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Physiology, Lecture #3Transport and Body Fluid

We have two types of passive transport: either a simple diffusion or a facilitated diffusion, In simple diffusion we either have pores between phospholipids or channels in the proteins in the cytoplasmic membrane, and through these pores and channels particles can cross the cytoplasmic membrane down their concentration gradient (From higher pressure to lower pressure).

And the rate of diffusion increases with increasing the difference between the two concentrations across the cytoplasmic membrane. For example if the rate of transport was 10 particles/sec and we doubled the concentration on the side of the higher concentration the rate will be doubled as well (20 particles/sec) so the rate is directly proportional to the concentration.

In the facilitated diffusion the rules are the same but there are specific proteins for specific particles due to the matching between the binding site of the protein and the shape of the particle, in other words a specific protein can only transport one kind of particles and no other (see figure), while in simple diffusion there is no differentiation. That means that any particle small enough to get in the pore will pass through without facing any rejection. When the particles bind to the binding site of the proteins it stimulates conformational changes that cause the protein to release the particle into the cell.

Let's take for example 50 proteins each with 2 binding sites so in general we have 100 binding sites, so if we have 50 they will pass and if we have 70 particles they will pass and the rate will increase, if we have 100 they will pass too and the rate will also be increased. However, if we have 110 particles only 100 pass and 10 remain outside the cytoplasmic membrane because all of the binding sites are preoccupied, so at this point we will reach that concentration which is equivalent to the number of the binding sites for that transporter and the rate of transport will not exceed that and that is called the saturation level (see fig), so if we have 150 particles only 100 particles/sec will pass through the membrane, if we have 600 only 100 particles/sec will pass through the membrane.

What about active transport?

Simple diffusiondown concentrationgradient

Rate oftransportof moleculeinto cell

Concentration of transportedmolecules in ECF

Carrier-mediated transport down concentration gradient(facilitated diffusion)

Low High

Tm

Active transport needs energy from an outside source rather than the kinetic motion itself, and it's very essential for our life, so if the active transport in our cells is blocked for any reason all of the cells will die. Active transport is divided to primary or secondary. Primary active transport uses ATP directly as a source of energy, an example of it is the sodium potassium pump while the secondary transport uses the energy produced by simple or facilitated diffusion (the flow of substances from high concentrations to lower concentration). Secondary transport is divided to:

1- Co-transport: When a passively transported particle goes from one side to the other it creates energy to take another particle in the same direction.

2- Counter transport: It is the opposite, passively transported particle go from A to B but the actively transported particles go from B to A.

So how can these active transporters play their role? How can they transport particles from lower concentrations to higher concentrations? These transporters are protein carriers with binding sites. Since they are carrying the particles on the binding site they should be able to carry particles from both sides (the higher and the lower concentration) but these proteins are specific enough to change the shape of the binding site when exposed to the other side. Let us explain that. Look at the figure of step 1, this is a protein, and the blue squares are particles, this protein is an active carrier for this particle. When it's exposed to the side with the lower concentration these sites will be accessible to bind that particle (the particle fits in the binding site).

When they bind, the protein will switch over and the channel will be opened to the other side then the particle will be released. After that the protein will go through conformational changes (the shape of the binding site changes) so the particle no longer fits in the binding site. In other words this active protein carrier was able to carry these particles from the lower to the higher concentration (Against their own concentration gradient) but it wasn't able to carry the same particle from the higher concentration to the lower concentration . Figure step 2.

Osmosis: Simple Diffusion of Water In this figure the membrane is permeable for both particles and water, in compartment B we have higher concentration of particles (lower concentration of water), while in A we have higher concentration of water (lower concentration of particles), and because this membrane is permeable for both molecules the particles will cross the membrane from B to A. At the same time the water will pass from A to B because it has higher concentration in A until we reach the equilibrium. The equilibrium means that the concentration of particles in A and B are equal, and the concentration of water is also equal at both sides. The transport will continue but the net transport will be zero. This is related to membranes which are permeable to both solute and solvent.

