Body Fluids and Membrane Transport

In Lecture Today:

- Body Water- Fluid Compartments- Cell membrane - Mechanisms of transport across the cell membrane:

• Diffusion, and rate of diffusion

- Osmosis- Molarity and Molality- Osmolality- Tonicity- Carrier-mediated transport

CHAPTER 6

About 60% of the adult human body is composed of water. In this water there are ions and substances essential for life either dissolved or floating. The water is inside of the cells as well as outside:

- Claude Bernard, 19th century French physiologist:

“milieu interieur ”

Body Water

CELL

Water inside the cells : Intracellular Fluid (ICF) ICF

Water outside the cells : Extracellular Fluid (ECF) ECF

In an Adult Male 70 Kg Body Weight:

Tissue % water Liters of waterSkin 72 9.07Muscle 75 22.1Skeleton 22 2.45Blood 83 4.65Fat 10 0.70All others - 3.03

Total Body Water(TBW) 42

Total Body WaterIn a normal adult male, total body water (TBW) is 60% of body weight. For example in a 70 Kg man TBW is 0.6 x 70 = 42 Kg or 42 L

Percentage Body Water

The fraction of body weight that is water varies with both the age and gender (male & female).

Female usually have more body fat which contains less water than other body tissues. The % water content of the body also decreases with age.

Body Fluid Compartments

The fluids in the body are compartmentalized. Body fluids can be either inside (ICF) or outside (ECF):

1- Intracellular Fluid (ICF) Compartment:

- About 40% of total body weight is ICF. There are more than 75 trillions of cell in human body. Each cell-type has its own unique structure. But concentration of most substances in the cell are similar from one cell to another. That is why the fluid inside all the cells of the body is considered as one compartment.

ICF

2- Extracellular Fluid (ECF) Compartment: All the fluids outside of the cells are collectively called ECF, which is about 20% of total body weight.

- ECF is composed of :

1- Interstitial fluid (ISF) = ECF fluid between the cells.2- Plasma = ECF of blood.3- Transcellular fluid* =

synovial = in jointsperitoneal = visceral fluidpericaridal = around the heart

intraocular = in the eyescerebrospinal fluid = within the brain

* Trancellular fluid can be considered as separate compartment.

Body Fluid Compartments

Cell Membrane

• Separates intracellular fluid from extracellular fluid.

• Composed primarily of phospholipids and proteins.

• Proteins may serve as carriers, channels and receptors.

Mechanisms of Transport Across Cell Membrane

• According to the means of transport there are two categories:

1- Non-carrier-mediated transport- Simple diffusion

2- Carrier-mediated transport- Facilitated diffusion- Active transport

Diffusion• Random motion of molecules due to their thermal energy

is called diffusion.

• Molecules in a solution tend to reach a uniform state. For example a drop of ink in a water container spreads uniformly.

Diffusion

Diffusion Through the Cell Membrane

• Two major groups of molecules can pass the phospholipid bilayer of cell membrane by simple diffusion:

1- Molecules that can dissolve in the lipid bilayer membrane, non-polar molecules such as:

O2, Hormones (Steroids)

2- Small polar molecules which are uncharged such as:

CO2, alcohol, and urea

Lipid Soluble

Small uncharged

Diffusion Through Protein Channels

• Small ions can use ion channels in the membrane:

Charged

Rate of Diffusion

• Rate of diffusion = number of diffusing molecules passing through the membrane per unit time.

• Rate of diffusion depends on:

1- Concentration difference across the membrane.

2- Permeability of the membrane to the diffusing molecule.

3- Surface area of the membrane.

4- Molecular weight of the diffusing molecule.

5- Distance.

6- Temperature.

Rate of diffusion Concentration gradient x Surface area x Temperature

MW x distance

Osmosis

• The net diffusion of water across the

membrane is called osmosis.

• Osmosis can occur only if the membrane

is semipermeable.

• Semipermeable means that the membrane

must be more permeable to water than the

solute dissolved in water.

Osmotic Pressure• What is osmotic pressure?

The force needed to prevent osmotic movement of water from one area to

another across a semipermeable membrane.

Molarity and Molality• Equivalent of one molecular weight (g) of a substance dissolved in water to

make a total one liter solution is called a Molar solution (1 M).

• When equivalent of one molecular weight (g) of a substance is added to

one liter (Kg) of water, this solution is called Molal solution (1 m).

Molal solution is a better indication of solute to solvent ratio, therefore it is a

better indicator of osmosis. However, in the body since the differences between

Molal and Molar concentration of solutes is very small, Molarity is often used.

Molarity and Molality

Osmolarity• Total molality of substances in a solution is called osmolality (Osm).

Our body fluids contain a mixture of various substances. Each substance

has a certain molar concentration. The total sum of these concentration is

measured in Osmolar (Osmoles/Liter)

e.g A solution containing 1 m glucose and 2 m fructose has osmolality

of 3 osmol/L (3 Osm).

• Electrolytes such as NaCl are ionized when in solution, therefore one molecule of NaCl in solution yields two ions. So 1 m of NaCl has osmolality of 2 Osm.

Tonicity

• Solutions that have the same total concentration of solutes and the same osmotic pressure as plasma* are said to be isotonic.

• Solutions that have a lower total concentration of solutes and a lower osmotic pressure than plasma are said to be hypotonic.

• Solutions that have a higher total concentration of solutes and a higher osmotic pressure than plasma are said to be hypertonic.

* In the body plasma has osmolarity of 0.28 Osm (280 mOsm).

Tonicity

Regulation of Blood Osmolarity• Blood osmolarity is maintained within a narrow range and when this

osmolarity changes several regulatory mechanisms come into action.

Carrier-Mediated transport• Unlike the simple diffusion, carrier-mediated transport shows:

1- Specificity

2- Competition

3- Saturation

Carrier-Mediated transport• There are two major types of carrier-mediated transport:

a) Facilitated diffusion: like simple diffusion facilitated diffusion is powered by thermal energy of the diffusing molecules. But the transport of molecules across the membrane is helped by a carrier protein. For example glucose is transported to the cells of the body by faciliteted difussion. the net transport is along the concentration gradient.

b) Active transport: Movement of molecules against their concentration gradient which requires energy (ATP). For example movement of calcium from inside to outside of the cell.

Facilitated Diffusion

Active Transport

a) Primary active transport: ATP is directly needed for the carrier protein in the following sequences:

1- Binding of molecule to the carrier protein

2- ATP is hydrolysed to provide energy for transport.

3- Carrier changes its shape and moves the molecule across the membrane.

a) Primary active transport:e.g Transport of Ca++ from inside to outside of the cell. Example 1

a) Primary active transport:e.g Na/K pump.

Example 2

b) Secondary active transport (Co-transport):

The energy required is obtained from downhill transport of Na+ into cell:

ECF ICF

NaNa

K K

NaNa

GlucoseGlucose

Active Transport

Na

Na

CELL

b) Secondary active transport (Co-transport):e.g Transport of glucose in kidney.

b) Secondary active transport (Co-transport):e.g Co-transport of Na+ and glucose.