Biotransport - Principles of Biomedical Engineering - page 34

buffer system is an important urinary buffer. These mechanisms are complemen-
tary to the long term (days) changes regulated by the kidneys. Essentially, kidneys
act like size-separation units for blood, making use of the different sizes of the
particles and filtering them. Kidneys change the extracellular fluid electrolytes to
maintain the pH at 7.4. Unlike the pH changes due to CO 2 , which the respiratory
system can produce (minutes), the renal compensation is slow. Further, there are
limitations to the acidity of urine. The kidneys cannot produce a urine pH of much
less than 4.4. Strong acids can be removed from the blood and excreted in the urine
by reacting with the basic salt of phosphoric acid in the urine without producing a
reduction in the pH of the urine, or by the addition of NH 3 (a base) to the urine.
Buffering by kidney occurs only when the components that change the pH are in
excess. If the pH change is due to a deficiency of a component other than carbonic
acid, pH regulation by kidneys is not possible unless that deficiency is restored by
supplementation.

2.3 Diffusion

Diffusion is a random movement of molecules, which results in molecules moving
from an area of higher concentration to an area of lower concentration. Once the
concentration becomes uniform, there is no net transfer of molecules from one re-
gion to the other although random movement continues to occur. Consider Figure
2.4 where two regions, L and R, containing a solvent (e.g., water) are separated by
a membrane, which is permeable to a solute A (e.g., common salt). If a high amount
of A is added into region R, then molecules start randomly moving, which results in
molecules crossing to region L. Since region L did not have any A, diffusion results
in the net gain of A.
The transfer of molecules by diffusion is one of the main processes through
which molecules move in many physiological processes. For example, the lipid
bilayer and basement membranes are permeable to water molecules and a few
uncharged small molecules like O 2 , CO 2 , N 2 , NH 3 , fatty acids, and some alco-
hols. These diffuse freely in and out of the cell. However, the lipid bilayer is not
permeable to ions such as K + , Na + , Ca 2+ (called cations as they migrate toward the
cathode), Cl − , HCO 3 − (called anions because they migrate toward the anode), or
small hydrophilic molecules like glucose, proteins, and RNA. Removal of blood

Figure 2.4 The diffusion process.