31. Osmoregulation

Question 1

Define the following terms:

Osmosis

Answer 1

The diffusion of water molecules
(solvent) across a semipermeable
membrane,
from a dilute solution to a concentrated solution.

Question 2

Osmotic pressure

Answer 2

The pressure required to prevent
solvent migration by osmosis
across a semi-permeable membrane.

Applying pressure to the
more concentrated solution
can prevent the movement of water to the
region of greater solute concentration.

Question 3

Osmole

Answer 3

Reflects the concentration of
osmotically active particles in solution.

1 osmole =
amount of solute that exerts an
osmotic pressure of 1 atm when
placed in 22.4 L of solution at 0 °C.

For substances that do not dissociate,
e.g. glucose, 1 osmole = 1 mole.

For substances that dissociate into
two osmotically active particles, e.g.
NaCl,
1 osmole = 1 mole/2 (i.e. 1 mole = 2 osmoles).

Question 4

Osmolarity

Answer 4

The number of osmoles (or mosmoles)
of solute in 1 L of solution, osm/L.

As it is temperature dependent,
it poses a potential source of inaccuracy.

Question 5

Osmolality

Answer 5

The number of osmoles (or mosmoles) in
1 kg of water (pure solvent), osm/kg.

It is not influenced by temperature and
is therefore more
accurate than osmolarity

Question 6

How do you calculate plasma osmolality?

Answer 6

A simple formula,
which sums up the major solutes,
may be used:
(2 × Na+) + glucose + urea

This adds up to approximately 290 mosmol/kg H2O.

Question 7

What are the colligative properties

of water?

Answer 7

These are properties of solutions
that depend on the number of solute particles,

but not on their nature,

i.e. they depend on the osmolarity of a solution:

> Lowering of vapour pressure
Elevation of boiling point
Depression of freezing point
Osmotic pressure.

Question 8

How do you calculate osmotic

pressure?

Answer 8

Dilute solutions behave in a 
similar way to ideal gases, 
i.e. osmotic pressure (P) 
is related to 
temperature (T) and volume (V) 
in the same way that an ideal gas is.

Applying the van’t Hoff equation:
PV = nRT

Where:
n = number of particles and
R = universal gas constant (n/V = osmolality),

we can calculate the value of
osmotic pressure of plasma as follows:

P = nRT /V

= 290 mosm/kg H2O × 8.32 J/K × 307 K
= 740 729.6 Pa
= 740.7 kPa
= 7.33 atm (5629.3 mmHg)

Question 9

How do you measure osmotic pressure?

Answer 9

Osmometers capable of
detecting temperature changes
of 0.002 °C are used.

They utilise one or more of
the colligative properties of water:

> 1 mole of a solute added to
1 kg of water will depress the freezing point
by 1.86 °C
(e.g. grit salt on the icy roads causes the ice to melt).

> The molar concentration of a solute
causes a directly proportional
reduction in vapour pressure (Raoult’s law).

Question 10

What is oncotic pressure (colloid osmotic pressure)?

Answer 10

Electrolytes account for more than
99% of plasma osmolality
and osmotic pressure.

Plasma proteins contribute to the
remaining <1%,

which is the colloid osmotic (oncotic) pressure,
and amount to 25–28 mmHg.

Despite its small number,
oncotic pressure is significant,
as it is the major determinant of
retention of fluid within the capillaries.

Question 11

What are the body’s osmoreceptors?

Answer 11

These are cells of the anterior hypothalamus,
located outside the blood– brain barrier.

They respond to changes in osmolality
and stimulate thirst and
the secretion of vasopressin.

Question 12

What are the actions of
vasopressin (antidiuretic
hormone, ADH)?

Answer 12

ADH stimulates V2 receptors on
collecting ducts,
which increases adenylate cyclase activity.

This causes fusion of pre-formed 
water channels on the
apical membrane resulting in 
increased permeability of 
the collecting ducts to water. 

Other ADH effects include:
> Stimulates thirst

> Release of factor VIII by the endothelium

> Platelet aggregation and degranulation

> Arteriolar vasoconstriction

> Glycogenolysis in the liver

> B rain neurotransmitter

> Secretion of ACTH from the anterior pituitary gland.

Question 13

What stimulates water intake?

Answer 13

Multiple factors are involved in regulating water intake.

1
> An increase in plasma osmolality
(with effective increase in
osmotic pressure)

stimulates osmoreceptors in the
anterior hypothalamus,
which in turn control thirst and
simulate us to drink.

ADH release also stimulates thirst.

2
> Extracellular fluid (ECF) volume
depletion stimulates the renin–angiotensin system.

The resultant increase in circulating
angiotensin II acts on a specialised receptor
in the diencephalon concerned with thirst.

Baroreceptors appear to be involved
as well when ECF volume is low.

3
> Dryness of the pharyngeal mucous membranes causes the sensation of thirst.

4
> Psychological and social factors also play a role.