vicky
Guest
From the dictionary:
"A model is a set of propositions or equations describing in simplifed form some aspects of our experience. Every model is based upon a theory, but the theory may not be stated in concise form".
Let's take a barrel full with water (or better- wine
) and make a hole in it. Wine will begin to flow out of the barrel. We can put a small bottle below the barrel and fill it with the spilling wine. We can now make theories and calculations for how long it will take to fill the bottle, taking into account the decrease in the barrell's pressure as wine level gets lower, atmospheric pressure, the size of the hole etc. etc.
The above example can be simplified by a "2-compartment model" the first compartment represents the barrel, the second the wine bottle. The connection between the two compartments represents the flow of wine from first compartment to the second one. For this example, if the bottle if filled in an exponential process, one can establish the "time constant" of the filling bottle. It can be measured each time the bottle's remaining volume is halfed, and of course compared to our theory.
One can build a multiple-compartment model for antibiotic level in the blood: The first compartment can represent the stomach (where we swallow the pills) with a time constant that represents how long it takes for the drug to be absorbed into the blood (Here's the second compartment). Other compartments can represent several tissue-types where the antibiotics will be delivered from the blood, then we can add a compartment for the kidneys which perhaps take away some of the drugs into the urinary system and so on. After that one needs to make experiments to establish all the time-constants, or flow-constants or whatever represents the connection between compartments. Then one can predict the dosage (how many pills a day and how much antibiotics each one contains) in order to keep a constant and efficient amount of drug in the tissues. Usually the results are simplified to fit a genrral average of population- the dosage is set according to body weight and the doctor tells you to how many pills to take every day etc. etc.
So, in a similar manner Haldane applied the same type of models for absorption of Nitrogen in tissues. Each tissue is represented in the model by a compartment. Of course, it is a model, not reality, and our body consists of more than 5,8, 16 or any other number of compartments we may add to the model, and perhaps the time-constants for these "tissues"/compartments may be dynamic, and the whole model may be influenced by various parameters, as Dr. Deco suggested (for example- alcohol, body physical fitness, tiredness, dehydration, and all the other DCS "risk factors"). But as long as the model makes a fair representation in order to make a prediction for the amount of Nitrogen in the body or actually in order to calculate the "M-values", NDL times and so on- then the model is a good one. By the way, models are supposed to be as simple as possible. If one makes a too-complicated model it is rather possible the model will fail because of having too many degrees of freedom that cannot be predicted. That's one of the nice things about models- they are neat and simple but make a good representation of reality. You will be surprised how many things in our life can be represented by very simple models...
"A model is a set of propositions or equations describing in simplifed form some aspects of our experience. Every model is based upon a theory, but the theory may not be stated in concise form".
Let's take a barrel full with water (or better- wine
) and make a hole in it. Wine will begin to flow out of the barrel. We can put a small bottle below the barrel and fill it with the spilling wine. We can now make theories and calculations for how long it will take to fill the bottle, taking into account the decrease in the barrell's pressure as wine level gets lower, atmospheric pressure, the size of the hole etc. etc.The above example can be simplified by a "2-compartment model" the first compartment represents the barrel, the second the wine bottle. The connection between the two compartments represents the flow of wine from first compartment to the second one. For this example, if the bottle if filled in an exponential process, one can establish the "time constant" of the filling bottle. It can be measured each time the bottle's remaining volume is halfed, and of course compared to our theory.
One can build a multiple-compartment model for antibiotic level in the blood: The first compartment can represent the stomach (where we swallow the pills) with a time constant that represents how long it takes for the drug to be absorbed into the blood (Here's the second compartment). Other compartments can represent several tissue-types where the antibiotics will be delivered from the blood, then we can add a compartment for the kidneys which perhaps take away some of the drugs into the urinary system and so on. After that one needs to make experiments to establish all the time-constants, or flow-constants or whatever represents the connection between compartments. Then one can predict the dosage (how many pills a day and how much antibiotics each one contains) in order to keep a constant and efficient amount of drug in the tissues. Usually the results are simplified to fit a genrral average of population- the dosage is set according to body weight and the doctor tells you to how many pills to take every day etc. etc.
So, in a similar manner Haldane applied the same type of models for absorption of Nitrogen in tissues. Each tissue is represented in the model by a compartment. Of course, it is a model, not reality, and our body consists of more than 5,8, 16 or any other number of compartments we may add to the model, and perhaps the time-constants for these "tissues"/compartments may be dynamic, and the whole model may be influenced by various parameters, as Dr. Deco suggested (for example- alcohol, body physical fitness, tiredness, dehydration, and all the other DCS "risk factors"). But as long as the model makes a fair representation in order to make a prediction for the amount of Nitrogen in the body or actually in order to calculate the "M-values", NDL times and so on- then the model is a good one. By the way, models are supposed to be as simple as possible. If one makes a too-complicated model it is rather possible the model will fail because of having too many degrees of freedom that cannot be predicted. That's one of the nice things about models- they are neat and simple but make a good representation of reality. You will be surprised how many things in our life can be represented by very simple models...
