For the Afternoon of Tuesday 9 November 2010
FOR THE AFTERNOON OF TUESDAY 9 NOVEMBER 2010
Essentials of Clinical Medicine / Tuesday 9 November 2010Session 17
Homeostasis
Group work
Total body water
1. What is the expected value for the normal total body water, expressed as fraction of body weight, in:
- a female of average weight
- a male of average weight
- a new born infant
- a male aged 70 years.
2. What may account for the differences you give?
Distribution of body water
3. How is body water distributed among compartments?
4. How, in general, would you measure the size of these compartments?
5. Outline the electrolyte composition of the various compartments of body fluids.
6. How far do the concentrations of these components change in normal physiological conditions? What might you expect to happen in severe exercise?
Cellular water and interstitial fluid
7. How is an equilibrium maintained between cellular water and interstitial fluid?
8. In open heart surgery, a cardioplegic solution may be used to stop the heart. The composition of St Thomas’ cardioplegic solution No.2 is:
component / concentration (mmol-1)Na+ / 120
K+ / 16
Mg2+ / 16
Ca2+ / 1.2
Cl- / 160.4
HCO3- / 10
(You are not expected to remember the detail of this composition. You will consider how cardioplegic solutions stop the heart during the CVS module.)
8.1 What effect will this solution have on membrane potential of cardiac myocytes?
8.2 Why does cell volume increase in the heart perfused with such a cardioplegic solution?
8.3 How might you consider altering the composition of a cardioplegic solution to prevent this increase in cell volume? (You are being asked to speculate here – there is no clear ‘correct’ answer.)
9. Central neurones, exposed to low osmolality solutions, release taurine. What effect will this release have on the expected cell swelling resulting from the low osmolality of the extracellular solution?
Interstitial fluid and plasma – Starling forces
10. What factors determine the equilibrium between interstitial fluid and plasma?
11. What will happen to this equilibrium if capillary permeability is increased, as in anaphylaxis?
12. What will happen to this equilibrium if venous hydrostatic pressures are increased, as in ventricular (heart) failure?
13. Liver disease may result in oedema. What factor(s) lead to oedema in such conditions?
14. Disease of the renal glomerulus (which filters blood as the first step in urine formation) may also lead to oedema. Why should this be the case?
Dehydration
15. Diarrhoeal diseases lead to loss of body water, often causing death through dehydration. What will happen to the various compartments of body water in cholera?
16. What will happen to a) urine output, b) blood pressure, c) body temperature, and d) haematocrit in someone suffering from a diarrhoeal disease?
17. Oral rehydration therapy was introduced ~30 years ago. Its introduction led to a world wide reduction in child deaths from diarrhoea from 4.6 million a year in 1980 to around 1.5million a year in 2000. The World Health Organisation and UNICEF recommend use of a solution with the following composition:
component / concentration (mmol-1)Na+ / 75
Cl- / 65
Glucose / 75
K+ / 20
citrate3- / 10
(You are not expected to remember the composition of this solution.)
17.1 Why does this solution contain glucose and Na+?
17.2 By what mechanism(s) are these transported through the intestinal mucosa?
17.3 Why does the solution contain K+ in addition to Na+?
17.4 By what mechanism does cholera (infection with Vibrio cholerae) generate high loss of salt and water through diarrhoea?
Hypovolaemic shock (You will learn more about shock in semester 2, so detailed understanding is not expected at this stage.)
18. What do you understand by the term hypovolaemic shock?
19. Hypovolaemia may be managed by giving intravenous fluids. What do you understand by a) a crystalloid solution and b) a colloid solution? How would these solutions be distributed through the body fluid compartments?
20. 0.9% NaCl solution is often described as isotonic. What does the term isotonic denote? Show, by calculating the concentration of NaCl in mmol.l-1, that 0.9% salineis isotonic. The molecular weight of NaCl is 58.5.
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