Refrigeration Applications - HVAC of the Human Body - Refrigeration for Non-Techies (56)

Horses sweat, gentlemen perspire, and ladies glow. Whatever it's called, we all recognize the benefit of evaporative cooling when it comes to keeping our cool. There is an instinctive understanding of the effect of relative humidity on the performance of our corporeal cooling; too high and the benefit is reduced due to lack of evaporation, whereas too low humidity causes other discomforts.

Many other HVAC subsystems in our bodies are closely analogous to HVAC plants in buildings, but are less evident. Most are related to blood flow. Blood is remarkably multifunctional because it provides heating and cooling like a hydronic system, it carries fuel and oxidants to the localized combustion chambers, and it removes the waste products of combustion. The fuel is a form of hydrocarbon (usually sugar or starch, which are both carbohydrates), and the oxidant is oxygen extracted from air and packed into transport canisters made of iron (hemaglobin). The transport canisters are built to order within our bones in a process controlled by the effluent treatment plant (kidneys) monitoring the efficiency of the waster transfer process.

The canisters are conveyed by the blood stream to local microfurnaces throughout the body, and the combustion of carbohydrate in these furnaces is so efficient that the only waster products are carbon dioxide gas and water. C0is returned to the oxidation plant in the same reusable canisters that delivered the oxygen - a very effective form of container recycling.

Heating or cooling is achieved by diverting the hydronic fluid toward or away from key areas. For example, if our core temperature is up, blood will divert to the body surface, and we radiate heat to the surroundings. If the core temperature falls, blood id directed away from the body surface toward the core to keep it warm. Since blood is required for other purposes such as carrying oxygen to the brain when a lot of thinking is required, or to the stomach when there is a heavy meal to digest, this temperature regulation can sometimes be subverted. This multifunction approach is also why we feel sleepy after eating a lot; blood flow can't do everything at once. 

Blood provides other functions, too. It is used to control the rate of extract ventilation provided by the lungs in an unusual way. The concentration of exhaust gas (C02) in the blood is regulated by monitoring the blood pH, and the rate of ventilation is modulated to maintain a steady pH level of 704, +0.05: if the pH falls it is presumed the C02 level is high, so the ventilation rate increases, and vice versa. This can cause control problems if the outdoor air already has a high C0concentration, putting the feedback loop out of synch and causing hyperventilation.

The lungs are an effective gas exchange plant. Over 300 million tiny pits with a total surface area of about 1,000ft(about 100m2) enable oxygen to flow from air to blood, while letting carbon dioxide pass in the opposite direction. To function effectively they require pre-treated air. Inlet air is warmed and filtered in the nasal cavity before passing to the gas exchange unit. If lung function is compromised by contaminants, the system is less affected by the adverse effects of excess C02. The system is self-compensating and applies an automatic offset to compensate for the reduced functionality.

Most of these HVAC components have built-in redundancy: two lungs, two nostrils, and two kidneys. The key component of the whole system, however, is the heart: the circulating pump. It provides about 3 billion strokes in its working life in a double-action configuration with one loop feeding the gas exchange plant and the other delivering oxygen-rich blood to the rest of the system at 1.25gpm (0.079L/s). Wherever your beliefs lie on the creation-evolution line, I think all HVAC engineers can agree with the Psalmist that we are "fearfully and wonderfully made."

Last modified: Tuesday, 3 January 2017, 3:02 PM