Dr. Basim Al-Ka'abi
Cellular Physiology
Third Lecture
1
Lecture Name: Body Fluids and Transport Across the Cell Membrane
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:
The concept of body fluids.
The way of measuring volumes of different fluid compartments.
An introduction to the types of transport across the cell membrane.
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
Third Lecture
2
Body fluids
-In the average young adult male, 18% of the body weight is protein and
related substances, 7% is mineral and 15% is fat; the remaining 60% is
water.
-Total body water is comprised of extracellular and intracellular fluids.
-The extracellular fluid, which is about 20% of total body weight can be
subdivided into 2 sub-compartments: Plasma or intravascular fluid (20%)
and interstitial fluid (80%).
-Whereas, the intracellular fluid accounts for about 40% of body weight.
-The percentage of total body water is greater in newborns and lean
persons, and is lower in adult females, elderly persons, or adults with large
amounts of adipose tissue.
-Because the plasma and interstitial fluids are separated only by highly
permeable capillary membranes, their ionic compositions are similar and
they are often considered together as one large compartment of
homogeneous fluid.
-The most important difference between plasma and interstitial fluid is the
higher concentration of proteins in the plasma, which exist because the
capillaries have a low permeability to the plasma proteins.
-Both extracellular and intracellular fluid contains nutrients that are needed
by the cells, including glucose, amino acids, oxygen and other nutrients.
-Extracellular fluid contains large quantities of sodium and chloride ions,
but only small amounts of potassium, magnesium and phosphate ions (the
major cation is Na
+
and the major anions are Cl
-
and HCO3
-
).
-In contrast, intracellular fluid contains large amounts of potassium and
phosphate ions, moderate amounts of Mg
+2
ions and few Ca
+2
ions (The
major cations are K
+
and Mg
+2
and the major anions are protein and organic
phosphates such as ATP, ADP and AMP).
Dr. Basim Al-Ka'abi
Cellular Physiology
Third Lecture
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-These differences in the ionic composition of the fluids cause a membrane
potential to develop across the two sides of the cell membrane which is
negative on the inside and positive on the outside.
Measurement of body fluid volumes
-The volume of a fluid compartment in the body can be measured by
placing a substance in the compartment, allowing it to disperse throughout
the compartment’s fluid and then analyzing the extent to which it has
become diluted in the compartment.
-For this method to be used properly, the substance must be uniformly
distributed in the compartment, and only in the compartment that is being
measured.
Substances used for major fluid compartments
Total body water (TBW)
-Is measured using substances that disperse throughout the body fluids,
such as a radioactive water (3H
2
O) or heavy water (2H
2
O).
Extracellular fluid (ECF)
-Is measured using substances that disperse in the plasma and interstitial
fluid but do not permeate the cell membrane, such as radioactive sodium,
inulin and thiosulfate.
Intracellular volume (ICF)
-Cannot be measured directly, but can be calculated as the difference
between total body water and extracellular volume (TBW – ECF volume).
Dr. Basim Al-Ka'abi
Cellular Physiology
Third Lecture
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Plasma volume
-Is measured by injecting substances that do not penetrate capillary
membranes and therefore remain in the vascular system, such as
radioactive albumin.
Interstitial fluid volume
-Cannot be measured directly but can be calculated as the difference
between extracellular fluid volume and plasma volume (ECF volume –
plasma volume).
Transport across the cell membrane
Simple diffusion
-Occurs down an electrochemical gradient (downhill), which is the net
movement of molecules through the cell membrane along chemical or
electrical gradients.
-Molecules migrate from a region of high concentration to one lower
concentration.
-This form of transport is not carrier-mediated and do not require metabolic
energy and hence it is passive.
-The rate of diffusion across the cell membrane is directly related to:
1. The electrical potential and chemical concentration differences across
the membrane.
2. The surface area of the membrane.
3. The permeability of the membrane for the solute which is inversely
related to the size of the solute and the membrane thickness.
Facilitated diffusion
-Occurs down an electrochemical gradient (downhill) similar to simple
diffusion.
Dr. Basim Al-Ka'abi
Cellular Physiology
Third Lecture
5
-Does not require metabolic energy and therefore is passive, but is more
rapid than simple diffusion because being carrier-mediated.
-Example of that is glucose transport in the muscle and adipose cells which
is mediated by insulin (carrier protein).
-In type I diabetes mellitus, glucose uptake use by muscle and adipose cells
is impaired due to insulin deficiency.