Physiological systems for Santos™ consist of all the life maintaining systems, which occur within a human. We primarily attempt to model the circulatory system, ventilatory system (lungs), body fluids system (kidneys), metabolism and temperature regulation. The initial focus was on creating the ability to simulate the vital signs of for Santos™.

Vital signs are physical signs that indicate an individual is alive. These signs can be observed, measured, and monitored to assess an individual's level of physical functioning. The vital signs are body temperature, pulse (heart rate), respiratory rate (breathing), and blood pressure.

12.1.2 Normal ranges of the vitals for the average healthy adult

(Medlineplus medical encyclopedia,2004 )

Temperature: 97.8-99.1 degrees Fahrenheit / average 98.6 degrees Fahrenheit

Breathing: 12-18 respirations (breaths) per minute
Pulse: 60-80 beats per minute (at rest)

Blood Pressure: Systolic: less than 120 mm (of mercury),Diastolic: less than 80 mm Hg

Significant progress has been made on modeling the circulatory system. The ventilatory system, temperature regulation and blood pressure are still in the early stages of development.

12.1.3 Introduction

The body requires energy to sustain its activities. Often the combustion engine has been compared to the human body. In the combustion engine gasoline and air are introduced into the cylinder, a spark ignites the mixture and the resulting explosion forces a piston to move. The engine is cooled to prevent overheating and the waste products are expelled. The engine is dependant on a continuous supply of gasoline and functions only in the presence of oxygen; hence its function is aerobic. The initial movement of the piston (self starter mechanism) is powered by the electrical energy stored in the battery. The starter works in the absence of oxygen, its function is anaerobic. The energy of the battery is limited and must be frequently recharged.

In the human machine the muscle fibers are the pistons, Adenosine triphosphate (ATP) is the gasoline, the waste products are constantly excreted through sweat, urine and feces. The energy produced tends to heat up the body. The core body temperature has to be maintained in range of 4^oC of 36.5^oC. To maintain the core body temperature the blood carries away the excess heat and transfers it to the skin, where it is transferred to the surrounding atmosphere through convection and radiation, aided by sweating.

Contraction is the muscles only active property. The main source of energy of energy for muscular contraction is the carbohydrates and fat from the food consumed. Special chemical compounds act as carriers of energy within the cell, ATP is one such substance, which is the cells primary source of energy for metabolic processes. ATP breaks down into Adenosine Diphosphate (ADP) with the release of energy. Various phosphate compounds represent different energy levels. ATP is the highest, with the loss of one of three phosphate radicals yields ADP, which yields Adenosine Monophosphate (AMP), inorganic phosphate is the lowest energy level. The stores of ATP are limited and have to be constantly regenerated. Glucose, fatty acids and amino acids are oxidized to produce ATP molecules. The regeneration of ATP from ADP and other waste products like lactic acid also requires oxygen. ATP replenishment is the result of oxygen consumption in the electron transport system of the mitochondria of the body’s cells.

Because energy expenditure is due to the breakdown of ATP, and because that ATP is replenished as the result of oxygen utilization in the mitochondria either during exercise or during recovery, there is a direct relationship between oxygen consumption and energy expenditure. Accurate measurements of the kilocalories of heat produced as the result of oxygen utilization, show that for normal subjects on a mixed diet of fat, carbohydrate and protein, expend about 5Kcal of energy for each liter of oxygen they consume (Lamb, 1978).

Figure 1. Simplified scheme of physiological response to physical activity (Lamb,1978)

12.1.4 Oxygen uptake

The oxygen consumed by the muscles in liberating energy is called the oxygen uptake.

During physical activity the oxygen uptake increases and the cardiac output also increases (Åstrand, 1970). The oxygen needed for aerobic processes is assumed to be supplied from the increased blood flow. The muscle myoglobin stores contribute little to the net oxygen used by the muscles (Barstow, 1987).

12.1.5 Energy

The energy supplied during the first few seconds of an activity is almost entirely anaerobic, it occurs in the absence of oxygen primarily by the breakdown of creatine phosphate and muslce glycogen or glucose (Lamb,1978) .Since this energy is anaerobic the oxygen from the blood is not yet required. As the duration of the activity increases, the amount of ADP increases, the result of ATP breakdown. This increase in ADP is the spark for aerobic metabolism to begin. (Lamb,1978).

In order to simulate the heart rate, the total energy used has to be split into the anaerobic and aerobic component at every time instant. The oxygen supplied by the blood is only for the aerobic component. It is not clearly known how much is anaerobic and how much is aerobic, for energy required for different activities and is still debated (Medbǿ,1989).

In addition to the energy required by the muscles, the body requires energy for its functions of life, like breathing, digestion etc. The Basal Metabolic Rate (BMR), is defined as the portion of human energy, used or involved in homeostasis, or merely living and breathing. This energy is excluding work activities, walking, standing, thinking, and digestion of foods. Typically measured on a daily basis and is estimated to represent between 60% and 70% of total minimum energy requirements. The BMR is constant and is primarily dependant on weight, height, age and sex. The BMR is calculated from the Harris-Benedict equations for men and women (Frankenfield,2003).

Men: kilocalories/day = 66 + 13.75(w) + 5.0(h) - 6.76(a)

Women: kilocalories/day = 655 + 9.56(w) + 1.85(h) - 4.68(a)

Where a is the age in years, w is the weight in kilograms and h is the height in centimeters.

Though the actual energy consumption varies during the course of the day, for calculating the heart rate it is assumed that the BMR is constant throughout the day. The BMR per second is calculated and added to the total energy requirement. The energy required for activity is calculated using the Santos™ energy module (Kim,2004)

Varying energy demands have different oxygen requirements which in turn cause the heart to either increase or decrease the heart rate, to supply the oxygen requirements. This is the underlying principle which has been used in the simulation of the heartrate.

There are three steps which in which the oxygen delivery meets the requirement to the muscles ,(1)the cardiovascular system increases its arterio-venous oxygen difference (Barstow,1987),(2) increases the stroke volume of the heart (Hermansen,1970) and (3)increases the number of beats per second.

12.1.6 Arterio-Veonous oxygen difference

The arterio-venous oxygen differnece is the difference between the oxygen carried by the arteries and the oxygen content of the venous system. The oxygen content of the arterial blood remains relatively constant 0.95-0.98(expressed as a ratio of concentration of oxygen per liter of arterial blood to the total oxygen capacity per liter of blood) (Margaria,1976) of at sea level. The oxygen stores of the venous blood rapidly decrease at the onset of physical activity,the time has been estimated to be around 15 seconds, therby increasing the arterio-venous oxygen difference (Barstow,1987).In the current state of the hear rate simulator the areterio-venous oxygen differnence is assumed to be constant at the maximum level of around 0.65(Margaria,1976).

12.1.7 Stroke Volume

Stroke volume is the volume of blood ejected from the left ventricle at each beat of the heart. As the physical activity increases the heart strives to increase the volume of blood pumped out. The stroke volume of the heart increases as the oxygen uptake increases to keep up with the oxygen delivery requirement. The stroke volume however reaches its maximum much quicker than the heart rate. This means that the maximum stroke volume is reached well before the maximum heart rate is reached.

A relationship between the percentage maximum stroke volume and the percent maximum oxygen uptake was derived by plotting a trend line from data in the paper by Hermansen Lars et al, 1970,”Cardiac output during sub maximal and maximal treadmill and bicycle exercise”, Journal of applied physiology 29(1): 82-86 (Hermansen,1970). The data was taken from the treadmill experiments since the treadmill data represented a larger percentage of muscle groups of the body in use.The data used is given in Table 1.