Membrane

Higher H2Oconcentration,lower soluteconcentration

Lower H2Oconcentration,higher soluteconcentration

H2O

Area A

Area B

If the membrane is permeable to the water but impermeable to the particles the osmosis phenomena will appear. Now in this figure, there is lower concentration of particles in A and higher in B so these particles will start to suck the water from side A to B until the concentration of particles become equal in both sides, but that will never be reached! Why is that? Let me give u another example.

Area A doesn’t have any particles just pure water and assume that the particles in B are NaCl. NaCl is not permeable through the membrane, so water start to pass from A to B until equilibrium is reached, but what is equilibrium? I have pure water in A and particles in B so I will never have equal conc. of particles in both sides because in A the particle conc. is zero. So when will the osmosis stop? The osmosis will stop when the weight of water in B is equal to the osmotic pressure, so the level of water in B will be higher and the amount of water in it will be more also.

Now what's the osmotic pressure? If we want to prepare 1 liter solution of NaCl which has the molarity of 1 mol /liter = 1 Molar, we weigh 58 g of NaCl which is equivalent to 1 mole (58 is the molecular weight of NaCl), then we dissolve them in a small amount of water (not in one liter) then when it's dissolved we add water until it reaches 1 liter, this solution will be 1 molar of NaCl.

What is the osmolarity of this solution? Osmolarity is equal to the molarity for any substance which doesn't dissociate in water, like glucose for example, so if I prepare 1 molar solution it's equal to 1 osmole but if that substance dissociate in water (for example NaCl) the osmolarity is doubled because NaCl dissociates to 2 particles, Na+ and Cl- , so if we prepare I Molar solution of NaCl its osmolarity will be 1*2=2 osmole/liter so there is a factor called dissociation factor for chemicals, if it's 1 the osmolarity is equal to the molarity, if it's 2 the osmolarity is double the molarity if it's 3 the osmolarity is triple the molarity. One osmole in the solution creates 19300 mmHg osmotic pressure.

Now back to the figure, we mentioned that the osmosis will stop when the weight of the water is equal to the osmotic pressure, so if we have one osmole in area B we need a weight of water which is equivalent to the osmotic pressure created by the one osmole which is 19300 mmHg, and that is really a huge number. If this is a glucose solution and we want to prepare 1 Molar of glucose (molecular weight for glucose is 180) we should weigh 180 grams of glucose then dissolve them in a small amount of water keep adding until 1 liter. Now u have 1 liter of glucose solution and 1 liter of NaCl solution, so where is the higher conc. of water? If we are talking about conc. there will be 180 grams of glucose in the first solution in 1 liter of water and 58 grams of NaCl in the second solution also in 1 liter of water, so the concept of conc. of water isn't real in osmosis!

BA

A B

The real thing is called the chemical potential. So what's the chemical potential of water? This membrane (see figure) is permeable for water molecules and impermeable for particles. (to keep it simple we assumed that there are 4 particles in A and 8 in B), the concept of osmosis means that the water will go from A to B, we want to answer why? It's a simple diffusion not an active one so its driven by its kinetic motion, the kinetic motion will be the driving force for that, so the molecules of water inside A and B are constantly moving, up, down, right, and left in these direction so they will strike the cytoplasmic membrane. When these particles move there is a probability that they will hit the holes in the cytoplasmic membrane and close it ( these are big particles), and because the particles in B are double the number of particles in A the probability that they close the holes is also doubled because all particles move constantly and randomly and all molecules (water and particles) have the same kinetic motion so the water just move in a fast way with random direction and if the hole is opened it will pass through and if it is closed (by the particles) it can't get through it.

