Physics and Physiology

TABLE OF CONTENTS

Barotrauma
 Oxygen Uptake
 Diving Oxygen needs
 Whales and DCS
 Diet and Ideal Body Weight

 

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I become concerned when a diver is advised to avoid diving deeper that 30 feet. The reason is depicted in the graph below which shows that the greatest change in lung volume occurs near the surface, where the risk for lung overpressure injury and Arterial Gas Embolism is the greatest. Lung overpressure injury can occur from as little as 4 feet of depth because of the Boyle's law relationship: P V = K, which states the the product of absolute pressure and volume is a constant. As pressure falls on ascent, the lung volume expands.

 

 

This diagram shows the percent change in lung volume for 10 foot changes in depth from 100 feet to the surface. Note that there is a 32.7% change in volume when ascending from 10 feet to the surface, while the change in volume when going from 100 to 90 feet is only 2%.

 

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 Although diving may seem to be a relaxing sport which requires little physical exertion, the truth is that a considerable amount of physical exertion can occur during diving, and good physical conditioning is essential for safe diving. Conditioning is measured by the amount of oxygen which a person can take up per minute when exercising at maximum capacity. This measure is called the Maximum Oxygen Uptake, and is an excellent measure of aerobic conditioning. The sensation of discomfort related to high levels of exercise depends on the percent of maximum capacity which the exercise requires. Working near 100% of maximum is very uncomfortable, and usually can only be sustained for a few minutes. Steady exercise can be done up to about 65% of maximum. At higher intensity, excess lactic acid is produced, and exercise is limited.

  The graph shows the amount of oxygen used as workload increases. There is a limit to the extent that oxygen uptake can increase, and at a high workload, maximum uptake no longer rises. Diver A working at intensity 'a' (about 70% of max) will be uncomfortable, but can tolerate the exercise. Diver B working at intensity 'a' (over 100% of max) will quickly become short of breath, and fatigue will occur within minutes. If this work load is needed to get to safety during a dive, diver A but not diver B will return safely. Diver B can only sustain steady exercise at intensity 'b'. You can improve your exercise capacity, and move the Maximum Oxygen Uptake higher with exercise conditioning. RETURN

 

 

 

The oxygen consumption curves shown in the previous discussion can be applied to the graphs below to determine capacity for a diver of different levels of conditioning. One knot is about 100 feet per minute of speed. Swimming at one knot requires an oxygen consumption of about 25 ml/kg/min (blue curve). If this is 70% of maximum, then maximum must be about 36 ml/kg/min. Equivalent Mets (multiples of resting oxygen consumption) are shown in the red graph. Swimming at a leisurely pace underwater is done at about 0.3 knots, which requires about 10 ml/kg/min. This is a tolerable pace even for a poorly conditioned diver. The excess capacity helps when swimming is needed to avoid problems. Then poor conditioning can result in unsafe diving. Graph is from: Diving Medicine A.A. Bove, Editor. W.B. Saunders Co, 1997, Ch 21

The blue curve on the left graph shows oxygen consumption in ml/kg/min for swimming speeds from 0.4 to 1.6 knots. 1 knot is about 100 feet per minute. Swimming against a 1 knot current required a good level of conditioning.

The red curve shows the same data based on Mets. One Met is equivalent to the oxygen consumption at rest (about 3.5 ml/kg/min)

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The formation of nitrogen bubbles in the tissues and blood causes numerous reactions and tissue injury which we know as decompression sickness (DCS). Various animal models of DCS have been developed, including cell cultures, tadpoles, mice, dogs, cats, goats, sheep and humans. Studies in these models over many years have allowed diving to become a safe and enjoyable sport , and a safe, although somewhat risky, occupation.

Although most air-breathing animals require a life support system underwater, some have adapted to the aquatic environment and can perform tremendous breath holding dives to depths below 1000 feet. Sperm Whales have been observed at depths of 800 meters ( 2600+ feet) , and porpoises have been observed to dive as deep as 300 meters (900+ feet). These dives are made on a single breath, and can last for over an hour in the case of the Whales, and over 30 minutes in the case of seals. How do these animals avoid decompression from these long, deep dives?

