- Units of measurement of metabolic rates
- Conditions for measuring basal metabolism
- Respirometric methods to measure tms and tmb
- Calorimetric methods for measuring tms and tmb
- Basal metabolism and body size
- Allometric equation of metabolic scaling
- Basal metabolism, circulation and respiration
- Basal metabolism and longevity
- Medical interest
- References
The basal metabolism can be defined as the set of chemical reactions in the body through which an animal spends the minimum amount of energy needed to maintain vital processes. This amount typically represents 50% or more of an animal's total energy budget.
Basal metabolism is quantified by standardized measures of energy expenditure per unit of time. The most common are the standard metabolic rate (TMS) and the basal metabolic rate (BMR).
Source: pixabay.com
TMS is measured in cold-blooded animals, such as most fish, mollusks, amphibians, and reptiles. TMB is measured in warm-blooded animals, such as birds and mammals.
Units of measurement of metabolic rates
TMS and BMR are usually expressed as O 2 consumption (ml), calories (cal), kilocalories (kcal), joules (J), kilojoules (kJ), or watts (W).
A calorie is defined as the amount of heat required to raise the temperature of 1 g of water by 1 ° C. One calorie equals 4,186 joules. The joule is the fundamental measure (SI, International System) of energy. The watt, which is equal to 1 joule per second, is the fundamental (SI) measure of energy transfer and transformation rates.
Conditions for measuring basal metabolism
To ensure that the values obtained by different studies are comparable, the measurement of TMS and BMR requires that the experimental animals be at rest and fasting. In the case of TMB, these animals are also required to be in their thermoneutral zone.
An animal is considered at rest if it is in the inactive phase of its normal daily cycle, without spontaneous movements, and without physical or psychological stress.
An animal is considered fasting if it is not digesting food in a way that generates heat.
An animal is considered in its thermoneutral zone if, during the experiments, it is kept in the temperature range within which its body heat production remains unchanged.
Respirometric methods to measure tms and tmb
- Volume or constant pressure respirometry. The animal is kept in a sealed container. The pressure changes due to the consumption of O 2 by the animal are measured at constant temperature by means of a manometer. The CO 2 produced by the animal is chemically eliminated using KOH or ascarite.
If a Warburg respirometer is used, the pressure change is measured by keeping the volume of the container constant. If a Gilson respirometer is used, the change in volume is measured by keeping the pressure constant.
- Gas analysis. Currently there is a great variety of laboratory instruments that allow direct quantification of O 2 and CO 2 concentrations. This instrument is very precise and allows automated determinations.
Calorimetric methods for measuring tms and tmb
- Bomb calorimetry. Energy consumption is estimated by comparing the heat produced by the combustion of a sample of uneaten food with the heat produced by the combustion of an equivalent sample of digested remains (feces and urine) of that food.
- Direct calorimetry. It consists of directly measuring the heat produced by the combustion flame of the sample.
- Indirect calorimetry. It measures heat production by comparing O 2 consumption and CO 2 production. It is based on Hess's law of constant sum of heat, which states that in a chemical reaction an amount of heat is released depending only on the nature of the reactants and products.
- Gradient calorimetry. If a heat flux Q passes through a material of thickness G, area A, and heat conductivity C, the result is a temperature gradient that increases with G and decreases with A and C. This makes it possible to calculate energy expenditure.
- Differential calorimetry. It measures the heat flux between a chamber containing the experimental animal and an adjacent unoccupied chamber. The two chambers are thermally insulated except for the surface that joins them, through which they exchange heat.
Basal metabolism and body size
TMS and BMR vary disproportionately with the size of the animals. This relationship is known as metabolic escalation. The concept can be easily understood by comparing two herbivorous mammals of very different sizes, such as the rabbit and the elephant.
If we quantify the foliage they eat for a week, we would find that the rabbit eats much less than the elephant. However, the mass of the foliage eaten by the first would be much greater than its own body mass, while in the case of the second it would be the other way around.
This disparity indicates that, proportionally to their size, the energy needs of both species are different. Studying hundreds of animal species shows that this particular observation is part of a general pattern of metabolic escalation quantifiable in terms of TMS and BMR.