To further explain that, if we take one second as our time limitation and focus on one hole for simplification, in one second the hole in side A will be opened once but on side B in 2 seconds it will be opened once so in side A the channels will be opened more, and in 2 seconds it will be opened twice and pass 2 water molecules to side B while the channel in side B will be opened only once in 2 seconds allowing only 1 water molecule to pass to side A. So the net result will be more water going through these channels from A to B not because of the conc. of the water but because of the opening probabilities.

Anyway, the point the Dr. wants us to understand is that the concept of conc. isn't 100% real. It's true if we are not measuring the conc. but we are measuring the number of particles in the water, so the number of particles will replace a number of water particles, so water particles will be less in higher concentration.

Body Fluid in Human Body: We mentioned that cells can't live without water; they can't survive or perform their functions without being in a solution. That's why 60% of our body weight is water, and that’s about 42 liters in human body (in adult male (70) kg). It is located in different areas or regions:

1- Extracellular body fluid (ECF): located outside the cell, either inside the vessels which is intravascular or outside the vessels which is interstitial. The amount of ECF is about the third of the total body water, so about 12 liters outside the cells. They are located in the plasma and interstitial compartments. The barrier between intravascular and interstitial is the capillaries (the layer of the epithelial cells of the capillaries) endothelial cells.

2- Intracellular body fluid (ICF): It makes the major compartment of body fluid and is located inside the cells. Even though the cell is very tiny but we are talking about trillions of cells, so if you collect the amount of water inside the cells you'll get about 28 liters in average. The barrier between ECF & ICF is the cytoplasmic membrane of the cells.

Through our life changes happen to the interstitial compartments and the plasma inside the cells, and those changes shouldn't stay in that region they should move, change their selves to keep the internal environment constant, to keep our life normal. Now, we are not talking about nutrients, vitamins, O2 or CO2, we are talking about water movement, we want to fix the volume of the water in our body, so it will not exceed or be less than those 42 liters, and we also should have normal osmolarity of the solution inside our body. So the goal of this lecture is to give u an idea on how can we maintain normal volume of water and normal osmolarity. What's the tonicity of the body fluid? The normal tonicity of body fluid is called isotonic or normotonic, the isotonic means the conc. or the osmolarity of the solution which keeps the cell in its normal volume. We said that one of the major functions of the plasma membrane is to keep cell fluidity, so if u put the cell in isotonic solution its volume will remain the same, but if u changed the osmolarity, and that's possible just by adding a little bit of NaCl or glucose or fatty acid to that solution u will change the osmolarity. The normal osmolarity of plasma which is called isotonic is equal to (280-300) milli osmole/ liter (memorize this number it’s very important).

If I add particles to the solution I will change that solution from isotonic to hypertonic. Hyper means more particles so the hypertonic solution is the solution which has more than 300 milli osmole/liter. If we put the cell in that hypertonic solution it will be shrunken (it will cause shrinking to the cell), because it will suck the water from ICF, so it isn't acceptable to have hypertonic solution (it's acceptable under normal ranges), but there is a way in which the homeostasis keeps the osmolarity normal. However if we take particles from the solution we will create a hypotonic solution, if we put the cell in this hypotonic solution the cell will be swollen (it will cause swelling to the cell) because the extra water outside will enter the cell, so more water will get inside the cell. The swelling and shrinking of the cell is a killer change for brain & other tissues. That's why the medical concept called dehydration isn't acceptable in medicine, if u have a dehydrated child for example you should seek help in hospitals, on the other hand if u have what's called water intoxication which is really swelling of the whole body, it's very dangerous for the brain and the body functions. So the target of homeostasis is to maintain the cell's normal function and performance by keeping it in isotonic solution. So how does this happen?

In general, there are 2 ways to keep the internal environment stable concerning variables such as water or other substances. If we need to increase that substance inside our body we either take it from outside (if we r talking about water then we drink water), or we metabolize that substance inside our bodies (cells and tissues) for example it is possible to synthesize water inside tissues. So we have 2 sources to increase a substance either from outside sources or from inside by metabolism). But if I want to decrease a substance whether it's water or any other substance I can do the same thing but in opposite direction, I will get rid of that substance by urine, feces, sweating, evaporation or breathing (I exhale that substance and expel it outside), or I can reduce the surface of that substance, how is that? In the human body there are spaces called storage places such as liver, muscles, brain and spleen.