This question is best studied by a mathematical analysis of nitrogen kinetics in a typical diving mammal. For large whales, the weight in tons is about the same number as the length in feet. So let us analyze a 100 foot long, 100 ton sperm whale, diving to a depth of 100 meters (about 320 feet) for 1 hour. This would require a long decompression for a human diver breathing a gas mix which at this depth would likely be a helium, nitrogen, oxygen mixture. Another interesting assumption is that the whale doesn't live on the surface, but usually stays in the 20-30 foot depth range, and surfaces every now and then to breathe. This slightly increased baseline pressure also gives the whale an opportunity to reduce the pressure difference durnig ascent from a dive. A few other numbers are needed to begin the calculation: assume the whale is 40% fat, and about 35% water, blood volume is 200 ml/kilogram of body weight, solubility of nitrogen in blood, water and fat are usually accepted values (.013, .0123, .067), lung volume is 50 ml/kilogram, the nitrogen half time for water is 30 minutes, and for fat is 120 minutes. Whales are adapted to allow their chest to collapse under pressure, as the lungs get smaller from the Boyle's law relationship. By a combination of blood shifts into the lung, and the collapse of the chest, the whale avoids lung squeeze which would occur in a human diving to these deep depths.

    The graph shows the increasing Nitrogen content of the three tissues during a 2 hour 100 meter dive. The yellow line shows fat nitrogen, blue, the water nitrogen in solid, and total capacity of nitrogen dashed, and red, the blood nitrogen, dashed is the total capacity. Total fat nitrogen capacity is 957 moles, not shown on the graph.

Based on Henry's law, the whale's fat could hold 945 moles of nitrogen at 100 meters depth, and the water could hold 154 moles of nitrogen. The blood could hold 93 mols of nitrogen when saturated at 100 meters. Neither the fat nor the water in tissues and organs of the body will become saturated during the 2 hour dive. There is not enough nitrogen in the lungs to fill the capacity for nitrogen, fat nitrogen will exceed base values by 32%. From other studies, the 120 minute half time tissue can safely oversaturate to 60% over base value. The nitrogen in fatty tissue will not supersaturate enough to cause bubbles. The same occurs for water in tissues and organs. The remaining problem is the blood, where most of the nitrogen could be rapidly transferred as the whale descends. Usually when diving with compressed air, the supersaturation of the slower tissue compartments determines the need for decompression stops. Because these compartments do not take up enough nitrogen to become a decompression problem, they do not have an effect on the decompression. The blood however may be an important problem during decompression. Blood is considered to be a rapid half time tissue. It takes up nitrogen rapidly from the lung because of the direct interchange of gases which occurs in the lung. The blood nitrogen capacity of the 100 ton sperm whale at 100 meters depth is calculated to be 93 mols, and the total amount of nitrogen in the lung is 142 mols. RETURN

 The blood will take up most of the nitrogen which is in the lungs, if the exchange of gas occurs normally. There is some question of whether nitrogen diffusion into the blood is slowed during diving because of the reduction of lung size from Boyle's law. If this is the case, the blood would be a slower uptake tissue. I tested the model with blood half times ranging from 3 to 30 minutes. The 5 minute tissue is usually assigned a safe surfacing supersaturation ratio of about 3.5. This means that the blood can be supersaturated to about 3.5 times surface capacity without concern for bubble formation and decompression sickness. If the blood takes up nitrogen rapidly, the supersaturation ratio achieved by the whale during ascent from 100 meters would be about 5.5, and would exceed the allowable ratio. Under these circumstances, bubbles would form in the blood. To avoid bubbling in the blood, the whale would have to ascend to about 30 meters and remain for about 8 minutes, the whale could then surface for a breath, and return to the base depth of 20-30 feet to avoid bubbling.

