16 - Osmolality introduction

Osmolality is a count of the number of particles in a fluid sample. The unit for counting is the mole which is equal to 6.02 x 10²³ particles (Avogadro's Number). Molarity is the number of particles of a particular substance in a volume of fluid (eg mmol/L) and molality is the number of particles disolved in a mass weight of fluid (mmol/kg). Osmolality is a count of the total number of osmotically active particles in a solution and is equal to the sum of the molalities of all the solutes present in that solution.


 For most biological systems the molarity and the molality of a solution are nearly exactly equal because the density of water is 1 kg/L. There is a slight difference between molality and molarity in plasma because of the non-aqueous components present such as proteins and lipids which make up about 6% of the total volume. Thus serum is only 94% water and the molality of a substance in serum is about 6% higher than its molarity. Except in unusual circumstances this difference is unimportant and the terms molarity to the molarity are often used interchangeably. Note that the size of the particle is unimportant so that a single ion, eg sodium, contributes as much to the serum osmolality as a single large protein molecule, eg albumin.


The osmolality of physiological fluids tends to be dominated by small molecules which are present in high concentrations. For example in serum, sodium, potassium, chloride, bicarbonate, urea and glucose are the only components present in high enough concentrations to individually affect the osmolality. Together these make up over 95% of total osmolality of serum. Large serum components contribute little to the overall osmolality.  For example the molar concentration of albumin, the most abundant serum protein, is only about 0.6 mmol/L. Only a few exogenous compounds such as ethanol, methanol, ethylene glycol and manitol can be present in the blood at high enough quantities to significantly affect the osmolality.

15 - Basal Metabolic Rate Mcqs with notes

1q: which of the following statements is true regarding basal metabolic rate ?


a. increased in starvation
b. it is not influenced by hormonal changes
c. it is not affected by dietary changes
d. decreased by 40 % in starvation
e. it is not affected by energy expenditure

answer only ‘d’ is the correct statement

Some important points about basal metabolic rate :


1. the energy expended by an individual during resting post-absorptive state is called Basal Metabolic Rate. It represents the energy required for normal body functions .


2. in starvation it is decreased by 40 % .


3. it is influenced by hormones like thyroid hormones and catecholamines .


4. it is affected by dietary component . eg: diet containing protein increases metabolic rate than same amount of carbohydrate and fat .


5. from 50 % to 70 % of the daily energy expenditure in sedentary individuals is attributable to BMR until very old age .


Factors affecting BMR are :


1. Age : BMR is higher in children than adults. With advancing age BMR gradually falls and in neonate BMR is high .
2. Sex : women have lower BMR than male
3. Surface area : BMR is directly proportional to body surface area .
4. Exercise : increases BMR .
5. Climate : in colder climates BMR is higher than in tropical climates.
6. Nutrition : BMR decreases 20-30 % in malnutrition ,starvation, wasting disease .
7. Body temperature : BMR rises by 14 % for each Celsius degree of elevation .
8. Hormones : circulating levels of hormones secreted by thyroid ,adrenal medulla ,anterior pituitary ( eg: GH ) , male sex hormones increase BMR .
9. Emotional state : sympathetic stimulation .
10. Pregnancy , lactation
11. Race
12. Sleep , drugs , barometric pressure etc . 


Download and read this article by American journal of nutrition which discusses about the various factors affecting the BMR on priority basis.


The MCQ above was asked in the PGIMER ( PGI ) chandigarh 2001 december paper .