The Capillaries: filtration



The function of the capillaries is to transport nutrients and oxygen to the tissues and to collect waste from these tissues. They do this by filtrating the blood.

A. Introduction:


The capillaries do not have a muscular wall. In fact they only have a single layer of endothelial cells.



Because of this thin wall, water and small molecules can easily filtrate through this porous layers of cells. But, the capillary membrane works as a filter and not as a leak; large molecules and cells can not pass through the membrane.


The blood pressure in the arterioles, just before the capillaries, is typically 30 mmHg. Remember that the blood pressure has decreased along the arterial system from about 120/80 to 30 mmHg (because of the arterioles). This pressure is usually called the hydrostatic pressure.



Normally, the pressure outside the capillaries, in the interstitial space, is about 0 mmHg.


Therefore, the pressure difference (=gradient) between inside and outside the capillary would be 30 mmHg. This is quite a lot.


In fact, if there were nothing else, we would quickly loose all our water (5 litres blood) into the much larger interstitial space; we would then develop massive oedema and die of cardiovascular shock.

Obviously, this is not the case and this is due to the capillary exchange system.



B. The Capillary Exchange System: (also called the Starling-exchange system):


This exchange system is really an exchange of water (and whatever is dissolved in it such as oxygen, ions and small nutrients).


This exchange system is determined by two pressures:

    1. the hydrostatic pressure; the blood pressure inside the capillaries (see above).
    2. the oncotic pressure. This is an osmotic pressure. The capillaries allow the filtration of water and small molecules. Large molecules (such as albumin) cannot pass the capillary wall. If particles cannot pass a wall but water can, then water will be transported and this is osmosis.
    3. what is osmosis?



The height of this oncotic pressure is determined by the number of particles that cannot filtrate through the membrane and is typically 25 mmHg.


The hydrostatic pressure is a pressure from inside the capillary to outside; it ‘pushes’ the water to go out of the capillary.


The oncotic pressure works in the opposite direction; it is a ‘sucking’ pressure. It reabsorbs water from outside to inside the capillary.



Because the hydrostatic pressure in the arterioles (30 mmHg) is a little bit higher than the oncotic pressure (25 mm Hg), the difference in the pressures at the beginning of the capillaries will also be a bit higher. Therefore, some water will go out of the capillary.


You can calculate the pressure difference between the two: hydrostatic – oncotic = (net) filtration pressure. In this case, the filtration pressure is 30-25 = 5 mmHg.


However, this is at the beginning of the capillary. At the end of the capillary, things have changed. The hydrostatic pressure has decreased (because of the capillary resistance). Now, at the end of the capillary, the hydrostatic pressure has decreased to about 20 mmHg.



But the oncotic pressure has not changed at the end of the capillary. This is because the number of particles that are unable to cross the capillary membrane has not decreased (they could not get out; remember?).


So, at the end of the capillary, the hydrostatic pressure (20 mmHg) is less than the oncotic pressure (25 mmHg). The net filtration pressure is now negative (20-25 = - 5 mmHg) which means water is reabsorbed (‘sucked’) into the capillary.


In conclusion, the water that leaves the capillaries at the beginning (close to the arterioles) is now reabsorbed at the end of the capillaries.


Capillary Exchance Slide Show


Because the water that goes out contains (small) nutrients and dissolved oxygen, this will ‘automatically’ flow to the cells. At the same time, water from the cells that contain waste and CO2, will flow back into the capillaries. The water at the beginning is exchanged with water at the end; hence the name of the system (exchange system!).


C. Technical Details:


The oncotic pressure is determined by the size of the dissolved particles that cannot pass the capillary membrane. This is approximately 50,000-60,000 molecular weight. This means that all the large proteins and all the blood cells cannot pass the membrane. The most common protein that cannot pass the membrane is albumin (molecular weight 69,000).



In the description above, we assumed that the interstitial pressure was 0 mmHg. Likewise, we also assumed that the oncotic pressure in the interstitial space is also 0 mmHg. But both these assumptions are not always true. For example, in the gut, after a meal, there are many food particles in the gut interstitial space.


If the interstitial hydrostatic and/or oncotic pressure are not zero, then one should first calculate the real hydrostatic pressure gradient (= the difference between the blood pressure and the interstitial pressure) and the real oncotic gradient ( = the difference between the blood oncotic pressure and the interstitial oncotic pressure) before calculating the filtration pressure. (See example )




The hydrostatic pressure is not (always) equal to the local blood pressure. It is about the same, in all parts of the body, when a person is lying flat. However, when a person is standing, the blood pressure in the legs is higher because of the weight of the blood column (an additional 5-10 mmHg).


The permeability of most capillaries works in the way described above. However, there are also capillaries in the body that are either much more permeable (such as the fenestrated capillaries in the gut and the kidneys and the sinusoidal capillaries in liver and bone marrow) or are much less permeable (such as the blood brain barrier in the brain).


D.Pathology: Oedema (the Americans say "Edema"!):


The capillary exchange system is not only important in normal daily life to keep our cells alive. The system also explains when something goes wrong and oedema (= tissue swelling) develops.



There are essentially three situations when something goes wrong in this system and oedema develops:

  • when the Oncotic Pressure becomes too low
  • when the Hydrostatic Pressure becomes too high
  • when the capillary membrane becomes too leaky.

3. Oncotic Pressure is too low.

This is the situation when the amount of particles in the blood that does not pass the capillary wall becomes too low. This is usually the case with albumen. Albumen is the most common protein in our blood. In the case of malnutrition (=chronic lack of food), the blood albumen is used for energy. This will decrease the albumen blood concentration, and hence the oncotic pressure.



Therefore, instead of a value of 25 mmHg, the oncotic pressure could drop to, say, 20 mmHg. That means that more fluid will leave the capillary and less will be reabsorbed, leading to accumulation of fluid in the tissue; oedema!

This is seen in cases of malnutrition as often seen in poor, underdeveloped, countries. Remember the small children on TV with a big belly but thin arms??? The bellies are big because they are filled with fluid (oedema in the peritoneal space (=belly), which is called ascites). These children are severely malnourished.



5. Hydrostatic Pressure is too high.

When the hydrostatic pressure is too high, then again more fluid will leave the capillary into the interstitial space. This can be caused either by a too high arterial pressure or a too high pressure in the veins. This condition often occurs when the heart is not working properly (swelling in the ankles for example).


6. A problem with the capillary membrane.

If the capillary does not function properly and becomes too leaky, then the oncotic pressure will be lower (because this is determined by the particles that do not cross the capillary wall). This can happen for example during an infection. A typical example is the sting of a bee. The bee injects a toxic substance that makes the capillary leak. This will cause a swelling at that location.


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