65. Calorimetric methods, the definition and importance of the respiratory quotient

Question 1

Calorimetry:1.

Answer 1

Direct calorimetry
– Method of Lavoiser and Laplace
– Complex direct calorimetry
– Compensation calorimetry
– Gradient calorimetry

2. Indirect calorimetry
   – Open system
   – Closed system

• Respiratory quotient

Question 2

direct calorimetry:

Answer 2

Lavoisier and Laplace had
already developed a calorimeter (direct calorimeter) in the 18th century, in which the
chamber, containing the animal, was surrounded by ice. The amounts of melted ice and the water being produced, were used for the estimation of heat generation. To melt 1 gram of ice, 0.334 kJ of energy is needed. A disadvantage of this method is that neither expired air, nor evaporation of the body are measured, and that the animal is in a chamber with high humidity.

-The emission of water vapour from the body is also taken into account by complex direct calorimetry. An advantage of this method is that energy turnover can also be measured under conditions of motion, or work, if the device is combined with a special equipment.
- In the outer casing of the heat isolated chamber, water of known temperature is circulated in a copper tube. Heat, emitted from the animal inside, warms up the circulating water. Then, temperature of the water leaving the system can be measured, and from this value, and from the velocity of the flow, heat production can be calculated (adiabatic calorimeter). - For the determination of total heat production, the amount of water evaporated from the body also has to be measured. For this purpose, water absorbent substances, e.g., sulphuric acid or native soda (sodium carbonate), are placed in the chamber, and the amount of evaporated water is calculated from the increase of their weight. The heat of water
vaporization is 0.585 kcal/g (2.4 kJ/g).

Question 3

compensation calorimeter,

Answer 3

• In a compensation calorimeter, there are two chambers connected together and placed in a space with constant temperature. The animal is put into one of the chambers, and an electric bulb with known output is set up in the second one.
• Heat, produced by the animal, warms up the air of the first chamber, and a heat-difference occurs between the two air-spaces. As a result, the lamp will be switched on, and remains functioning until heat becomes equalized between the two sides. Energy, used up by the bulb and measured by means of an electric meter, will be the same as the output of heat produced by the animal.

Question 4

gradient calorimeter

Answer 4

A gradient calorimeter is a double-walled chamber, the inner wall of which is a good conductor of heat, and the outer wall of which is
kept at constant temperature. The amount of heat dissipated through the wall of the chamber is measured in a way that the heat-difference
between the two walls is registered. The most recent gradient calorimeters can detect heat dissipated by both radiation, convection and conduction. They make simple, quick and reliable calorimetric measurement possible.

Question 5

Indirect calorimetry:

Answer 5

• There are two major types of calorimeter devices used for these measurements: open-system ones and closed-system ones.
• The advantage of the method is that the animal can stay in its natural environment, it can move freely, therefore the method can provide accurate, reliable measurements, which can be used also in practice. Nevertheless, a disadvantage is considered to be the restricted ability of the method in measuring only O2 -dependent energy production. During heavy physical activity, however, oxygen debt takes place in the organism. The amount of released energy is more than the amount of the consumed oxygen. (After finishing working, when oxygen debt is paid back, more oxygen is consumed).

Question 6

Open-system calorimetry:

Answer 6

The experimental animal breathes in oxygen from the environment, while the amount and composition of the expired air is measured. The amount of O2 breathed in per minute is calculated after the analysis of the air.
The process contains three major steps:
– 1. collecting expired air,
– 2. measurement of the volume of expired air,
– 3. determination of O2 , CH4 (ruminants) and CO2 values.
-The most commonly used device for this purpose is the Douglas bag, which can be fastened to the back of the animal, thus there is a possibility to determine the exchange of gases also during activity.

Question 7

Closed-system:

Answer 7

It is typical of indirect calorimetry that the experimental person or animal is entirely isolated from atmospheric air, it takes inspirations from a spirometer, and also exhales into it. The most well-known equipment of this kind, used in human practice, is the Krogh’s device. Oxygen is added to the system previously, then, the amount of consumed oxygen is measured during the experimental period. The device has valves which prevent inhaled and exhaled air from being mixed with each other.
-Carbon dioxide and water contents of the exhaled air are absorbed in natron (a mixture of NaOH and CaO), therefore, only the exhaled oxygen can get back to the gas reservoir of the device. An indicator directly shows the amount of consumed oxygen, which can also be read from the curve drawn by an attached kymograph.

-The amount of absorbed CO2 cannot be measured inthe Krogh’s device. However, in the Knipping’s device, both oxygen consumption and CO2 production can be monitored. Here, CO2 is absorbed by KOH, from which bound CO2 can be liberated by acids at the end of the experiment, and its amount can be determined. In other devices, consumed oxygen is continuously replaced to maintain a constant O2 concentration in the system, and the amount of added oxygen is measured here.

Question 8

Respiratory quotient (RQ):

Answer 8

The respiratory quotient (respiratory exchange rate) is defined as the volume of production of carbon dioxide within a time unit divided by the volume of consumed oxygen during the same period of time.

• The range of RQ normally varies between 0.7 and 1.0.
• The actual amount of heat produced depends on what kind of nutrient is combusted by the inspired oxygen in a particular period.

Question 9

Respiratory quotient (RQ): range

Answer 9

Hyperventilation: value of RQ reaches 2
• metabolic acidosis: RQ increases
• fattening (CHO -> fat): RQ > 1
• starving (amino-acid -> CHO): RQ < 0.5

-Calculated (estimated) RQ values are all approximate, because CO2 production and O2
consumption can be modified by several factors other than metabolic ones. Thus, in
hyperventilation, expiration of CO2 increases up to a certain level (value of RQ reaches 2), while
after finishing physical exercise, RQ falls to 0.5 or even below it (because of the paying back of
oxygen debt). In metabolic acidosis, RQ increases, because respiratory compensation of
acidosis increases the amount of expired CO2.

Question 10

The formula for oxidation of glucose is :

Answer 10

C6H12O6 + 6 O2 = 6 CO2 + 6 H2O

Question 11

Oxidation of 1 mol of glucose

Answer 11

Oxidation of 1 mol of glucose leads to the production of 6 moles of CO2 and the utilization of 6 moles of oxygen. Therefore, RQ=6/6, this value for carbohydrates equals to 1.

Question 12

combustion of fats

Answer 12

for the combustion of fats gives a different result. If there are only fats oxidized in the body, consumption of 163 moles of oxygen leads to the production of 114 moles of CO2, the value of RQ will be 0.7.

Question 13

During fattening

Answer 13

Value of RQ can be higher than 1, if carbohydrate-fat transition occurs in the body.
During fattening, RQ values of even 1.31-1.6 can be detected, since an oxygen-rich compound is converted to an oxygen-poor substance, thus, the amount of utilized oxygen, taken from the environment, decreases.
-In the case of carbohydrate production during glyconeogenesis from glycerol or amino acids
(compounds poorer in oxygen), value of RQ can decrease to even 0.4 (e.g., during fasting).

Question 14

Complex indirect and direct calorimetry:

Answer 14

• The experimental animal is placed into a closed, heat isolated chamber. The degree of heat production of the animal can be calculated from data collected by measuring the increase of water temperature, and the velocity of the water outflow.
• Similar to indirect calorimetry, O2 consumption and CO2 production of the animal can also be determined, since the system is equipped with a O2 container and absorbents for CO2 and water. Natron-lime is used for the absorption of carbon dioxide, and sulphuric acid is used for the absorption of water vapour. This device, providing exact and reliable data, is mainly used in research.