5.
Why? Because of the resistance of the capillary wall to the blood flow, just like in the arterioles.

6.
At the same time, outside the capillaries, in the interstitial space, the pressure is much lower; about 0 mmHg.

7.
Therefore, the pressure difference (=gradient) between inside and outside the capillary would be 20-30 mmHg. This is quite a lot.

8.
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.

9.
Obviously, this does not happen and this is due to the oncotic pressure and the capillary exchange system.

3.
Furthermore, in contrast to the hydrostatic pressure, the oncotic pressure is constant throughout the length of the capillaries.

4.
Now, we are going to ‘play’ these two pressures against each other!

5.
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.

6.
The oncotic pressure works in the opposite direction; it is a ‘sucking’ pressure. It absorbs water from outside to inside the capillary.

1.
Now it gets interesting!

2.
There are two pressures influencing the flow of fluid inside the capillaries.

3.
The hydrostatic pressure pushes the blood fluid out of the capillaries.

4.
And the oncotic pressure ‘sucks’ the fluid from outside back to inside the capillaries!

5.
You can now calculate the pressure difference between the two pressures:

Hydrostatic Pressure – Oncotic Pressure = Filtration pressure.

6.
So, at the beginning of the capillary, the filtration pressure is:

30-25 =
5 mmHg.

7.
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.

8.
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.

9.
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?).

10.
So, at the end of the capillary, the hydrostatic pressure (20 mmHg) is less than the oncotic pressure (25 mmHg).

13.
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!).

1.
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).

2.
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

3.
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 ->).

4.
The hydrostatic pressure is not always the same in all parts of the body. It is about the same, everywhere, 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).

5.
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).