Compartmental Modeling - Introduction to Biomedical Engineering - page 374

7.3.4 Cell Volume and Osmosis
The cell regulates its volume by controlling the internal osmolarity through primary active
transport mechanisms using ATP-driven pumps for sodium, potassium, calcium, chlorine,
hydrogen, and other ions. The most important ATP-driven pump is the

pump. Sixty
to 70 percent of a cell's energy consumption is devoted to ATP pumps. Within the cell are
anions (such as chlorine, proteins, nucleic acids, sulfate ions, and phosphate ions) and cations
(such as potassium, calcium, and sodium). As we will see, some of these ions move freely
through the membrane, some appear impermeable, and others are impermeable. The ATP
pumps move ions through the membrane to maintain nonzero concentration gradients for
many ions at steady-state. 9 Transport of ions is achieved using carrier proteins, which differs
from diffusion. These pumps are used to keep the intracellular osmolarity of the cell equal to
extracellular osmolarity and to maintain cell volume. A secondary active transport of ions not
involved in ATP-driven pumps is a by-product of ion concentration gradients created
through the primary active transport.
Any change in the steady-state ion concentrations causes water to be drawn into or be
withdrawn from the cell by osmosis. Most mammalian cells have an osmolarity equal to
plasma, approximately 300 mOsm. Because of its frailty, the cell membrane cannot survive
even minimal pressure differences between the intracellular and extracellular fluids. The
ATP pumps keep the pressure difference equal to zero and control the cell volume.
Two solutions with the same osmolarity are called isotonic. A solution that has a lower
osmolarity to another is called hypotonic, and a solution that has a higher osmolarity to
another is called hypertonic. If the osmolarities are not equal, water moves from the lower
osmolarity side to the higher osmolarity side until the osmolarities are equal. Water will
leave a cell that is hypotonic to the extracellular solution, which decreases the volume until
an isotonic condition is achieved. Water will enter a cell if it is hypertonic to the extra-
cellular solution, which increases the volume until an isotonic condition is achieved. ATP
pumps also restore the cell volume to its original state.
Cell volume is typically maintained during extracellular osmolarity changes by adjusting
the intracellular osmolarity. When the extracellular osmolarity is hypotonic with respect to
the intracellular osmolarity, the following occur:

Na-K

1. Cell volume immediately increases due to osmosis.
2. The ATP pumps drive ions out of the cell to restore cell volume while maintaining the
osmotic balance.

When the extracellular osmolarity is hypertonic with respect to the intracellular osmo-
larity, the following occur:

1. Cell volume immediately decreases due to osmosis.
2. The ATP pumps drive ions into the cell to restore cell volume while maintaining the
osmotic balance.

At steady-state, the ionic concentrations across the cell membrane are maintained by ATP
pumps that create electrical and concentration gradients. The electrical gradient will be

9 Note that the system is not in equilibrium, since the pump uses energy to maintain the concentrations.
We refer to this situation as steady-state.