Let's take an example… If you eat a sugary meal the level of glucose in your body will be increased in an unacceptable way because u can't burn that glucose immediately so it will remain in your body and that is not acceptable, the normal plasma glucose level in blood is 70

mg/dL to 99 mg/dL but that normal level changes all the time because of eating or drinking glucose-rich foods so what happens inside our body?

There are special hormones that take the glucose and put it in the storage places of our body to keep it away from the plasma (to keep the vital sign which is the glucose level normal), and whenever we need that glucose we don't have need to take it from outside sources, a signal is released telling the stored glucose that we need it and so it is released and utilized especially when we are hungry.

So the osmolarity of the body fluid keeps changing all the time but that doesn’t cause a dangerous situation for the cells, how is that possible? First of all we have 42 liters of water in our body, but every day we all take some amount of water in by drinking and some amounts are created by synthesizing processes inside the body, this amount is about 2.3 liter /day (majority of it will be taken as drinking water) but because we have 42 liters we don't need these 2.3, they are extra because it's higher than the normal volume so by the end of the day u will lose them back by different routes, the main one is the urine which excrete about 1.4 liter/day of water, also you excrete some water with feces and sweat and there is a significant amount of water that will be lost as insensible loss of water about (500-700 ) ml/ day, that means you lose water without feeling it (in contrast you do feel it when you sweat or urinate) you lose that insensibly whether you are sleeping or walking or sitting, by doing so; the extra amount of water we took per day will be lost. So, normal volume of body fluid will be maintained.

What if we have only 40 liters of water in our body in a given day you will not drink 2.3 liters, instead you will drink 4.3 liters, 2 liters will replace the loss and 2.3 liters will do whatever their job is; which is washing the internal environment, giving extra nutrition and oxygen to the tissues, taking the waste back and washing everything inside your body.

We said that the ECF makes about the third of the whole water weight in our bodies, and the plasma volume is 5 liter (the blood volume is 5 liters), and interstitial compartment has about 8-9 liters. However the ICF compartment has the volume of 29 liters that means there is a way to measure the volume of fluids in the body. What is the way to measure the fluid in the body? There is a direct way, it isn't useable for human beings but u can use it for animals. If there is a dead animal we can put it in a closed compartment and heat that animal until all of the water evaporates then we condensate that water and measure the volume of it so it isn't practical.

The indirect way we use a principle called indicator dilution principle, it is simple and easy, let's say we have three glasses of water, in the first one we have 1 liter of pure water and in the second 5 liters and in the third 10 liter and I add one drop of ink to each of them, (the ink here is the indicator), now when we observe the density of the ink in each container, we see that the 1 liter container is more dense than the second and the second is more dense than the third, why is that? Because the dilution factor for that drop of ink depends on the volume of water.

This idea was taken to measure the body fluid in human body, and we used an indicator (tracer), examples for these traces are Tritium, Deuterium, NaCl, Enolein, and albumin and so on, let's take the NaCl as a simple example for you, you need to know that Na is located outside the cells, there's no way that Na gets inside the cells in significant amounts, no matter how much salt you eat and the concentration of Na outside the the cell is 150 millimolar,

while inside the cells it's only 5 millimolar up to 7 or 10 millimolar, so now I take an amount of Na and I label it (labeling means mark the tracers to distinguish between what we gave to the person and what was already located inside his/her body when I take the plasma sample, and it can be done by many ways; coloring, specific chemical structure of these tracers or by radioactive material in small non-dangerous amounts or dosages), now we prepare a solution of NaCl (let's assume that the volume of NaCl was 1 mill and this is the initial volume of the tracer ) which is labeled by radioactive material, then I measure the radioactivity in it by a device, the number I get represents the initial concentration of NaCl, now I give this solution to a human being intravenously, after a few minutes it will be distributed evenly inside the body but not inside the cells, (in other words the Na will be evenly distributed in the ECF compartment not in ICF), now we take a sample of the plasma and I measure the radioactivity of that sample (it will be much less than initial radioactivity because now the Na was distributed all over the ECF (about 9 liters)) and this number represents the radioactivity of the Na we gave to the person, then we use this simple equation: Vi Ci = Vf Cf

