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     Diffusion can be defined as the movement of particles from a region of high concentration to a
region of low concentration across a concentration gradient. So therefore diffusion is the process 
in which a gas or a substance that is in a liquid expands to fill all the volume equally. 
  Cause of movement:
The reason by which the expansion of the gas or the substance in the liquid is due to the motion of its 
particles. The particles of a substance dissolved in a solvent whether they are molecules or even just 
an atom are in a continuous random movement. A given particle in the dissolved substance is equally likely 
to move out of the area or also into the area in which there is a high concentration of the particle 
present. Due to the higher number of particles in the area of the high concentration, the total 
number of particles moving towards the lower concentration is greater than the total number of 
particles moving towards the area with the higher concentration. This is called a 'net flux' of solute
particles from areas of high to areas of low concentration.  

 Diffusion in physics
  'Ficks Law' in physics is used in physics to describe diffusion and is used to find the diffusion 
coefficient. Fick’s First Law helps to show the amount of flux that is present  under steady states. 
It describes the laws which describe diffusion in which  particles which he refers to as Flux
goes from regions of high concentration to regions of low concentration across a concentration gradient.
He was able to find this law by measuring the number of Particle in a substance (X),that crosses an area
per second and using this knowledge he was able to find the equation: (Number of X particles )/(Area × Per Second). 
The resulting figure can be called the flux that is present in a system of diffusion.

Types of Diffusion
 There is two type of diffusion that occurs in a human body.both of these are passive process so no energy is required 
for them. the two types are: 
     - Simple Diffusion.
     - Facilitated Diffusion.
Simple Diffusion:
 Simple diffusion can be described as the unassisted net movement of small, non polar substances down their concentration 
across a selectively permeable membrane. An example of this in the human body would be the exchange of oxygen and carbon
carbon dioxide between blood and body tissues.
Facilitated Diffusion:
 Facilitated diffusion is the movement of ions and small, polar molecules down their concentration gradient assisted across a selectively 
permeable membrane by a transport protein. Facilitated diffusion can then be again divided into two parts:
Channel mediated diffusion is the movement of ion down its concentrationgradienj through a protein channel. examples of this in the 
human body would be when Na ions move through Na ion channel into cell.

Carrier-mediated is the movement of small, polar molecules down its concentration gradient by a carrier protein. An example in cells is 
the transport of glucose into cells by glucose repairs.

Diffusion in medicine
 Diffusion MRI  is a magnetic resonance imaging (MRI) method which was first used in the mid-1980s. It allows us to find the 
diffusion process of molecules, mainly water, in biological tissues in the human body, in the living body  and non-invasively. Molecular diffusion in tissues
is not free, but reflects interactions with many obstacles,  such as macromolecules, fibers, membranes, etc. Water molecule diffusion patterns
can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state.


 Bio by John Laughlin
 GANONG'S Review of medical physiology 
 Junqueira's basic histology Text and atlas 13th edition