Circulatory Organization and Microcirculation
Terminologically, the circulatory system is often separated from the lymphatic system. The human circulatory system is closed and forms two main circuits of circulation:
- The pulmonary circuit is responsible for gas exchange: in lung capillaries, blood releases carbon dioxide and becomes oxygenated.
- The systemic circuit performs a trophic function, delivering nutrients and oxygen to tissues while removing metabolites.
The general sequence of blood flow is as follows: blood is directed from the heart through arteries to the microcirculatory bed. The classical pathway within the microcirculatory bed involves the transition from arterioles to capillaries, and then to venules. However, an alternative route also exists—the shunting of blood through arteriovenous anastomoses (AVAs) bypassing the capillary bed. The return of blood to the heart occurs via the venous system.
Hemodynamic Parameters: Pressure and Velocity
Blood in the body is divided into circulating and pooled volumes. At rest, 1.0 to 1.5 liters are excluded from active blood flow and are located in blood depots (spleen, liver, skin).
Blood movement is driven by a pressure gradient that gradually drops from the aorta to the venae cavae due to blood viscosity and friction against the vessel wall:
- In the aorta, pressure drops insignificantly (from 100 to 99 mm Hg).
- A sharp drop occurs in the arterioles (a decrease of more than 40 mm Hg).
- Capillaries show the most intensive pressure drop per unit length due to their narrow lumen.
- In the venae cavae and right atrium, pressure can become negative (down to –2 mm Hg). Venous return is ensured by the suction effect during atrial relaxation rather than residual kinetic energy.
When assessing blood flow, two types of velocity are distinguished:
- Linear velocity depends on the total cross-sectional area of the vessels. It is maximal in the aorta (45 cm/s) and minimal in capillaries (0.5 mm/s). In veins, it increases again as vessels merge.
- Volumetric flow rate remains constant at any level of the vascular tree and is about 5 L/min.
Temporal Hemodynamic Parameters
The transit time of blood through different compartments varies greatly. In the systemic circuit, blood spends about 30 seconds; in the pulmonary circuit, approximately 10 seconds. The average time for a complete cycle is 40 seconds, although some blood may pass faster through shunts (in 20–25 s), and some significantly slower.
The greatest blood delay is observed in the microcirculatory bed (about 16 s). The maximum residence time occurs in venules (about 9.5 s). The physiological significance of this phenomenon is that it is precisely within venules that conditions are created for leukocyte emigration into surrounding tissues.
Embryonic Vascular Development
The circulatory system develops from the mesenchyme. The process of primary angiogenesis begins in the wall of the yolk sac, then extends to the chorion, and only later into the body of the embryo itself.
Vascular formation passes through the stage of so-called "blood islands." Cellular clusters form within the mesenchyme and subsequently differentiate. The peripheral cells of the island flatten, giving rise to the endothelium (the primary vessel wall), while the central cells round up and become primary blood cells.
Isolated primary vessels gradually fuse to form a single closed network. Subsequent histogenesis of the vessel wall (the development of muscular and elastic layers) directly depends on local hemodynamic conditions. Pressure, pulsation, and flow velocity determine whether a specific vessel transforms into an artery, vein, or capillary.
Organization of the Lymphatic System
The lymphatic system permeates most body organs, except for the epithelium, cartilage, brain, red bone marrow, eyeball, and placenta.
Structurally, the bed is organized from the periphery to the center:
- Lymphatic capillaries begin with blind ends. Their main task is the filtration of tissue fluid and the removal of excess water, metabolites, and foreign particles.
- Lymph nodes interrupt the course of vessels. They contain lymphocytes that trap and neutralize antigens. In oncology, lymph nodes are of immense importance: if a node fails to destroy malignant cells, a secondary tumor focus (metastasis) forms within it.
- Large trunks and ducts collect lymph and return it to the venous bed. The largest—the thoracic duct (ductus thoracicus)—drains 75% of the lymph into the left venous angle. The right lymphatic duct collects the remainder and empties into the right venous angle.