Where Vi = initial volume of the tracer Ci = initial concentration of the tracer (radio activity) Vf = final volume (which we want to calculate, in this experiment it's the ECF volume) Cf = final concentration (the radio activity of the plasma sample)

We can also use heavy water which is different from real water, and give it to a human being to drink, but before drinking the heavy water we measure the volume and concentration of it, which is 100 % in this case, then after few minutes (45 min) the heavy water will be distributed inside the cells and outside the cells because its water, there is no rejection from the cell to the water, it will accept that water. After 45 min if I take any sample from that human being and I measure the concentration for that sample, the final concentration will be known, and we already know the initial concentration so we can calculate the final volume using (Vi Ci = Vf Cf), which represents the total body water in that human being. Now we can calculate the intracellular fluid compartment by measuring the total body water and ECF and subtract the ECF from the total body water. (Total body water - ECF = ICF)

How does the intravascular and extra vascular fluid communicate? This process is called the starling capillary circulation. It means that the plasma move out from capillaries to interstitial compartment then interstitial fluid move back to capillaries.

This is a capillary (the figure) it has only one layer of endothelial cells, but if u look carefully between those cells there are pores in which water passes through. The more pores the vessel has the more the water can get out, this criteria is called the permeability of the capillaries. If you compare the permeability of capillaries with the permeability of arteries and arterioles (it is 100 times more); so these small tiny capillaries are the end of vascular system, and they are huge in network and their surface area is huge and the pores in them are huge in numbers; so the flow from the plasma outside is easy but it's not that simple. If I have a pore I can get water from out to in or from in to out, I should have a driving force that determines which way the water will go. We have only 2 types of driving force these 2 forces are: 1- Hydrostatic pressure 2- Osmotic pressure.

What kind of hydrostatic pressure and osmotic pressure are in the capillaries? Let's take a capillary, u know that there is blood pressure created by the heart pump which pumps the blood, it will create the pressure inside the vascular system and this pressure is the highest in the aorta, then it goes down till it reaches almost zero. This is a capillary (look at the figure); before the capillary we have an arteriole, this capillary is divided in two halves: the first one is called the arterial part and the other one is the venous part. What's the difference? There are pores all over the capillary, if u count these in the arterial part compared to the venous part, u will find that it's more in the venous part. So in other words, the permeability of the venous part is higher. Now when the plasma go through these capillaries, it has a hydrostatic pressure, if you measure the hydrostatic pressure in the capillaries in the arterial part it is almost 30 mmHg , but if you measure the hydrostatic pressure at the venous end of the capillary it's only 10 mmHg . Hydrostatic pressure is a pushing force, so the pushing force of plasma located in 30 mmHg (arterial end) is higher than 10 mmHg (venous part), so if you look carefully ,this kind of pressure will force the plasma to go through these pores outside the capillaries, so

there is a force that causes the water to go out by 30 mmHg where as in the other end the plasma will go out by force of 10mmHg.