For example, the average BMR (2200 J / h) of 100 g mammals is not ten times, but only 5.5 times, greater than the average BMR (400 J / h) of 10 g mammals. Similarly, the average mammalian BMR of 400 g (4940 J / h) is not four times, but only 2.7 times, greater than the average mammalian BMR of 100 g.
Allometric equation of metabolic scaling
The relationship TMS (or TMB), represented by T, and the body mass, represented by M, of an animal can be described by the classical equation of biological allometry, T = a × M b, in which a and b are constants.
The fit to this equation explains mathematically why the TMS and the BMR do not vary proportionally to the mass of the animals. Applying logarithms to both sides, the equation can be expressed as follows
log (T) = log (a) + b × log (M), log (a) and b can be estimated by linear regression analysis between experimental values of log (T) and log (M) of multiple species of an animal group. The constant log (a) is the cut-off point of the regression line on the vertical axis. For its part, b, which is the slope of said line, is the allometric constant.
It has been found that the average allometric constant of many animal groups tends to be close to 0.7. In the case of log (a), the higher its values, the higher the metabolic rates of the animal group under analysis.
Basal metabolism, circulation and respiration
The lack of proportionality of TMS and BMR with respect to size causes that small animals have higher O 2 needs per gram of body mass than large animals. For example, the energy expenditure rate of one gram of whale tissue is much lower than that of one gram of homologous mouse tissue.
Large and small mammals have hearts and lungs of similar sizes in relation to their body mass. For this reason, the contraction rates of the heart and lungs of the latter need to be much higher than those of the former in order to carry enough O 2 to the tissues.
For example, the number of heart beats per minute is 40 in an elephant, 70 in an adult human, and 580 in a mouse. Similarly, humans breathe about 12 times and mice about 100 times per minute.
Within the same species, these patterns are also observed between individuals of different sizes. For example, in adult humans the brain is responsible for approximately 20% of the total metabolic expenditure, while in children aged 4 to 5, this expenditure reaches 50%.
Basal metabolism and longevity
In mammals, brain and body sizes and basal metabolism are related to longevity by the equation
L = 5.5 × C 0.54 × M -0.34 × T -0.42, Where L is longevity in months, C is brain mass in grams, M is body mass in grams, and T is BMR in calories per gram per hour.
The exponent for C indicates that mammalian longevity has a positive association with brain size. The exponent of M indicates that longevity has a negative association with body mass. The exponent of T indicates that longevity has a negative association with the speed of metabolism.
This relationship, although with different exponents, is also applicable to birds. However, they tend to live longer than mammals of similar body mass.
Medical interest
Women's BMR can double during pregnancy. This is due to the increase in oxygen consumption caused by the growth of the fetus and uterine structures, and by the greater development of maternal circulation and renal function.
The diagnosis of hyperthyroidism can be confirmed by increased oxygen consumption, that is, a high BMR. In about 80% of cases of overactive thyroid, BMR is at least 15% higher than normal. However, a high BMR can also be caused by other diseases.
References
- Guyton, AC, Hall, JE 2001. Treatise on medical physiology. McGraw-Hill Interamericana, Mexico.
- Hill, RW, Wyse, GA, Anderson, M. 2012. Animal Physiology. Sinauer Associates, Sunderland.
- Lighton, JRB 2008. Measuring metabolic rates - a manual for scientists. Oxford University Press, Oxford.
- Lof, M., Olausson, H., Bostrom, K., Janerot-Sjöberg, B., Sohlstrom, A., Forsum, E. 2005. Changes in basal metabolic rate during pregnancy in relation to changes in body weight and composition, cardiac output, insulinlike growth factor I, and thyroid hormones and in relation to fetal growth. American Journal of Clinical Nutrition, 81, 678–85.
- Randall, D., Burggren, W., French, K. 1998. Animal physiology - mechanisms and adaptations. McGraw-Hill Interamericana, Madrid.
- Solomon, SJ, Kurzer, MS, Calloway, DH 1982. Menstrual cycle and basal metabolic rate in women. American Journal of Clinical Nutrition, 36, 611–616.
- Willmer, P., Stone, G., Johnston, I. 2005. Environmental physiology of animals. Blackwell, Oxford.