Microcirculation and Starling forces

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.

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.

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.

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.

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.

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.