could be approached from a medical aspect, particularly with a pathogenic point of view. Students often find it easier to grasp the relevance of particular systems if they know what happens when these systems fail. This also links well with other parts of the syllabus and introduces the topics in a wider social context. It allows students to explore these areas biochemically, anatomically, physiologically and ethically.
The activity ‘Diving into danger’ (see below) introduces a real life scenario where the build-up of carbon dioxide is fatal and allows them to use this to discuss the need to excrete the gas. This can be followed through with hereditary urea cycle disorders, as a means of showing the need to excrete urea and links well with later topics e.g. 6.1.2, Patterns of Inheritance.
The Ravicti website is useful to use in order to explore these genetic disorders is.
The role of the liver (5.1.2b) is linked to the urea cycle disorders and is a good way of introducing the urea cycle. Glycogen storage will connect with 5.1.4, homeostasis and 2.1.2f but can be discussed in the context of glycogen storage diseases. This stresses the role of the liver but also helps to demarcate the glucose related terms that many students mix up.
The American Liver Foundation (see resource) looks at Type I Glycogen Storage Disease, including a description of the disease and how it's diagnosed.
Following through the pathological theme, the importance of the basement membrane in ultrafiltration can be discussed in the context of Goodpastures Syndrome where the body’s immune system attacks the basement membrane. This links with autoimmune diseases (4.1.1k). The importance of the sodium-glucose co-transporter in selective reabsorption (5.1.2c) is emphasised by looking at inhibition of this protein, which is used to treat diabetes. This is a neat link with 5.1.4. The Australian Prescriber article (see resource) shows the action of sodium-glucose co transporter inhibitors.
Students often find it difficult to visualise how ADH attaches to kidney cells and elicits a response. The focal point of ADH could be taught in the context of its receptor which allows students to appreciate that ADH binds to the basolateral membrane and causes aquaporin movement to the luminal membrane. A defect in this receptor leads to diabetes insipidus which can be contrasted with diabetes mellitus as a link with 5.1.4e.
An interesting way of discussing the need for carbon dioxide excretion can be seen with divers. Students could discuss the difficulties that divers could face in terms of blood gas concentrations and then use an article to prompt further discussion and introduction to the need to excrete carbon dioxide e.g.
Article 1 resource; ‘Dave Shaw died from carbon dioxide black-out’. This article covers the death of a famous diver from carbon dioxide build up.
This could be connected with lung (3.1.1e) and haemoglobin function (3.1.2i) by using another diving scenario discussed in detail in Article 2 (see resource).
This topic can utilise many teaching methodologies, independent and team work as well as incorporating extensive numeracy and literacy aspects.
This can be discussed using hydrostatic pressure values especially for students who are mathematically orientated. The Diagrams resource provides figures and clear diagrams related to selective reabsorption.
Graphs (see Dialysis prep resource) can be used to summarise the roles of the different parts of the nephron (M1.3).
There are many animations that summarise various processes including the role of the loop of Henle ((5.1.2cii). The animation within the resource has no sound (could add your own!) but is a simple way of connecting the role of the loop to the vasa recta.
Indeed some Animations could be used to inspire students to work in groups and come up with their own animations/videos to illustrate renal dialysis and various aspects of kidney failure (5.1.2e). The Dialysis for kidney disease video is a good starting point to give students inspiration (see resource).
Students must produce pamphlets for kidney patients, discussing the possible causes and treatments. This is a good way of summarising the role of the kidney.
Ideas can be found by looking at the National Kidney Foundation website (see resource).
Kidney transplants can be discussed in a social context by looking at the illegal trade in transplants. See Guardian article resource.
Ethical consideration can also be given to anabolic steroid use and topical sports issues can be discussed and linked with ‘Winning without cheating’ (see activity below).
Models can be used to illustrate key principles. This is shown in the ‘Filtering out the waste’ activity. Models are also useful to demonstrate the pregnancy test strip with cut out antibodies for the various stages, an important feature to prevent confusion of the different types of antibodies involved.
Flow charts should be used for the role of ADH (5.1.2d) to emphasise the chronological events of its stimulation.
Simple annotation/labelling can be given added interest as illustrated with the ‘Let’s piece it together’ activity (in Curriculum content section).
Students often become confused with the consecutive steps involved in ultrafiltration and can fail to see the relevance of each structure. This can be reduced by making a model of the glomerulus.
Use a clear straw for the endothelium, putting slits in it to represent the fenestrations. The straw should be bent into a U shape. Use a broken abrasive wool or similar mesh and place it around the outside of the straw to represent the glomerular basement membrane. The structure can be held in a rubber glove (on the students hand) to represent podocytes. Red dyed water can then be poured into the straw to represent blood and students can discuss what is happening as the water moves through the slits, wool and in between the fingers of the glove.
To show the importance of urine testing in sport, students can be given gas chromatograms of various ‘samples’ and asked to interpret them alongside standard chromatograms/peak data, to show if anabolic steroids have been taken. This links with PAG6.
Standard chromatograms can be found within the resources.
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