How does the whale avoid decompression sickness on deep, long dives? The body fat and water do not receive enough nitrogen to become saturated to the point of bubble formation. The blood is likely to be protected by the collapse of the lungs during descent, with a reduction of the capacity to transfer gases from lung to blood. Under these circumstances, there is not enough nitrogen to be distributed to blood and tissues, and the supersaturation needed to cause bubble formation is prevented. RETURN

 

 

 

 

 

DIET AND IDEAL BODY WEIGHT

There are many different diets designed to accomplish specific goals. If your weight is normal, you are an active adult, and need to eat healthy for diving, you need to know some of the basics about nutrition. To understand nutrition, you need to understand the basic structure and calorie value of foods. We usually refer to the energy value of foods in calories. Physics texts define a calorie as the amount of energy needed to raise a gram of water one degree centigrade, and a kilocalorie as the amount of energy needed to raise a kilogram of water one degree centigrade. In food science, the kilocalorie is referred to as one Calorie (with a capital C). One food Calorie is equivalent to 1000 physical calories. Energy requirements for daily activity depend on the amount of work you do, your age, body size and metabolic rate. An average man doing a day's work in an office uses about 30 Calories per kilogram (13.6 Calories per pound) of body weight, and an average women uses about 25 Calories per kilogram (11.4 Calories per pound) of body weight per day. Individuals who have jobs requiring physical exertion have higher energy demands. Most dietitians recommend a balance of 20% protein, 30% fat and 50% carbohydrate (CHO) for daily energy needs. The table below shows the distribution of calories for a 170 pound man and a 140 pound woman, both leading sedentary lives. RETURN

 DAILY CALORIE NEEDS

   Wgt lbs  Tot Cals  fat - 30%  cho - 50%  prot - 20%
 Man  170  2312  694  1156  462
 Woman  140  1596  479  798  319

 DAILY FOOD NEEDS

   Wgt lb  Tot Cals  Gms Fat  Gms cho  Gms Prot
 Man  170  2312  77  289  115
 Woman  140  1596  53  249  80

 

 The amount of each nutrient in grams is provided for the same calorie distribution in the second table (above). If you want to reduce the amount of fat in your diet, you need to replace the calories with another food type. In most low fat foods, the substitute is carbohydrate in the form of sugar, but the proportions often leave you with the same or a greater number of calories. It is ironic that a high carbohydrate diet can cause an elevation of triglycerides (fats) in the blood, and can have an effect opposite to what is expected from the diet change. RETURN

If you are contemplating a special diet, you should determine whether there is adequate information to justify a large deviation from the recommended balance of foods. Although requirements for essential fatty acids (the building blocks of fat) are only 1-2% of total caloric intake, a very low fat diet may deprive you of these essential fatty acids, and cause a loss of normal nutritional requirements. If you are concerned about blood fats and the risk for atherosclerosis, a modest reduction of saturated fat (animal fats, and tropical oils like coconut and palm) in the diet might be reasonable, but in some individuals, excess carbohydrates and excess fats can raise blood lipid levels. A long distance runner training daily will require a high calorie intake which is usually made up of increased carbohydrates. If you reduce total food intake below your daily energy needs, you will lose weight.

Ideal Body Weight
How do you determine your ideal body weight. There are charts to look up you ideal weight, but you can calculate your ideal body weight using the body mass index. Body mass index is calculated from height and weight. If you meet the BMI guidelines you are at your ideal body weight. BMI should be about 23. BMI is calculated as:
Weight in Kilograms/square of height in meters.


BMI = wgt/(hgt*hgt)
Kilograms = pounds/2.2
Meters = inches/39.37
If you use pounds and inches:
BMI = 704 * wgt/(hgt*hgt)
To find your ideal weight, use a BMI of 23. The ideal weight is:
ideal weight = hgt*hgt/30

Example: Ideal weight for a man who is 5'11" (71 inches) is:
71*71/30 = 168
Ideal weight for a women who is 5'5" (65 inches) is:
65*65/30 = 141

A man who weighs 168 pounds and is 5'11" tall is at his ideal body weight. Variation occurs in the ideal weight based on body build.

Maintaining an ideal body weight by moderation in total food intake, while following the recommended balance of food components, is still the best and safest means of achieving a healthy life style for diving. When you perform a greater amount of work, you can compensate by increasing food intake to obtain additional energy. To be ready for the demands of diving, you should add an exercise program to your nutritional program. A rough rule of thumb is that a mile traveled on foot consumes about 100 calories of energy regardless of the time it takes. A cup of dry cereal, or a cup of 1% milk is equivalent to 100 calories. So a two-mile walk will account for most of the calories in your breakfast. If you are aware of your usual daily dietary needs, and add as your activity dictates, you should maintain a stable weight, and be ready for the physical demands of diving. RETURN