There are special proteins in the plasma called plasma proteins (8% of plasma), and when we say proteins we know that they are big particles, they can't go through these pores, only one type of protein can go through which is very small, only a small particle of it. Now because we have these protein particles inside the capillaries they create osmotic pressure, if we measure the osmotic pressure inside the capillary created by these proteins (this pressure has a special name called oncotic colloid pressure) if we measure it, it is 28 mmHg osmotic pressure. This force will not push the water out it will try to suck the water from outside; because if you look at the interstitial compartment, it has a solution and if u look at intravascular compartment it also has a solution, they are almost identical in composition with one exception is that the proteins are not present in the interstitial fluid; so the osmosis inside the capillary is more than the osmosis outside the capillary so these 28 mmHg try to pull water back. Now if u measure pressure in the interstitium, we have cells there, if you put a device which measure pressure between cells, u will get a negative reading. So in other words; between cells there is a negative pressure this pressure is the hydrostatic pressure, the amount of that pressure is -3 mmHg. Some small particles of proteins can leave the capillary but not all, so when they are out they will create an osmotic pressure outside the capillary; the amount of osmotic pressure outside the capillary (in the interstitial fluid) is around 8 mmHg. Now I think u should answer this simple question, if you look at this minus pressure which is located in the interstitial compartment will it give a chance to suck the water from capillary or will it push it back? It's a minus so it's suction, so this suction will create an extra flow of water through the pores. Let's talk about the 8 mmHg osmosis, small particles of proteins in the interstitium; this creates suction for the water from the capillary once more. Now let's summarize that: For the arterial end:-30 mmHg pushing the water out 28 mmHg pushing the water back -3 mmHg pushing the water out 8 mmHg pushing the water out In the arterial end we have 3 forces pushing the plasma out, where as one force is trying to suck the water back in. so in math (30+3+8+)-28 =13 mmHg is the net filtration pressure which cause the water to go outside the capillary (with the consideration that any force that push the water out takes a positive sign while forces that suck the water back has a minus sign).

For the venous end:-10 mmHg pushing the water out

28 mmHg pushing the water back -3 mmHg pushing the water out 8 mmHg pushing the water out Now in the venous part I have 10 mmHg pushing the water outside and -3 also pushing it out and 8 also but the osmotic pressure -28 so in math (10+8 +3)-28=-7 mmHg .So this conclusion means god gave us that nice function of capillary which is to create it in two halves: arterial half which has different forces, but net filtration pressure in that half is to push the plasma out. Why? Because we need the plasma to supply tissues with the food and oxygen and so on.

The plasma now is out in the interstitial compartment, if that happen and it doesn’t go back that will cause a disaster. Every second you will have more plasma out and less plasma inside the capillaries and that will cause death! However, as we said that we need to keep the plasma volume constant in the interstitial compartment and in the intracellular fluid. So this amount of plasma, which left the capillary to give the oxygen and nutrition to cells and to wash this area from toxic materials, when it is close to the venous part there is a force to suck it back, this force is equal to -7 mmHg, and we mentioned that the permeability of the venous part is more than the arterial part. So I don’t need the same force to get the water back in so the -7 mmHg is enough to get most of the plasma back.

If I filtrate 100 ml of plasma per sec to feed the tissue, and then calculate the plasma that goes back in the capillary, it's less than 100 so there is a small amount of plasma remaining outside the capillary all the time. Even if it is not significant it is not acceptable in life, it is a tiny amount of plasma, (let's say 0.5 ml/hr) which will be around 12 ml per day it will be 24 ml per 2 days and so on. To avoid that there is another special structure inside the capillary circulation, in all these capillary in your body there is special lymph structure going side by side with capillaries. The function of this lymph vessel is not to keep any amount of plasma accumulated in the interstitial compartment, so it will suck it back and it will eventually return it to the plasma. Another function of lymph vessels is that it doesn’t allow these small particles of protein to be accumulated more and more and increase the pressure outside capillaries, if it is more than 8 mmHg it will cause a disaster. So this lymph vessel will take the extra water which was left from the capillary back to the plasma circulation and it will take the extra amount of protein which was left outside the capillary.

Forgive us for any mistakes...Best of luck to all of you....

Done By: Anwar Durrah Bayan Al-Shaikh

Special Thanks to: Bayan Al-Omari Aminah Al-Farraj