Microcirculation and Starling forces

Definitions & Key takeaways

The microcirculation refers to the network of small blood vessels that deliver oxygen and nutrients to individual cells and remove waste products. Starling forces are the physical forces that determine the movement of fluid between capillaries and tissue fluid.

The two major starling forces are hydrostatic pressure and oncotic pressure. Hydrostatic pressure is the force exerted by blood inside the capillary or in the interstitial space. It is generated by the contraction of the heart muscle, which squeezes blood through the arteries and pushes it into the capillaries. On the other hand, the oncotic pressure is created by proteins (mainly albumin) in the blood, which pulls water out of tissue fluid into capillaries.

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Introduction0:00–0:28

With microcirculation and Starling forces, microcirculation refers to the blood that flows through the smallest vessels in the circulatory system called capillaries.
And Starling forces, named after British physiologist Ernest Starling, sometimes called Starling pressures, are the forces that drive the exchange of fluid through the walls of the capillaries.

Microcirculation0:28–1:33

The capillaries have a single layer of endothelial cells lining their walls with clefts between these cells. Normally, blood flows into smaller and smaller arteries, eventually reaching the arterioles, the metarterioles, and then the capillaries.
In the capillary bed, due to the capillary’s thin walls and clefts, substances like nutrients or waste products can move from the blood into surrounding tissues and vice-versa.
After the capillaries, blood moves into venules, and then finally into veins. Intertwined with these capillaries are the lymphatic capillaries, which return interstitial fluid and proteins to the vascular system.
Lymphatic capillaries can also empty into larger lymphatic vessels and eventually into the thoracic duct, which empties lymphatic fluid directly into the large veins.
So, arterioles, metarterioles, capillaries, venules, and lymphatic vessels together make up the microcirculation. Now, the arterioles that come before the capillaries act as floodgates, regulating blood flow into the capillaries.

Arterioles1:33–3:12

So if the arterioles constrict, the resistance increases, and if they dilate, the resistance decreases. Therefore, the arterioles generally determine total peripheral resistance, or the amount of resistance opposing blood flow.
This means arterioles play a key role in regulating the blood flow to an organ. Now, there are 2 mechanisms that help them do their job, intrinsic and extrinsic control.
Intrinsic control of blood flow is based on the level of metabolites in the surrounding tissue. For example, adenosine and carbon dioxide will cause nearby arterioles to dilate.
Another type of intrinsic control is autoregulation, and it's when the flow of blood is kept steady against changing arterial pressure.
So, a sudden drop of pressure can reduce the movement of blood towards an organ, but the arterioles, autoregulate, by dilating, which reduces resistance to maintain blood flow.
Then there's active hyperemia, and it's when an organ becomes more metabolically active, its perfusion goes up to meet the increased demand.
So, when a jogger is running, the blood flow to her leg muscles go up. Now, with extrinsic control is based on the sympathetic nervous system and endocrine system which can decrease or increase vascular smooth muscle contraction, constricting or dilating the arterioles.
Now, moving along the microcirculation to the capillaries. Here substances can cross the capillaries in three ways; there's simple diffusion, vesicular transport, and osmosis.

Capillaries3:12–5:08

But overall, the most common is simple diffusion. Normally, some substances can diffuse through the clefts between the endothelial cells, but only if they’re water soluble.
So, molecules like ions, glucose, and amino acids can readily pass through these openings. But there are others, like proteins, that are too big to fit through these clefts, so they have to cross in little membrane bubbles called vesicles.
The exceptions are the capillary walls in the kidney and intestine which are fenestrated, meaning they have large pores that allow some proteins to cross unimpeded.
Alternatively, lipid-soluble solutes and gasses like oxygen and carbon dioxide, can just diffuse across the capillary walls.
Additionally, the rate of diffusion of water-soluble substances and lipid-soluble gases are not the same. It all comes down to the total surface area available for them to cross.
So water-soluble molecules like glucose, are limited to the clefts, while something like oxygen can diffuse across any surface of the endothelial membranes.
This is why oxygen can diffuse into tissues faster than glucose. Now, while all these exchanges are occurring along the capillaries, another substance, water, is also making its way across, specifically, through the endothelial clefts.
It generally occurs by osmosis, no, not the makers of this video you’re watching, but the movement of water across a semipermeable membrane from an area of low solute concentration to an area with high solute concentration.

