Dr. Basim Al-Ka'abi
Cellular Physiology
Fourth Lecture
1
Lecture Name: Transport Across the Cell Membrane, Osmosis and Edema
Lecturer Name: Dr. Basim A. Al-Ka'abi
Department: Medical Physiology
Stage: First Stage Medical Students
Lecture Objectives:
By the end of this lecture students the should be able to know the following:
Continuation of cell membrane transport types.
Meaning of osmosis.
Isotonic, hypertonic and hypotonic types of fluids.
Edema and its various causes.
References:
Barrett, K et al. (2018). Ganong's Review of Medical Physiology. Twenty
sixth edition. USA.
Guyton, A and Hall, J (2015). Text Book of Medical Physiology.
Thirteenth edition. Philadelphia, USA.
Dr. Basim Al-Ka'abi
Cellular Physiology
Fourth Lecture
2
Transport across the cell membrane (cont'd)
Primary active transport
-Occurs against an electrochemical gradient (uphill).
-Requires direct input of metabolic energy in form of ATP so it is active.
-Examples of it:
(a) Na
+
/K
+
ATPase (or Na
+
pump) in the cell membrane transports Na
+
from intracellular to extracellular fluid and K
+
from extracellular to
intracellular fluid. It maintains low intracellular Na
+
and high intracellular
K
+
.
-Both Na
+
and K
+
are transported against their electrochemical gradients
and the energy is provided from ATP.
(b) Ca
+2
ATPase (or Ca
+2
pump) in the sarcoplasmic reticulum or cell
membrane that transports Ca
+2
against its electrochemical gradient.
(c) H
+
/K
+
ATPase (or proton pump) in gastric parietal cells that transports
H
+
into lumen of the stomach against its electrochemical gradient.
Secondary active transport
-In this transport; a transport of two or more solutes is coupled.
-One of the solutes (usually Na
+
) is transported downhill and provides
energy for the uphill transport of the other solute.
-Metabolic energy is not provided directly, but indirectly from the Na
+
gradient, which is maintained across cell membranes. Thus, inhibition of
Na
+
/K
+
ATPase will decrease transport of Na
+
out of the cell, decrease the
transmembrane Na
+
gradient, and eventually inhibit secondary active
transport.
-If the solutes move in the same direction across the cell membrane, it is
called co-transport or symport. Example of that Na
+
/glucose cotransport in
the small intestine.
Dr. Basim Al-Ka'abi
Cellular Physiology
Fourth Lecture
3
-If the solutes move in opposite directions across the cell membranes, it is
called counter transport or antiport. Example of that is Na
+
/Ca
+
exchange
or Na
+
/H
+
exchange.
Osmosis
-Is the flow of water across a semipermeable membrane from a solution
with low solute concentration to a solution with high solute concentration.
-Cell membranes in the body are highly permeable to water and whenever
there is a higher concentration of solute on one side of the membrane, water
rapidly diffuses across the membrane toward the region of higher solute
concentration.
-The osmotic pressure increases when the solute concentration increases;
the higher the osmotic pressure of a solution, the greater the water flow
into it.
-Osmolarity is the concentration of osmotically active particles in
a solution.
Isotonic, hypertonic and hypotonic fluids
-A solution is said to be isotonic if no osmotic force develops across the
cell membrane when a normal cell is placed in the solution.
-This means that an isotonic solution has the same osmolarity as the cell
and that the cells will not shrink or swell when placed in this solution.
-Example of isotonic solutions is 0.9% NaCl (normal saline).
-A solution is said to be hypertonic when it contains a higher osmotic
concentration of substances than does the cell.
-In this case, osmotic force develops that causes water to flow out of the
cell into the solution, thereby greatly concentrating intracellular fluid and
shrinking the cell.
Dr. Basim Al-Ka'abi
Cellular Physiology
Fourth Lecture
4
-Example of hypertonic solutions is D5NSS (5% Dextrose in normal saline
solution).
-The solution is said to be hypotonic if the osmotic concentration of
substances in the solution is less than their concentration in the cell.
-An osmotic force develops immediately when the cell is exposed to the
solution, causes water to flow by osmosis into the cell until the intracellular
fluid has about the same concentration as the extracellular fluid, or until
the cell bursts from excessive swelling.
-Example of hypotonic solutions is 0.45% NaCl (half strength normal
saline).
Edema
-Edema refers to the presence of excess fluid in the body tissues.
-In most instances, edema occur mainly in the extracellular fluid
compartment, but it can involve intracellular fluids as well.
Intracellular edema
-Can occur when the cell membrane is damaged or when there is
inadequate nutrition to the cells.
-When this happens, sodium ions are no longer efficiently pumped out of
the cells, and the excess sodium ions inside the cells cause osmosis of water
into the cells.
Extracellular edema
-Is more common than intracellular edema. Occurs with accumulation of
fluid in the interstitial spaces.
-There are two general causes of extracellular edema:
1. Abnormal leakage of fluid from the plasma to the interstitial spaces
across the capillaries.
Dr. Basim Al-Ka'abi
Cellular Physiology
Fourth Lecture
5
2. Failure of the lymphatics to return fluid from the interstitium back to
the blood.
-The most common clinical cause of interstitial fluid accumulation is
excessive capillary fluid filtration.
Factors causing interstitial fluid edema
1. Increased capillary filtration coefficient, which allows leakage of fluid
and plasma proteins through the capillary membranes, this can occur as
a result of allergic reactions, bacterial infections, and toxic substances
that injure the capillary membranes.
2. Increased capillary hydrostatic pressure, as in heart failure, when the
heart fails to pump blood normally from the veins into the arteries
resulting in an excess flow of blood from the arteries into the capillaries.
3. Decreased plasma colloid oncotic pressure, which can occur because of
failure of the liver to produce sufficient quantities of plasma proteins,
loss of large amounts of proteins into the urine in certain kidney
diseases, loss of large quantities of proteins through burned areas of the
skin, or in case of starvation.
4. Increased interstitial fluid colloid oncotic pressure, which will draw
fluid out of the plasma into the tissue spaces. This results from
lymphatic blockage, which prevents the return of proteins from
interstitial spaces to the blood.