Role of the Heart and Cushioning Vessels
The heart functions as the primary pump of the vascular system, rhythmically ejecting blood into the aorta.
Cushioning vessels include the aorta as well as large and medium-sized elastic arteries. Due to the high distensibility (elasticity) of their walls, they dampen pressure pulsations. Their main task is to convert intermittent cardiac output into a continuous and smooth blood flow. This mechanism is often referred to as the Windkessel effect (compression chamber effect).
Resistance Vessels ("Main Taps")
Resistance vessels include the pre-capillary pathways—small arteries and arterioles. Their walls contain abundant smooth muscle fibers.
- They exhibit a pronounced myogenic basal tone and account for the major share of vascular resistance.
- The greatest drop in blood pressure within the vascular bed occurs in the arterioles (more significant than in capillaries due to the greater aggregate length of arterioles).
- The tone of these vessels is regulated by the sympathetic nervous system and biologically active substances (metabolites, hormones).
Arterioles are traditionally considered the "main taps" of the circulatory system because they control tissue blood supply.
A special role is played by pre-capillary sphincters—small smooth muscle structures that determine how many capillaries are functioning (perfused) at any given moment. Their tone depends primarily on local metabolites (mostly vasodilators). By contracting, the sphincter closes the lumen and limits blood flow into the capillary bed.
Exchange and Shunt Vessels
Exchange vessels are the capillaries. Transcapillary exchange of gases and nutrients between the blood and surrounding tissues occurs precisely here. The main mechanisms of this exchange are filtration, diffusion, and osmosis.
Shunt vessels (arteriovenous anastomoses) function as direct bypass channels. They allow arterial blood to flow directly into the venous system, bypassing the capillary bed.
Capacitance Vessels and Blood Return
Capacitance vessels are represented by the venous bed. Their role in creating resistance to blood flow is minimal. However, they are capable of altering their capacity by changing their shape and lumen diameter. Even a minor change in the venous lumen leads to a significant redistribution of blood volume in the body.
Return vessels (small, medium, and large veins) are responsible for delivering blood back to the heart. They can accommodate up to 20% of all circulating blood. Venous hemodynamics is determined by blood volume, central venous pressure (CVP), and the magnitude of venous return. In the venae cavae, pressure is minimal and can reach negative values (-3 to -5 mm Hg). These factors ultimately determine the subsequent stroke volume.