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Purpose: The function of the capillaries is to transport nutrients and oxygen from the blood to all the tissues in the body and to collect waste from these tissues back into the blood.
The capillaries do not have a muscular wall. In fact, they only have a single layer of cells. These are called endothelialcells.
The blood pressure in the arterioles, at the start of 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). Furthermore, the blood flow is no longer pulsatile. This pressure is called the hydrostatic pressure.
At the beginning of the capillary therefore, the hydrostatic pressure is 30 mmHg. But at the end of the capillary, this hydrostatic pressure has decreased to about 20 mmHg.
Why? Because of the resistance of the capillary wall to the blood flow, just like in the arterioles.
At the same time, outside the capillaries, in the interstitial space, the pressure is much lower; about 0 mmHg.
Therefore, the pressure difference (=gradient) between inside and outside the capillary would be 20-30 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 larger interstitial space (typically 10-15 litres); we would then develop massive oedema and die of cardiovascular shock.
Obviously, this does not happen and this is due to the oncotic pressure and the capillary exchange system.
1.
The oncotic pressure is an osmotic pressure. This is because the capillaries allow the filtration of water and small molecules but not of large molecules (such as albumin). If particles cannot pass a wall but water can, then water is transported and this is osmosis.
(Remember osmosis? A.2.3. Passive Transport Systems)
The height of the oncotic pressure is determined by the number of particles that cannot filtrate through the membrane. In general, this is typically about 25 mmHg.
Furthermore, in contrast to the hydrostatic pressure, the oncotic pressure is constant throughout the length of the capillaries.
Remember that the hydrostatic pressure is a pressure from inside the capillary to the 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 absorbs water from outside to inside the capillary.
There are two pressures influencing the flow of fluid inside the capillaries.
The hydrostatic pressure pushes the blood fluid out of the capillaries.
You can now calculate the pressure difference between the two pressures:
Hydrostatic Pressure – Oncotic Pressure = Filtration pressure.
So, at the beginning of the capillary, the filtration pressure is:
30-25 = 5 mmHg.
This means, that some water will flow from the inside of the capillary to outside in the interstitial space.
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.
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 automatically 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!).
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 weights. 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).
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 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 sinusoidalcapillaries in liver and bone marrow) or are much less permeable (such as the blood brain barrier in the brain).
There are essentially three situations when something goes wrong in this system and oedema develops:
Oncotic Pressure is too low.
This is the situation when the amount of particles in the blood, that cannot 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.
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).
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 amount of 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.
