Direct Calorimetry

Direct Calorimetry
  • The chemical changes that occur when carbohydrate or fat oxidized during respiration in the body are identical overall to the chemical changes, when the chemicals are burnt in air.
  • This is the principle of direct calorimetry. The amount of energy released or expanded is measured by the heat produced.
  • In this method the amount of heat produced by the body is measured directly.
  • At Water-Rosa, Benedict perfected the human respiration calorimeter. To measure the heat production in an individual, a specially constructed chamber, called a respiration calorimeter is used.
  • It is determined through the relation between energy output and oxygen consumed in a person in the calorimeter.
  • Respiration calorimeter consists of an airtight copper chamber insulated by wooden wall with air space in between.
  • Enough provision for sitting or to lie down is made through a folding bed, chair and table.
  • Food is provided through the opening at one end and the excreta is removed through another opening.
  • The chamber is very well arranged for the individual to relax, read or write.
  • It is insulated to prevent the entry and exit of heat through the walls.
  • The chamber is ventilated by a current of air; the CO2 and water given off are removed by soda lime and sulphuric acid respectively.
  • For oxygen supply, a known amount of oxygen is introduced to the chamber through a gas metre with an air current.
  • The oxygen consumption and CO2 production are calculated using a formula.
  • The heat produced is also measured by circulating a current of water through copper pipes and measured the quantity of water that has been circulated through the chamber and also the difference between the temperature of the water entering and leaving the chamber.
Respiratory Quotient
  • Respiratory Quotient is the ratio between the volume of CO2 given out and the volume of O2 consumed by the human subjects.
Respiratory Quotient (RQ) - Volume of CO2 produced
Volume of O2 consumed
  • The RQ varies with the type of food being oxidized.
  • When glucose is oxidized RQ is 1, for a fatty acid it is 0.7 and for protein it is 0.8.
  • Under resting condition with no food for 12-14 hours the RQ is 0.82.
  • In this condition carbohydrate, fats and protein present in the body are utilized.
Last modified: Saturday, 28 April 2012, 7:15 AM