Human Physiology, 14th edition (2016)

134 Chapter 6

the opposite direction, but not as frequently. As a result, there

will be a net movement from the region of higher to the region

of lower concentration until the concentration difference no

longer exists. This net movement is called net diffusion. Net

diffusion is a physical process that occurs whenever there is

a concentration difference across a membrane and the mem-

brane is permeable to the diffusing substance.

The mean diffusion time increases very rapidly with the

square of the distance that the diffusing molecules or ions must

travel. According to some calculations, this produces a mean

diffusion time of (a) 10^2 7 sec. to cross a plasma membrane

(10 nm); (b) 1.6  3  10 2 6 sec. to cross a synapse (40 nm); and

(c) 1 to 2  3  10 2 3 sec. to cross the two squamous epithelial cells

that separate air from blood in the lungs (1–2  m m). Notice that

A solution consists of the solvent, water, and solute mol-

ecules that are dissolved in the water. The molecules of a

solution (solvent and solute) are in a constant state of random

motion as a result of their thermal (heat) energy. If there is a

concentration difference, or concentration gradient, between

two regions of a solution, this random motion tends to elimi-

nate the concentration difference as the molecules become

more diffusely spread out ( fig. 6.3 ). Hence, this random molec-

ular motion is known as diffusion. In terms of the second law

of thermodynamics, the concentration difference represents an

unstable state of high organization (low entropy) that changes

to produce a uniformly distributed solution with maximum dis-

organization (high entropy).

As a result of random molecular motion, molecules in

the part of the solution with a higher concentration will enter

the area of lower concentration. Molecules will also move in

Figure 6.3 Diffusion of a solute. ( a ) Net diffusion

occurs when there is a concentration difference (or concentration

gradient) between two regions of a solution, provided that the

membrane separating these regions is permeable to the diffusing

substance. ( b ) Diffusion tends to equalize the concentrations

of these regions, and thus to eliminate the concentration

differences.

Higher

concentration

Lower

concentration

Equal concentrations

(a) (b)

Net diffusion No net diffusion

CLINICAL APPLICATION

In the kidneys, blood is filtered through pores in capillary

walls to produce a filtrate that will become urine. Wastes

and other dissolved molecules pass through the pores,

but blood cells and proteins are held back. The molecules

needed by the body are then reabsorbed from the filtrate

back into the blood by transport processes. Wastes gener-

ally remain in the filtrate and are excreted in the urine. When

the kidneys fail to perform this function, the wastes must

be removed from the blood artificially by a process called

dialysis ( fig. 6.4 ). In dialysis, there is a net diffusion of waste

molecules from the blood across a semipermeable mem-

brane into a fluid called a dialysate. In hemodialysis, blood

is channeled to an external filtering system. In peritoneal

dialysis, a dialysate is introduced into the abdominal cavity

and the person’s own peritoneal membrane serves as the

semipermeable membrane.

Figure 6.4 Diffusion through a dialysis membrane. A

dialysis membrane is an artificially semipermeable membrane

with tiny pores of a certain size. Proteins inside the dialysis bag

are too large to get through the pores (bent arrows), but the

small, diffusible molecules and ions are able to fit through the

pores and diffuse (solid, straight arrows) from higher to lower

concentration out of the bag and into the surrounding fluid.

Glucose can also fit through the pores, but because it is present

at the same concentration outside of the bag, there is no net

diffusion (double dashed arrows).

Proteins

Small, diffusable molecules and ions

Dialysis membrane

Glucose

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