Starling Forces5:08–5:44

And the net movement of water is determined by Starling forces, sometimes called Starling pressures, named after British physiologist Ernest Starling, who formulated the Starling Equation.
And it goes something like this: Where Jv is fluid movement, Kf is the filtration coefficient, Pc is capillary hydrostatic pressure, Pi is interstitial hydrostatic pressure, σ is reflection coefficient, πc is capillary oncotic pressure, and πi is interstitial oncotic pressure.

Hydrostatic and Oncotic Pressure5:44–6:49

The equation simply states that there are two types of forces acting within, and outside the capillaries, that determine fluid movement.
Let's start with hydrostatic pressures, which is described as the pressure that is exerted by a fluid in an enclosed space, in this case, inside the capillary, or the interstitial space.
Here we have, Pc or capillary hydrostatic pressure, which pushes fluid out of the capillaries. And then the Pi or interstitial hydrostatic pressure, which pushes fluids into the capillaries.
Then there are the oncotic pressures, sometimes called colloid osmotic pressures. And it’s caused by proteins like albumin, which pull fluids towards them.
Here we have the πc or capillary oncotic pressure, which pulls fluids into the capillary. In addition to the πi or interstitial oncotic pressure, which pulls fluids into the interstitial space.
But since there’s usually little protein in the interstitial space, the interstitial oncotic pressure in normal conditions is close to zero.
Next, let’s look at Kf or filtration coefficient, which is not a force. But rather, determines the water permeability of the capillary walls.

Filtration Coefficient6:49–7:24

It varies among different types of capillaries, and it depends on the size of the clefts or fenestrations. So, there are bigger fenestrations in glomerular capillaries, which let more water molecules through, and as a result, it has a high filtration coefficient.
Cerebral capillaries, in comparison, have no fenestrations, so they have a low filtration coefficient. Lastly, there’s the σ or reflection coefficient, and it describes the capillary’s permeability to proteins.

Reflection Coefficient7:24–8:55

A coefficient of 1 means the capillary is impermeable to protein, whereas a 0 means it is completely permeable. So, when the reflection coefficient is 1, it means that plasma proteins, like albumin are super concentrated in capillary blood, and exert a great osmotic pressure.
So, an increase in capillary protein concentration will lead to increases in capillary oncotic pressure, and therefore decrease the movement of water towards the interstitial space.
To determine if the fluid is going into or out of the capillaries. We can use a simplified version of the equation where we ignore Kf and σ.
Let's say that we are measuring the net pressure in the arterial end of the capillary. The Pc or capillary hydrostatic pressure is 37 mmHg and Pi or interstitial hydrostatic pressure is 1 mmHg.
Moving along, πc or capillary oncotic pressure is 50 mmHg, while πi or interstitial oncotic pressure is 0 mmHg. We get a net pressure of -14 mmHg.
Since the number is negative, it means water will move into the capillary. If the number is positive, it means the water will move out.
Okay, there are many conditions that alter these variables so as an example, let’s look at some causes of edema. In heart failure, blood can’t get pumped out efficiently, so it backs up into the capillaries.

Altering Conditions8:55–9:53

This increases capillary hydrostatic pressure (Pc) causing fluid to “leak out”, causing edema in the lower limbs. When there’s an infection, toxins, or burns, it increases the filtration coefficient (Kf), the clefts become bigger letting more water through, which also leads to edema.
What’s more, when there's lymphatic blockage like in filariasis, proteins leak out into the interstitium which increases interstitial oncotic pressure (πi) causing lymphedema.
In other instances, like in nephrotic syndrome, liver failure, and protein malnutrition there’s a decrease in plasma protein which decreases capillary oncotic pressure (πc), and this also causes edema.
Alright, as a quick recap, the microcirculation is made up of the arterioles, metarterioles, capillaries, venules, and lymphatic vessels .

Review9:53–10:39

And it's in the capillaries, where the exchange of substances happens. The exchanges can be through simple diffusion, vesicular transport, and osmosis.
Water crosses the capillary clefts with the use of the Starling forces. And the four Starling forces are, hydrostatic pressure in the capillary, hydrostatic pressure in the interstitium, oncotic pressure in the capillary, and oncotic pressure in the interstitium.
When the net force is positive, water moves out of the capillaries, and when it’s negative, water moves into the capillaries.