1014MSC - Fluid compartments, diffusion, osmosis & active transport

Fluid distribution in the body are distributed into these two regions

- Intracellular Compartment (ICF)

- Extracellular Compartment (ECF)

Intracellular Compartment (ICF)

- inside of the cells contained by a plasma membrane

- contains 2/3 of body fluid

- 40% of body weight

Extracellular Compartment (ECF)

- outside of the cells, contained in blood vessels or just sitting between the cells in the tissues

- contains 1/3 of body fluid

a. Interstitial spaces (14% of body weight)

b. Plasma (vascular) compartment (5% body weight)

c. Transcellular compartment (1% body weight)

Body Water

- is distributed in several compartments

- varies with sex, age, depending on proportions of muscle and fat

- makes up 45% –75% of total body weight:

• ~75% infants,

• ~50% women

• ~60% men (= ~40L for 70-75 kg male*)

Intracellular Water

not freely moveable within the body unlike blood and tissue (which can be somewhat moveable)

ICF is separated from the

ECF by cell (plasma) membranes

ICF contains

high potassium (K) and low sodium (Na)

ECF can be divided into two major compartments

- Plasma

- Interstitial fluid

Plasma

separated by capillary endothelium from interstitial (or tissue) fluid

Interstitial fluid bathes

Cells and is drained from the interstitial by lymphatic vessels

ECF examples

- cerebrospinal fluid

- aqueous humour in eye

- digestive secretions

- synovial fluid (joint fluid)

- renal tubular fluid

- urine

Tears, urine, and sweat are all derived from

blood plasma and the water which is absorbed from the GI tract is moved about in plasma

Electrolytes

are salts which dissociate into ions in aqueous solution (e.g. NaCl -> Na+ + Cl-)

Electrolyte concentrations differ

between the fluid compartments, as do the concentrations of protein and glucose

Electrolytes contribute most to total solute concentration of body fluids, this therefore

Determines osmolarity and water movements across membranes and between compartments

Movements of molecules occurs via

- Simple diffusion

- Facilitated diffusion (requires protein transporters)

- Active transport (requires protein transporters and consumption of energy)

Molecular Kinetic Theory

- molecules in fluid and gases move continuously, movements are random

- molecules are continually colliding with each other & walls of container and there is no loss of kinetic (moving) energy after collisions. 

- The hotter the molecules, the faster the movement

- The larger the molecules, the slower the movement

Rule 1: The net effect for the molecules is

movement from high concentration to low concentration (in fluids or in gases)

Rule 2: Diffusion does not require energy, but with energy

it will be quicker (e.g. heat soln, you stir it..)

Rule 3: Affected by distance

diffusion is rapid over microscopic distances, slow over macroscopic distances.

In living forms, diffusion is crucial to the

body and cell function (e.g. movement of respiratory gases)

Can also occur on the cellular level

e.g. Maintenance of high ICF (K+) & high ECF (Na+)

A change in [ ] equates to a change in

Pressure

If tube (airway) is restricted (e.g. in Asthma)

it means there's a lower volume of air going in and out

If recoil is lost (emphysema)

If diaphragm can't be moved up or down, it means the pressure and [ ] in the compartments can't be controlled, meaning they loose ability to take air in (inhale) and exhale

Diffusion uses the

kinetic energy of molecular movement and does not require an outside energy source

Molecules diffuse from an area of

higher concentration to an area of lower concentration

Diffusion is faster

- along higher [ ] gradients

- over shorter distances

- at higher temps.

- for smaller molecules

Diffusion continues until [ ]

come to an equilibrium,

Diffusion can take place in an

Open system or across a partition that separates the two systems

Simple diffusion (kinetic energy)

Net movement of molecules from an area of their higher [ ] to an area of their lower [ ], that is down their [ ] gradient

Examples of simple diffusion

Fats, oxygen, carbon dioxide - more through lipid bilayer of the membrane

Facilitated diffusion (kinetic energy)

Same as simple diffusion, except diffusing substance is attached to a lipid-soluble membrane carrier protein (carrier-mediated facilitated diffusion) or moves through a membrane channel (channel-mediated facilitated diffusion)

Examples of facilitated diffusion

Glucose and some ions move into cells

Osmosis (kinetic energy)

