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.