Diffusion of water through a selectively permeable membrane

Osmosis examples

Movement of water into and out of cells directly through the lipid bilayer of the membrane or via membrane channels (aquaporins)

Osmosis is simply,

The diffusion of water across a semi-permeable membrane from high water to low water (High H2O to low H2O)

This is equivalent to saying that

Water moves from a soln. of low [solute] to a soln. of high [solute] - solutes occupy spaces o high [solute] means less water per uni volume

As the water moves across the membrane into a compartment,

It generates a pressure within that compartment, it pushes it outwards - this is called osmotic pressure

The magnitude of the osmotic pressure is

Equal to the hydrostatic pressure needed to oppose the water movement across the membrane

What controls the movement of water across membranes?

- diffusion through a cell membrane, limited opportunity via gaps in lipid bilayer

- diffusion through channels called aquaporins, integral proteins with tiny channels that allow H2O to pass, but essentially nothing else

- pinocytosis (fluid based endocytosis), essentially enclosing water in a membrane bound 'parcel' that is internalised

Osmosis is determined by the

Number of active particles in a solution

The no. of particles is not always the same as the no. of

Molecules because some molecules dissociate into particles (e.g NaCl - Na+ & Cl-)

Osmolarity is what determines whether osmosis occurs between

Compartments and is dependent on the no. of particles in the soln. and whether they are ionic or non-ionic

In summary, osmosis is dependent on the

Total solute [ ] of a soln.

Unit of osmolarity

Osmol (ICF has an osmolarity at around 300 mOsm)

REMEMBER: Osmolarity increases as

Solute [ ] increases

Glucose molecules do not dissociate into

Smaller particles w/in a soln.; 1 molar = 1 Osm

A molecule of NaCl in soln. tends to

Dissociate into two particles, Na+ and Cl- ions (1 mole of NaCl in soln. will produce approx. 2 osmoles)

Actual measurement of osmolarity will be closer to 1.8x then 2x because

Biological systems are not absolute

Electrolytes dissociate due to

The pull of water

Tonicity describes how a soln. affects cell volume; there is not effect

If there is no net movement of water, but it changes cell volume if there is a net movement of water in, or out

Tonicity describes how cells

Respond when immersed in different solns., and is dependent on what can cross the cell membrane and what can't

Three different types of tonicity based on [ ] of soln.

- Isotonic

- Hypertonic

- Hypotonic

Isotonic

Equal tonicity with the ICT (no swelling or shrinking)

Hypertonic

Higher tonicity than ICF (cells will lose water, i.e. shrink)

Hypotonic

Lower tonicity than ICF (cells will gain water, swell)

Water moves out of cell because the [solute] in the ECF is higher

Hypertonic

Water moves into cells because the [solute] in the cell is higher than in the ECF

Hypotonic

No water movement because the [solute] in the cell and ECF are equal

Isotonic

Movement of water between plasma and the interstitium (role of osmolarity between the cells that make up capillary tubes)

The osmotic principles are identical and since blood is sent everywhere to serve all cells, it is the osmolarity of blood which determines cell volume

Capillary permeability can vary greatly, however,

Most capillaries have tight junctions, permeable to water and ions, but tight to proteins and other large molecules

In some places, junctions are permanently "open", allowing proteins to

Move in and out

For example, in the brains, permeability is

Restricted

Increased capillary permeability is the hallmark of the

Inflammatory response, mediated largely by histamines, released from damaged tissues - now fluid and protein are lost to tissues, forming an oedema

Colloid osmotic pressure (COP) is associated with

Cells and in blood plasma

COP is also known as

"Oncotic" pressure

Oncotic pressure refers to

the form of osmotic pressure exerted by proteins in a blood vessel's plasma (blood/liquid) that usually tends to pull water into the circulatory system. It is the opposing force to hydrostatic pressure.

In cells, proteins cannot cross intact membranes,

They are non-penetrating solutes

High intracellular [protein] generate a positive intracellular pressure

Cells "pop" if pricked

Blood contains large amounts of

Plasma proteins

Plasma proteins don't generally cross

Capillary endothelial walls, and therefore they "attract" water from the interstitial space (ICF)

Plasma COP is important in the

Regulation of water distributed between plasma and interstitial fluid

Movement occurs mostly between endothelial cells;

Not to be confused with movement across cell membranes